2. Nondegradation Assessment
CITY OF
CHANHASSEN
7700 Market Boulevard
PO Box 147
Chanhassen, MN 55317
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Phone: 952.227.1100
Fax: 952.227.1110
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Phone: 952.227.1125
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Web Site
VNfW.ci .chan hassen.mn. us
[2]
MEMORANDUM
TO: Todd Gerhardt, City Manager
FROM:
Terrance Jeffery, Water Resources Coordinator
DATE:
D~'
January 15, 2008
SUBJ:
Nondegradation Assessment
SUMMARY
Staff is requesting the Planning Commission review the City's Nondegradation
Assessment, hold a public hearing, review comments, and make a
recommendation to the City Council to approve distribution of the report to the
Minnesota Pollution Control Agency in compliance with the NPDES Phase II
permit as revised June 1,2006.
BACKGROUND
In June of 2006, the Minnesota Pollution Control Agency (PCA) issued Permit
No. MNR040000 - General Permit Authorization to Discharge Stormwater
Associated with Municipal Separate Storm Sewer Systems (MS4). Among other
items, this permit identified 30 select cities which are required to develop a
nondegradation assessment and implementation plan within a certain time frame.
These communities are referred to as "Selected MS4s". The selection of these
communities was based upon population growth experienced or projected to be
experienced for three time frames: 1990 to 2000; from 2000 to 2003 and from
2000 to 2020. The City of Chanhassen is required to submit their nondegradation
assessment plan within 20 months (February 1, 2008) from the date of issuance of
the above-referenced permit.
Wenck Associates, Inc. was contracted to complete the Nondegradation Plan. A
draft of the plan has been attached for your review. Staff and a representative
from Wenck will be making a short presentation on the assessment findings and
subsequent timeline of events.
NONDEGRADA TION REQUIREMENTS
The selected MS4s are required to assess changes in base loading of three
parameters to their waters from 1988 loading levels. These three parameters are
total stormwater volume, total suspended solids (TSS) and total phosphorus (TP).
If new or expanded discharges are found to exist or to be likely to exist, the City
of Chanhassen must develop a nondegradation plan which will need to be
The City 01 Chanhassen · A growing community with clean lakes, quality schools, a charming downtown, thriving businesses, winding trails, and beautiful parks. A great place to live, work, and play.
Todd Gerhardt
Nondegradation Assessment
January 15, 2008
Page 2 of 3
developed and submitted to the PCA identifying what reasonable and practical best management
practices (BMPs) the City will implement to reduce these discharges to the 1988 levels. These
BMPs will be incorporated as revisions to the existing Storm Water Pollution Prevention Plan
(SWPPP) for the City of Chanhassen.
SUMMARY OF NONDEGRADA TION FINDINGS
The City of Chanhassen has already implemented a number of policies and practices which have
aided in the reduction of pollutant loading to area waters. These include street sweeping,
reforestation measures, tree preservation, planning tools such as density transfer and the Bluff
Creek Overlay District, and others. As a result, the City is projected to meet non degradation
requirements for total phosphorus and total suspended solids for the City as a whole. However,
there are individual watersheds which will not meet nondegradation requirements for TSS and
TP. These areas include Lake Ann which has experienced minimal development to date but,
based upon proposed land uses through 2020, will undergo land use changes which will affect
the loading of Lake Ann. The Riley, Purgatory and Bluff Creek Watersheds will also not meet
nondegradation requirements. This is not inconsistent with the fact that many of these waters are
listed as impaired by the PCA. The PCA has yet to determine if the assessments will be applied
on a citywide basis or on a per watershed basis.
The City will not meet the nondegradation requirements for total stormwater volume. As areas
within Chanhassen are converted to urbanized land use, the amount of impervious surface
increases and there is a net reduction in native vegetation. Vegetation acts to intercept rainfall
before it reaches the ground and becomes runoff. This process is referred to as abstraction. This
increase in impervious surface and decrease in native vegetation results in an increase in total
runoff volume.
IMPLICA TIONS FOR CHANHASSEN
The City is required to submit a Nondegradation Plan to the MPCA by February 1, 2008. Prior
to submittal to the PCA, the City is required to publicly notice the proposed SWPPP revisions as
related to the Nondegradation Report and Implementation Plan and the Nondegradation Report
and Implementation Plan must be made available to the public and to local water authorities,
including the Lower Minnesota River Watershed District, the Riley-Purgatory-Bluff Creek
Watershed District, the Minnehaha Creek Watershed District, and the Carver County Watershed
Management Organization.
By permit directive, the City must "consider alternatives, explain which alternatives have been
studied but rejected and why, and propose alternatives that are reasonable and practical. The
report must give high priority to BMPs that address impacts of future growth such as ordinances
for new develol?ment."
Todd Gerhardt
Nondegradation Assessment
January 15,2008
Page 3 of 3
Upon approval of the Nondegradation Plan by the PCA, the City will need to begin revising their
SWPPP to incorporate changes in practice, policy and education necessary to bring those areas
which will not meet nondegradation requirements to a level consistent with 1988 loading. The
staff will continue to work with local water authorities, City Council and Planning Commission
to determine how to best achieve the desired outcomes. The proposed SWPPP revisions within
the nondegradation plan direct the City to begin the due diligence necessary to determine which,
if any, code changes are necessary; what existing practices can be incorporated into the SWPPP;
what additions or revisions to educational programming are needed and what partnership
opportunities are available; and what changes to street, pond and structural BMP maintenance
and operations are needed. The revised NPDES permit and SWPPP will then be provided to the
agencies for their review.
RECOMMENDA TION
Staff recommends that the Planning Commission adopt the following motion:
"The Chanhassen Planning Commission recommends that the City Council approve the
distribution of the Nondegradation Assessment to the Minnesota Pollution Control Agency in
compliance with the NPDES Phase II permit as revised June 1,2006".
A TT ACHMENTS
1. Chanhassen's Draft Nondegradation Assessment prepared by Wenck Associates dated
December 2007.
g:\eng\terry\npdes phase ii\nondeg\pc 011507 _rev.doc
Wenck File #1481-03
Prepared for:
CITY OF CHANHASSEN, MINNESOTA
Prepared by:
WENCK ASSOCIATES, INC.
1800 Pioneer Creek Center
P.O. Box 249
Maple Plain, Minnesota 55359-0249
(763) 479-4200
Chanhassen
Nondegradation
Assessment
ReviewD RA F T
December 2007
~Wenck
Table of Contents
1.0 INTRODUCTION .................................. .;................................................................... ....... 3
1.1 Introduction.......................................... .................................................................. ......... 3
2.0 LAND USE AND P8 MODELING ....................................................................................4
2.1 P8 Model Construction and Organization....................................................................... 4
2.2 Land Use Assessment ................................ ...................................................... ............... 5
2.3 Imperviousness Assessment................ ....... .................................................... ............... 10
2.4 Pervious Curve Numbers .............................................................................................. 12
2.5 Other P8 Inputs.......... ......................... ................................................................ .......... 13
2.6 Model Validation and Assumptions.... .......................................................... ..... ........... 13
3.0 CURRENT WATER QUALITY TREATMENT CAPACITY ........................................ 15
3.1 Applicable Rules, Codes and Ordinances ..................................................................... 15
3.1.1 Chanhassen Codes and Ordinances ...................................................................... 15
3.1.2 Riley-Purgatory Creek-Bluff Creek Watershed District....................................... 15
3.1.3 Minnehaha Creek Watershed District ................................................................... 16
3.1.4 Lower Minnesota Watershed District ................................................................... 16
3.2 Treatment Device Quantification............ ...................................................................... 16
3 .3 Year Built For Developments............ ........................ ......................................... .......... 16
3.4 Application of Rules ..................................................................................................... 18
3.5 Street Sweeping...................................................................................................... ...... 19
4.0 MODEL RESULTS......... .................................................................................... ............. 20
4.1 Organization............................................................... .................................... ................ 20
4.2 Load Changes.......................................................... .............................................. ........ 20
4.3 Current BMP Treatment......................................... ...................................................... 22
4.3.1 Development Rules......................................... ...................................................... 22
4.3.2 Street Sweeping.................................................................................................... 23
4.3 .3 Volume......................................................................... ......................................... 24
4.3.4 TSS Loading............ ..... ....... ................................................................................. 24
4.3.5 TP Loading............... ........... ................................... ............................................... 25
4.4 Model Sensitivity............... ........................................................................................... 25
5.0 SUMMARY AND PROPOSED BMPS ........................................................................... 27
5.1 Total Suspended Solids..... ...... ......... ............................. ........................... ..................... 27
5.1.1 Regulatory Program ..................................,........... .................... ....... ...................... 27
5.1.2 Street Sweeping .................................................................................................... 28
5.1.3 Lake Ann Subwatershed ...................................... ................. ................................ 28
5.1.4 Other Activities........................................................................ ............................. 29
5.2 Total Phosphorus ............................................................................ .............................. 29
5.2.1 Regulatory Program................................................................ ..... ......................... 29
5.2.2 Street Sweeping.................................................................... ................................ 29
5.2.3 Lake Ann and Riley-Purgatory-Bluff Creek Subwatersheds................................ 30
5.2.4 Other Activities............................................................................ ......................... 30
5 .3 Volume........... ...... ............... ....... ................................... ................................................ 30
5.3.1 Regulatory Program ............. .............................................. ................................... 31
5.3.2 Education Program... ........... .................................................................................. 33
5.3.3 Structural Improvements..... ............................ ...................................................... 33
5.3.4 N on structural Improvements... ....................... ...................................................... 34
6.0 SWPPP MODIFICATIONS.......... ................................................................................... 36
6.1 Total Suspended Solids.............................................................................. ................... 36
6.1.1 Regulatory Program .............................................................................................. 36
6.1.2 Street Sweeping .................................................................................................... 36
6.1.3 Lake Ann Subwatershed ............. ................................ .............. ............................ 36
6.1.4 Other Activities.. ......................... ............................. ................. ..... ....................... 37
6.2 Total Phosphorus....... ............................ ........................ ................. .............................. 37
6.2.1 Regulatory Program.................................. ............ ...................... .......................... 37
6.2.2 Street Sweeping................................................................... ........................ ......... 37
6.2.3 Lake Ann and Riley Purgatory Creek Subwatersheds .......................................... 37
6.2.4 Other Activities................................................................... ................... ............... 38
6.3 Volume............................................................................................................. ............. 38
6.3.1 Regulatory Program. ........................................................... ................... ............... 38
6.3.2 Education Program... ......... .................................................................................... 38
6.3.3 Structural Improvements ...... ................................................................................. 39
6.3.4 N on structural Improvements .......................... ...................................................... 39
TABLES
Table 2.1. Current status of UAA analyses in the City of Chanhassen. ........................................ 4
Table 2.2. Land use categories for the City of Chanhassen. .......................................................... 5
Table 2.3. Land use acreages in the City of Chanhassen for the three key time periods............... 5
Table 2.4. Impervious fractions by land use from the UAAs, LANDSAT analysis, and those
fractions used in this study........... ......................... ................... ........................ ..... 10
Table 2.5. Digitized impervious and pervious acres for commercial and industrial areas in Eden
Prairie. ................................................................................. ................... ............... 10
Table 2.6. Total change in upland imperviousness in the City of Chanhassen. .......................... 12
Table 2.7. Curve numbers by land use and soil type. ..................................................................12
Table 2.8. Runoff and loading by curve number from a test watershed in P8............................. 13
Table 2.9. Monitored and predicted volume and water quality for Purgatory Creek. ................. 14
Table 3.1. Estimated areas that developed under watershed district rules by subwatershed....... 18
Table 3.2. Riley-Purgatory Bluff Watershed District rules. ........................................................ 18
Table 3.3. Minnehaha Creek Watershed District rules. ............................................................... 18
Table 3.4. Lower Minnesota River Watershed District rules. ..................................................... 19
Table 3.5. Measured mass of material removed from street sweeping in the City of Chanhassen.
........................... ........................... ......... ..................... .............. ............................. 19
Table 4.1 . Raw increases in imperviousness, volume, TSS, and TP for the City of Chanhassen
from 1990 to 2020... ................................... .......... ........ .......... ................. .............. 21
Table 4.2. Predicted TSS and TP load increases between 1990 and 2020 and estimated removal
due to development rules. ............................ ....................... .................... .............. 22
Table 4.3. Particle size breakdown by NURP particle size class for three samples taken from
street sweepings in Eden Prairie. .......................................................................... 23
Table 4.4. TSS and TP removal as a result of street sweeping in the City of Chanhassen.......... 23
Table 4.5. Predicted volume increases in the City of Chanhassen between 1990 and 2020. ...... 24
Table 4.6. Predicted TSS load increases between 1990 and 2020 in comparison to BMP
removals. ...... ......................................................................................................... 24
Table 4.7. Predicted TP load increases between 1990 and 2020 in comparison to BMP removals.
...............................................................................................................................25
Table 4.8. Rice Marsh subwatershed TSS loading and removals for the 50th and 90th percentiles
of the NURP studies...................... ........ ............... ................................................. 26
Table 4.9. Rice Marsh subwatershed TP loading and removals for the 50th and 90th percentiles
of the NURP studies.................... .............................. ............................................ 26
FIGURES
Figure 2.1. Subwatershed Map ...................................................................... ..... ...................... ......6
Figure 2.2. 1990 Land Use....... ...................................................................... ........................... ...... 7
Figure 2.3. 2000 Land Use....... ...................................................................... .......................... .......8
Figure 2.4. 2020 Land use.............................................................................. ............................. ....9
Figure 2.5. MPCA 1998 Landsat Imagery (% Impervious)..........................................................11
Figure 3.1. Hennepin County Year Built by Rule Change ...........................................................17
Figure 5.1. Annual Stormwater Volume That Could Be Captured Through
Abstraction/Infiltration ................... ........ ...... ... ....................... ................... .................... .... ........... .31
TECHNICAL APPENDICES
A East Chaska Creek
B Lake Ann
C Lake Lucy
D Lake Mitchell
E Lake Riley
F Lake Susan
G Lotus Lake
H Lower Minnesota River
I Minnehaha Creek WD
J Rice Marsh Lake
K Riley Purgatory Bluff Creek WD
L Silver Lake
M Sweepings Analysis
N Analytical Methods Detail
Executive Summary
This study is completed in response to the Nondegradation Assessment requirement of the
Minnesota Pollution Control Agency as part of the City's National Pollutant Discharge
Elimination System (NPDES) Phase II permit. The purpose of this study is to assess changes in
storm water runoff volume, total suspended solids (TSS) and total phosphorus (TP) loading in the
City of Chanhassen since 1988, and to predict how land change expected to occur between now
and 2020 would impact those parameters. This analysis is to be used to identify Best
Management Practices to be undertaken to return to 1988 or better conditions.
A key component of this analysis is an assessment of the change in impervious surface due to
land use change. The percent impervious area varies in the City, with most subwatersheds
between 20 and 35 percent impervious.
1990
16%
2000
20%
2020
28%
This analysis estimated the pollutant loads and stormwater volumes for three key years - 1990 (a
proxy for 1988), 2000, and 2020. Much of the land use change in Chanhassen since 1988
occurred under regulation to protect water quality. Additionally, the City currently undertakes
Best Management Practices (BMPs) to improve water quality. The pollutant load and volume
reductions associated with those BMPs were estimated to determine the net impact to stormwater
volume and TSSffP load since 1988 and predicted to occur between now and 2020.
Removal bv BMP Net
Total Total Total Develop- Removal (-)
Parameter Load Load Increase Street or Net
1990 2020 1990-2020 ment Sweeping Increase
Rules (+)
Total 1,274,100 2,060,167 786,067 933,108 254,446 -401,488
Suspended lbs/yr Ibs/yr lbs/yr lbs/yr lbs/yr lbs/yr
Solids
Total 4,555 7,243 2,688 3,073 589 -974
Phosphorus lbs/vr Ibs/yr lbs/yr lbs/yr lbs/yr lbs/yr
Runoff 5,488 8,324 2,836 None None +2,836
Volume acre-feet acre-feet acre-feet estimated estimated acre-feet!
/year /year /year year
For the period 1990-2020, this study estimates the City will remove about 400,000 more pounds
ofTSS than created since 1988, and 1,880 pounds more TP than created. For these two
parameters, the City meets the MPCA requirement of no net increase in TSS and TP over 1988
conditions. However, land use change will generate 2,836 acre-feet of new runoff volume.
To address this increase in stormwater volume, the City has identified several new BMPs that
will over time either reduce stormwater volume or minimize the adverse effects of additional
City of Chanhassen
Nondegradation Assessment
1
D R AFT December 07
stormwater volume. Managing stormwater volume has the additional benefit of reducing TSS
and TP loading by reducing the amount of runoff picking up and transporting pollutants to
downstream waters. These BMPs include:
. A new requirement on development and redevelopment to abstract (remove from runoff by
infiltration, evapotranspiration, capture and reuse, etc.) a portion of annual runoff volume;
· Reducing additional stormwater runoff volume through evapotranspiration by developing
and implementing a reforestation program;
· Retrofitting volume management BMPs where opportunities arise, such as on City street
reconstruction projects; and
· Mitigating volume impacts by stream restoration, erosion control projects, and shoreline
restoration.
City of Chanhassen
Nondegradation Assessment
2
DR AFT December 07
1.0 INTRODUCTION
1.1 Introduction
This study is completed in response to the Nondegradation Assessment requirement of the
Minnesota Pollution Control Agency as part of the City's National Pollutant Discharge
Elimination System (NPDES) Phase II permit. The purpose of this study is to assess changes in
stormwater runoff volume, total suspended solids (TSS) and total phosphorus (TP) loading in the
City of Chanhassen since 1988, and to predict how land change expected to occur between now
and 2020 would impact those parameters. This analysis is to be used to identify Best
Management Practices (BMPs) to be undertaken to return to 1988 or better conditions.
A nondegradation assessment consists of three key steps:
1. An assessment of change in land use and imperviousness in the city from 1988 to present
and out to 2020 conditions.
2. An associated pollutant load change with the land use change for total phosphorus, total
suspended solids, and water volume.
3. Identification of current of necessary BMPs to return to 1988 conditions or better.
This study is organized as follows:
1. A description of the City, land use change since 1988, and change expected to occur
between now and 2020.
2. A description of the method used to calculate imperviousness and the results.
3. A description of the various input parameters used to model pollutant loading and
stormwater runoff volumes.
4. A review of the existing BMPs designed to reduce pollutant loading, including a history
of watershed and city rules and ordinances regulating development and redevelopment;
the maintenance of sump manholes that collect material washed into the storm sewer
system from the landscape; and the street sweeping program.
5. A detailed presentation of the results of water quality and quantity modeling reported by
subwatershed in the city. These results estimate the amount of new TSS and TP loading
and stormwater runoff volumes resulting from land use change in the City.
6. An analysis of the amount of pollutant loading removed by the BMPs described above.
7. The net impact of those BMPs in reducing the increased TSS and TP load and new
stormwater volume.
8. Identification of new BMPs the City could undertake to return to 1988 conditions or
better, or mitigate the impacts of increases.
9. Proposed modifications to the City's Storm Water Pollution Prevention Plan to plan to
implement those BMPs.
City ofChanhassen
Nondegradation Assessment
3
D R AFT December 07
2.0 LAND USE AND P8 MODELING
2.1 P8 Model Construction and Organization
Modeling for much of the City has already been completed through the preparation of Use
Attainability Assessments (UAAs) for many of the lake watersheds in Chanhassen by the Riley-
Purgatory-Bluff Creek Watershed District (Table 2.1). The UAAs include both watershed load
modeling and in-lake response modeling. The UAAs are used to identify potential improvement
projects and estimate the costs to restore the waterbody to meet selected water quality goals.
UAAs have not been completed for Lake Minnewashta nor Rice Lake.
T bl 2 1 C
fUAA
. h C' fCh h
a e . . urrent status 0 analyses m t e Ity 0 an assen.
Watershed Water Body Status
Riley-Purgatory-Bluff Creek Mitchell Lake Completed
Rice Marsh Lake Completed
Lake Riley Completed
Lake Susan Completed
Lotus Lake Completed
Lake Lucy Completed
Lake Ann Completed
Silver Lake Completed
Minnehaha Creek Lake Minnewashta Not Completed
Lower Minnesota Rice Lake Not Completed
The UAAs include a P8 model developed for each of the lake watersheds. P8 - Program for
Predicting Polluting Particle Passage thru Pits, Puddles, & Ponds - is a public domain
(http://wwwalker.net/p8/). industry standard model developed to assess pollutant loading in
urban watersheds. P8 was developed using National Urban Runoff Program (NURP) data and
provides loading estimates based on data collected as a part of the NURP program. P8 was
designed to assess the effectiveness of BMP implementation in reducing runoff loads from
impervious surfaces and provides a tool for evaluating other non structural practices such as street
sweeping. These models represent the best tools available for predicting watershed loads under
current conditions.
The models provided by the watershed districts were the basis for the City's assessment. For the
assessment, the P8 model subwatersheds were maintained as well as the device configurations.
However, imperviousness and the pervious curve numbers were updated based on evaluations of
land use, LANDSAT data, and aerial photo interpretation. Selection of these input variables is
described later in this report. It is important to note the modeling is not focused on absolute
values, rather the purpose is to assess relative change among the time periods. The modeling
does need to be close to real world values to assess the impact of current activities on loading.
The modeling was developed for this purpose and is not intended to be a calibrated loading
model.
City of Chanhassen
Nondegradation Assessment
4
D R AFT December 07
The analysis was conducted according to the receiving water including ten lakes (Figure 2.1).
Although the analysis focused on changes in loading to the lakes, the subwatershed detail was
maintained in the model for future evaluation of projects on water quality.
2.2 Land Use Assessment
Land use data was collected from the City of Chanhassen, which used the same land use
categories to classify land use in 1990, 2000, and planned 2020. These data were combined into
a representative group of categories (Table 2.2, Figures 2.2, 2.3 and 2.4). 1990 data was selected
as a surrogate for 1988 because land use data was readily available for 1990 and not 1988. Little
change occurred in the City between 1988 and 1990. The resultant coverages are the best
estimate of land use for the three critical time periods including 1990,2000 (present) and 2020.
These data were compiled for the City of Chanhassen (Table 2.3). The 2020 land use coverage
assumes build-out conditions and that no agricultural or vacant land will remain in the City. A
near doubling of the land area used as single-family residential is expected between 1990 and
2020, along with significant increases in industrial and office uses. Large-lot rural residential
uses are also expected to be converted to other, more intense uses.
T bI 2 3 L d
. th Ct f Ch h
~thth kf
'd
a e . . an use acrea esm e uyo an assen or e ree ey . lme perlO s.
Land Use Categories 1990 Land use (acres) 2000 Land Use (acres) 2020 Land Use (acres)
Agriculture 2,880 1,639 0
Commercial 246 246 310
Industrial 327 557 848
Maior Highway 443 447 794
Multi Family Residential 60 96 517
Office/Institutional 58 98 392
Open Water 1,636 1,636 1,636
Parks 1,461 1,762 1,967
Rural Residential 1,568 1,425 1,055
Railway 104 104 108
Single Family Residential 2,447 3,641 4,713
Vacant 1,112 691 2
Wetlands type 1,2,6,7,8 591 591 591
Wetlands type 3,4,5 1,647 1,647 1,647
TOTAL 14,581 14,581 14,581
City of Chanhassen
Nondegradation Assessment
5
D R AFT December 07
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2.3 Imperviousness Assessment
To estimate the imperviousness in the City, assumed percent imperviousness by land use from
the watershed district supplied models and LANDSAT imagery were assessed (Table 2.4; Figure
2.5). The LANDSAT impervious fraction is an area-weighted average of imperviousness for all
pixels or partial pixels falling in each land use classification. The values were in relative
agreement except for commercial and industrial areas. Aerial photos were analyzed to determine
the imperviousness of these two land uses. The selected values for the modeling were chosen
based on aerial photo interpretation or based on an evaluation of LANDSAT estimated
imperviousness as well as knowledge of the area and to provide a conservative approach.
Table 2.4. Impervious fractions by land use from the UAAs, LANDSAT analysis, and those fractions used in
h' d
t IS stu Iy.
LANDSAT Impervious UAA Impervious Selected Impervious
Land Use Fractions Fractions Fractions
Agriculture 0.11 0.03 0.05
Commercial 0.58 0.85 0.67
Industriall 0.51 0.75 0.55
Multi-Family Residential 0.33 0.65 0.55
Parks and Recreation Areas 0.07 0.02 0.10
Office/Institutional 0.35 0.50 0.35
Railway 0.09 N/A 0.20
Roadway 0.36 0.50 0.50
Rural Residential 0.09 N/A 0.09
Single Family Residential 0.20 0.30 0.25
Vacant 0.11 0.03 0.05
h
Verified through aerI31 photo analysIs.
N/A = Not available
An aerial photo assessment of impervious areas in the adjacent City of Eden Prairie evaluated a
total of 376 commercial acres as well as 628 industrial acres (Table 2.5). Impervious and
pervious areas were digitized to estimate the area of impervious surfaces in these land use
categories. Although they represent a very small area, parking lot islands were considered
impervious because of soil compaction in these areas. Commercial areas were determined to be
approximately 67% impervious while industrial areas were determined to be 46% impervious.
Commercial development in Chanhassen is very similar to Eden Prairie. However, the
LANDSAT impervious fraction for industrial was 51 %, and 75% was used in the UAAs. This
study used 55% as the impervious fraction for industrial uses based on the LANDSAT data.
T bl 2 5 D'"f d'
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~
'1 d'd t'l
. Ed Pr' 'e.
a e . . l2J,llZe ImpervIous an . pervIous acres or commercia an m us rI3 areas m en alrI
Category Commercial Industrial
Acres Percent Acres Percent
Pervious 123 33% 341 54%
Impervious 253 67% 287 46%
TOTAL 376 100% 628 100%
City of Chanhassen
Nondegradation Assessment
10
DR AFT December 07
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Upland areas (areas excluding lake and wetland surface areas) in the City went from 16%
impervious in 1990 to a projected imperviousness of 28% in 2020 (Table 2.6). The Lake
Mitchell and Rice Marsh lake watersheds demonstrated the greatest impervious fractions with
54% and 39% impervious areas respectively.
T bl 2 6 T t I h
I d'
th C't fCh h
a e . . o a c an~e In uplan ImperVIOusness In e IlY 0 an assen.
Watershed Watershed Percent Imperviousl
ID Area (acres)! 1990 2000 2020
Lake Ann 130 7 9 15
Chaska Creek 177 18 23 31
Lotus Lake 998 24 24 24
Lower MN River 509 15 15 20
Lake Lucy 707 13 17 23
Minnehaha Creek WD 2,047 18 19 21
Lake Mitchell 129 31 55 54
Rice Marsh Lake 586 32 36 39
Lake Riley 683 11 15 25
Riley Purgatory Bluff Creek WD 3,532 10 15 30
Silver Lake 113 20 25 25
Lake Susan 1,082 22 29 36
TOTAL 10,694 16 20 28
II
Does not Include wetland and lake surface areas.
2.4 Pervious Curve Numbers
The second key piece of information for the P8 model is a curve number for the pervious areas in
the model. The SCS curve number reflects an area-weighted-average of the pervious areas
considering soil types, land use and hydrologic groups. Table 2.7 lists the values used in the P8
modeling as a function of land use, hydrologic condition, and soil group.
T bl 2 7 C
b bid
d 'It
a e .. urve num ers JY. an use an so. type.
Hydrolo~ic Soil Grou
Land Use A B C D AID BID B/C AIB AlC Unclassified
Agricultural 2 49 69 79 84 66.5 76.5 74 59 64 70.25
Roadway 2 49 69 79 84 66.5 76.5 74 59 64 70.25
Single Family Residential 2 39 61 74 80 59.5 70.5 67.5 50 56.5 63.5
Park and Recreation 1 39 61 74 80 59.5 70.5 67.5 50 56.5 63.5
Commercial 2 49 69 79 84 66.5 76.5 74 59 64 70.25
Multi-Family Residential 1 39 61 74 80 59.5 70.5 67.5 50 56.5 63.5
Vacant 1 39 61 74 80 59.5 70.5 67.5 50 56.5 63.5
Industrial 3 68 79 86 89 78.5 84 82.5 73.5 77 80.5
Railway 3 68 79 86 89 78.5 84 82.5 73.5 77 80.5
OfficelInstitutional 1 39 61 74 80 59.5 70.5 67.5 50 56.5 63.5
I . .
SIngle famIly reSidentIal, park and recreatIon, multIfarruly reSidentIal, vacant, and office/InstItutIonal were
considered to be in good condition.
2 Agricultural, commercial, and highways and roadways were considered to be in fair condition.
3 Industrial and rail were considered to be in poor condition.
City of Chanhassen
Nondegradation Assessment
12
D R AFT December 07
2.5 Other P8 Inputs
Other model input is the particle, precipitation and temperature file. The particle file used was
the P8 default NURP50 particle file. Both the precipitation and temperature files used were
obtained from UAA models. All models were run for the same 10 year period (1992 to 2002)
with 5 passes through the precipitation file.
2.6 Model Validation and Assumptions
Modeling was conducted for all loading in the City, including loading from other NPDES Phase
II permit holders. Other permit holders include MnlDOT, Carver County, and Hennepin County.
These loadings were included in this assessment.
The modeling approach presented here is a conservative approach to assess nondegradation. The
P8 model was developed for urban watersheds. However, one of the critical aspects of this
modeling is the change in loading that occurs as land is converted from open or agriculture to
developed land. To test the P8 model's handling of open space, a test watershed (100 acres) was
run for an average precipitation year (Table 2.8). Curve numbers in the model for open space
and agriculture typically ranged from 60 to 70. Because of the low runoff (<1.4 inches), the
loading rates for open land with these curve numbers is very low. This may be artificially
lowering the runoff from these areas, especially if the current land use is agriculture. However,
to maintain a conservative approach, these numbers were maintained in the model.
T bl 2 8 R ff did' b
b f
h d' P8
a e . . uno an oa lOll: )y curve num er rom a test waters e In
TSS load TP load
eN Runoff (in) (IbsJaclvr) TSS (ppm) (IbsJaclyr) TP (ppm)
50 0.3 , 0.7 12 0.01 0.127
55 0.5 2 17 0.01 0.138
60 0.7 4 22 0.02 0.150
65 1.0 7 29 0.04 0.167
70 1.4 13 40 0.06 0.193
75 1.9 24 55 0.10 0.226
80 2.7 44 71 0.16 0.263
85 4.0 82 89 0.28 0.305
90 6.3 163 114 0.52 0.363
95 11.0 366 148 1.09 0.440
Impervious 26.1 649 110 2.09 0.354
(NURP 50)
Impervious 26.1 1947 330 4.43 0.750
(NURP 90)
To validate the model, model results were compared to stream data collected as a part of the
Watershed Outlet Monitoring Program (WOMP; Table 2.9). For the WOMP data, a close to
average year precipitation was selected to evaluate. Data are presented as flow weighted means
or on a unit area basis. This makes the data comparable even though the model was only run for
the City of Eden Prairie portion of the watersheds. The model typically over-predicted runoff
and loading for the Purgatory Creek watershed in Eden Prairie. Although the model does not
City of Chanhassen
Nondegradation Assessment
13
DR AFT December 07
account for receiving water processes, it represents a conservative estimate of loading from the
land.
Table 2.9. Monitored and redicted volume and water uatit Creek.
Runoff Flow TSS load Flow
Watershed Year (in) Weighted (Ibs/aclyr) Weighted TP
TSS (m ) ( )
Purgatory 2004 7 23 38 160
Creek
TP load
(Ibs/aclyr)
0.26
Modeled 10 93 217 314
Data calculated from WOMP station for entire watershed
2Data calculated for City of Eden Prairie portion of watershed using NURP50 file.
0.73
City of Chanhassen
Nondegradation Assessment
14
D R AFT December 07
3.0 CURRENT WATER QUALITY
TREATMENT CAPACITY
Understanding the BMPs and water quality treatment devices in the watershed is vital in
assessing load changes in the City between 1988 and present. Additionally, assessing whether
the current practices are sufficient for protecting water quality into the future can help guide City
activities to assure compliance with nondegradation.
3.1 Applicable Rules, Codes and Ordinances
Applicable watershed district rules and City codes and ordinances that address water quality
were reviewed to determine the current treatment capacity in the City resulting from regulation.
3.1.1 Chanhassen Codes and Ordinances
Wetlands. Provides for management of wetlands and wetland impacts by management
classification as determined by a functions and values assessment. Ordinance specifies
allowable hydrologic change in wetlands, and buffer and setback standards by classification.
Shore land Management. Establishes structure, septic system, and drain field setbacks, and
minimum lot sizes; regulates grading and filling activities and alteration of natural vegetation in
the shoreland management area; and regulates onsite sewerage systems in the shoreland area.
Also, specifies maximum impervious coverage by land use type and regulates placement and
design of roads, driveways and parking areas.
Bluff Creek Overlay District. Contains a number of regulations to protect the Bluff Creek
Corridor, wetlands, bluffs, trees, and general environmental quality in the Overlay District. This
ordinance encourages minimization of new impervious surface, cluster development, and other
Low Impact Design elements, and requires a Natural Habitat Restoration Plan mitigating impacts
to natural habitat areas in the Overlay District.
Surface Water Management. Sets forth water quantity and quality requirements. Rate control
and NURP-Ievel treatment is required on new developments. Additional treatment requirements
are established for developments that discharge to higher waterbodies. Where developments
discharge to Riley or Purgatory Creek, new runoff is prohibited from increasing the flow rate in
the creek.
3.1.2 Riley-Purgatory Creek-Bluff Creek Watershed District
The District's regulatory program was formally established in 1996. Projects to alter or disturb
land surfaces in excess of one acre, fill within the lOG-year floodplain, or fill within a wetland
are required to meet NURP design requirements and include skimming of floatable materials.
City of Chanhassen
Nondegradation Assessment
15
DR AFT December 07
No specific rate or volume control is required. According to Bob Obermeyer, former engineer
for the District, standards have been in place since 1977 for projects altering more than one acre.
The early emphasis was simply on erosion control, but in the early 80s started looking for
sedimentation control and treatment. The design criteria used was to provide sufficient surface
settling area to remove a 0.1 mm particle based on a 10-year, 1.65" in 30 minutes storm event
(Obermeyer, pers. corresp. 6/8/06, 7/12/06).
3.1.3 Minnehaha Creek Watershed District
The District's regulatory program was formally established in 1974 and has been periodically
revised since that time. The standards in place since 1988 apply to all land alterations except for
residential developments ofless than two acres. Wet detention basins meeting NURP design
requirements are required, except that developments less than 40 acres can choose to purchase
regional storm water storage credits. The District's current regulatory program requires all
projects to incorporate Best Management Practices, including site design, structural, and non-
structural practices. Water quality requirements depend on the land use and size of project.
Generally, NURP-level treatment is required for developments of 8 acres or more.
3.1.4 Lower Minnesota Watershed District
The District's regulatory program was formally established in 1999. A Runoff Management
Plan (RMP) is required for residential projects in excess of 5 acres or nonresidential projects in
excess of one acre, or projects meeting other criteria. Runoff from the site after development
must not exceed the peak runoff from the site predevelopment for the 20-percent chance (5-year)
and I-percent chance (lOO-year) storm event. The one LMWD lake in Chanhassen, Rice Lake,
is a Level IV lake, requiring runoff treatment prior to discharge to NURP standards except that
dead storage must be provided for a 2-inch event rather than a 2.5-inch event.
3.2 Treatment Device Quantification
The City of Chanhassen's pond and wetland database was reviewed to assess the ability to
quantify the treatment capacity in the City. The most important data includes pond dead storage
or bottom and surface areas. Flood pool storage is also useful but not critical. Pond surface
areas are available, but it is not known if all ponds are included. For the Riley-Purgatory-Bluff
Creek Watershed, the data are available in the P8 model supplied by the District. Consequently,
the treatment capacity was estimated based on the year the lot was developed and the District or
City rules in place at the time.
3.3 Year Built For Developments
The first step in identifying the rules under which development occurred, is to determine the date
when a lot was developed. Hennepin and Carver Counties maintain a GIS layer that includes the
development year for a particular tax lot (Figure 3.1). The City was then broken into areas
based on the period in which different watershed rules were in place. Realizing that some
development can still occur without review (less than 5 acres), each subwatershed was evaluated
City of Chanhassen
Nondegradation Assessment
16
DR AFT December 07
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to estimate the area of development that occurred under the watershed district rules (Table 3.1).
This was accomplished by evaluating the dates the lots were built in relation to neighboring lots.
If several lots were built together, the area was assumed to develop under the rules.
T bl 3 1 Eft d
th t d
d d
h d d. t . t I b b t h d
a e . . sIma e areas a eve ope un er wa ers e IS rlc ru es )y su wa ers e .
Watershed Estimated Number Estimated Number Fraction Built
ID Of New Lots Built Under Rules Under Rules Notes
Lake Ann 2 2 1.00
Chaska Creek 20 20 1.00
Lotus Lake 0.65 Estimated
Lower MN River 16 10 0.63
Lake Lucy 267 251 0.94
Minnehaha Creek WD 0.70 Estimated
Lake Mitchell 13 13 1.00
Rice Marsh Lake 295 282 0.96
Lake Riley 314 290 0.92
Riley Purgatory Bluff
Creek WD 0.80 Estimated
Silver Lake 73 66 0.90
Lake Susan 0.90 Estimated
3.4 Application of Rules
Watershed rules were in place in the early 1980s, however the key rules are those in place after
1988. Since the early 1990s the pond design was the National Urban Runoff Program (NURP)
standard and is assumed to provide a 50 to 60% total phosphorus reduction and a 70 to 80% total
suspended solids reduction.
T bl 3 2 Ril Po
BI ff W
h dD.
a e . . ey- r atory. u aters e Istnct ru es.
TP% TSS % Volume %
Year Built Reduction Reduction Reduction Notes
Pre 1988 0 0 0 Some WQ reduction starting in early to mid
80s
1988-1992 50-60% (1) 70-80% (I) 0 Provide dead storage for the lO-year 30 min
event (1.65 inches)
1993-2007 50-60% (2) 70-80% (2) 0 Rules - NURP. Provide dead storage for the
2.5 inch, 24-hr event
1.
Assumed based on the rule
2 Assumed based on NURP standards
Table 3.3. Minnehaha Creek Watershed District rules.
TP% TSS% Volume %
Year Built Reduction Reduction Reduction Notes
Pre 1988 50-60% (IJ 70-80% (IJ 0
1988-1992 50-60% (I) 70-80% (I) 0
1993-2007 50-60% (2) 70-80% (2) 0 Requirement depends on type of land use and
development size
1.
Assumed based on the rule
2 Assumed based on NURP standards
City of Chanhassen
Nondegradation Assessment
18
D R AFT December 07
Table 3.4. Lower Minnesota River Watershed District rules.
TP% TSS% Volume %
Year Built Reduction Reduction Reduction Notes
Pre 1988 0 0 0 Some WQ reduction starting in early to mid
80s
1988-1992 50-60% (1) 70-80% (I) 0 Provide dead storage for the lO-year 30 min
event (1.65 inches)
1993-2007 50-60% (2) 70-80% (2) 0 Rules - NURP. Provide dead storage for the
2.5 inch, 24-hr event
1,
Assumed based on the rule
2 Assumed based on NURP standards
3.5 Street Sweeping
The City of Chanhassen has a street sweeping program and maintains detailed records of
material removed from streets. The City removed between 1 and 6 million pounds of raw
material annually between 2004 and 2006 with an average removal of 3.3 million pounds (Table
3.5).
Table 3.5. Measured mass of material removed from street sweeping in the City of Chanhassen.
Tons Pounds Material
Swept (Dry Removed
Year Weil!:ht)
2003
2004 1,472 2,944,400
2005 506 1,011,500
2006 2,975 5,950,000
Average 1,651 3,301,967
City ofChanhassen
Nondegradation Assessment
19
DR AFT December 07
4.0 MODEL RESULTS
4.1 Organization
Model results were organized on a receiving water basis with the City's ten lakes as the focus
(Figure 2.1). Model results are presented in for these subwatersheds. Those subwatersheds that
drain out of the City or drain to creeks that ultimately drain out of the City without entering a
lake were grouped together. The details of the P8 models supplied by the watershed districts
were maintained and are provided in the Technical Appendices.
4.2 Load Changes
Changes to impervious area, volume, TSS, and TP for the City of Chanhassen between 1990 and
2020 are presented in Table 4.1. Between 1990 and 2020 the City is expected to add about 1,250
impervious acres resulting in an increased discharge of over 2,800 acre-feet of water. The
increased discharge was predicted to include an additional 786,000 pounds of TSS and 2,690
pounds of total phosphorus.
City of Chanhassen
Nondegradation Assessment
20
DR AFT December 07
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Table 4.1. R
TSS. and TP for the City of Chanh
fj
1990 to 2020
to.>
....
--.. ----------- ------- -- ----------7. -------7 --- --- -- - ---
Watershed Area Impervious Area
In (acres) (acres) Flow (ac. ft. per year) TSS (lbs per year) TP (lbs per' ear)
1990 2000 2020 1990 2000 2020 1990 2000 2020 1990 2000 2020
Lake Ann 130 9 11 19 36 41 64 7,363 8,635 13,803 27 31 49
Chaska
Creek 177 31 41 54 99 118 155 22,905 28,622 38,448 79 98 131
Lotus Lake 998 236 241 240 668 682 682 162,366 166,222 166,271 555 567 568
Lower MN
River 509 75 76 103 239 242 299 54,705 55,417 71,558 191 193 246
Lake Lucy 707 91 117 165 285 347 460 68,607 85,955 117,550 235 292 395
Minnehaha
Creek WD 2,047 368 386 429 1,106 1,146 1,237 261,172 272,234 297,523 901 937 1,020
Lake
Mitchell 129 40 70 70 116 193 193 28,824 49,546 49,487 142 241 241
Rice Marsh
Lake 586 187 209 231 517 567 622 129,130 142,930 157,846 437 532 532
Lake Riley 683 78 99 171 279 318 483 61,091 72,480 118,182 307 359 572
Riley
Purgatory
Bluff Creek
WD 3,532 358 528 1,076 1,364 1,731 2,977 290,670 394,652 739,018 1,038 1,378 2,508
Silver Lake 113 23 28 28 66 78 78 15,771 19,095 19,092 54 65 65
Lake Susan 1,082 236 319 392 713 908 1,074 171,498 225,324 271,391 588 765 916
TOTAL 10,694 1,732 2,127 2,979 5,488 6,370 8,324 1,274,100 1,521,111 2,060,167 4,555 5,458 7,243
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4.3 Current BMP Treatment
The primary BMPs in the City are load reductions through the application of development rules
and street sweeping. These BMPs were evaluated to estimate the pollutant removal and volume
reduction each were currently providing and could be expected to provide in the future. The
reductions were summed to estimate the total amount of pollutant removal and applied to the
load increase resulting from land use change to determine the net change in pollutant load and
stormwater volume between 1990 and 2020.
4.3.1 Development Rules
To evaluate the impact of application of development rules, each parcel in the city was assigned
a treatment level depending on when it was built and the treatment rules that were in place at that
time. For each subwatershed, raw and treated TSS and TP loads were calculated for each
subwatershed based on land use in 1990,2000, and 2020.
The reduction attributed to development rules was calculated as follows. Say a parcel that was
vacant in 1990 generated a 30 pound per year TSS load. After development, it generated a raw
load of 100 pounds per year TSS. Treatment rules were applied to the raw post-development
load: 100 pounds * 80 percent removal = 80 pounds removed per year. This parcel generated a
raw load increase of 70 pounds per year, but treatment resulted in 80 pounds per year TSS
removal. Thus development can result in a net decrease in TSS and TP load.
Treatment rules have been in place since 1988, so development after 1988 was assumed to be
treated to NURP standards (80 percent removal TSS, 60 percent removal TP). However, as
discussed above, some development may not have required treatment. The estimated removal
due to rules was calculated as the raw load increase times the 80 percent NURP removal rate,
times the estimated fraction of development built under the rules shown in Table 3.1.
Table 4.2. Predicted TSS and TP load increases between 1990 and 2020 and estimated removal due to
d I I
eve opment ru es.
Total TSS Development Total TP Development
Increase Rules Increase Rules
Watershed 1990 - 2020 TSS Removal 1990 - 2020 TP Removal
ID (Ibs/vr) (Ibs/vr) (Ibs/vr) (Ibs/vr)
Lake Ann 6,439 3,975 22 11
Chaska Creek 15,544 17,095 51 88
Lotus Lake 3,905 14,135 13 184
Lower MN River 16,853 16,079 55 88
Lake Lucy 48,943 66,698 160 214
Minnehaha Creek WD 36,351 51,212 119 165
Lake Mitchell 20,663 34,025 99 162
Rice Marsh Lake 28,715 45,232 95 339
Lake Riley 57,091 68,975 265 245
Riley Purgatory Bluff Creek WD 448,348 469,214 1,470 1187
Silver Lake 3,321 4,829 11 27
Lake Susan 99,893 141,641 328 362
TOTAL 786,066 933,108 2,688 3,073
City of Chanhassen
Nondegradation Assessment
22
DR AFT December 07
4.3.2 Street Sweeping
The City of Chanhassen has a street sweeping program and maintains detailed records of
material removed from streets. Much of the volume of material that accumulates on streets is
trash and organic material such as leaves, and sand and sediment particles that are too heavy to
become suspended in stormwater as TSS. To estimate the fraction of material removed by street
sweeping that would likely have become a part of the TSS load, three samples of street
sweepings taken in nearby Eden Prairie were analyzed and an actual particle size distribution
developed. Based on that analysis, it is estimated that about 5.5 percent of the volume of street
sweepings was of a particle size that fell into the P8 TSS particle size class distribution (see
Table 4.3). That percent was applied to the average annual volume of street sweepings removed
to obtain an estimate of TSS removal from sweeping. To calculate the estimated annual TP
removal by street sweeping, the P8 fraction of TP associated with TSS (3,850 mglkg) was
applied to the 3.3 percent that fell into the particle size distribution PO% to P50%. A literature
value of 300 mglkg was applied to the 2.2 percent that fell into the P80% class. The annual
average removal of TSS and TP was allocated among the watersheds based on the impervious
acres in that subwatershed, assuming that the subwatersheds with more impervious acres had
more road surface that were swept (Table 4.4).
Table 4.3. Particle size breakdown by NURP particle size class for three samples taken from street sweepings
in Eden Prairie.
NURP % of Sample Particle Class
Particle Class Sample 1 Sample 2 Sample 3 Average %
PO% 0.0 0.0 0.0 0.00
PlO% 1.6 1.6 1.1 1.43
P30% 0.5 0.5 0.0 0.33
P50% 2.2 1.7 0.8 1.57
P80% 2.7 2.7 1.1 2.17
Total 2.7 6.5 3.0 5.50
T bl 4 4 TSS d TP
It f t t
. th C't f Ch h
a e . . an remova as a resu o s ree swee lID\! ID e llV 0 an assen.
Impervious Watershed TSS Removal TP Removal
Watershed ID Area in 2000 (acres) (pounds) (pounds)
Lake Ann 11 951 2
Chaska Creek 41 3,521 9
Lotus Lake 241 20,579 50
Lower MN River 76 6,504 16
Lake Lucy 117 9,986 24
Minnehaha Creek WD 386 32,983 80
Lake Mitchell 70 6,019 15
Rice Marsh Lake 209 17,834 43
Lake Riley 99 8,481 21
Riley Purgatory Bluff Creek
WD 528 45,118 110
Silver Lake 28 2,385 6
Lake Susan 319 27,247 66
TOTAL 2,127 181,608 441
City of Chanhassen
Nondegradation Assessment
23
DR AFT December 07
4.3.3 Volume
Estimated volume increases for the major subwatersheds in the City are presented in Table 4.5.
Although there is potential for a net loss from evaporation from constructed ponds, no BMPs or
loss have been quantified for this study.
T bl 4 5 P d' t d
. th C't f Ch h
b t
1990 d 2020.
a e . . re IC e vo ume Increases In e uyo an assen e ween an
Watershed Total Flow Increase
Watershed ID Area (acres) 1990 - 2020 (ac.-ft./ yr)
Lake Ann 130 28
Chaska Creek 177 56
Lotus Lake 998 14
Lower MN River 509 60
Lake Lucy 707 175
Minnehaha Creek WD 2,047 131
Lake Mitchell 129 76
Rice Marsh Lake 586 105
Lake Riley 683 204
Riley Pur~atory Bluff Creek WD 3,532 1,614
Silver Lake 113 12
Lake Susan 1,082 361
TOTAL 10,694 2,836
4.3.4 TSS Loading
The total predicted increase in TSS loading from 1990 to 2020 was compared to removals by
active BMPs in the City to assess compliance with nondegradation. Except for the Lake Ann
subwatershed, annual TSS removals exceeded the estimated increase between 1990 and 2020
(Table 4.6).
T bl 46 Pr d' d TSS I d'
b
1990 d 2020 .
BMP
a e . . e lete oa Increases etween an In comparison to remova s.
Total TSS TSS Removal Net
Increase 1990-2000 2000-2020 Removal (-) or
Watershed 1990 - 2020 Development Development Street Cleaning Net Increase (+)
ID (Ibs/yr) Rules (lbs) Rules (lbs) (lbs) (lbs)
Lake Ann 6,439 179 3,796 1,619 +845
Chaska Creek 15,544 4,610 12,485 4,646 -6,197
Lotus Lake 3,905 5,775 8,360 20,516 -30,746
Lower MN River 16,853 338 15,741 8,772 -7,998
Lake Lucy 48,943 16,459 50,239 14,124 -31,879
Minnehaha Creek
WD 36,351 10,796 40,415 36,649 -51.509
Lake Mitchell 20,663 17,927 16,098 6,010 -19,372
Rice Marsh Lake 28,715 14,769 30,463 19,708 -36,225
Lake Riley 57,091 13,242 55,733 14,635 -26,519
Riley Purgatory
Bluff Creek WD 448,348 70,377 398,837 91,889 -112,755
Silver Lake 3,321 1,599 3,229 2,376 -3,884
Lake Susan 99,893 48,567 93,074 33,502 -75,251
TOTAL 786,066 204,637 728,471 254,446 -401,488
City of Chanhassen
Nondegradation Assessment
24
D R AFT December 07
4.3.5 TP Loading
The total predicted increase in TP loading from 1990 to 2020 was compared to removals by
active BMPs in the City to assess compliance with nondegradation. For all of the subwatersheds
except Lake Ann and Riley-Purgatory-Bluff Creek, annual TP removals exceeded the estimated
increase between 1990 and 2020 (Table 4.7).
T bl 47 Pr d' t d TP I d'
b t
1990 d 2020 .
t BMP
a e . . e IC e oa lDcreases e ween an lD comparIson 0 remova s.
Total TP TP Removal Net
Increase 1990-2000 2000-2020 Removal (-) or
Watershed 1990 - 2020 Development Development Street Cleaning Net Increase (+)
ID (Ibs/yr) Rules (Ibs) Rules (Ibs) (Ibs) (Ibs)
Lake Ann 22 1 10 4 +8
Chaska Creek 51 38 50 11 -47
Lotus Lake 13 77 107 48 -219
Lower MN River 55 22 67 20 -54
Lake Lucy 160 66 149 33 -87
Minnehaha Creek
WD 119 43 122 85 -131
Lake Mitchell 99 85 77 14 -76
Rice Marsh Lake 95 171 168 46 -291
Lake Riley 265 53 193 34 -14
Riley Purgatory
Bluff Creek WD 1,470 201 986 213 +70
Silver Lake 11 14 14 6 -22
Lake Susan 328 128 234 78 -111
TOTAL 2,688 898 2.174 589 -974
4.4 Model Sensitivity
Because the model is not calibrated and is only used for comparison purposes, it is useful to test
the sensitivity of model to selected inputs, especially as we compare model results to real-world
measured removals.
The first parameter that may affect the results of the model is the selection of the particle file.
The current modeling is based on the NURP50 file, which represents the median concentrations
from the NURP studies. To assess the potential effects of underestimating runoff concentrations,
the Rice Marsh subwatershed was analyzed using both the median runoff concentrations
(NURP50) and 90th percentile runoff concentrations (NURP90).
Results of the 50th and 90th percentile runs are presented in Table 4.8 and Table 4.9. Even if the
actual concentrations were more similar to 90tfi percentile concentrations for both TSS and TP,
the City of Chanhassen would be meeting the nondegradation requirement.
City of Chanhassen
Nondegradation Assessment
25
DR AFT December 07
Table 4.8. Rice Marsh subwatershed TSS loading and removals for the 50th and 90th percentiles of the
NURP studies.
Total TSS TSS Removal Net
Increase Removal (-) or
Watershed 1990 - 2020 Development Street Net Increase (+)
ID (Ibs/vr.) Rules (Ibs) Cleaning (lbs) (lbs)
Rice Marsh
NURP 50 27,193 45,232 17,834 -35,873
Rice Marsh
NURP 90 81,577 114,250 17,834 -50,507
Table 4.9. Rice Marsh subwatershed TP loading and removals for the 50th and 90th percentiles of the NURP
studies.
Total TP TP Removal Net
Increase Removal (-) or
Watershed 1990 - 2020 Development Street Net Increase (+)
ID (lbs/vr) Rules (lbs) Cleaning (lbs) (lbs)
Rice Marsh
NURP 50 95 339 43 -287
Rice Marsh
NURP 90 202 702 43 -543
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5.0 SUMMARY AND PROPOSED BMPS
The City of Chanhassen, in conjunction with the Riley-Purgatory- Bluff Creek and Minnehaha
Creek Watershed Districts, has been actively managing water quality for many years. These
activities have included developing and enforcing rules to minimize water quality impacts from
development; building and maintaining BMPs throughout the city; and actively sweeping city
streets to remove as much material as possible from the impervious surfaces. These activities
have been very effective in the mitigation of increased TSS and TP loads from the watershed.
The increase in impervious surface between 1990 and 2000 has resulted in increased stormwater
runoff volume, and that volume is expected to continue to increase with further development.
Volume is difficult to remove from the watershed, and it is unlikely that the City will be able to
attain 1988 volume conditions. However, the effects of increased runoff volume can be
minimized or mitigated through stream bank and shoreline stabilization programs, infiltration,
and rate control. Consequently, the best approach to addressing volume is through aggressive
abstraction and mitigation and opportunistic volume control.
5.1 Special and Impaired Waters
Bluff Creek was added to the State of Minnesota 303( d) list of Impaired Waters in 2002 for
excess turbidity and on 2004 based on its fish community index of biotic integrity score.
TMDLs have not yet been completed for those impairments. Lotus Lake was added to the
303(d) list in 2002 for excess nutrients, and Lake Riley in 2002 for excess nutrients and mercury-
fish consumption. Seminary Fen and Assumption Creek are Outstanding Resource Value
Waters.
5.2 Total Suspended Solids
The combination of development rules and street sweeping has proved effective in reducing the
volume of total suspended solids to a level well below that estimated to have been contributed in
1988. Most ofthat removal is a result of application of development rules and other regulations,
but almost one-quarter is achieved through the aggressive street maintenance program. Each
subwatershed except Lake Ann meets the Nondegradation Total Suspended Solids requirement
by removing more TSS through regulation and BMPs than the new load generated since 1988
(see Table 4.6). It should be noted that if any future TMDLs for any water resource to which
Chanhassen discharges requires additional TSS load reduction, then the City may be required to
remove additional TSS or TP load beyond simply meeting the Nondegradation TSS and TP
requirement.
5.2.1 Regulatory Program
Development rules will continue to require land-altering activities to provide at least NURP-Ievel
removal (80%) of Total Suspended Solids prior to discharge. In accordance with its NPDES
City of Chanhassen
Nondegradation Assessment
27
DR AFT December 07
permit, the City annually inspects 20 percent of those detention ponds that are publicly-owned to
determine maintenance needs and evaluate whether they continue to perform at the level to
which they were designed. City ordinance requires owners of private stormwater facilities to
perform regular maintenance and to operate those facilities so they continue to perform at the
level to which they were designed. Developer's Agreements specify required maintenance. The
City's Stormwater Management Plan designates Assumption Creek and Seminary Fen as
"Preserve" classification water resources, requiring a higher level of treatment for discharges to
those waters, including temperature controls.
5.2.2 Street Sweeping
This maintenance program removes not only smaller particles that contribute to Total Suspended
Solids, but also larger particles of inorganic and organic material that can contribute to
downstream clarity and water quality issues as well as obstruct pipes, create sediment deltas at
outlets, and aggrade in streams and impair biotic functions. Street sweeping also improves
traffic safety and general aesthetics by removing unsafe or unsightly materials from the streets.
The City will continue to sweep streets once per year -in the spring to remove sand and salt
residue from winter de-icing plus any spring leaves and seeds and old organic material. Other
street sweeping is performed throughout the year as necessary.
Recent research in Wisconsin (Bannerman 2007) indicates that the incremental water quality
benefit of increased street sweeping is small unless the frequency of sweeping is very aggressive.
Street sweeping practices will be refined to improve the water quality and overall benefit of
street sweeping:
1. Spring street sweeping will be performed as early as possible to remove material from the
street before the spring rains.
2. Increased street sweeping frequency may be of benefit in sensitive locations, such as in the
Lake Ann subwatershed or where there is noticeable aggradation in streams or below
outfalls or where a UAA or TMDL indicates that efforts should be maximized to remove
pollutant loading. The City will evaluate the potential benefits of more frequent street
sweeping in sensitive areas.
3. Timing of street sweeping will be evaluated to maximize its efficacy, for example,
sweeping prior to hydrant flushing.
5.2.3 Lake Ann Subwatershed
The Lake Ann subwatershed is projected to generate 845 pounds more TSS in 2020 than was
generated in 1990. This sub watershed is very small, and has a low watershed to lake area ratio.
The lake also has good water quality and clarity. There are several outfalls into Lake Ann. The
City will investigate the possibility of retrofitting those outfalls with sump manholes or other
devices to collect sediment prior to discharge into the lake. The City will also look for
opportunities to reduce runoff by increasing abstraction in the subwatershed, and evaluate land
use policies governing development for opportunities to reduce the creation of new TSS.
City of Chanhassen
Nondegradation Assessment
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D R AFT December 07
5.2.4 Other Activities
The volume management activities described in section 5.3 below such as increased infiltration,
biofiltration, and runoff volume controls may further reduce TSS and TP loading by pre-treating
or reducing the amount of runoff volume conveying TSS and TP to waters.
5.3 Total Phosphorus
As with TSS, the combination of development rules, manhole cleanout, and street sweeping has
proved effective in reducing the volume of total phosphorus to a level below that estimated to
have been contributed in 1988. Most of that removal is a result of application of development
rules, but almost 20 percent is achieved through the aggressive street maintenance program.
Each subwatershed meets the Nondegradation Total Phosphorus requirement by removing more
TP through regulation and BMPs than the new load generated since 1988 (see Table 4.7). It
should be noted that if any future TMDLs for any water resource to which land in Chanhassen
drains requires additional TP or TSS load reduction, then the City may be required to remove
additional TP or TSS load beyond simply meeting the Nondegradation TP and TSS requirement.
5.3.1 Regulatory Program
Development rules will continue to require land-altering activities to provide at least NURP-Ievel
removal (60%) of Total Phosphorus prior to discharge. In accordance with its NPDES permit,
the City annually inspects 20 percent of those detention ponds that are publicly-owned to
determine maintenance needs and evaluate whether they continue to perform at the level to
which they were designed. City ordinance requires owners of private stormwater facilities to
perform regular maintenance and to operate those facilities so they continue to perform at the
level to which they were designed. Developer's Agreements specify required maintenance. The
City's Stormwater Management Plan designates Assumption Creek and Seminary Fen as
"Preserve" classification water resources, requiring a higher level of treatment for discharges to
those waters, including temperature controls.
5.3.2 Street Sweeping
This maintenance program removes not only smaller particles that are a fraction of the TSS load,
but also larger particles of organic material that can contribute to downstream clarity and water
quality issues. Street sweeping also improves traffic safety and general aesthetics by removing
unsafe or unsightly materials from the streets, and reduces the amount of material on the street
that can clog catch basin covers and limit street drainage.
The City will continue to sweep streets once per year -in the spring to remove sand and salt
residue from winter de-icing plus any spring leaves and seeds and old organic material. Other
street sweeping is performed throughout the year as necessary.
Recent research in Wisconsin (Bannerman 2007) indicates that the incremental water quality
benefit of increased street sweeping is small unless the frequency of sweeping is very aggressive.
City of Chanhassen
Nondegradation Assessment
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D R AFT December 07
Street sweeping practices will be refined to improve the water quality and overall benefit of
street sweeping:
1. Spring street sweeping will be performed as early as possible to remove material from the
street before the spring rains.
2. Increased street sweeping frequency may be of benefit in sensitive locations, where there is
noticeable aggradation in streams or below outfalls or where a UAA or TMDL indicates
that efforts should be maximized to remove pollutant loading. The City will evaluate the
potential benefits of more frequent street sweeping in sensitive areas.
3. Timing of street sweeping will be evaluated to maximize its efficacy, for example,
sweeping prior to hydrant flushing.
5.3.3 Lake Ann and Riley-Purgatory-Bluff Creek Subwatersheds
The Lake Ann subwatershed is projected to generate 8 pounds more TP in 2020 than was
generated in 1990, with 70 additional pounds TP estimated in the Riley-Purgatory-Bluff Creek
subwatershed.
The Lake Ann subwatershed is very small, and has a low watershed to lake area ratio. The lake
also has good water quality and clarity. There are several outfalls into Lake Ann. Reduction in
TSS loading from those outfalls as described above will also reduce phosphorus loading. The
City will investigate the possibility of retrofitting those outfalls with sump manholes or other
devices to collect sediment prior to discharge into the lake. The City will also look for
opportunities to reduce runoff by increasing abstraction in the subwatershed, and evaluate land
use policies governing development for opportunities to reduce the creation of new TP.
The Riley-Purgatory-BluffCreek subwatershed encompasses about one-third of the city's area.
Impervious cover is expected to double between 2000 and 2020. A significant portion of this
subwatershed falls within the City's Bluff Creek Overlay District, described in Section 3.1.1
above. This ordinance contains a number of zoning and planning tools that will be applied to
development and redevelopment in this overlay district as part of the City's overall policy to
protect the Bluff Creek Corridor and its wetlands, bluffs, wooded areas, and water resources. If
the regulatory program is not successful in limiting new TP the City will incorporate TP
reduction BMPs into City projects as opportunities arise.
5.3.4 Other Activities
The volume management activities described in section 5.3 below such as increased infiltration,
biofiltration, and runoff volume controls may further reduce TP and TSS loading by pre-treating
or reducing the amount of runoff volume conveying TP and TSS to waters.
5.4 Volume
This report estimates that stormwater volume in the city will increase by about 50 percent
between 1990 and 2020, with most of that volume increase occurring between 2000 and 2020.
City of Chanhassen
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DR AFT December 07
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A secondary but important benefit of abstraction is that volume is no longer being contributed to
runoff, and is not picking up and transporting pollutants to downstream waters. Research
conducted by Prof. Robert Pitt (Pitt 1998) in the City of Milwaukee suggests that 25 percent of
the annual total phosphorus load is conveyed in the first one-half inch of rainfall, and 15 percent
of the annual TSS load. For the first inch of rainfall, that load is 65 percent of annual TP and 50
percent of annual TSS. In the Twin Cities, 90 percent of all rainfall events are one inch or less.
The most common form of abstraction is infiltration. A growing number of watershed
management organizations are enacting infiltration rules, with one-half inch infiltration from
new impervious surface common. The Ramsey-Washington County Watershed District and
Capitol Region Watershed Districts recently enacted a one inch from new impervious surface
rule, and the Minnehaha Creek Watershed District is considering one inch as well.
Adopting an abstraction requirement will reduce the new volume of runoff from developing and
redeveloping areas. Chanhassen soils are mapped as generally being of soil hydrologic group B.
These soils readily infiltrate. However, do to the highly urbanized land use throughout much of
Chanhassen actual infiltration rates may be diminished from those anticipated for the mapped
soil unit. There are pockets of less permeable C and D soils. Most development and
redevelopment should be able to successfully incorporate infiltration.
An abstraction requirement for new development will only address new volume predicted to
occur between its promulgation and 2020 - about 1,950 acre-feet. However, in the very long-
term, as existing development redevelops, the new volume generated between 1990 and 2000
and prior will slowly be mitigated as redevelopment and reconstruction occurs.
The projected new storm water volume between 2000 and 2020 is 1,950 acre-feet. A one-half
inch abstraction requirement might reduce new stormwater volume by an estimated 65 percent,
or 1,270 acre-feet, leaving an estimated net new stormwater volume of 680 acre-feet.
The City will work with the Watershed Districts to review and consider adopting a volume
management requirement for new development and redevelopment.
Soil Management Plan requirement. An assumption in calculating stormwater volume to be
captured from an abstraction requirement is that the part of the site that is pervious and
infiltrating stormwater pre-construction will be as pervious and infiltrate at least as much
stormwater volume post-construction. In reality, construction activities such as mass grading,
road and utility construction, and structure construction lead to soil compaction and loss of
permeability.
To maximize the effectiveness of an infiltration requirement, the City will consider revising its
ordinances to require developers to submit a Soil Management Plan that details how the
developer plans to 1) minimize soil compaction from construction activities, 2) phase
construction activities to minimize exposed soils, and 3) restore site permeability through actions
such as post-construction soil ripping or soil amendment.
City of Chanhassen
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D R AFT December 07
Encourage Low Impact Development (LID). Low Impact Development (LID), Integrated
Management Practices (IMP), and Better Site Design principles and practices encourage
developers to consider stormwater impacts of new development early in the planning process.
The City's zoning code includes provisions for Planned Unit Developments that may vary from
strict application of zoning requirements to make it easier to tailor a development to the terrain
and minimize stormwater impacts such as new volume. The City will review its zoning code and
Planned Unit Development process to evaluate existing zoning or development incentives to
encourage developers to creatively apply LID, IMP and better site design principles to minimize
new stormwater volumes and pollutant loading and to consider revisions to its zoning code and
PUD process.
5.4.2 Education Program
The City has in place an education and outreach program to provide information to various
audiences on a wide variety of stormwater management and water quality issues. This program
will target both residential property owners and commercial property owners on ways to
incorporate small infiltration practices on their property. Small-scale practices such as rain
gardens, conversion of turf to native vegetation, pond and wetland buffers, installation of rain
barrels and cisterns, and use of pervious pavement and pavers, can add up to significant water
quality and stormwater reductions when broadly applied. The City will utilize its currently
existing educational programming as well as explore cooperative opportunities with outside
agencies and will also develop targeted information for developers on incorporating Low Impact
Development principles and water quality and volume management into development and
redevelopment.
5.4.3 Structural Improvements
Structural BMPs to manage storm water volume will be constructed as stand-alone projects or
incorporated into other projects such as street reconstruction as opportunities arise. These BMPs
store and infiltrate or evapotranspirate storm water at the street, block, or neighborhood scale.
The cost per acre-foot for these types of BMPs can be significant as stand-alone projects, but
may be more cost-effective when combined with other improvements such as a neighborhood
street reconstruction project. These types of BMPs also will have the benefit of further reducing
pollutant loading.
Where it is difficult or too costly to significantly reduce new stormwater volume, downstream
water resources will be protected from adverse effects through restoration and stabilization
projects.
Infiltration basins. An infiltration basin is a shallow, vegetated basin designed to hold and
infiltrate stormwater. Some evapotranspiration also occurs. Infiltration basins also have an
ongoing significant maintenance cost. Over time sediment discharged into the basins
accumulates and reduces the infiltration capacity. This sediment must be removed periodically
and permeability restored through soil ripping, soil amendment, etc.
City of Chanhassen
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D R AFT December 07
Constructed wetlands. Constructed wetlands can decrease stormwater volume through
evapotranspiration from wetland vegetation as well as evaporation from open water stored in the
wetland.
Underground infiltration. New technologies are available to construct infiltration devices under
large paved areas such as parking lots or streets Integrated Management Practices (IMP), and
Better site Design principles and practices. There is an ongoing annual cost to remove and
dispose of accumulated material from the device.
Streambank stabilization. The new volume of storm water generated through land use change is
conveyed through the system of lakes, wetlands, and small channels into the major streams that
drain the City: Bluff Creek and Riley Creek and to a lesser extent Assumption Creek. This
additional volume increases peak flow rates and elevations in the streams, and has the potential
to destabilize and erode streambanks. The increased peak flows and streambank erosion also
may stress biological communities in the streams.
These adverse impacts can be mitigated by strengthening streambanks and restoring areas of
existing erosion, as well as considering structural amendments to the stream channel such as rock
vanes. Habitat lost to past impacts can be mitigated with this restoration by re-vegetating
streambanks, adding step pools, and enhancing substrate.
The City and the watershed districts have performed general inventories of the major streams to
identify areas of existing erosion. The City is currently taking steps to stabilize a number of the
identified erosion areas and will continue to work in partnership with the watershed districts to
undertake a program of stream restoration. These projects would range from simple spot repairs
of streambanks to potential total restoration of the stream. These projects will combine both
stabilization and habitat enhancement.
5.4.4 Nonstructural Improvements
Reforestation. Urban trees are an integral part of the storm water management system. Trees
intercept rain and snow fall, storing event volume for later evaporation. Trees also use surface
storage for transpiration, removing additional storm water volume from the stormwater system.
Trees have other benefits as well. Trees increase property values, provide habitat, take in C02
and release oxygen, enhance neighborhood aesthetics, and decrease home heating and cooling
costs. The City recognizes this value by requiring, by ordinance, replacement of trees lost to
construction or land alteration.
Numerous studies have been conducted estimating the volume ofrainfall that trees intercept (for
example, Xiong et al 2000, McPherson 2005) This annual volume is dependant on the type and
species of tree, and whether the tree is isolated, such as an urban street tree, or part of a forest
canopy. For example, evergreen trees can intercept and store more stormwater than deciduous
trees because their narrow leaves provide a greater leaf surface area. The volume also depends
on the intensity of the storm, with less intense events resulting in more interception. On average,
however, literature suggests that an isolated deciduous tree can intercept approximately 25
percent of the annual volume of rainfall that falls upon it. Combined with the daily water use for
City of Chanhassen
Nondegradation Assessment
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DR AFT December 07
transpiration (Wullschleger 1998), it is estimated that a single deciduous tree can
evapotranspirate 2,000 - 3,000 gallons of stormwater per year. A coniferous tree can
evapotranspirate an even greater amount.
It is estimated that about 10,000 trees could evapotranspirate about 100 acre-feet of stormwater
volume. The City will evaluate its existing reforestation program and current preservation
practices to reduce the amount of rainfall that runs off the landscape.
WetlandlUpland Restoration. The City has restored dozens of acres of wetlands and will
continue to look for additional opportunities to restore wetland areas and the associated upland.
Wetland vegetation is very efficient at transpiring significant volumes of water, reducing the
overall volume discharged to water resources downstream. Native vegetation planted in restored
uplands can increase the infiltration capacity of poorer soils by increasing organic matter in the
soil and by creating long root channels to convey water deep into the soil.
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6.0 SWPPP MODIFICATIONS
The following are the proposed modifications to the City of Chanhassen SWPPP to address the
findings of this analysis.
6.1 Total Suspended Solids
The City currently meets the Total Suspended Solids requirement. Modifications to the SWPPP
are confined to some refinement of the BMPs for those activities that reduce TSS throughout the
City, and specifically in the Lake Ann subwatershed.
6.1.1 Regulatory Program
Development rules will continue to require land-altering activities to provide at least NURP-Ievel
removal (80%) of Total Suspended Solids prior to discharge. The City's Surface Water
Management ordinance specifies design criteria, including NURP-Ievel removals. BMP 5b-l
Regulatory Mechanism to Address Post Construction Runoff from New Development and
Redevelopment provides for review and update of ordinances to ensure they are adequate and
consistent with storm water management plan requirements. No additional update is necessary.
6.1.2 Street Sweeping
Increased street sweeping frequency may be of benefit in sensitive locations, where there is
noticeable aggradation in streams or below outfalls or where a UAA or TMDL indicates that
efforts should be maximized to remove pollutant loading. BMP 6a-2 Street Sweeping will be
revised to add an evaluation of locations where additional street sweeping would be most
effective. Based on that evaluation the City may perform additional sweeping in those sensitive
areas.
6.1.3 Lake Ann Subwatershed
BMP 5a-l will be revised to include an evaluation of land use policies for the Lake Ann
watershed that would minimize increase in TSS and also complement the Comprehensive Land
Use plan promoting the dedication of park land and preservation of buffers and open space
within the watershed. Such policies may include park expansion, increased setbacks or buffers,
tiered density, promotion of cluster housing, or restrictions on the amount of land in anyone
phase of construction. The City will continue to actively manage the forested area within Lake
Ann Park to maximize canopy cover as well as look for additional opportunities in Lake Ann
Park for abstraction.
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Opportunities for additional stormwater treatment, abstraction and infiltration will be identified
within the Lake Ann watershed and the City will work with adjacent property owners to facilitate
these opportunities.
6.1.4 Other Activities
The volume management activities described in section 5.3 above such as increased infiltration,
biofiltration, and runoff volume controls may further reduce TSS loading by pre-treating or
reducing the amount of runoff volume conveying TSS to waters. BMP 5a-1 Development and
Implementation of Structural and/or Non-structural BMPs will be amended to incorporate these
practices into city construction projects.
6.2 Total Phosphorus
The City currently meets the Total Phosphorus requirement. Modifications to the SWPPP are
confined to some refinement of the BMPs for those activities that reduce TP throughout the City,
and specifically in the Lake Ann and Riley-Purgatory-Bluff Creek subwatersheds.
6.2.1 Regulatory Program
Development rules will continue to require land-altering activities to provide at least NURP-level
removal (60%) of Total Phosphorus prior to discharge. The City's Surface Water Management
ordinance specifies design criteria, including NURP-level removals. BMP 5b-1 Regulatory
Mechanism to Address Post Construction Runoff from New Development and Redevelopment
provides for review and update of ordinances to ensure they are adequate and consistent with
storm water management plan requirements. No additional update is necessary.
6.2.2 Street Sweeping
Increased street sweeping frequency may be of benefit in sensitive locations, where there is
noticeable aggradation in streams or below outfalls or where a UAA or TMDL indicates that
efforts should be maximized to remove pollutant loading. BMP 6a-2 Street Sweeping will be
revised to add an evaluation of locations where additional street sweeping would be most
effective. Based on that evaluation the City may perform additional sweeping in those sensitive
areas. The City will also review the coordination and timing of street sweeping activities with
other City management activities to maximize efficacy of sweepings.
6.2.3 Lake Ann and Riley Purgatory Bluff Creek Subwatersheds
The BMPS identified in Section 6.1.3 above would also reduce TP load to Lake Ann. A
significant portion of this subwatershed falls within the City's Bluff Creek Overlay District,
described in Section 3.1.1 above. This ordinance contains a number of zoning and planning tools
that will be applied to development and redevelopment in this overlay district as part of the
City's overall policy to protect the Bluff Creek Corridor and its wetlands, bluffs, wooded areas,
and water resources. BMP 5b-1 Regulatory Mechanism to Address Post Construction Runoff
from New Development and Redevelopment will be revised to provide for a review of the
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Overlay District ordinance to determine whether revisions are necessary to provide additional
protection.
6.2.4 Other Activities
The volume management activities described in section 5.3 above such as increased infiltration,
biofiltration, and runoff volume controls may further reduce TP loading by pre-treating or
reducing the amount of runoff volume conveying TP to waters. BMP 5a-l Development and
Implementation of Structural and/or Non-structural BMPs will be amended to incorporate these
practices into city construction projects.
6.3 Volume
The City will work cooperatively with the three watershed districts with land in the city to
coordinate regulatory volume management BMPs with watershed district rules.
6.3.1 Regulatory Program
Abstraction requirement BMP 5b-l Regulatory Mechanism to Address Post Construction Runoff
from New Development and Redevelopment will be revised to provide for review and
amendment of local controls and the local water management plan to evaluate and implement an
abstraction/infiltration requirement for new development and redevelopment that is consistent
with watershed district requirements.
Soil Management Plan requirement. BMP 5b-l Regulatory Mechanism to Address Post
Construction Runoff from New Development and Redevelopment will be revised to provide for
review and amendment of local controls and the local water management plan to evaluate and
implement a soil management plan requirement for new development and redevelopment.
Evaluate regulatory means to encourage Better Site Design. BMP 5b-l Regulatory Mechanism
to Address Post Construction Runoff from New Development and Redevelopment will be
revised to provide for a review of City Ordinances and procedures to identify means such as
zoning or development incentives to encourage developers to incorporate Low Impact
Development (LID), Integrated Management Practices (IMP) or Better Site Design principles to
minimize new stormwater volumes and pollutant loading.
6.3.2 Education Program
The City undertakes an education and outreach program to providing education to various
audiences on a wide variety of stormwater management and water quality issues. These are
described in various BMPs implementing Minimum Measure 1: Public Education and Outreach.
These BMPs adequately generally describe the education and outreach program. Specific topics
to be covered in these activities vary from year to year. The annual list of topics will include
information on small practices for water quality and volume management such as rain gardens,
conversion of turf to native vegetation, installation of rain barrels and cisterns, and use of
pervious pavement and pavers. Topics will also include developer education regarding
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storm water management. Other educational and outreach activities will include continuation of
the existing Arbor Day tree sale and promotion of the Carver County Soil and Water
Conservation District's seedling pack purchase program. Current methods to communicate with
the public will be evaluated and revised as necessary.
6.3.3 Structural Improvements
Structural Projects. Structural BMPs will be constructed as stand-alone projects or incorporated
into other projects such as street reconstruction as opportunities arise. The highest need is for the
reduction of new stormwater volume generated since 1988. Most structural volume-management
improvements are not cost-effective as stand-alone projects, but may be more cost-effective if
they can be incorporated into other projects such as street reconstruction. BMP 5a-l
Development and Implementation of Structural and/or Non-structural BMPs will be revised to
incorporate pollutant loading and volume management BMPs on city projects.
Mitigation Projects. Where it is difficult to significantly reduce new storm water volume,
downstream water resources can be protected from adverse effects through restoration and
stabilization projects. BMP 5a-l Development and Implementation of Structural and/or Non-
structural BMPs will be revised to incorporate development of a stream and shoreline restoration
program to mitigate past erosion, enhance habitat, and prevent future erosion. The City will
continue identifying eroded gullies, escarpment and wash areas and begin to cooperatively
mitigate these areas whenever possible with the Watershed Districts. This program may include
city or watershed district projects, and assistance to property owners in completing such projects.
6.3.4 Nonstructural Improvements
Reforestation. BMP 5a-l Development and Implementation of Structural and/or Non-structural
BMPs will be revised to include a goal to provide opportunities for resident to purchase or plant
trees for increased storm water abstraction based upon the existing spring tree sale held annually
by the City and the tree coupon program the City currently operates.
The City will continue its policy of preserving existing woodland areas where opportunities arise
through density transfer, conservation easements, park dedication and other planning tools.
BMP 5b-l Regulatory Mechanism to Address Post Construction Runoff from New Development
and Redevelopment describes the ordinance review process the City will undertake to determine
if its regulatory program is adequate to effectively prevent or reduce nonpoint source pollution.
As part of that review process the City will review its existing Landscaping and Tree
Preservation Requirements and revise if necessary to promote tree preservation and reforestation.
WetlandlUpland Restoration. BMP 5a-l Development and Implementation of Structural and/or
Non-structural BMPs will be revised to note that the city will continue to look for opportunities
to undertake wetland and upland restorations as volume management BMPs that are part of an
overall volume management program.
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References
Bannerman, R.T. et al. 1993. Sources of pollutants in Wisconsin storm water. Wisconsin Department of
Natural Resources. Water, Science and Technology. 28 (3-5): 241-259.
Bannerman, R.T. 2007. Reducing the uncertainty in the calculations of street cleaner performance for
Wisconsin municipalities. Wisconsin Department of Natural Resources. Unpublished research.
Frelich, Lee E. 1992. Predicting dimensional relationships for Twin Cities shade trees. S1. Paul, MN:
University of Minnesota, Department of Forest Resources.
McPherson, G. et al. 2005. Municipal forest benefits and costs in five US cities. Journal of Forestry.
December 2005: 411-416.
Minnesota Department of Transportation. 2005. The cost and effectiveness of stormwater management
practices. Report No. MNIRC - 2005-23. S1. Paul, MN: MnDOT Research Services Section.
Minnesota Pollution Control Agency. 2000. Protecting Water Quality in Urban Areas. St. Paul, MN. "
http://www.pca.state.mn.us/water/oubs/sw-bmomanual.html>>
Minnesota Pollution Control Agency. 2006. Minnesota Stormwater Manual. S1. Paul, MN. <<
http://www.pca.state.mn.us/water/stormwater/stormwater-manual.html>>
Obermeyer, B. Riley-Purgatory Creek-Bluff Creek and Nine Mile Creek Watershed District engineer.
Pers. corresp. 6/8/06, 7/12/06
Pitt, Robert E. April 29-30, 1998. Course notes presented at the workshop: Storm Water Quality
Management through the Use of Detention Basins. Earl Brown Continuing Education Center,
Univ. of Minnesota, S1. Paul, Minn.
Pitt, R., J. Lantrip, and R. Harrison. 1999. Infiltration through disturbed urban soils and compost-
amended soil effects on runoff quality and quantity. Report No. EPAl6001 R-001016 US EPA. National
Risk Management Research Laboratory, Cincinnati, OH.
USEPA. 1999. Preliminary data summary of urban storm water best management practices. Report No.
EPA-821-R-99-012
Wullschleger, S. D., F.e. Meinzer, and R.A. Vertessy. 1998. A review of whole-plant water use studies
in trees. Tree Physiology. 18:499-512.
Xiao, Q., et al. 2000. Winter rainfall interception by two mature open-grown trees in Davis, California.
Hydrological Processes. 14:763-784.
Xiao, Q., et al. 2000. A new approach to modeling tree rainfall interception. Journal of Geophysical
Research. 105(D23):29,173-29,188.
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