HomeMy WebLinkAboutCity Council Packet 09-11-2007 SpecialAgenda
City of Plymouth
Special City Council Meeting
Tuesday, September 11, 2007
5:30 p.m.
Medicine Lake Room
1. Call to Order
2. Review draft non -degradation plan
3. Update with City Manager
4. Adjourn
j '< CITY C)F PLYMOUTH
r
CITY CQUNCIL AGENDA REPORT , `
DATE: September 5, 2007 for the City Council Study Session of September 11, 2007
TO: Laurie Ahrens, City Manager through
Doran Cote, P.E., Director of Public Works
FROM: Derek Asche, Water Resources Manager b A
SUBJECT: NON -DEGRADATION REPORT FOR
MUNICPAL SEPARATE STORM SEWER (MS4) GENERAL PERMIT
CITY PROJECT 7108E
BACKGROUND: Storm water discharging from the City of Plymouth is permitted through the
Mimlesota Pollution Control Agency by the .Municipal Separate Storm Sewer (MS4) General Permit
MNR040000). The purpose of the MS4 Permit is to establish conditions for discharging storm water
and other related discharges to waters of the State. As outlined in the permit, the City of Plymouth is a
selected" MS4 which requires a more intensive analysis of storm water discharges. Selected MS4s
must submit proposed' changes to our Storm Water Pollution Prevention Plan (SWPPP) including a
Loading Assessment and Non -degradation Report by October 2007. Specifically, the Loading
Assessment requires a water quality analysis to assess the change in storm water discharge loading for
our permitted area by modeling changes in average annual flow volume, total suspended solids, and
phosphorous from 1988 to present and present to 2020. Selected MS4s that have significant new or
expanded discharges. are required to complete a Nondegradation Report.
The City has conducted its Load Assessment and determined storm water discharges have expanded
from 1988 to the present for average annual flow volume (54%) and phosphorous (12%). Total
suspended solid discharges have decreased by 20%, likely due to a significant increase in water quality
ponding since 1988. Based on the Load Assessment results, the City is required to complete a
Nondegradation Report which will incorporate best management practices (BMPs) to mitigate the
effects of increased average armual flow volume and phosphorous including BMPs to mitigate the
effects of new and re -development. Additionally, BMPs are required to address the.negative impacts
of increased storm water discharge volumes having the potential to affect designated uses of wetlands
and streams.
Historically, BMPs such as ponding, rain gardens, and wetland buffers have been required to eliminate
increased rate, total suspended solid, and phosphorous contributions from development or re-
development projects at the time of plan submittal. The conclusions of the loading assessment indicate
our current BMP policies together with planned capital improvement projects will likely meet
phosphorous .noir-degradation requirements, however, water volume discharges are still expected to
expand. Therefore, additional BMPs to control water volume are required to be stated in the Non -
degradation Report. Additional BMPs, such as soil ripping, soil amendments, or disconnection of
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impervious surfaces to the drainage system would be expected to impact development or re-
development proposals including the cost to develop land in the City of Plymouth.
The Load Assessment Report and Non -degradation Report are requirements of the Minnesota Pollution
Control Agency and must be completed before the City can finish the Surface Water Management
section of the Comprehensive Plan. The Load Assessment Report.has been approved by Council and a
final version of the Non -Degradation Report is expected to be presented for City Council approval on
September 25, -2007. The final Non -degradation Report will include specific recommendations for
BMPs to address water volume and those recommendations will be incorporated into the Surface
Water Management Plan. By proposing BMPs that will reduce water volume and phosphorous loading
to 1988 levels, the City will be in compliance with the requirements of the Minnesota Pollution
Control Agency.
Attachments: Agenda
Acceptable Best Management Practices
DRAFT Capital Improvement Projects 2008-2012
DRAFT Non -degradation Report
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City Council Study Session
September 11, 2007
6 pm
Non -Degradation Report
City Project 7108E
1. Overview
a. Municipal Separate Storm Sewer (MS4) permit
b. Requirements of non -degradation cities
2. Non -degradation report
a. Current requirements for development/re-development
i. Water Quantity
1. NURP Ponds °
ii. Water Quality
1. Additional BMPs
iii. CIP projects
b. Possible new requirements for development/re-development
i. Water Quantity
ii. Water Quality
c. Other items?
0:\Engineering\PROJECTS\2000 - 2009\7108E\Memos\City Council Study Session agenda.doc
City of Plymouth
Acceptable Best Management Practices (BMPs)
To improve the quality of surface water runoff in the City of Plymouth, acceptable BMPs
have been listed below. Choose at least one BMP from each category below for inclusion
in your project:
Reduction of Discharge/Runoff Reduction/Volume Control
Reduce impervious area
Porous pavement
Rain garden
Compost amendment to soil
Other
Stormwater Treatment
NURP Pond
Ribbon Curb
Restore capacity to existing treatment systems
Other
Floatable/Oil Removal
Skimmers
Hydrocarbon absorbing pillows
Other
Sediment Control
Rip Rap
Inlet protection
Slope stabilization
Vegetated swale
Other
Nonstructural BMPs
Street Sweeping
Reinforced Turf
Narrow streets
Green Roof
Other
The City reserves the right to require additional BMPs where necessary. For questions
regarding BMP implementation, contact Derek Asche, Water Resources Technician at
763-509-5526.
O:\Lngineering\WTRRCS RCWccptable_BMPs. doc
City of Plymouth, Minnesota
2010
Maintain Water Quality Ponds
Capital Improvenlent P167/1
06 -WR -002
f
90,000
2008 thru 2012
Water Quality & Drainage
a 1
n/a 25,600
Stream Bank Repairs: Plymouth Creek Water Quality & Drainage 07 -WR -001 n/a 40,000
PROJECTS YYEARR
1 1
07 -WR -004
Project Name Department Project # Priority Project Cost
n/a 250,000
Bass Lake Outlet Water Quality & Drainage 08 -WR -005
2008
200,000
Total for 2010
Erosion Repair - Wood Creek @ E. Medicine Lake Rd Water Quality & Drainage 06 -WR -001 n/a 140,000
Maintain Water Quality Ponds . Water Quality & Drainage 06 -WR -002 n/a 30,000
Hedberg Aggregate/Nathan Lane Drainage Site Water Quality & Drainage 06 -WR -003 n/a 45,000
Plymouth Creek Water Quality Pond @ West Med Park Water Quality & Drainage 06 -WR -005 n/a 925,000
County Road 61/County Road 9 Erosion Site Water Quality & Drainage 06 -WR -006 n/a 75,000
Stream Bank Repairs: Plymouth Creek Water Quality & Drainage 07 -WR -001 n/a 50,000
TH 55 Drainage Channel and Wetland Cleaning Water Quality & Drainage 07 -WR -002 n/a 440,000
Wild Wings Development Wetlands Water Quality & Drainage 07 -WR -003 n/a 175,000
Storm Sewer System Repairs Water Quality & Drainage 07 -WR -004 n/a 0
Timber Creek Improvements Water Quality & Drainage 08 -WR -002 n/a 350,000
2008 Drainage Improvements Water Quality & Drainage 08 -WR -007 n/a 200,000
Total for 2008 2,430,000
2009
Maintain Water Quality Ponds Water Quality & Drainage 06 -WR -002 n/a 60,000
Plymouth Creek Water Quality Pond @ West Med Park Water Quality & Drainage 06 -WR -005 n/a 36,300
Stream Bank Repairs: Plymouth Creek Water Quality & Drainage 07 -WR -001 n/a 440,000
Storm Sewer System Repairs Water Quality & Drainage 07 -WR -004 n/a 0
Unspecified Drainage Improvements Water Quality & Drainage 08 -WR -003 n/a 250,000
Ranchview/Medina Road Wetland Water Quality & Drainage 08 -WR -004 n/a 125,000
Bass Lake Outlet Water Quality & Drainage 08 -WR -005 n/a 35,000
Total for 2009 946,300
2010
Maintain Water Quality Ponds Water Quality & Drainage 06 -WR -002 n/a 90,000
Plymouth Creek Water Quality Pond @ West Med Park Water Quality & Drainage 06 -WR -005 n/a 25,600
Stream Bank Repairs: Plymouth Creek Water Quality & Drainage 07 -WR -001 n/a 40,000
Storm Sewer System Repairs Water Quality & Drainage 07 -WR -004 n/a 0
Unspecified Drainage Improvements Water Quality & Drainage 08 -WR -003 n/a 250,000
Bass Lake Outlet Water Quality & Drainage 08 -WR -005 n/a 200,000
Total for 2010 605,600
2011
Maintain Water Quality Ponds Water Quality & Drainage 06 -WR -002 n/a 120,000
Erosion Repair - Conor Meadows Water Quality & Drainage 06 -WR -004 n/a 250,000
Plymouth Creek Water Quality Pond @ West Med Park Water Quality & Drainage 06 -WR -005 n/a 13,100
Stream Bank Repairs: Plymouth Creek Water Quality & Drainage 07 -WR -001 n/a 340,000
Storm Sewer System Repairs Water Quality & Drainage 07 -WR -004 n/a 0
Unspecified Drainage Improvements Water Quality & Drainage 08 -WR -003 n/a 250,000
Turtle Lake Outlet Water Quality & Drainage 08 -WR -006 n/a 35,000
45th Avenue and Nathan SE Corner Water Quality & Drainage 10 -WR -001 n/a 50,000
Total for 2011 1,058,100
Prodziceci Using the Plan -11 Capital PlanningSoftiicu•e Page 1 DI'ecinesdap, June 27, 200.7
Project Name Department Project # Priority Project.Cost
2012
Maintain Water Quality Ponds Water Quality & Drainage 06 -WR -002 nla 150,000
Stream Bank Repairs: Plymouth Creek Wator Quality & Drainage 07 -WR -001 nla 300,000
Unspecified Drainage Improvements Water Quality & Drainage 08 -WR -003 nla 250,000
45th Avenue and Nathan SE Corner Water Quality & Drainage 10 -WR -001 nla 300,000
Total 1'm• 2012 1,000,000
GRAND TOTAL 6,040,000
Report criteria:
All Project Types
Project Status: Active
Department: Water Quality & Drainage
All Categories
All Priority Levels
All Source Types
All Project # data
All data
All data
All data
Prodked Using the Plan -11 Capilal Planning Soflirarc Page 2 1,Yednesday, June 27, 2007
Nondegradation Report Submittal to the
Minnesota Pollution Control Agency for
Selected MS4 Permit Requirements
Prepared for
City of Plymouth
Submitted by
Barr Engineering Company
August 2007
Executive Surma
The Minnesota Pollution Control Agency (MPCA) revised the General National Pollutant Discharge
Elimination System/State Disposal System (NPDES/SDS) Pen -nit MNR040000 (Permit) for the city
of Plymouth to Discharge Storm Water Associated with Municipal Separate Storm Sewer Systems
MS4), effective June 1, 2006. Plymouth had previously completed a Stonn Water Pollution
Prevention Program (SWPPP) to address the six minimum control measures required by the previous
permit. This report has been developed to address modifications to the SWPPP for measures that
may be necessary to meet the new, applicable requirements of Appendices C and D in the re -issued
permit. Appendix C covers discharges to wetlands that are applicable to the city of Plymouth.
Appendix D covers the nondegradation requirements for Selected MS4s (30 pennittees including the
city of Plymouth), including the development of a loading assessment and nondegradation report.
For the loading assessment, the Simple Method was used to deterinine the pollutant loadings and
runoff volumes from each of the land uses within each watershed and the P8 Model was used to
account for the effects of Best Management Practice (BMP) implementation for the time periods of
interest in the Permit conditions. The loading assessment modeling results were summarized for
each of the city's four major watersheds to show the Simple Method loading and volume estimates
for each time period, as well as the total phosphorus (TP) and total suspended solids (TSS) loading
and volume estimates after applying the P8 model design criteria for BMP implementation, based on
the ordinances and design standards that were in place when the various developments occurred.
The results show that, without BMPs, the total average annual flow volume from the city has
increased significantly since 1988 and would continue to increase substantially by 2020, without
implementation of infiltration practices. Current and future implementation of BMPs will provide
adequate treatment for TP and TSS in runoff to the city's receiving waters.
The loading assessment was completed assuming that future BMP implementation would follow the
city's current policies and standards. The results of the loading assessment provide conservatively
low estimates for water quantity and quality improvement associated with BMP implementation
because it assumes the minimal NURP Bond BMP design requirements. Since 2002, the city has
required volume control and additional storm water treatment beyond the NURP design
requirements, typically in the form of rain water gardens (or bioretention basins), reduced impervious
areas, porous pavement, or compost amendment to soil. In northwest Plymouth, it is the city's policy
that the Elm Creek stream flows will be limited to pre -development in -stream rates. In the future, the
Draft Plymouth Nondegradalion Reporl.doc
city intends to implement infiltration practices to the maximum extent practicable to mitigate the
volume and loading increases, from the loadings assessment estimates, as much as .possible. As a
result, the city will update its development review policies, standards and procedures, as cited in the
SWPPP. This approach will ensure the following:
Receiving water quality should be improved for lakes, wetlands and streams in Plymouth
Channel erosion and stream morphology changes will be controlled
Further protection will be provided for the physical and biological integrity of the stream and
wetland corridors
Controlled bounce and duration of inundation in the city's wetlands and preservation of the
functions and values for each type of wetland classification
Wherever practicable, the infiltration requirements will be applied to redevelopment projects,
that are greater than 2.5 acres or will add 1 or more acres. of additional impervious surface, to
mitigate past increases in storm water runoff volume and further improve receiving water
quality and habitat
A review of the Hennepin County Soil Survey shows that between 36 and 49 percent of the land area
within the Plymouth watersheds may not have suitable soils for infiltration practices. In addition,
infiltration practices may be further restricted by proximity to drinking water wells, the seasonal high
groundwater level and runoff from stone water hotspots, such as loading docks, fueling and vehicle
maintenance areas. As a result, future implementation of infiltration practices will need to include
flexibility for new developments and redevelopments that have site constraints that would otherwise
limit the BMP feasibility or cost-effectiveness and/or lose excessive amounts of useable space due to
larger infiltration storage volume requirements.
Draft Plymouth Nondegradation Report.doc ii
City of Plymouth Nondegradation Report Submittal to the Minnesota
Pollution Control Agency for Selected MS4 Permit Requirements
Table of Contents
ExecutiveSummary....................................................................................................................................... i
1.0 Introduction 6
1.1 MS4 Pen -nit Requirements..........................................................................................................6
2.2 Watershed Imperviousness Detennination...............................................................................23
1.1.1 Loading Assessment......................................................................................................... 6
1.1.2 Nondegradation Report....................................................................................I............. 6
1.1.3 Proposed SWPPP Modifications and Submittals to MPCA.......................................... 7
1.1.4 Discharges to Wetlands.................................................................................................. 8
1.1.5 Discharges Affecting Source Water Protection Areas ................................................... 8
1.2 Discussion of MPCA Guidance.................................................................................................. 8
1.2.1 Responses to Comments............................................................ :................................... 8
2.4 Results and Discussion..............................................................................................................39
1.2.1.1 Modeling Approach and Complexity............................................................. 9
41
1.2.1:2 Average Annual Flow Volume.................................................................... 10
2.4.3 Total Suspended Solids................................................................................................45
1.2.1.3 Wetlands.......................................................................................................14
3.0 Nondegradation Report........................................................................................................................47
1.2.1.4 Special Waters Considerations..................................................................... 1.5
1.2.2 Guidance Manual for MS4s......................................................................................... 15
1.2.2.1 Loading Assessment..................................................................................... 16
1.2.2.2 Nondegradation Report ............................................................................... 18
1.3 Plymouth Storm Water Management Planning and Water Quality Improvement Projects...... 19
2.0 Loading Assessment............................................................................................................................ 21
2.1 Land Use/Land Cover Compilation.......................................................................................... 21
2.2 Watershed Imperviousness Detennination...............................................................................23
2.2.1 1988 Imperviousness Determinations...................................................................:......23
2.2.2 2007 Imperviousness Detenninations.......................................................................... 23
2.2.3 2020 Imperviousness Detenninations.......................................................................... 27
2.2.4 Surnmary of Land Use/Land Cover by Watershed...................................................... 27
2.3 Modeling Approach and Methodology for Loading Estimates.................................................28
2.3.1 Average Annual Flow Volume.................................................................................... 35
2.3.2 Total Phosphorus......................................................................................................... 36
2.3.3 Total Suspended Solids................................................................................................ 37
2.3.4 BMP Implementation Modeling ............................... :.................................................. 38
2.4 Results and Discussion..............................................................................................................39
2.4.1 Average Annual Flow Volume.................................................................................... 41
2.4.2 Total Phosphorus......................................................................................................... 44
2.4.3 Total Suspended Solids................................................................................................45
3.0 Nondegradation Report........................................................................................................................47
Draft Plymouth Nondegradation Report.doe iii
3.1 Future Conditions Loading Assessment...................................................................................47
3.1.1 Other BMPs and Considerations Not Included in the Loading Assessment ................ 47
3.1.2 Implications of Impaired Waters for Addressing Loading Assessment ...............:....... 49
3.2 BMP Selection Considerations for New Development............................................................. 50
3.2.1 Receiving Water Quality .............................................................................................. 50
3.2.2 Stream Morphology/Channel Erosion......................................................................... 51
3.2.3 Wetlands....................................................................................................................... 52
3.2.4 Source Water Protection Areas.................................................................................... 54
3.3 Retrofit and Mitigation BMP Options.........................................................:............................ 54
3.4 Cost/Benefit, Social and Environmental Considerations.......................................................... 55
4.0 Proposed SWPPP Modifications..........................................................................................................56
5.0 Comments on Proposed SWPPP Modifications...............................................................:.................. 58
5.1 Public and Local Water Authority Comments on Proposed SWPPP Modifications................58
5.1.1 Public Comments on Proposed SWPPP Modifications ................................................ 58
5.1.2 Local Water Authority Comments on Proposed SWPPP Modifications ..................... 58
5.1.2.1 Bassett Creek Water. Management Commission (BCWMC) ....................... 58
5.1.2.2 Elm Creek Watershed Management Commission (ECWMC)..................... 58
5.1.2.3 Minnehaha Creek Watershed District(MCWD).......................................... 58
5.1.2.4 Shingle Creek Watershed Management Commission (SCWMC)................ 58
5.2 Record of Decision on the Comments...................................................................................... 58
References...................................................................................................................................:............... 59
List of Tables
Table 2-1 Land Use and Land Cover (LULL) Classes.................................................................... 21
Table 2-2 Plymouth Land Use/Land Cover (LULC) for 1988, 2007 and 2020 .............................. 22
Table 2-3 Average Imperviousness by Land Use Type for Plymouth based on 2000 Landuse and
ImperviousnessData....................................................................................................... 27
Table 2-4 Plymouth Land Use/Land Cover (LULC) by Watershed for 1988, 2007 and 2020 ......... 29
Table 2-5 Comparison of Modeling Attributes/Capabilities by Selection Criteria .......................... 32
Table 2-6 Plymouth Nondegradation Loading Assessment Summary ............................................. 42
List of Figures
Figure 2-1 Plymouth Land Use/Land Cover (LULC) for 1988 ......................................................... 24
Figure 2-2 Plymouth Land Use/Land Cover (LULC) for 2006 ......................................................... 25
Figure 2-3 Plymouth Land Use/Land Cover (LULC) for 2020 ......................................................... 26
Draft Plymouth Nondegradation ReporLdoc iv
Figure 2-4 Map of Impaired Waters, BMP Treatment Areas and Watershed Discharge Nodes ........ 40
Figure 2-5 Plymouth Loading Assessment—City-Wide Average Annual Flow Volume.................. 43
Figure 2-6 Plymouth Loading Assessment—Average Annual Flow Volume by Watershed w/ BMP
Implementation................................................................................................................ 43
Figure 2-7 Plymouth Loading Assessment—City-Wide Total Phosphorus Loading ......................... 45
Figure 2-8 Plymouth Loading Assessment—City-Wide Total Suspended Solids Loading............... 46
List of Appendices
ti
Appendix A Lake Water Quality Trend Analyses
Draft Plymouth Nondegradation Report.doc v
1.0 introduction
1.1 MS4 Permit Requirements
The Minnesota Pollution Control Agency (MPGA) revised the General NPDES/SDS Permit'
NINR040000 (Pen -nit) for the city of Plymouth to Discharge Storm Water Associated with Municipal
Separate Storm Sewer Systems (MS4), effective June 1, 2006. Plymouth had previously completed a
Storm Water Pollution Prevention Program (SWPPP) to address the six minimum control measures
required by the previous permit. This report has been developed.to address modifications to the
SWPPP for measures that may be necessary to meet the new, applicable requirements of Appendices
C and D in the re -issued permit. Appendix C covers discharges to wetlands that are applicable to the
city of Plymouth. Appendix D covers the nondegradation requirements for Selected MS4s (30
pennittees including the city of Plymouth), including the development of a loading assessment and
nondegradation report. The following sections describe the sections of the perrnit that are relevant
for the city of Plymouth.
1.1.1 Loading Assessment
Each Selected MS4 must assess the change in storm water discharge loading for its permitted area
using a pollutant loading water quality model that, at minimum, addresses changes in average annual
flow volume, total suspended solids (TSS), and phosphorus (TP). This.modeling should be based on
two time periods: from 1988 to the present, and from the present to 2020. The Selected MS4s must
use a simple model, or another more complex model that they find to be more appropriate, that
addresses the parameters of concern. This may include a model that the Selected MS4 has already
used. Other assessment methods may be used if they can be shown to be as effective at quantifying
the increase in loading as the modeling methods. The models and/or other methods will be used as
part of the assessment to develop the Nondegradation Report, to help in selecting appropriate best
management practices (BMPs) that address nondegradation, to determine whether, additional control
pleasures can reasonably be taken to reduce pollutant loading.
1.1.2 Nondegradation Report
Selected MS4s that have significant new or expanded discharges are required to complete a
Nondegradation Report and, upon approval, to incorporate its findings on BMPs that address
nondegradation into their SWPPP. The BMPs should address changes in pollutant loadings as far as
is reasonable and practical through future development. Additionally, the BMPs shall address, as far
as is reasonable and practical, the negative impacts of increased stonn water discharge volumes that
Drart Plymouth Nondegradation Report.doc 6
cause increased depth and duration of inundation of wetlands having the potential for a significant
adverse impact to a designated use of the wetland, or changes in stream morphology that have the
potential for a significant adverse impact to a designated use of the streams.
The Nondegradation Report must include consideration of the Loading Assessment, which must
include analysis of flow and may include removal of pollutants by BMPs already initiated. For
purposes of the permit, 1988 levels consistently attained means runoff that would have been
produced under approximately average conditions of rainfall. Local storm water management plans
and other pertinent factors may also be considered. BMPs implemented by other parties may be
considered when those BMPs affect the storm water from the area of the Selected MS4. If the
pollutant loadings cannot be reduced to levels consistently attained in 1988, the Nondegradation
Report must describe reasonable and practical BMPs that the Selected MS4 plans to incorporate into
a modified SWPPP. The Selected MS4 must consider alternatives, explain which alternatives have
been studied but rejected and why, and propose alternatives that are reasonable and practical. The
Nondegradation Report must give high priority to BMPs that address impacts of future growth, such
as ordinances for new development. Where increases in pollutant loading have already occurred due
to past development, the Nondegradation Report must consider retrofit and mitigation options
BMPs) that the Selected MS4 detennines to be reasonable, practical and appropriate for the
community. The Selected MS4 is responsible for developing any site specific cost/benefit, social, and
environmental information that the Selected MS4 wishes to bring to the Agency's attention. The
Selected MS4 must incorporate the BMPs into a modified SWPPP and include an implementation
schedule that addresses new development and retrofit BMPs it proposes to implement.
1.1.3 Proposed SWPPP Modifications and Submittals to MPCA
Prior to submittal to the MPGA, the proposed SWPPP modifications to address nondegradation will
be public noticed at the local level. Each Selected MS4 shall also submit its SWPPP modifications to
address nondegradation to the appropriate local water authority (e.g. watershed organizations or
county water planning authority) in time to allow for their review and comment. The Nondegradation
Report explaining the proposed BMPs and the entire SWPPP must be made available to the public
and local water authority upon request.
Selected MS4s must submit their proposed changes to the SWPPP, reports addressing nondegradation
for all waters, together with other supporting documents; to the MPGA in accordance with the
schedule in Appendix E of the pen -nit. This submittal must include:
Draft Plymouth Nondegradation Report.doc
1. The Loading Assessment;
2. The Nondegradation Report;
3. The proposed SWPPP modifications to address nondegradation;
4. The public and local water authority comments on the proposed SWPPP modifications to
address nondegradation, with a Record of Decision on the comments; and
5. An application to modify the permit.
1.1.4 Discharges to Wetlands
The permit does not authorize physical alterations to wetlands, or other discharge adversely affecting
wetlands, if the alteration will have a significant adverse impact to the designated uses of a wetland.
Any physical alterations to wetlands that will cause a potential for a significant adverse impact to a
designated use must be 'implemented in accordance with the avoidance, minimization and mitigation
requirements of Minn. R. 7050.0186 and other applicable rules.
1.1.5 Discharges Affecting Source Water Protection Areas
BMPs shall be incorporated into the SWPPP to protect any of the following drinking water sources
that the MS4 discharge may affect, and a snap of these sources shall be included with the SWPPP, if
they have been mapped:
1. Wells and source waters for drinking water supply management areas identified as vulnerable
under Minn. R. 4720.5205, 4-720.5210, and 4720.5330, and
2. Source water protection areas for surface intakes identified in the source water assessments
conducted by or for the Minnesota Department of Health under the federal Safe Drinking
Water Act.
1.2 Discussion of MPCA Guidance
1.2.1 Responses to Comments
Following the close of the comment period on the draft permit, the MPCA issued responses to
comments received through April 15, 2005 on the Pennit. To provide further guidance on
Draft Plymouth Nondegradation Report.doc 8
i
compliance with the Permit requirements, this section describes responses to comments that pertain
to the following subjects:
Loading Assessment modeling approach and complexity
Addressing volume as a parameter of concern for the Loading Assessment and
Nondegradation Report
Nondegradation requirements for Wetlands
Nondegradation requirements for Special Waters
1.2.1.1 Modeling Approach and Complexity
In response to several comments regarding the modeling approach and complexity required for the
Loading Assessment described in the Pennit, the MPCA stated that the Loading Assessment should
include changes to pollutant loadings associated with changes due to past land use changes and
changes due to anticipated land use changes. The Loading Assessment is intended to be used as a
planning tool to compare 1988 levels to present and 2020 levels of discharge. It is to be presented as
comparative results (increase), not absolute (accurate) flow, total suspended solids (TSS), and phosphorus
discharge levels from the MS4. It is acceptable for MS4s to do more extensive modeling for design of
BMPs; but it should be explained.
The Permit does not, however, specifically require that BMPs be factored into the Loading
Assessment, but the MPCA clearly states that BMP analysis could be provided if any Selected MS4
so desires. The assessment can include changes due to BMPs that have already been implemented, if
increase in the loading since 1988 is explicitly stated, as well as changes due to BMPs that are
planned to be implemented and written into the MS4's ordinances or other regulatory mechanisms.
MPCA further states that the Loading Assessment was developed after considerable discussion,
including discussion with consultants, cities, and the League of Minnesota Cities. It was determined
that to limit costs the nature of the assessment must be limited. The MPCA chose not to include
treatment options in this requirement since the level of modeling must be significantly increased to
model treatment. Many communities will not be conducting other modeling, therefore. this
requirement will be a cost that needs careful distinction between what is desirable and what is
required. The MPCA chose a level that will prevent undue burden while still developing useful
information.
Draft Plymouth Nondegradation Report.doc
The Loading Assessment is comparable to an influent analysis, while the Nondegradation Report
addresses the actual discharges of storm water to receiving water. The permittees are allowed to show
reduction in discharge or to make other arguments they believe are appropriate in the development of
the Nondegradation Report. A detailed Loading Assessment can support the Nondegradation Report.
Under the provisions of Minn: R. 7050.0185, subp. 4, the MPGA must "determine whether additional
control measures beyond those required by subpart 3 can reasonably be taken to minimize the impact
of the discharge on the receiving water. " The MPCA does not have absolute numeric or other criteria
that it will use in snaking this determination for each of the Selected MS4s. The criterion of
reasonableness" requires flexibility and site specific determinations. Reasonableness determinations
will therefore be made on a case-by-case basis. Site specific variations in situation, funding,
population, and receiving water will be as critical to the determination of reasonableness as a specific
increase in loading. Additionally, the MPCA must note that the required analysis and documentation
for the Nondegradation Plans are relative, not absolute, in nature. For example, the Loading
Assessments required by the permit are net changes; we do not request the actual pollutant loading,
just estimates of the relative quantity of the change.
1.2.1.2 Average Annual Flow Volume
In response to several comments regarding the requirement for addressing volume as a parameter of
concern for the Loading Assessment and Nondegradation Report described in the Permit, the MPCA
stated that permit and guidance were revised to include more specifics on how flow volume will be
addressed in BMPs and the Nondegradation Report. The responses were qualified by first stating that
when an MS4 develops a Nondegradation Report, site specific objections, costs and other considerations
can be raised, which the MPCA must consider in its determinations. Reasonable measures, not any and all
measures, must be installed. For this permit, the reasonableness of volume control policy is not general
and applicable for all MS4s, but is determined on an individual, site specific basis. In some situations the
problems created by increased flow volume can be reduced and minimized by effective implementation of
appropriate BMPs based on site specific conditions.
The MPCA asserts that based on the following statutory definition (Minn. Stat. § 115.01 Definitions
Subd. 13. Pollution of water, waterpollution, pollute the water.) and actual environmental impacts,
volume may qualify as water pollution under many specific conditions:
Pollution of water, " "water pollution, " or 'pollute the water "means: (a) the discharge of any
pollutant into any waters of the state or the contamination of any waters of the state so as to create a
nuisance or render such waters unclean, or noxious, or impure so as to be actually or potentially
Draft Plymouth Nondegradation Reporrt.doc 10
harmful or detrimental or injurious to public health, safety or welfare, to domestic, agricultural,
commercial, industrial, recreational or other legitimate uses, or to livestock, animals, birds, fish or
other aquatic life; or (b) the alteration made or induced by, human activity of the chemical, physical,
biological, or radiological integrity of ivaters of the state.
MPCA staff looked at the rules that are applicable to nondegradation (Minn. R. 7050.0185) and
studied the concept of increased loading of one or snore pollutants as used in the rule. They
determined that the rule directs the MPCA to consider the adverse effects of increased flow volume,
and where effects are adverse, to consider flow volume as a pollutant. It is not volume per se that was
asked to be addressed but the -change in volume related to MS4 development. Additionally, it is well
known that increases in flow can have a variety of negative environmental impacts. A discussion of
the reasoning for the inclusion of volume of storm water as a pollutant was provided in excerpts from
Chapter I 1 of the Minnesota 2001-2005 Nonpoint Source Management Program Plan. These excerpts
are summarized below:
Hydromodification, which involves changes in flow patterns in natural waterways such as
rivers or streams and wetlands, is the second leading cause of impairment of fresh waters.
Removal of perennial vegetation led to a decrease in infiltration and an increase in the
volume of runoff. Exposing soils to wind and water increased sediment loads carried by
runoff. Impervious surfaces and artificial drainage systems increased the volume of runoff
and accelerated the rate at which water was removed from the landscape. Impervious surfaces
in urban areas also transported runoff more rapidly and in greater volumes than before
development.
Minn. Stat. § 155.0 1, subd. 13 (b) defines pollution of waters as "the alteration made or
induced by human activity of the chemical, physical, biological, or radiological integrity of
waters of the state". The basis for this statute is that human activity, such as
hydromodification, affects these waters in many adverse ways. Under natural conditions and
at bank -full capacity, studies have shown that streams can handle a flow approximately equal
to the 1.5- to 2 -year frequency peak discharge within their banks (Rosgen, 1994; Leopold el
al., 1964). After urbanization, increased runoff can cause bank -full flow to be exceeded
several times each year. In addition to increased flooding, this condition causes previously
stable channels to erode and widen. Much of the eroded material becomes bed load and can
smother bottomdwelling organisms.
Draft Plymouth Nondegradation Rehort.doc I I
In this process, stream habitat diversity is damaged or lost. Water that was once slowed by
bends, pools, and woody debris in the water column moves faster and with greater volume
cutting into the bed and eroding the banks. This faster flowing water carries with it an
increased sediment load, some of which is deposited in the downstream reaches. Many fish
and invertebrate species cannot use substrates that are laden with excessive silt for
reproduction, feeding, or cover. Riffles and pools become scarce or absent as the stream is
converted from riffle, run, pool sequences to long runs or pipes. Not only is habitat diversity
affected but the stream hydrology becomes inherently less stable. As water leaves the system
faster, the natural hydrologic timing is altered. The overall effect is an increase in the
intensity of the high flows and decreased duration of low flow events. If the water is stored to
prevent increased peak flows, then the flow duration is extended. Streams in which the
surrounding vegetation has been removed or altered are usually compromised by an increase
in the amount of silt -laden runoff. Also, water temperatures within the stream may rise as the
overhead canopy is removed exposing the stream to full sunlight.
Urbanization also changes the extent and duration of inundation in wetlands, which can
modify the established wetland vegetation. Measures to control discharges to wetlands must
control the peaks and volume of flow to wetlands, if they are to be protected. This also means
that reduced surface and ground water flow caused by diversion to storm sewers is also an
area of concern, especially for sensitive wetlands.
Urbanizing areas increase runoff from small events in greater proportion than large events.
This is important because, in Minnesota, more than 90% of the precipitation events are less
than 1.0 inch. These rainfall events also account for approximately 65% of the cumulative
runoff quantity in urban areas and proportionately large amounts of the pollutant loading
associated with these rainfall events (Pitt, 1998). While the significance of large flood events
should not be underestimated, the smaller flows with an approximately nine month to two-
year return period frequency, are probably as important or more important to overall water
quality. These flows can be very erosive and can be the major source of increased pollutant
loading. Pollutant loading is more closely associated with total runoff volume than with peak
runoff rates. Utilizing methods to maintain volumes and peaks closer to those that originally
shaped the channel can reduce the channel reshaping process in a watershed. Examples of
appropriate management techniques are the volume reduction that results from the use of
swales instead of curb and gutter, reduced impervious surfaces or infiltration structures.
Wetland and upland vegetation can affect or be significantly affected by hydrologic changes.
Draft Plymouth Nondegradation Report.doc 12
For example, drainage can obviously change the vegetation at a site, but increased water that
drains from a project area into an off-site drainage basin can impact trees and other
vegetation, including wetland vegetation. In such cases, water itself is the damaging agent
even if it is clean. The increase in water level, both surface and subsurface, can result in the
death of roots. Roots require oxygen from the air, and saturated soils create an anaerobic
condition that will eventually kill the roots. A case in point is a tamarack swamp that receives
water from several developments. As water levels increase through the swamp, the increased
flow depth results in the death of many of the tamarack trees, even though they are tolerant of
wet conditions. In Minnesota, we have several tree species that tolerate short periods of
flooding, but we should be encouraging diversity and be inindful of sensitive areas
downstream. Likewise vegetation in upland areas can change the infiltration capacity or
evapotranspiration capacity of a watershed. By using native plantings that have denser
canopies and/or deeper root networks the storage capacity of the upland areas are
significantly increased reducing run-off volumes, especially in the smaller storms.
Addressing average annual flow volume in the nondegradation plan may show that the modeling
effort indicates a significant increase in flow from 1988. This is an indication to the MPCA that your
loading of one or more pollutants has increased, and the plan will need to address what is reasonable
and practical to get the flow back to 1988 levels. Alternatively, you may wish to demonstrate that
your flow increase has not resulted in water quality degradation and therefore does not need to be
addressed.. The MPCA has found flow volume to be related to significant degradation, therefore
claims to the contrary will be carefully scrutinized. To address flow volume some of the options
include consideration of BMPs for flows existing before 1988, BMPs for flows developed since
1988, and limitations on future flows. The MPCA notes that the 1.0 inch event is about the 90ti,
percentile event for 24 hour storm on an average annual basis, and that this represents 67% of the
cumulative volume of precipitation. This means that runoff reduction often can be related to BMPs
that reduce flow from events smaller than 1.0 inches in depth. If properly' designed the BMPs could
also treat some percentage of flow related to larger events without loss of effectiveness for reasons
such as re -suspension. Depending on development patterns, zoning, soils, water table, and other
factors, many communities may be able to meet the non degradation goal of returning the flow to
pre -1988 levels. Treatment BMPs that reduce flow include infiltration basins, trenches, bio -retention,
enhanced swales, evapo-transpiration, disconnection of impervious surfaces, reduced
imperviousness,'filterstrips, and variations and combinations of these and other BMPs.
Draft Plymouth Nondegradation Rcport.doc 13
In some instances, a community may not be able to reduce the flows to 1988 levels. If so, the basis
for this conclusion should be explained. For example the current problems may be related to past
development patterns, past or present zoning, soils, water table, and other factors that may be
pertinent. In establishing the case, any cost information that is available, especially site specific
information, should be provided. The MPCA must consider the potential impact of the discharge on
the receiving water and cumulative impacts of multiple discharges. While MS4s are not required to
develop information on this aspect of the analysis, they may find it beneficial to supply information
that supports their position.
1.2.1.3 Wetlands
In response to several comments and questions regarding the designated uses and nondegradation
requirements for wetlands in the Permit, the MPCA clarified that the terns "designated uses" of the
permit relate to MPCA rules and requirements and are set by MPCA through notice and comment
rulemaking under state law and any changes to designated uses would have to be made through notice and
comment rulemaking. The MPCA has included, in guidance, the pertinent parts of those rules to help
describe the context of these terns. The permit and rules are under MPCA authority and the pennit
implements the rules.
Under this NPDES permit, the permittee is required to comply with conditions that are established to
protect the water quality standards of wetlands as listed in Minn. R. 7050. One of the purposes of the
NPDES pennit is to establish requirements or conditions that the permittee must operate under in order to
assure compliance with the water quality standards. While the WCA for LGUs does regulate the activities
that cause draining, filling and some excavation to certain wetlands, the WCA does allow for ten
categories of exemptions to these requirements, does not -have jurisdiction over all wetlands that are
considered waters of the state, and does allow the LGU to vary wetland sequencing requirements if a local
wetland plan is developed. The permittee must recognize the nondegradation standards for wetlands and
the required mitigation sequence of Minn. R. 7050.0186 to mitigate for degradation of wetlands, apply to
All wetlands that are considered waters of the state. The MPCA water quality standards provide more
comprehensive water quality protection for all wetlands in'Minnesota than is required of the LGU to
implement under WCA. Application of the WCA by the LGU will provide comparable wetland protection
to wetland impacts in many to most cases and the WCA determination would also satisfy the Minn. R.
7050.0186 determination. However, in the few projects where the requirements of the WCA are not as
comprehensive as MPCA water quality standards, then the requirements of the NPDES pennit will
require an LGU to make a detennination that will also satisfy Minn. R. 7050.0186. Considering those
Draft Plymouth Nondegradation Report.doc 14
exceptions, allowing the permittee to only reference the WCA requirements for wetland protection would
not be adequate to assure compliance with the NPDES permit for all cases.
The MPCA does not anticipate that it will review and snake a separate determination (a duplicate
effort) regarding the evaluation of the sequence mitigation requirements when that determination has
been conducted by the permittee. MPCA enforcement of the NPDES permit requirements of Minn. R.
7050.0186 regarding wetland impacts associated with a component of the storm water system should
only be necessary if the LGU does not apply the permit requirements to their determinations. A
separate determination by the pennittee under the NPDES requirements that a wetland alteration
activity satisfy Minn. R. 7050.0186 sequencing is only initiated when the WCA requirements exempt
or consider the wetland or the activity nonjurisdictional or if the local wetland plan designation of the
wetland does not require full sequence evaluation for impacts of a wetland alteration. It should be
noted the WCA also recognizes that there may be other agencies or programs that have regulatory
jurisdiction regarding wetland impacting activities. The WCA rules contained in Minn. R. 8420.0105,
item B state that WCA rule is in addition to other regulations including those of the United States
Army Corps of Engineers, United States Department of Agriculture, Minnesota state agencies,
watershed districts, and local governments. Also, specifically the WCA requires that the person
conducting an activity in a wetland under,an exemption ensure the activity is conducted in
compliance with all other applicable federal, state, and local requirements (see Minn. R. 8420.0115).
1.2.1.4 Special Waters Considerations
The evaluation for special waters is contained in Appendix C and the evaluation of other waters is
contained in Appendix D. The test for ORVWs is that feasible and prudent alternatives must be used.
The test for other waters is reasonable and practical BMPs to be implemented. These analyses have a
different criteria and standard of judgment with a long history of precedent that must be considered.
The exact format of the evaluation is not described, but this distinction should be kept in mind as
evaluations are planned; the MPCA will also address this in guidance.
1.2.2 Guidance Manual for MS4s
The purpose of this draft report.(MPCA, 2006) is to provide guidance for MS4s to comply with the
Permit requirements, including the nondegradation policy. Nondegradation is achieved if 1988 levels
of flow and pollutants can be maintained. If it is not feasible for a Selected MS4 to demonstrate that
it has achieved 1988 levels of flow and pollutants, the MPCA must find if additional measures
BMPs) are "reasonable and practical" (Minn. R. 7050.0185). These measures are in addition to the
minimum measures of the permit. The MPCA will review required submittals such as the loading
Draft Plymouth Nondegradation Report.doc 15
assessments, and other information such as water plans, population growth data and development plans to
detennine appropriate measures. During the review, the MPCA will consider what additional control
measures would be reasonable to reduce the impact on the receiving water in light of the relative
importance of the economic and social impacts. The objective is to allow the MPCA to make an
informed, public decision that reasonably balances additional BMP costs against the adverse impact on
the environment posed by the new or expanded discharge.
Under Minn. R. 7050.0185, the MPCA is free to consider whatever information is available while the
MS4 has the opportunity, albeit the burden, to demonstrate to the MPCA why expanded discharges are
necessary to accommodate important economic or social development and what treatment is reasonable
and practical. This burden is appropriately placed upon the MS4 since the discharger is in the position to
know the relative costs and benefits of the proposed actions. The MPCA must consider the economic and
social development of the community; this means the houses, jobs, taxes, recreational opportunities, and
other impacts on the public at large that will result from development. Therefore, the MS4 should point
out to the MPCA how and why the public has benefited from the development that created the new or
expanded significant discharge, and why the public costs associated with the proposed BMPs are
reasonable.
1.2.2.1 Loading Assessment
Loading Assessment modeling Must be conducted for the entire MS4, not for individual watersheds or
areas unless the MS4 will model these for their own interests. Some communities may wish to use models
that address peak flows, or site specific increased loading. While this snakes some sense in teens of
overall plan development, it is not required by the permit; it is an option that the MPCA encourages but
does not require. Modeling examples of methods that may be acceptable include but are not limited to the
following:
The Simple Method
PONDNET
SLAMM
P8 Urban Catchment Model
XP-SWMM
Draft Plymouth Nondegradation Report.doc 16
Modeling or assessment methods will be used to estimate increases in loading based on two time periods,
1988 to current development and current to projected (2020 or ultimate, whichever is first) development.
Modeling may also be used to help in the decision malting process of determining appropriate BMPs to
implement to bring those discharges back to 1988 levels, or maintaining those levels into the future if
they are not already exceeded. Use of the models in this manner is not required but is encouraged.
The MPCA expects that the model will produce relative values. For this effort, the MPCA is more
concerned with the average annual increases than about specific event increases. It is not as important for
this particular requirement of the permit to get the actual loads correct as it is to model consistently,
showing the relative change in loads rather than the actual loads. Also note, the permit does not require
the development of annual rainfall tables or calculation of hydrographs and/or store and release
calculation.
All models need to be adapted for use in the specific circumstances of each MS4. Gather available
information on land use/imperviousness and other pertinent facts from conditions that existed or will exist
from 1988 to 2020. Selection of the appropriate method is often dependant on the readily available or
collectable data as well as on the outputs or results required. Since the MPCA's goal is to show relative
increases or decreases in loading, a simple method can be used rather than a more complex model. MS4-s
may still want to use models that are more complex for your own purposes. The permit requirement is to
consistently model between time periods so that the result can be objectively compared. An MS4 may
want to select a model that can model BMPs to show removal from various practices that you may have
installed or that you may want to install. This is not necessary for compliance with the permit, but makes
sense when it comes to justifying your nondegradation plan. The model does not need to calculate design
features such as hydrographs, but can show removal rates based on design criteria which can be just as
useful for planning purposes. Design calculations may need to be run before implementation but often
these can be run on a much smaller scale. Runoff and loading factors should be developed based on
available information. BMP modeling, while optional, can be used in plan development and could
consider BMP measures taken since 1988 to present and proposed BMP measures for present to 2020 or
ultimate development conditions. The MPCA has examples of how the "simple method" can be applied
to every community in the metro area.
The modeler must provide an explanation of assumptions and calculation methods. The inputs will need
to be listed and the values shown. All values will. need to be explicitly stated. The modeler must also
provide an explanation of assumptions and calculation used in the model, whether they are inherent to the.
model or assigned by the user. The exact algorithms must be shown. The results.of the model must be
Draft Plymouth Nondegradation Report.doe 17
examined to demonstrate reasonable results from the model runs. Outlier values that do not seem in line
with reasonable results must be explained or discussed in enough detail to help the MPCA decide the
significance of the results.
1.2.2.2 Nondegradation Report
Based on the modeling, local storm water management plans, and other pertinent factors, permittees must
develop a Nondegradation Report to get new or expanded discharges back to 1988 levels. Where
increases in runoff or pollutant loading has occurred due to new or expanded discharges from storm water
runoff, the Nondegradation Report must include retrofit and mitigation options (BMPs) that the permittee
has determined to be reasonable and practical to be included in the permittee's SWPPP.
Each Selected MS4 will submit its SWPPP, including BMPs proposed to be included, to the appropriate
water authority, watershed organizations or county water planning authority, for their review and
comment. The Nondegradation Report, as the basis for the SWPPP, will also be available to the water
authority. The intention is that these groups will work together to create a Nondegradation Report that is
acceptable to the public and other affected parties. As required in the permit, the proposed SWPPP, as
based on the Nondegradation Report, will be public noticed at the local level for public participation.
The Nondegradation Report explains the decisions made by the permittee regarding the incorporation of
BMPs into their SWPPP to meet the.nondegradation requirements. The purpose of the Nondegradation
Report is "to allow the MPCA to make an informed, public decision that reasonably balances additional
BMP costs against the adverse impact on the environment posed by the new or expanded discharge"
Minn. R. 7050.0185). The report is an explanation of the nondegradation implementation plan proposed
to be adopted by the MS4 community, explaining why some measures have been rejected and why the
measures taken are reasonable and practicable given the circumstances for the community they serve.
To help the MPGA determine if discharge loads should be allowed to increase, Selected MS4s must
submit pertinent information that demonstrates how potentially adverse water quality impacts from a new
or expanded discharge have been addressed. The goal of the Nondegradation Report is to demonstrate
what additional control measures would be reasonable to reduce the impact on the receiving water in light
of the relative importance of the environmental, economic and social impacts. The Report should explain
all aspects of the proposed Report that the permittee intends to implement. It is understood that the
SWPPP itself may have already addressed some specific aspects of nondegradation, and it may be
beneficial to note these in the Report. The Report should also address the alternatives that have been
studied but rejected. It is not necessary to include all rejected alternatives, but it will be very important to
Draft Plymouth Nondegradation Report.doc 18
establish the general thinking regarding why some option have been rejected and the basis for such
rel ect> on.
1.3 Plymouth Storm Water Management Planning and Water
Quality Improvement Projects
In addition to its SWPPP (Plymouth, 2006), the City of Plymouth has completed, participated in, and
implemented several storm water management planning and water quality improvement projects
since 1988. These projects are summarized below:
Plymouth Wetland. Inventory and Ordinance Development (1994)
Water Resources Management Plan (WRMP, 2000)—Established water quality goals and
BMP implementation requirements for all new development within the city.
Medicine Lake Implementation and Management Plan (2001)—Medicine Lake is a regional
water resource in the Bassett Creek watershed.
Hydrologic and Hydraulic Study of the 2020 Urban Expansion Area (2001)—The Urban
Expansion Area is the MUSA expansion area in the northwestern portion of the city.
Parkers Lake Management and Implementation Plan
Schmidt Lake Management Plan
Gleason Lake Management Plan
Aquatic Vegetation Management Plans for Medicine, Parkers, Bass and Schmidt Lakes
Natural Resources Inventory and Minnesota Land Cover Classification System Mapping
2006)—Divided and classified the city into appropriate land cover types, assessed the
relative ecological quality of the remaining natural areas, and recommended potential
corridors and management considerations.
Capital Improvement Plan projects specifically designed for water quality improvement and
stabilization/erosion control
The City has continued work on the following ongoing water quality monitoring and
improvement programs and projects:
Draft Plymouth Nondepradation Repoa.doc 19
o Medicine Lake Aquatic Plant Surveys and Curlyleaf Pondweed Control
o Parkers Lake Aquatic Plant Surveys and Eurasian Watermilfoi] Control
o Lawn fertilizer application control, erosion control, shoreland zoning, and animal
control ordinances `
o Engineering Guidelines for Developers
o Expanded street sweeping program
o Funding for small residential rain garden and other BMP implementation
o Citizen Lake Monitoring Program (CLMP)
o Goose Management Program
o Pond Maintenance Policy
o Purple loosestrife bio -control project
o Development Reviews
o Storm water education presentations, outreach, meetings and training
o Wellhead Protection Program
Draft Plymouth Nondegradation Report.doc 20
2.0 Loadinq Assessment
2.1 Land Use/Land Coves- Compilation
An important parameter for estimating historical TP and TSS loading and stormwater runoff volumes
is an accurate determination of land use and land cover (LULC) for the city of Plymouth for the years
of interest. These LULC data are available in Geographic Information System (GIS) data format for
various years in the Twin City Metropolitan area, but due to land use changes in Plymouth, the
LULC data available does not reflect the development status of the City during all ofthe years
specifically analyzed for this study.
To meet the Permit requirements, it will be necessary to estimate average annual runoff volumes, TP
and TSS loadings for 1988 (the base year), 2007 (existing conditions), and 2020. To get a consistent
comparison of LULC for all three years using the data that were available, a generalized LULC
classification system was developed. The LULC classes used are shown in Table 2-1.
Table 2-1 Land Use and Land Cover (LULC) Classes
Class Name Description
Agriculture (Cultivated). Row Crops, Small Grains
Agriculture (Non -
Cultivated) Hay/Pasture
Airport Airlake Airport
Commercial Commercial areas and corporate campuses
Forest Forested areas within parks or undeveloped areas
Grassland Non forested open space including most developed parks
Highway Controlled and limited access highways
Industrial Manufacturing, utilities, etc
Institutional Schools, churches, City buildings
Rural Residential Large lot housing, mostly outside of the MUSA boundary
Single Family Residential Single family homes with lots size typically less than an acre
Multi -Family Residential Duplexes, townhouses, apartments, condominiums, etc
Water Wetlands, Lakes, Detentions Ponds
LULC for the city of Plymouth (excluding County and State right-of-ways) for the 1988,
2007 and 2020 are summarized. in Table 2-2. Table 2-2 shows that agriculture and some
forest and grassland has been or will be replaced by residential, commercial, industrial and
some institutional land uses during the two time periods.
Draft Plymouth Nondegradation Report.doc 21
Table 2-2 Plymouth Land Use/Land Corer (LULC) for 1988, 2007 and 2020
LULC
Area (acres) by year
1988 2007 2020
Agriculture (Cultivated) 981 249 0
Agriculture (Pasture/Hay) 1,322 " 640 0
Highway 21 24 23
Commercial 383 1,218 1,102
Industrial 1,197 1,648 1,813
Institutional 806 912 786
Multi -Family Residential 811 1,816 2,029
Single Family Residential 6,107 8,829 9,495
Rural Residential` 0 0 990
Forest 1,131 716 276
Grassland 3,666 1,175 765
Water 5,203 4,397 4,346
Total 21,627 21,625 21,625
Total Developed 9,325 14,447 16,238
Area Imperviousness 2,686 4,743 5,076
Developed Area
Percent Impervousness 28.8% 32.8% 31.3%
For the 1988 and 2007 LULC, only the "developed" portions of rural residential properties were
Identfied by the available landuse data. These smaller areas (they do not include the entire parcel)
were assumed to have similar imperviousness single family residential areas. For 2020, the City's
guide plan designated Rural Residential as the entire parcel. Therefore, a lower impervousness and a
seperate land use class was assumed.
Sources used to derive the data for 1988 and 2007 include the 1984 and 1990 Metropolitan
Council Landuse GIS data, USGS National Land Cover Database (NLCD, 1992),
Hennepin County Parcel Data, 1991 and 2006 aerial photography. City of Plymouth's
Existing Conditions (2007) Landuse GIS layer and the City Wetland Inventory and parcel -
based development information was obtained from City of Plymouth Planning Department
staff were also used. "The data used to estimate the 2020 LULC included the City's 2020
Guide Plan, 2007 LULC data and input from the City's Engineering and Planning staff.
Figures 2-1, 2-2 and 2-3 show the LULC developed for 1988, 2007 and 2020, respectively.
Most of the non -urban land cover was estimated using the 1992 USGS NLCD land cover
data.
Draft Plymouth Nondegradation Report.doc 22
2.2 Watershed Imperviousness Determination
Another parameter that is required to develop estimates of average annual runoff volume, TP
and TSS loadings is imperviousness. Imperviousness was estimated using satellite -derived
LandSat) data developed by the University of Minnesota for the MPCA. These data are
available for the entire Twin Cities Metropolitan areas for the years 1986, 1991, 1998, 2000
and 2002. Since imperviousness data was not specifically available for 1988 or 2006
additional data development and calculations were necessary for the city of Plymouth
2.2.1 1988 Imperviousness Determinations.
The estimate of the amount of imperviousness in 1988 in Plymouth required that the developed area
of the City be identified. Developed areas were identified using the 1988 LULC discussed in Section
2.1. The land classes identified as Residential (all types), Commercial, Industrial, Highway and
Institutional were considered developed. Once these areas were identified, imperviousness was
determined by overlaying the developed area of Plymouth in 1988 with the 199.1 LandSat-derived
estimates of imperviousness.
2.2.2 2007 Imperviousness Determinations
LandSat-derived imperviousness data was not available for 2006. As a result, the areas that
developed between 2002 (when the latest imperviousness data layer is available) and 2007 were
identified and imperviousness calculated separately from the pre -2002 development shown on the
2002 LandSat-derived coverage. Parcel -based built -on data were used to identify those areas that
were developed between 2002 and 2007.. Aerial photography from the period of 2002 to 2006 were
also used in this effort. Landuse classification for areas developed between 2002 and 2007 were
estimated by overlaying the 2007 LULC GIS layer with the layer outling 2002-2007 developments.
Areas developed on or before 2002 were flagged and imperviousness was calculated using the 2002
imperviousness data. Imperviousness in areas of Plymouth developed after 2002 was estimated using
a relationship that was developed between land use and imperviousness. City-wide average
imperviousness was developed for each of the specific land use types using the 2000 Metropolitan
Council Land Use coverage and the 2000 LandSat-derived imperviousness estimates. The 2000 data
was used because it was the only year were land use and impervousness data were both available.
The average imperviousness values for each land use type are summarized in Table 2-3.
Draft Plymouth Nondegradation Report.doc 23
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2020
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Table 2-3 Average Imperviousness by Land Use Type for Plymouth based on 2000 Landuse
and Imperviousness Data
Land Use Class Percent
Imperviousness
Commercial 62.5%
Highway 48.9%
Industrial 65.5%
Institutional 35.0%
Multi -Family Residential 40.7%
Single -Family Residential 20.8%
The total imperviousness is the sum of the impervious area for areas developed on or before 2002
and those developed after 2002.
2.2.3 2020 Imperviousness Determinations
For the areas developed as of 2007, the impervousness used for that year were used in the estimate of
imperviousness. For areas identified as developing after 2007, the imperviousness was estimated
using the estimated imperviousness summarized in Table 2-3. It should be noted that for rural
residential areas, an imperviousness of 7 per was used based on an analysis of Rural Residential
properties previously done on this Metropolitan Council category within the City of Lakeville. The 7
percent value reflects the parcel -wide nature of land use shown in the City's Guide Plan.
2.2.4 Summary of Land Use/Land Cover by Watershed
ArcMap GIS was used to intersect the four major watershed divides with the LULC and
imperviousness data for 1988, 2007, and 2020. The City was futher divided based on the
jurisdictional extent of the MS4 permit. Therefore, Hennepin County and MnDOT right-of-way were
removed from the analysis area. The data were summarized by the following land use characteristics
to develop inputs for estimating runoff volume, TP and TSS loading:
Urban Area (includes Airport, Commercial/Corporate Campus, Highway, Industrial,
Institutional, Multi -Family Residential, Single Family Residential, Rural Residential)
Percent of Urban area that is impervious
Grassland
Forest
Draft Plymouth Nondcgradation Repon.doc 27
Cultivated Agriculture
Hay/Pasture Agriculture
Water
The division of agriculture between cultivated and hay/pasture was estimated using agricultural land
cover types in Plymouth. from the 1992 USGS NLCD LULC data. In 1992, 43 percent of the
agricultural area was cultivated and 57 percent was hay/pasture. The land use/land cover
characteristics are summarized for the four major watersheds (excluding County and State right-of-
ways) in Table 2-4
2.3 Modeling Approach and Methodology for Loading Estimates
Complex models used to answer simple questions are not advantageous and simple models that do
not model important or required physical processes are not useful. In keeping with the Permit
conditions and guidance discussed in Section 1.2, our modeling approach was developed based on
the following requirements:
The loading assessment should include changes to pollutant loadings associated with changes
due to past land use changes and changes due to anticipated land use changes
The modeling will produce relative values, as the MPCA is more concerned with the average
annual increases than about specific event increases. It is not as important to get the actual loads
correct as it is to model consistently, showing the relative change in loads rather than the actual
loads
The assessment can include changes due to BMPs that have already been implemented, if
increase in the loading since 1988 is explicitly stated, as well as changes due to BMPs that
are planned to be implemented and written into the MS4's ordinances or other regulatory.
mechanisms
Draft Plymouth Nondegradation Report.doc 28
Table 2-4 Plymouth Land Use/Land Cover (LULC) by Watershed for 1988, 2007 and 2020
Area (acres) by year
WMO LULC 1988 2007 2020
Bassett Creek Agriculture (Cultivated) 320 2 0
Draft Plymouth Nondegradation Report.doc 29
Agriculture (Pasture/Hay) 295 3 0
Highway 13 15 14
Commercial 281 796 786
Industrial 1,082 1,347 1,467
Institutional 629 623 542
Multi -Family Residential 552 1,032 1,103
Single Family Residential 3,118 4,337 4,447
Rural Residential'` 0 0 0
Forest 294 131 121
Grassland 1,821 512 345
Water 2,717 2,324 2,296
Total 11,123 11,121 11,121
Total Developed 5,676 8,150 8,360
Area Imperviousness 1,870 3,006 3,079
Developed Area
Percent Impervousness 33.0% 36.9% 36.7%
Area (acres) by year
WMO LULC 1988 2007 2020
Elm Creek Agriculture (Cultivated) 482 223 0
Agriculture (Pasture/Hay) 688 442 0
Highway 1 1 1
Commercial 11 11.7 24
Industrial 18 0 0
Institutional 0 80 128
Multi -Family Residential 0 173 257
Single Family Residential 189 542 971
Rural Residential'` 0 0 494
Forest 425 299 65
Grassland 167 249 207
Water 756 611 591
Total 2,738 2,738 2,738
Total Developed 219 914 1,875
Area Imperviousness 39 243 415
Developed Area
Percent lmpervousness 17.7% 26.6% 22.1
Area (acres) by year
WMO LULC 1988 2007 2020
Minnehaha
Creek Agriculture (Cultivated) 93 0 0
Agriculture (Pasture/Hay) 34 0 0
Highway 2 2 2
Draft Plymouth Nondegradation Report.doc 29
Commercial 35 116 125
Industrial 33 49 50
Institutional 171 159 90
Multi -Family Residential 200 217 218
Single Family Residential 1,906 2,347 2,429.
Rural Residential* 0 0 0
Forest 33 33 19
Grassland 425 84 79
Water 641 564 560
Total 3,572 3,572 3,572
Total Developed 2,347 2,891 2,914
Area Imperviousness 496 653 660
Developed Area
6 6 6
Percent Impervousness 21.1% 22.6% 22.6%
WMO LULC 1988 2007 2020
Shingle Creek Agriculture (Cultivated) 86 24- 0
Agriculture (Pasture/Hay) 305 195 0
Highway 6 6 6
Commercial 56 188 167
Industrial 63 252 296
Institutional 5 51 26
Multi -Family Residential 60 393 451
Single Family Residential 894 1,602 1,648
Rural Residential* 0 0 496
Forest 379 253 71
Grassland 1,254 331 134
Water 1,088 899 900
Total 4,194 4,194 4,194
Total Developed 1,084 2,493 3,090
Area Imperviousness 282 841 930
Developed Area
Percent Impervousness 26.0% 33.7% 30.1
For the 1988 and 2007 LULC, only the "developed" portions of rural residential properties were
Identfied by the available landuse data. These smaller areas (they do not include the entire parcel)
were assumed to have similar imperviousness single family residential areas. For 2020, the City's
guide plan designated Rural Residential as the entire parcel. Therefore, a lower impervousness and a
seperate land use class was assumed.
Draft Plymouth Nondegradation Report.doc 30
O The model does not need to calculate -design features such as hydrographs, but can show
removal rates based on design criteria, which can be just as useful for planning purposes.
Design calculations may need to be run before implementation but often these can be run on a
much smaller scale.
Currently, there are several water quality models available for simulating urban runoff and the
treatment effectiveness of BMPs. Table 2-5 presents a qualitative comparison of several of the .
important attributes associated with some of the more common runoff water quality model
capabilities based on the various selection criteria. The compiled model attributes and capabilities
come primarily from peer-reviewed manuals (U.S. EPA, 1997; Burton and Pitt, 2001), with
additional updated information based on our own experience and professional judgment. The water
quality models included in the table are generally listed in increasing order of complexity (from left
to right). For each attribute or selection criteria the models are categorized by possessing low,
medium (intermediate) or high capabilities. Those capabilities that are not incorporated into a
particular model, or were not applicable, were also indicated. Our approach for model selection for
this assessment involved comparison of the advantages and limitations of the various models as they
pertain to the Permit requirements, available data, and objectives of the city.
Table 2-1. shows that the only limitation with the P8 model, as it relates to the modeling requirements
for the loading assessment, is that it is not intended to be used to determine pollutant loadings from
non -urban land uses. However, the Simple Method, PONDNET and GWLF can be used to determine
pollutant loadings from both urban and non -urban land uses. Both the Simple Method and
PONDNET are typically used on an annual time scale. Table 2-5 also shows that the Simple Method,
PONDNET and GWLF lack the ability to model the BMPs that would typically be considered for
implementation by the City (such as vegetated drainage ways, extended detention,
infiltration/filtration practices and street sweeping). SLAMM lacks a. snowmelt runoff routine, does
not have any capabilities for including baseflow in BMP analysis, and does not have the model
output features contained in the P8 model. XP-SWMM is more complex, but is not in the public
domain, is significantly more expensive, and BMP modeling is more cumbersome, less accurate and
less intuitive than the P8 model.
Drall Plymouth Nondegradation Reporl.doe 31
Table 2-5 Comparison of Modeling Attributes/Capabilities by Selection Criteria
Criteria/Attributes
Simple
Method
PONDNDT SLAMM P8 GWLF XP -
SWMM
Time Scale Annual H H
Single Event H H H
Continuous H H H H
Hydrology Runoff L L H H H H
Baseflow L H H
Snowmelt H H
Pollutant Sediment (TSS) H H H H H
Loading
Constituents) Nutrients H H H H H H
Pollutant
Loading (Land
Uses)
Urban H H H H H H
Agricultural H H H
Pollutant Transport L L L H
Routing Erosion H H
Transformation L
Hydraulic Flow Routing/Diversions L L H
Model Output Statistics L L L H L H
Graphics L H M H
Hydro/Pollutographs H H
Format Options L L H H H H
Sensitivity Analysis H
Input Data Requirements L L M M M H
Calibration L L L M L H
Default Data L H H H H M
User Interface L L H H H H
GIS Compatibility L L M L M
BMPs-General Evaluation H M H L H
Design Criteria H L H H
Specific BMPs Ponds/Wetlands H H H H
Extended Detention M . H H
Infiltration/Filtration H H M
Street Sweeping H H M
Others H H L
Documentation Peer Acceptance H H H H H H
Technical Support L L M H L H
Cost Software L L M L L H
Use L L M M M H
H High M — Medium (Intermediate) L — Low -- Not Incorporatecl (.Not Applicable)
Draft Plymouth Nondegradation Report.doc 32
For this loading assessment, we have chosen to use the Simple Method to detennine the pollutant
loadings and runoff volumes from each of the land uses within each watershed and then use the P8
model to account for the effects of BMP implementation for the time periods of interest in the Permit
conditions. In addition to the discussion associated with Table 2-5, the following information
provides further justification for choosing the Simple Method/P8 model combination for the loading
assessment modeling, in comparison to SLAMM, PONDNET, XP-SWMM, or some combination
thereof:
The Simple Method inputs can be directly derived within GIS
PONDNET does not model TSS loadings and is only intended for modeling TP within wet
detention ponds
SLAMM is more detailed than P8 with respect to distinguishing source loading areas (such as
driveways, parking lots, lawns, etc.), but P8 exceeds the capabilities of SLAMM when it
comes to networking of watersheds/BMPs and many of the graphics and advanced output
features
P8 provides routines for performing sensitivity analyses and can also be run in design mode
to determine required sizes of BMP(s) to meet treatment criteria
P8 has the highest peer acceptance in Minnesota for urban runoff and BMP water quality
modeling and enhancements have been supported by the MPCA
P8 is free, user-friendly and easy to learn with its menu driven system
P8 allows for some GIS compatibility via ASCII text file import of watershed data and export
of results
P8 models actual hourly precipitation and climatic data as it occurs, with its associated
antecedent moisture conditions, while SLAMM only reads in the total precipitation and
duration of each rainfall event and does not model actual runoff events in real-time with their
associated antecedent moisture conditions
Unlike SLAMM, P8 allows for hydrologic calibration within the program and can be
calibrated/validated to time series runoff events continuously simulated from climatic data
Draft Plymouth Nondegradation Report.doc 33
While the City of Plymouth has conducted a significant amount of monitoring of stormwater runoff
and receiving water quality/quantity, none of the studies included monitoring of runoff from
individual land uses or specific land cover types. P8 Models have been developed, and calibrated
with the available data, for portions of the city as part of diagnostic -feasibility studies. However,
these studies were completed in the mid-1990s and the P8 Models are not representative of either
1988 or current (2007) land use conditions, they include natural wetlands in the modeling network,
and do not include all of the individual BMPs for each developed site within the watershed (typically
due to a lack of site-specific BMP information for each site and the size limitation of the model).
Since the presence of natural wetlands in the modeled drainage systems would affect the downstream
water and pollutant loadings, it would not accurately distinguish between the expected treatment
levels or provide a truly relative comparison between the predicted loadings, with and without the
presence of the watershed BMPs.
Following the initial assessment of TSS, TP and volume contributions with the Simple Method, we
assessed the benefit that current BMP implementation has had on the flow, TP and TSS loadings
within the city limits using the P8 water quality modeling for developments based on P8 model
design criteria examples that are indicative of the ordinances and design standards that were in place
by the City, the watershed management organizations, the Wetland Conservation Act and the MPCA
when development occurred. Based on the available data, combining the Simple Method and P8
Model for the loading assessment ensures full compliance with the Permit requirements, for the
following reasons:
The Simple Method ensures that a consistent method for calculating average annual volumes
and loadings will be applied to all land uses to produce relative values across the two times
periods of interest, as discussed in the Permit and Guidance Manual (see Sections 1.1.1 and
1.2.2.1 of this report)
The P8 Model simulations of volume and pollutant loading reductions associated with BMP
implementation, according to the various ordinances and design standards that were in place
when development occurred, is consistent with the Pennit conditions and Guidance Manual
and provides a consistent method for calculating relative removal rates as suggested in.
Section 1.2.2.1 (which includes the following excerpts from the Guidance Manual, "The
model ... can show removal rates based on design criteria... Design calculations may need to
be run before implementation but often these can be run on a much smaller scale.)
Draft Plymouth Nondegradation Report.doc 34
O Excludes. the effects that natural wetlands would have on improving the storm water quality
within each watershed, which ensures that the loading assessment estimates that include
BMPs (discussed in Section 2.4) do not take credit for treatment by natural wetlands
The city will not have to revise and update existing P8 models to exclude the effects of
natural wetlands or collect significantly more data on every BMP to develop new P8 models
for the rest of the city, which would represent significantly more cost for a product that
would not provide a "distinction between what is desirable and what is required. The MPCA
chose a level [in its loading assessment requirements] that will prevent undue burden while
still developing useful information." (MPCA Guidance Manual, 2006)
The loading assessment modeling results were summarized for each of the four major watersheds to
show the Simple Method loading and volume estimates for each time period, as well as the loading,
and volume estimates after applying the P8 model design criteria examples, based on the ordinances
and design standards that were in place when the various developments occurred.
2.3.1 Average Annual Flow Volume
The conversion of land areas to agricultural and urban land uses leads to changes in watershed
hydrology and pollutant load rates. The areal increase in impervious surfaces in urban areas over
undeveloped rural and natural land uses leads to greater surface water runoff volumes. The increased
runoff coupled with human activities increases the types of pollutants and delivery rate of these
pollutants to surface waters. Impermeable surfaces shed water as surface runoff, as do agricultural
practices that convert natural land cover, which reduces the infiltration and evapotranspiration
components of the hydrologic cycle. Surface runoff in urbanized areas is generally. directed to storm
sewers and other conveyance systems to rapidly move the large volumes to receiving waters and
prevent flooding. This section provides a general discussion about the methodology used to quantify
the amount of runoff from the various land uses in the Plymouth watersheds during the two time
periods of interest for the Permit conditions.
As previously discussed, the Simple Method was used to estimate the average annual runoff volumes,
which in turn, are also used to calculate the TP and TSS loadings, for the various land uses present
within the Plymouth watersheds. In the urbanized portion of each watershed, average annual runoff
volume was calculated using the following relationships (as described in Schueler, 1987):
Annual Runoff Caeff tient [RCJ = 0.05 + ((0.009) x (hnpeiwious Fraction) x 100)
Draft Plymouth Nondegradation Report.doc 35
Annual Runoff Volamae (acre-feet) = RC x Annual Rainfall (inches) x Urban Area !acres) / 12
As previously discussed, there is no monitoring data available for runoff volumes or quality from
individual land uses or specific land cover types within the city. The annual runoff coefficients
percentage of rainfall resulting in runoff) were determined for each of the land uses based on a
review of the available literature. Reckhow et al. (1980) summarized the TP and water yield
monitoring results of several published monitoring studies throughout the country that were specific
to individual land uses or land cover types. All of the available water runoff and rainfall volume data
were taken from Reckhow et al. (1980) and used to determine the median runoff coefficient for the
hay/pasture land use category. The median runoff coefficient for the hay/pasture land use category
was 0.11. MCES (2004) had monitored the Crow River watershed and determined that the average
annual runoff coefficient was 0.17 between 2000 and 2004. Since, at that scale, the watershed area
has predominantly been cultivated, the long-term average runoff coefficient for the cultivated land is
expected to be a conservatively low estimate. For the forested land use, curve number methodology,
assuming good ground cover, was applied to the long-term Twin City rainfall records to estimate that
the relative event -based cumulative runoff coefficient was 0.03. It was assumed that grassland would
exhibit the same runoff coefficient as forestland. Each of these runoff coefficients, for non -urban
land uses, show good relative agreement with the urban pervious runoff coefficient of 0.05 shown
above (taken from Schueler [1987]). For the non -urbanized portion of each watershed, average
annual runoff volume was calculated for each land use category using the following relationship:
Annual Runoff Volume (acre-feet) = RC x Annual Rainfall (inches) x Land Use Area (acres) / 12
The agricultural areas of the city, specified by the Metropolitan Council land use coverage for 2020,
did not divide out pasture/hay and cultivated. Therefore, the division of agricultural land between
pasture/hay and cultivated (predominately row crops) was estimated using the breakdown from the
USGS National Land Cover Inventory (1991). The break down was 57% for pasture/hay and 43%
for cultivated land cover.
2.3.2 Total Phosphorus
As previously discussed, there is no monitoring data available for runoff volumes or quality from
individual land uses or specific land cover types within the city. In the urbanized portion of each
watershed the TP runoff concentrations were calculated using the following relationships, developed
from several Minnesota urban runoff water quality monitoring studies, for the Detailed Assessment of
Phosphorus Sources to Minnesota Watersheds (Bair Engineering Company, 2004), and peer-reviewed
by MPCA staff:
Draft Plymouth Nondegradation Report.doc 36
TP Runoff Concentration (ug/L) = 1075 - 14.4 x bnpervious (%) — 5.7 x Annual Rainfall (inches)
Phosphorus loading from urbanized portion of each watershed was then calculated according to the
following equation (Barr Engineering Company, 2004):
Urban TP Load (lbs.) = Concentration (ug/L) x Annual Runoff Volume (acre feet) x 0.00272
The TP contributions from non -urban land uses were based on Reckhow et al. (1980), which
summarized the TP export coefficients produced from several: published monitoring studies
throughout the country that were specific to individual land uses or land cover types. All of the
available TP export coefficient data were taken from Reckhow et al. (1980) and used to determine
the median export coefficients for the cultivated, hay/pasture and forested land use categories. The
median TP export coefficients for the cultivated, hay/pasture and forested land use categories were
0.78, 0.54 and 0.09 lbs/ac/yr, respectively. The TP export coefficient for agricultural lands is likely
appropriate for Plymouth, since MCES (2004) has observed annual TP yields in the range of 0.30 to
0.66 lbs/ac/yr (from 2001-2004) at its Crow River watershed monitoring station near Rockford,
which has 85% of the watershed in agricultural land uses. It was assumed that grassland would
exhibit the same TP export coefficient as forestland. The average annual phosphorus loading from
each land use in each watershed was then calculated according to the following equation:
TP Load (lbs.) =Land Use Area (acres) x TP Export Coefficient (lbs,/ac/yr)
2.3.3 .Total Suspended Solids
As previously discussed, there is no monitoring data available for runoff volumes or quality from
individual land uses or specific land cover types within the city. In the urbanized portion of each
watershed the TSS average annual runoff concentration was assumed to be 100 mg/L, based on the
median Nationwide Urban Runoff Program (NURP) studies TSS concentration cited by Athayede et
al. (1983).
TSS loading from urbanized portion of each watershed was then calculated according to the
following equation:
Urban TSS Load (lbs.) = Concentration (Ing/L) x Annzidl Rwigff'Volume (acre-feet) x 2.72
The TSS contributions from non -urban land uses were based on several literature sources (MCES,
2004; DeByle and Packer, 1972; Harms et al., 1974; Webber and Elrick, 1967; Sonzogni et al.,
1980), which summarized the TSS export coefficients produced from several published monitoring
Draft Plymouth Nondegradation Report.doc 37
studies throughout the country that were specific to individual land uses or land cover types. All of
the available TSS export coefficient data were taken from the literature sources and used to
determine the median export coefficients for the hay/pasture and forested land use categories. The
median TSS export coefficients for the hay/pasture and forested land use categories were 25 and 5
lbs/ac/yr, respectively. It was assumed that grassland would exhibit the same TSS export coefficient
as forestland. MCES (2004) had monitored the Crow River watershed and determined that the
average annual TSS export was 70 lbs/ac/yr between 2000 and 2004. Since, at that scale, the
watershed area has predominantly been cultivated and the sediment delivery ratio would be estimated
to be 0.05 (based on the watershed area and USDA [1972]), the long-term average TSS export
coefficient for the cultivated land would be expected to be 1,400 lbs/ac/yr, as a conservatively low
estimate. The average annual TSS loading from each land use in each watershed was then calculated
according to the following equation:
TSS.Load (lbs.) = Land Use Area (acres) x TSS Export Coefficient (lbs/ac/yr)
2.3.4 BMP Implementation Modeling
As previously discussed, P8 water quality modeling was used to assess the benefit that current, and
expected future, BMP implementation would have on the flow, TP and TSS loadings within the city
limits for developments based on the ordinances and design standards that control the treatment
efficiency of the BMPs when development occurs. No post -construction BMP treatment standards
were in place prior to 1991 when the city adopted development requirements consistent with the
NURP wet detention pond design standards (Walker, 1987; MPCA, 1989). The NURP pond BMP
design requirements have controlled the treatment efficiency of the BMPs associated with each new
development since 1991 and will be the minimum design requirements that control the treatment .
efficiency for BMPs that are implemented through 2020 for all of the watersheds in the city. The
NURP design scenario was run in P8 for a hypothetical low-density residential development with
25% imperviousness and a commercial development with 80% imperviousness to obtain a range of
treatment efficiencies, as well as the average efficiency, that would be expected for the same design
standard. For the NURP design scenario, the P8 Model estimated average TP and TSS load
reductions of 56% and 87%, respectively. It was assumed that no volume reduction would be
realized from implementation of the NURP design requirements.
In a few cases, development plans may have been approved with a BMP design that did not fully
meet the ordinances or design standards that were in place at the time that development occurred.
Most of these cases involved developments that could not meet the NURP design requirement, but
Draft Plymouth Nondegradation Repon.doc 38
could either meet an equivalent level of treatment with other types of BMPs or could attain the
NURP design requirements by contributing to the construction of regional treatment basins. As a
result, the city developed a parcel -based GIS coverage that showed all of the developments in the city
and the time that the development was approved. This coverage was intersected with the watershed
boundaries to determine the total areas of development that complied with the NURP BMP design
standard that controlled the treatment efficiency for the time periods that are of interest in the Permit
conditions (see Figure 2-4).
It is also noted that since 2003, the Shingle Creek Watershed Management Commission has had a
standard that calls for infiltration of one-half inch of the impervious surface runoff within 72 hours.
In northwest Plymouth, it is the city's policy (Policy 1.1.1 in the WRMP) that the Elm Creek stream
flows will be limited to pre -development in -stream rates. Since 2002, the city has used Policy 2.3, in
its WRMP, to require volume control and additional stone water treatment beyond the NURP design
requirements, typically in the form of rain water gardens (or bioretention basins), reduced impervious
areas, porous pavement, or compost amendment to soil. The expected improvement in runoff water
quality and quantity resulting from this policy is not reflected in the results of the loading assessment
because it is not clear how often infiltration practices can reasonably be expected to function
properly and be practicable in all future developments or redevelopments. The city intends to
implement infiltration practices to the maximum extent practicable, but a review of the Hennepin
County Soil Survey shows that between 36 and 49 percent of the land area within the Plymouth
watersheds may not have suitable soils for infiltration practices. `In addition, infiltration practices
may be further restricted by proximity' to drinking water wells, the seasonal high groundwater level
and runoff from storm water hotspots, such as loading docks, fueling and vehicle maintenance areas.
As a result;
I
the NURP design scenario has been used to conservatively represent the results of the
loading assessment, following BMP implementation.
2.4 Results and Discussion
Table 2-6 shows the results of the loading, assessment modeling, which were summarized for each of
the four major watersheds to show the Simple Method loading and volume estimates for each time
period (without BMPs), as well as the loading and volume estimates after applying the P8 model
NURP design criteria example (with BMPs), based on the parcel -based GIS coverage that showed all
of the developments in the city and the BMP design standard that was.implemented for each
development.
Draft Plymouth Nondegradation Report.doe 39
0
3,500 7,000
Feet
1991 Aerial Phob
Significant Watershed Discharge Nodes
NURP Pond BMP Trealment Areas
Water Management Organization Boundaries
County and State ROW
City Umits
Percale (2007)
Impaired Streams
Impairment
Biota -Fish
Dissotved Oxygen
Impaired Lakes
Impairment -
Hg
st Hg, Nutrients
Nutrients
Figure 2-4
MAP OF IMPAIRED WATERS, BMP TREATMENTAREAS
AND WATERSHED DISCHARGE NODES
Nondegradation Load Assessment
City of Plymouth, MN
2.4.1 Average Annual Flow Volume
Table 2-6 and Figure 2-5 show that the total average annual flow volume from the city has increased
significantly since 1988 and would continue to increase slightly by 2020, with implementation of the
proposed BMPs. A review of the development data, from the parcel -based GIS coverage,. revealed
that between 16 and 20% of the area within the Plymouth portion of the Bassett Creek, Minnehaha
Creek and Shingle Creek watersheds had developed without implementation of any post -construction
BMPs and very little to no portion of the Elm Creek watershed would have developed without
implementation of NURP ponds by 2007. Since it was assumed that no volume reduction would be
realized from implementation of the NURP design requirements, Table 2-6 and Figure 2-5 show that
there has been an approximate 54% overall average annual flow volume increase between 1988 and
2007 that can be attributed to development within all of the city's watersheds. The results also show
that the projected volume increase between 2007 and 2020 will be limited to less than five percent.
Table 2-6 and Figure 2-6 show that more than half of the volume increase estimated for the period
between 1988 and 2007 can be attributed to development within the Bassett Creek watershed, while
there were also significant percentage increases in volume within the Elm and Shingle Creek
watersheds during the same time period. Figure 2-1 shows that much of the Plymouth portion of the
Minnehaha Creek watershed was developed by 1988 so the flow increased less than 25% between
1988 and 2007. Table 2-6 and Figure 2-6 show that planned development in the future will not
significantly increase the average annual flow volumes in most of the watersheds in the city,
compared to present conditions. The average annual flow volume in the Elm Creek watershed would
increase by approximately 29 percent between 2007 and 2020, but flow will not increase by more
than 8 percent in any of the remaining watersheds in the city. The average annual flow volume
increase projected for the period between 2007 and 2020 is 8, 2, and 1 percent in the Shingle, Bassett
and Minnehaha Creek watersheds, respectively.
Draft Plymouth Nondegradation Report.doc 41.
Table 2-6 Plymouth -Nondegradation Loading Assessment Summary
Without BMPs
WATERSHED
WATERSHED TOTAL RUNOFF (acre-feet) WATERSHED TP YIELD (LBS)
1988 2007 2020
WATERSHED TSS YIELD (LBQ,
1988 2007 2020198820072020
Bassett Creek 5,190 7,677 7,832 6,274 7,867 8,067 1,776,847 2,080,579 2,123,177
Elm Creek 542 900 1,165 1,000 1,386 1,725 726,132 501,716 312,779
Minnehaha Creek 1,462 1,804 1,819 2,397 2,852 2,874 508,529 488,904 493,327
Shingle Creek 992 2,266 2,445 1,472 2,654 2,878 340,786 629,518 661,869
TOTAL 8,186 12,647 13,262 11,143 14,759 15,543 3,352,295 3,700,717 3,591,152
With BMPs
WATERSHED
Bassett Creek 5,190 7,677 7,832 6,274 6,902 6,979 1,776,847 1,541,682 1,543,990
Elm Creek 542 900 1,165 1,000 980 1,052 726,132 374,582 99,924
Minnehaha Creek 1,462 1,804 1,819 2,397 2,595 2,604 508,529 410,561 411,039
Shingle Creek 992 2,266 2,445 1,472 1,999 2,178 340,786 347,700 348,624
TOTAL 8,186 12,647 13,262 11,143 12,476 12,813 3,352,295 2,674,525 2,403,576
Figure 2-5 Plymouth Loading Assessment—City-Wide Average Annual Flow Volume
14,0rN
12, DOJ
10 000
ar
A
o DDO
LL5 0r0
Q
4,000
r
2, D00
U
1988 2007 2020
El ,Aitthout BIdPs a t, ith POPS
Figure 2-6 Plymouth Loading Assessment -Average Annual Flow Volume by Watershed w/
BMP Implementation
14,000 — — --
12,000.
10,000
E
v
2,DDO
0
1988 2007 2020
ElBasse«Creek 13 Elm Creek utAnnehahaCreek rJShingleCreek
Draft Plymouth Nondcgradation Report.doc 43
i 12647 v 1284J.s
yp
h ,F,
dd
6
rr
ME
1
r,:4
Y iYa `%131:? yG'M1'i2y 3 Yi-
t1
J;'1`
g(SCl7
u`w, 't e fir. 'n. • T
TI
1988 2007 2020
El ,Aitthout BIdPs a t, ith POPS
Figure 2-6 Plymouth Loading Assessment -Average Annual Flow Volume by Watershed w/
BMP Implementation
14,000 — — --
12,000.
10,000
E
v
2,DDO
0
1988 2007 2020
ElBasse«Creek 13 Elm Creek utAnnehahaCreek rJShingleCreek
Draft Plymouth Nondcgradation Report.doc 43
2.1.2 Total Phosphorus
Table 2-6 and Figure 2-7 show that the average annual total phosphorus loading from the city has
increased since 1988 and would continue to increase by 2020, without implementation of BMPs.
Table 2-6 shows that implementation of the NURP ponds within each of the city's watersheds has
offset most of the increased phosphorus load between 1988 and 2007. BMP implementation between
1988 and 2007 limited the overall average annual total phosphorus loading increase from 32 to 12
percent within the city. As previously discussed, a small portion (between 16 and 20 percent) of the
Bassett Creek, Shingle Creek and Minnehaha Creek watersheds had developed without
implementation of any post -construction BMPs by 1991. Table 2-6 indicates that BMP
implementation and conversion of agricultural land has reduced the current total phosphorus loading
in the Elm Creek watershed relative to 1988 conditions.
Table 2-6 and Figure 2-7 show that continued implementation of the NURP practices, as planned in
the future, will continue to offset the increases in total phosphorus loading between 2007 and 2020
and will limit the overall TP load increase to 3 percent (from 12,476 to 12,813 lbs.), compared to the
total load estimate for 2007. Table 2-6 shows that continued implementation of the NURP practices
in the future will continue to limit the TP loading increase projected for all of the watersheds in the
city, compared to present conditions. Overall, the average annual TP load from the city in 2020, with
BMP implementation, is approximately 15 percent higher than the TP load estimate from 1988.
Table 2-6 shows that the total flow volume estimated for 2020 conditions is 62 percent higher than
the total volume estimate for 1988, on a city-wide basis. Comparing the results of the TP load and
average annual flow volume estimates indicates that the flow -weighted mean phosphorus
concentration in storm water runoff within the city will be 29 percent lower in 2020, compared to
1988 conditions.
Draft Plymouth Nondegradation Report.doc 44
Figure 2-7
18,060
16,060
M 14,060
ac
0 12,060
J
N
O
a 'IO,U6U
NO
a-
6 8,000
6,000
am
c
4,OQfl
2,DCO
0
Plymouth Loading Assessment—City-Wide Total Phosphorus Loading
1988 2007 2020
o`dt+iih04A 6w1PS M'PA,th BI•aPS
2.4.3 Total Suspended Solids
Table 2-6 and Figure 2-8 show that the average annual TSS loading from the city has increased since
1988 and would be higher by 2020, without implementation of BMPs. Table 2-6 and Figure 2-8
show that implementation of the NURP ponds within each of the city's watersheds has offset the
increased TSS load between 1988 and 2007 and resulted in an overall average annual TSS loading
reduction in the city, compared to 1988 conditions. The results also indicate that, with the exception
of the Shingle Creek watershed, BMP implementation and conversion of agricultural land has
reduced the current TSS loading in all of the watersheds between 1988 and 2007.
Table 2-6 and Figure 2-8 show that continued implementation of the NURP practices, as planned in
the future, will continue to offset any estimated increases in watershed TSS loading between 2007
and 2020 and will result in an overall TSS load decrease of 10 percent (from 2,674,525 to 2,403,576
lbs.), compared to the total load estimate for 2007. The results show that continued implementation
of the NURP practices in the future, combined with conversion of agricultural land, will reduce the
Draft Plymouth Nonde.t•adation Reporrt.doc 1 45
overall TSS loading from the city by 28 percent between 1988 and 2020. Table 2-6 and Figure 2-8
show that, with BMP implementation, the TSS "loadings estimated for 2020 conditions are lower than
the 1988 estimates for all of the watersheds, except for Shingle Creek where the TSS load is
estimated to be two percent higher.
Figure 2-8 Plymouth Loading Assess ment—Ci'ty-Wide Total Suspended Solids Loading
9,000,070
500.000
DJ
N
0 ' N
c
2 2400.000
ce7U)
C
1 X00.000
n7
500.000
0
1988 2007 2020
o a-lth-OL tBI-APS ®'VJItI16N11PS
Draft Plymouth Nondegradation Report.doc 46
3.0 Nondegradation Report
3.1 Future Conditions Loading Assessment
Table 2-7 and Figure 2-5 show that the total average annual flow volume from the city has increased
significantly since 1988 and would continue to increase slightly by 2020, with implementation of the
proposed BMPs. As discussed in Section 2.4, the results of the loading assessment, following
implementation of BMPs, indicates that the overall average annual flow volume from the city in 2020
is 62 percent higher than the flow volume estimate from 1988 but implementation of infiltration
practices will offset some of the increases in flow volume between 2007 and 2020.
Table 2-6 and Figure 2-7 show, that continued implementation of the NURP practices, as planned in
the future, will continue to offset the increases in total phosphorus loading between 2007 and 2020
and will limit the overall TP load increase to 3 percent, compared to present conditions. Overall, the
average annual TP load from the city in 2020, with BMP implementation, is approximately 15
percent higher than the TP load estimate from 1988. Comparing the results of the TP load and
average annual flow volume estimates indicates that the flow -weighted mean phosphorus
concentration in storm water runoff within the city will be 29 percent lower in 2020, compared to
1988 conditions.
As discussed in Section 2.3.4, the results of the loading assessment provide conservatively low
estimates for water quantity and quality improvement associated with BMP implementation because
it simply assumes NURP pond BMP design requirements. Since 2002, the city has used Policy 2.3,
in its WRMP, to require volume control and additional storm water treatment beyond the NURP
design requirements, typically in the form of rain water gardens (or bioretention basins), reduced
impervious areas, porous pavement, or compost amendment to soil. In northwest Plymouth, it is the
city's policy (Policy 1'.1.1 in the WRMP) that the Elm Creek stream flows will be limited to pre -
development in -stream rates. In the future, the city intends to implement infiltration practices to the
maximum extent practicable to mitigate the volume and loading increases, from the loadings
assessment (shown in the Table 2-6), as.much as possible.
3.1.1 Other BMPs and Considerations Not Included in the Loading Assessment
The results of the Loading Assessment with BMP implementation present the estimated volumes and
pollutant loading estimates associated with the minimum structural BMP requirements for new
developments. There are several other structural and nonstructural BMPs that have been, or will
continue to be, implemented in the city that, collectively, would also be expected to make significant
Draft Plymouth Nondegradation Report.doc 47
reductions in volume and pollutant loadings beyond those indicated in Table 2-6. These BMPs
include the following:
Capital Improvement Plan projects specifically designed for water quality improvement and
stabilization/erosion control
The lawn fertilizer phosphorus ban
Street sweeping program
Goose Management Program
Public education/participation/outreach
Illicit discharge detection and elimination
Pollution prevention/good housekeeping measures for municipal operations
Public nuisance ordinance controlling pet waste
Shoreland zoning ordinances controlling setbacks and requiring buffers for all public waters
and wetlands
Internal load reductions associated with control of exotic imacrophytes
In addition, there are other assumptions that were made about BMP implementation considered in the
Loading Assessment that were especially conservative, which meant that the 2007 and 2020 loadings,
with BMP implementation, shown in Table 2-6 were higher for the following reasons:
There is increased seepage to groundwater from storm water pretreatment measures and wet
detention ponds
Disconnection of impervious surfaces from drainageways
Additional treatment associated with future redevelopment projects was not considered in the
analysis
Draft Plymouth Nondegradation Report.doc 48
3.1.2 Implications of Impaired Waters for Addressing Loading Assessment
Figure 2-4 shows the receiving waters in the city that the MPCA has included in its 2006 impaired
waters listings because they do not meet the MPCA's water quality standards. Figure 2-4 shows that
Medicine, Schmidt, Pomerleau, Bass and Pike Lakes are each listed for excess nutrients. The
MPCA's water quality standard that pertains to lakes for excess nutrients, requires that the average
summer (May -September) total phosphorus concentration be maintained at or below 40 µg/L. While
it is expected that the MPGA will adopt a new shallow lakes standard of 60 µg/L, a review of the data
shown in Appendix A, along with a review of the available data for Lost and Gleason Lakes,
indicates that, except for Parkers Lake, most of the remaining city lakes have total phosphorus
concentrations that will exceed the applicable water quality standards for nutrients.
The Environmental Protection Agency (EPA) requires that the MPCA develop and submit Total
Maximum Daily Load,(TMDL) studies for each water -body that they have on the impaired waters
list. TMDL studies are used to determine what the maximum allowable pollutant loadings are for
each water body without exceeding the water quality standards. The allowable pollutant loading is
then allocated to each of the NPDES-permitted (including MS4s) and non-regulated sources of
pollutants in the watershed. At a minimum, TMDL studies completed for Medicine, Schmidt,
Pornerleau, Bass and Pike Lakes are likely to require the city to provide further reductions in total
phosphorus loadings to these water bodies to comply with the MPCA's water quality standards.
Figure 2-4 shows that Elm Creek is listed for dissolved oxygen and Bass Creek is on the impaired
waters list for biota -fish. The listing for Elm Creek may be the result of excess nutrient inputs, while
the listing for Bass Creek may be due to poor water quality or poor fish habitat associated with flows
or excess turbidity. As a result, pollutant load allocations associated with the creek impairments
would also require the city to reduce nutrient and/or sediment loadings, while possibly addressing
flow volume and peak runoff rates.
It is conceivable that the pollutant load allocations developed as part of future TMDL studies will
dictate that the city will need to provide further loading, reductions, beyond those currently projected
in the nondegradation load assessment. As a result, this Nondegradation Report will also consider all
reasonable and practical BMPs given the potential implications of future TMDL allocations
associated with the impaired waters that are receiving storm water discharge. Since the portions of
the city in both the Bassett Creek and Minnehaha Creek watersheds are essentially fully developed,
this Nondegradation Report will also consider reasonable and practical BMPs for redevelopments as
well as new development projects.
Draft Plymouth Nondegradation Report.doc 49
3,2 BMP Selection Considerations for New Development
The loading assessment indicates that implementation of watershed BMPs, in the past and planned
for the future, will ensure that the TSS loads from the city and the TP runoff concentrations will not
increase between 1988 and 2020. As a result, the following sections of the Nondegradation Report
discuss how BMPs, incorporated into a modified SWPPP, will address and mitigate the projected
increases in average annual flow volume and TP loading, as far as is reasonable and practical.
The following sections also include discussion about the BMP selection considerations, the
alternatives that were evaluated, and the basis for the selected BMP approach for both new
developinent and retrofits of existing development. The BMP selection considerations primarily
consist of receiving water quality, stream morphology/channel erosion, wetlands and source water
protection.
3.2.1 Receiving Water Quality
As discussed in Sections 2.4.2, 2.4.3 and 3.1, the loading assessment indicates that past
implementation of BMPs had limited increases in the overall TP load and reduced the TSS loads
from the city between 1988 and 2007. The loading assessment also indicates that implementation of
the BMPs planned for the future, will ensure that the overall TP and TSS loads from the city will
experience a limited increase or no increase at all between 1988 and 2020. For both time periods,
BMP implementation will ensure that the flow volume will increase at a higher rate than the TP and
TSS loads in every watershed. As a result, the runoff and receiving water quality will be improved
for TP and TSS, even for individual watersheds that will have increasing TP and TSS loads between
1988 and 2020.
As previously discussed, the results of the loading assessment indicate that, city-wide, the estimated
total phosphorus loading is 12_percent higher in 2007 relative to the 1988 condition, after accounting
for BMP implementation. However, the average annual flow volume has increased by 55 percent
during the same time period. As a result, the flow -weighted total phosphorus concentration of the
runoff entering the receiving waters in the city would have decreased during the period between 1988
and 2007 and the receiving water quality would be expected to improve during the same time period.
Appendix A illustrates the results of the statistical trend analyses completed for lake water quality in
each of the city lakes with a sufficient period of record of historical total phosphorus concentrations.
Insufficient long -tern total phosphorus data was available for Gleason and Lost Lakes, possibly
Draft Plymouth Nondegradation Report.doc 50
hindered by a lack of public access, so they were excluded froin the analysis. The statistical
significance of each trend was determined by comparing the Mann Kendall statistic to the critical
value for a range of confidence intervals. The estimated Sen's slope provides the magnitude of trend,
in Etg/L/yea.r, for those lakes exhibiting statistically significant trends. The results in Appendix A
show that statistically significant trends did not exist for Schmidt, Pomerleau, Parkers, Bass and
Mooney Lakes. Medicine Lake had a statistically significant improving trend for total phosphorus at
the 95 percent confidence interval with the Sen's slope indicating that the total phosphorus
concentration was dropping at the rate of 0.46 µg/L per year.
The results of the trends analyses are consistent with what would be expected for receiving water
quality given the fact the flow -weighted total phosphorus concentration in the runoff entering the
receiving waters, city-wide, would have decreased during the period between 1988 and 2007 and the
receiving water quality would not be expected to degrade during the same time period. As a result,
the trend analyses indicate that the NURP pond level of BMP treatment (for areas shown in Figure 2-
4) has likely ensured that the city can demonstrate that they have not degraded the receiving water
quality for lakes, streams and wetlands due to new or expanded discharges of stornwater. Therefore,.
the remainder of the Nondegradation Report focuses more attention on addressing the increases in
average annual runoff volume and its potential impacts on stream morphology and wetlands due to
increased depth and duration of inundation. v
3.2.2 Stream Morphology/Channel Erosion
While much of the storm water runoff generated in the Plymouth watersheds is conveyed to the lakes,
streams and wetlands via storm sewer, there are some open channels within these watersheds, in
addition to the streams, that could be subject to channel erosion due to increased flow volumes. hi
general, the open channels with culvert crossings experiencing increases in flow volume would still
have controlled the peak flows since the policy in the Plymouth WRMP would have restricted
increases in culvert flow capacity due to new development. As a result, the city has only observed
channel erosion along some creek segments and at storm sewer outfalls with poor energy dissipation
in the past. Current and past Capital Improvement Plan projects have been specifically designed
and/or constructed for erosion repair and stream bank stabilization based on the estimated peak flow
rates and volumes.
Both the Plymouth WRMP and the zoning ordinances require minimum structural setbacks and
storrnwater management along stream corridors, which will provide further protection for the
physical and. biological integrity within each watershed. Imperviousness is limited to 25 percent in
Draft Plymouth Nondegradation Report.doc 51
the Shoreland Management Overlay District.. In addition, Plymouth will continue to educate
I andowners and residents of existing developments about the importance of maintaining existing
stream buffers.
To mitigate past increases in volume and minimize volume from future development, the city intends
to implement infiltration practices to the maximum extent practicable. The Shingle Creek Watershed
Management Commission has had a standard that calls for infiltration of one-half inch of the
impervious surface runoff within 72 hours. In northwest Plymouth, it is the city's policy (Policy
1.1.1 in the WRMP) that the Elm Creek stream flows will be limited to pre -development in -stream
rates. Since 2002, the city has used Policy 2.3, in its WRMP, to require volume control and
additional storm water treatment beyond the NURP design requirements, typically in the forni of rain
water gardens (or bioretention basins), reduced impervious areas, porous pavement, or compost
amendment to soil. The expected improvement in runoff water quality and quantity, resulting from
this policy is not reflected in the results of the loading assessment because it is not clear how often
infiltration practices can reasonably be expected to function properly and be practicable for the
extenuating circumstances associated with all future developments or redevelopments. A review of
the Hennepin County Soil Survey shows that between 36 and 49 percent of the land area within the
Plymouth watersheds may not have suitable soils.for infiltration practices. In addition, infiltration
practices may be further restricted by proximity to drinking water wells, the seasonal high
groundwater level and runoff from storm water hotspots, such as loading docks, fueling and vehicle
maintenance areas.
Future implementation of infiltration practices, wherever practicable, represents the BMP alternative
that will typically be most reasonable and practical. However, the city's policies will need to include
flexibility for new developments and redevelopments that have site constraints that would otherwise
limit the BMP feasibility or cost-effectiveness and/or lose excessive amounts of useable space due to
larger infiltration storage volume requirements.
3.2.3 Wetlands
This section addresses, as far as is reasonable and practical, the potential negative impacts of
increased storm water discharge volumes that have caused increased depth and duration of inundation
of wetlands having the potential for a significant adverse impact to a designated use of the wetland.
The Permit uses terms such as "designated uses" and/or "functions and values" which come from
MPCA rules. The term "significant adverse impact" in the Permit is based on the existing water
Draft Plymouth Nondegradation Report.doc 52
q.uality standards and applicable rules. The term implies "significant adverse impact to a designated
use" of the water, as defined in water quality standards. The following rules apply to wetland
mitigation. Wetland mitigation maintains nondegradation of wetland designated uses. The wetland
mitigative sequence incorporates the following principles in descending order of priority:
1. Avoid the impact altogether by not taking a certain action or parts of an action;
2. Minimize the impact by limiting the degree or magnitude of the action and its
implementation, and by taking affirmative actions to rectify the impact and reduce or
eliminate the impact over time; and
3. Mitigate the unavoidable impact to the designated uses of a wetland by compensation.
Compensatory mitigation shall be accomplished in the following descending order of priority
of replacement:
a. Restoration of a previously diminished wetland; and
b. Creation of a wetland.
If compensatory mitigation is accomplished by restoration or creation, the replacement wetland shall
be of the same type and in the same watershed as the impacted wetland, to the extent prudent and
feasible. Compensatory mitigation shall be completed before or concurrent with the actual physical
alteration of the wetland affected by the proposed project to the extent prudent and feasible.
The City of Plymouth Wetland Inventory and Ordinance Development (1994) and Section 21670 of
the Plymouth Zoning Ordinance has been developed in confonnance with Minnesota Rules
8420.0650 and references the requirements of the Wetland Conservation Act (WCA). The Wetland
Inventory and Zoning Ordinance discusses the methods and results of the functions and values
assessment, the setback and buffer requirements for the wetlands within the city. The city requires
full sequence evaluation for impacts of a wetland alteration and provides wetland mitigation and
replacement requirements.
The management strategies that preserve the functions and values of wetlands in the city include
wetland buffer requirements for each classification and storm water management requirements
intended to maintain or minimize changes to bounce for a 2 -year storm event, according to the
requirements of each wetland classification.
Draft Plymouth Nondegradation Report.doc 53
As discussed in Section 1.2.1.3, the WCA does allow for ten categories of exemptions to the requirements
and does not have jurisdiction over all wetlands that are considered waters of the state. In the few
projects where the requirements of the WCA are not as comprehensive as MPCA water quality standards,
then the requirements of the NPDES permit will require an LGU to snake a determination that will also
satisfy Minn. R. 7050.0186. As a result, Plymouth will reference both the WCA and Minn. R. 7050.0186
requirements for wetland protection in the SWPPP.
The city intends to implement infiltration practices to the maximum extent practicable. Combining
the infiltration requirements with the city's policies and ordinances for setbacks, buffers and storm
water management represents the most reasonable and practical means of preventing significant
adverse impacts to the designated use of wetlands in the city of Plymouth.
3.2.4 Source Water Protection Areas
All 14 of the Plymouth drinking water wells have low source water susceptibility, low aquifer
sensitivity, and the source water protection areas in the city have variable vulnerability. As a result,
the city will define the appropriate measures that will reduce the threat to drinking water to the
maximum extent practicable. These measures will be developed in accordance with the Minnesota
Department of Health's, Evaluating Proposed Storm Water Infiltration Projects in Vulnerable
Wellhead Protection Areas, and the MPCA's, Minnesota Stormwater Manual guidance for potential
stormwater hotspots. Infiltration practices will not be allowed within the 1 -year time -of -travel
emergency response zone) Drinking Water Supply Management Area (DWSMA).
3.3 Retrofit and Mitigation BMP Options
The city's Capital Improvements Plan includes seventeen water quality and drainage projects, to be
implemented -between 2007 and 2011, that have been planned for improving the condition of parks,
wetlands, and watersheds based on past studies. Wetland restorations, native plantings, bank
stabilization, infiltration practices, and other BMP construction projects are in various stages of
implementation and will continue to be considered during planning to mitigate volume and pollutant
loading increases and improve water quality throughout the city of Plymouth.
The city currently applies its water quality policies to redevelopments that are greater than 2.5 acres
or create more than 1 acre of additional impervious surfaces. As a result, it is reasonable and
practical to require infiltration practices to redevelopment projects, wherever practicable, to mitigate
past increases in storm water runoff volume and further improve receiving water quality and habitat.
Draft Plymouth Nondegradation Report.doc 54
Variances to some of the requirements may occasionally be granted in the case of hardships or when
site conditions do not allow for proper BMP implementation.
3.4 Cost/Benefit, Social and Environmental Considerations
Kuo et al. (1988) determined that extended wet detention ponds provided the most cost-effective
performance, compared to infiltration trenches and porous pavements, to control stonn water quantity
and quality. Weiss et al. (2007) determined that constructed wetlands provide the most cost-effective
treatment for TSS and TP, compared to wet basins, sand filters, bioretention filters and infiltration
trenches, if land acquisition costs are ignored. If land acquisition costs are factored into the analysis,
wet basins would typically become more cost-effective in comparison to constructed wetland and
bioretention systems. Ignoring land acquisition costs, Wossink and Hunt (2003) determined that the
following BMPs would be expected to have decreasing levels of cost-effectiveness for treatment of
TP loadings: bioretention in sandy soils, stonnwater wetlands or wet ponds, bioretention in clay
soils, and sand filters.
The city intends to implement infiltration practices to the maximum extent practicable, but a review
of the Hennepin County Soil Survey shows that between 36 and 49 percent of the land area within
the Plymouth watersheds may not have suitable soils for infiltration practices. In addition,
infiltration practices may be further restricted by proximity to drinking water wells, the seasonal high
groundwater level and runoff from storm water hotspots, such as loading docks, fueling and vehicle
maintenance areas. P8 modeling results for development requirements involving 0.5 -inch of
infiltration storage volume over the surface of all newly created impervious areas estimates a volume
reduction of 79% for the site drainage compared to the same development condition without
infiltration. The P8 modeling results also indicate that additional volume reductions start to diminish
more significantly as infiltration storage volumes exceed 0.5 -inch. As a result, future
implementation of infiltration practices will need to include flexibility for new developments and
redevelopments that have site constraints that would otherwise limit the BMP feasibility or,cost-
effectiveness and/or lose excessive amounts of useable space due to larger infiltration storage volume
requirements.
Draft Plymouth Nondeoradation Report.doc 55
4.0 Proposed SWPPP Modifications
This section describes the modifications that are proposed for City of Plymouth SWPPP, based on the
results of the loading assessment and discussion in the nondegradation report.
The loading assessment and nondegradation report were completed assuming that future BMP
implementation would follow the NURP design criteria, at a )minimum. As a result, the city will
update its development review policies, standards and procedures, as cited in the SWPPP: This
approach will ensure the following:
Receiving water quality should be improved for lakes, wetlands and streams in Plymouth
Channel erosion and stream morphology changes. will be controlled
Further protection will be provided for the physical and biological integrity of the stream and
wetland corridors
Controlled bounce and duration of inundation in the city's wetlands and preservation of the
functions and values for each type of wetland classification
Wherever possible, the rules will be applied to redevelopment projects to )mitigate past
increases in storm water runoff volume and further improve receiving water quality and
habitat
In addition, the SWPPP will be modified to discuss further protection for the impaired waters within
the city or downstream of the city as TMDLs and their associated implementation plans are
developed.
In the few projects where the requirements of the WCA are not as comprehensive as MPCA water quality
standards, then the requirements of the NPDES permit will require an LGU to make a determination that
will also satisfy Minn. R. 7050.0186. As a result, Plymouth should reference both the WCA and Minn. R.
7050.0186 requirements for wetland protection in Section 21670 of the Plymouth Zoning Ordinance and
the SWPPP.
The SWPPP will show where the vulnerable wellhead protection areas are within the city and define
the measures that will reduce the threat to. drinking water to the maximum extent practicable. These
measures will be developed in accordance with the Minnesota Department of Health's, Evaluating
Draft Plymouth Nondegradation Report.doc 56
Proposed Storm Water Infiltration Projects in Vulnerable Wellhead Protection Areas, and the
MPCA's, Minnesota Stormwater Manual guidance for potential stormwater hotspots.
Draft Plymouth Nondegradation Reporl.doc 57
5.0 Comments on Proposed SWPPP Modifications
5.1 Public and Local Water Authority Comments on Proposed
SWPPP Modifications
Prior to submittal to the MPGA, the proposed SWPPP modifications to address nondegradation were
public noticed at the local level as required in the Permit. The City of Plymouth also submitted its
proposed SWPPP modifications to address nondegradation to the appropriate local water authorities (the
Bassett Creek Water Management Commission, Elm Creek Watershed Management Commission,
Minnehaha Creek Watershed District, and the Shingle Creek Watershed Management Commission) in
time to allow for their review and comment. The Nondegradation Report explaining the proposed BMPs
and the entire SWPPP was also made available to the public and local water authorities. The following
sections summarize the comments received from the public and the local water authorities on the
proposed SWPPP modifications and the Nondegradation Report. Copies of comment letters are included
as attachments to this report.
5.1.1 Public Comments on Proposed SWPPP Modifications
5.1.2 Local Water Authority Comments on Proposed SWPPP Modifications
5.1.2.1 Bassett Creek Water Management Commission (BCWMC)
5.1.2.2 Elm Creek Watershed Management Commission (ECWMC)
5.1.2.3 Minnehaha Creek Watershed District (MCWD)
5.1.2.4 Shingle Creek Watershed Management Commission (SCWMC)
5.2 Record of Decision on the Comments
Draft Plymouth Nondegradation Report.doc 58
References
Athayede, D.N., R.P. Healy and R. Field. 1983. Results of the Nationwide Urban Runoff Program,
Volume I — Final Report. U.S. EPA, Water Planning Division. NTIS PB84-185552.
Barr Engineering Company. 2004. Detailed Assessment of Phosphorus Sources to Minnesota
Watersheds: Prepared for the Minnesota Pollution Control Agency.
Burton, Jr., G.A. and R.E. Pitt. 2001. Stornwater Effects Handbook: A Toolbox for Watershed
Managers, Scientists, and Engineers. ISBN 0873719247. Lewis Publishers. CRC Press, LLC.
Kuo, C.Y., K.A. Kelly, and G.V. Loganathan. 1988. Planning of Urban Best Management Practices.
Journal of the American Water Resources Association. 24(1): 125-132.
Metropolitan Council Environmental Services (MCES). 2004. Crow River Monitoring Station
Information.
Minnesota Pollution Control Agency (MPGA). 1989. Protecting Water Quality in Urban Areas. Best
Management Practices for Minnesota.
Minnesota Pollution Control Agency (MPGA). 2006. Draft Guidance Manual for Small Municipal
Separate Stoi n Sewer Systems (MS4s). For General Pen -nit Number MNR040000.
Reckhow, K.H., M.N. Beaulac, and J.T. Simpson. 1980. Modeling Phosphorus Loading and Lake
Response Under Uncertainty: A Manual and Compilation of.Export Coefficients. EPA 440/5-80-
011.
Schueler, T.R. 1987. Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban
BMPs. Metropolitan Washington Council of Governments.
USDA, 1972. Sediment sources, yields, and delivery ratios. National Engineering Handbook, Section 3
Sedimentation.
U.S. Environmental Protection Agency (EPA). 1997. Compendium of Tools for Watershed Assessment
and TMDL Development. EPA841-B-97-006.
Walker, W.W., Jr. 1987. Design Calculations for Wet Detention Ponds. Prepared for St. Paul Water
Utility. St. Paul, MN.
Weiss, P.T., J.S. Gulliver, and A.J. Erickson. 2007. Cost and Pollutant Removal of Stonn-Water
Treatment Practices. Journal of Water Resources Planning and Management. 133(3): 218-229.
Wossink, A. and B. Hunt. 2003. The Economics of Structural Stonnwater BMPs in North Carolina.
WRRI Research Report Number 344.
Draft Plymouth Nondegradation Report.doe 59
Appendix,A
Lake Water Quality Trend Analyses
v.1.56. CAS# n/a
SEN'S SLOPE ESTIMATOR
S chmidt
O
lei
Apr 1995 Jan 2001
Constituent: TP (ug/L)
Date: 3/27/07
Facility: Lake Trend Analysis
Time: 4:51 PM
WQStat Plus"
n = 107
Slope = -0.342
units per year.
Mann Kendall
normal approx. _
0.936
A] a Critical Signif,
0.01 2.575 No
0.05 1.96 No
0.1 1.645 No
0.2 1.282 No
WNW
Data File: SCI -MD --1
View: SCHM TP
v.1.56. CAS# n/a
4
0
r.
U
0
15C
IN
51
SEN'S SLOPE ESTIMATOR
Medicine
Jul 1966
Constituent: TP (ug/L)
Date: 3/27/07
Aug 1986
WQStat Plus'"
n=225
Slope = -0.46
units per year.
Mann Kendall
normal approx. _
2.463
0.01 -2.575 No
0.05 -1.96 Down
0.1 -1.645 Down
0.2 -1.282 Down
Oct 2006
Facility: Lake Trend Analysis Data File: MEDICI--1
Time: 4:53 PM View: MED_TP
i
s . es N Z•.
Jul 1966
Constituent: TP (ug/L)
Date: 3/27/07
Aug 1986
WQStat Plus'"
n=225
Slope = -0.46
units per year.
Mann Kendall
normal approx. _
2.463
0.01 -2.575 No
0.05 -1.96 Down
0.1 -1.645 Down
0.2 -1.282 Down
Oct 2006
Facility: Lake Trend Analysis Data File: MEDICI--1
Time: 4:53 PM View: MED_TP
v.1.56. CAS# n/a
a
I
1
SEN'S SLOPE ESTIMATOR
Pomerleau
A,
May 1996 Dec 1999
WQStat Plus"
n=28
Slope = 0.076
units per year.
Mann Kendall
statistic = 37
2fi
0.01 131 No
0.05 101 No
0.1 85 No
267 N
Jul 2003
Constituent: TP (ug/L) Facility: Lake Trend Analysis Data File: POMERL--1
Date: 3/27/07 Time: 4:57 PM View: POM TP
v.1.56. CAS# n/a
SEN'S SLOPE ESTIMATOR
Parkers
x
Jul 1980
Constituent: TP (ug/]L)
Date: 3/28/07
Jul 1993
W QStat Pl us'TM
n=11
Mann Kendall
statistic = 8
Alpha .ri
0.01 34 No
0.05 27 No
0.1 23 No
0.2 1$ No
Jul 2006
Facility: Lake Trend Analysis Data File: PARKS -1
Time: 8:47 AM View: PARK TP
v.1.56. CAS# n/a
v
0
N
0
1
SEN'S SLOPE ESTIMATOR
Bass
Jul 1980 Jul 1993
wQstat Plash"
Mann Kendall
statistic = 11
Alpha
0.01 21 No
0.05 17 No
0.1 15 No
0.2 12 No
Jul 2006
Constituent: TP (ug/L) Facility: Lake Trend Analysis Data File: BASS—S—.1
Date: 3/28/07 Time: 8:45 AM View: BASS TP
v.1.56. CAS# n/a
SEN'S SLOPE ESTIMATOR
Mooney
Jul 1988
Constituent: TP (ug/L)
Mau 1994
Facility: Lake Trend Analysis
W QStat Pl usn'
n=18
Mann Kendall
statistic = 16
Aloha xi ' a 1 Signif,
0.01 68 No
0.05 53 No
0.1 45 No
0,2 36 No
Oct 1999
Data File: MOON)Y-1
Date: 3/27/07 Time: 4:48 PM View: MOONT_TP