SECTION I 1
ORR•SCHELEN•MAYERON a ASSOCIATES, INC.
IConsulting Engineers
Land Surveyors
February 24 , 1975
' Honorable Mayor and City Council
City of Shorewood
20630 Manor Road
' Shorewood, Minnesota 55331
Mayor and Councilmembers :
Enclosed is the Comprehensive Storm Water Study prepared as a
guide for the orderly development of Shorewood' s storm sewer
system. Recommendations are based on utilization of existing
' drainage patterns with consideration for present and future
development.
' We wish to acknowledge appreciation to officials of the City
of Shorewood and Staff, and also various engineering consult-
ants and other Governmental Agencies for information provided
during the compilation of this study.
In the event that additional information or discussion is
desired, we will be most happy to meet with the City' s Repre-
sentatives at their convenience.
Yours very truly,
' ORR-SCHELEN-MAYERON
& ASSOCIATES, INC.
B. A. Mittelsteadt
' BAM/jet
Enclosure
1 2021 East Hennepin Avenue • Suite 239 • Minneapolis, Minnesota 55413 . 612/331-8660
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I hereby certify that this plan, specification,
or report was prepared by me or under my direct
supervision and that I am a Registered Profes-
sional Engineer under the laws of the State of
Minnesota.
(A •O� osi ,
C. L. W zier
Date: February 24 , 1975 Reg. No. 5918
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TABLE OF CONTENTS
Page
Letter to Council
Certification
Table of Contents 1
Section I
Synopsis 3
Recommendations 5
Introduction 6
' Scope of Study 7
Environmental Assessment 9
' Basis of Design 10
Rainfall Data 10
1 Rainfall Design 11
1 Storm Water Runoff 12
Drainage Districts 14
' Adjacent Community Participation 16
Cost Estimate Summary 17
Assessment Policy 18
Section II
11 Drainage District Location Map
1 Individual Drainage Districts
(1) Purgatory Creek 20
(2) Carson Bay 25
(3) Lake William 29
(4) St. Alban' s Bay 32
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Section II Continued
1 (5) Christmas Lake 35
(6) Galpin Lake 39
(7) Gideon Bay 43
(8) Lake Mary 46
(9) Central 49
1 (10) South-Central 51
(11) Lake Minnewashta 53
(12) North-Central 55
1 (13) West-Central 58
(14) Lake Virginia 61
(15) West 64
(16) Islands 66
Appendix
Glossary of Terms
Shorewood Zoning Map, September 1973
Shorewood Wetlands Ordinance
Department of Natural Resources Letter
Re: Established Lake Elevations
Environmental Control Manhole Detail
United States Weather Service ' s Technical Paper 40
1 Basic Flood Hydrograph
Construction Cost Estimate
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� SECTION
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1 SYNOPSIS
1 This Comprehensive Storm Water Study was prepared as a guide
for orderly development of Shorewood' s Storm Sewer System. With-
out such a plan, drainage problems will occur as construction of
1 streets and buildings cause more and more surface runoff.
Shorewood' s Wetlands Ordinance and the Minnehaha Creek Water-
shed Requirements were used as guidelines in the proposed plan .
Wetlands and ponding areas were utilized to minimize storm sewer
costs and provide settling basins . Natural channels should be
' continued wherever possible to augment settling of non-soluble
polluting materials . These channels could be compatible with , and
' may be incorporated into a greenway system.
The determination of storm water runoff is based on rainfall
records , soil type, slope conditions , building cover and street
' area.
Shorewood is divided into 16 natural drainage districts .
Proposed storm sewer follows general street patterns ; however, in
unplatted areas it was necessary to assume development along nat-
ural drainage routes.
' In several of these districts , areas from adjacent communi-
ties drain naturally into Shorewood. In some areas drainage from
' Shorewood flows into other communities . It is necessary to formu-
late cost-sharing agreements with those communities prior to con-
struction of storm sewer which serves more than one community.
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IA storm sewer assessment policy for Shorewood is suggested
based on the total cost of the system as it relates to the contri-
Ibuting area.
Studies of each drainage district--existing conditions and
final storm sewer plan--is presented in Section II of this report.
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fECO" MFMDAT I ONS
' The following recommendations are presented to the City of
Shorewood.
RECOMMENDED :
11 That the City Council adopt this Comprehensive Storm Water
Study as a guide for a storm sewer program. Further, that any
development be in conformance with this plan .
RECOMMENDED:
' That a program be initiated to reduce the quantity of pollut-
ing materials entering the lakes .
RECOMMENDED :
That ponding areas and necessary easements he procurred so
that the storm sewer plan may develop without problems of land
availability.
' RECOMMENDED :
That a storm sewer assessment policy be adopted based on cur-
rent cost estimates , land use and assessable area.
RECOMMENDED :
IThat the areas of common storm water runoff be discussed
with the communities affected. Agreements should be reached prior
' to construction of storm sewer in those areas .
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INTRODUCTION
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The City of Shorewood, located immediately south of Lake
' Minnetonka, is generally within the Lake Minnetonka Watershed.
Lake Minnetonka outlets to the east via Minnehaha Creek to the
' Mississippi. River in Minneapolis . The southeast portion of the
community, however, outlets southeast via Purgatory Creek to the
Minnesota River. This comprehensive drainage plan outlines Shore-
wood' s natural drainage areas and provides for a storm sewer pro-
gram to meet Shorewood's present and ultimate needs .
' As a community develops , changes occur in its natural drain-
' age patterns . Without a drainage plan , development can cause ero-
sion, flooding, pollution of the water, and filling or changing of
' the existing water courses . Grading of the land and construction
of streets , parking areas and buildings forces storm water, pre-
viously absorbed into the ground, through street drainage or storm
' sewer pipe to storage areas or outlets. Storm water must be con-
trolled to prevent flooding or erosion of natural drainage ways .
' Also, storm water runoff picks up polluting materials which may
contaminate receiving waters if not properly managed.
11 Storms occur in varying intensities and duration . A short,
heavy rainfall causes flooding at collection points. A longer
duration storm causes low areas to flood. The drainage plan de-
llsign must strike a balance between the probability of occurance of
storm intensity and the economics of a storm sewer system which
' provides adequate control of the runoff.
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SCOPE OF STUDY
The entire watershed in which Shorewood is included was in-
vestigated. This includes adjacent communities which naturally
' drain into Shorewood, as well as those into which storm water from
Shorewood flows .
' Individual drainage districts are delineated and identified
on the Location Map at the beginning of Section II . A study of
each drainage area--existing conditions and final storm sewer plan
--also follows in Section II of this report.
Shorewood' s zoning ordinance was used to determine anticipat-
ed storm water runoff for each type of zoning. Commercial proper-
ties , with greater land cover of buildings and parking areas will
' cause more runoff than residential properties with more natural
ground to absorb rainfall .
Areas determined as wetlands according to the Shorewood
' Wetlands Ordinance are included in the drainage plan. This or-
dinance, as well as the Minnehaha Creek Watershed District Rules
' and Regulations were incorporated into this study.
Department of Natural Resources supplied established normal
Iwater levels and flood elevations for lakes within Shorewood's
Watershed. By a study of existing topographic conditions , normal
water and high water elevations for ponding areas and wetlands
11 used as storage basins in this plan were determined. Normal water
elevation is defined as the level of the outlet control structure
' for ponding areas , or natural overflow for wetlands . High water
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elevation is the highest level of water anticipated in a 100 year
storm. High water elevations are established to regulate develop-
ment in areas adjacent to lakes, ponding areas and wetlands. This
will prevent construction where flooding may occur, or filling in
wetland areas.
' Minimum basement building elevations should be established at
least 1 foot above high water to provide a freeboard for flooding.
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E NVI RONMF ITAL ASSESS1'EMT
The development of the storm sewer system will protect the
existing environment by removing or storing storm water as more
' and more surface runoff is created by construction of impervious
cover and grading of the natural ground. Storm water runoff car-
ries with it many contaminants which have a possible polluting ef-
fect on the receiving waters . For this reason, the plan allows
water to discharge into wetlands areas , or small ponding areas ,
' rather than directly into lakes , wherever possible. In cases of
low areas adjacent to lakes , environmental settlement manholes are
' used to collect waste materials . (See detail in Appendix)
' Where water follows a natural overland drainage channel ,
these natural channels should be utilized wherever possible. In
some areas greenbelts could follow these channels as a compatible
use . Runoff is controlled by storage in ponds and wetlands to re-
duce instantaneous flows and erosion in these channels.
A program to remove polluting materials before they enter the
storm sewer system is presently the most feasible method of storm
' water pollution control . Street sweeping, leaf and grass pickup,
cleaning of catchbasins and minimal use of de-icing agents are
' important.
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' BASIS OF DESIG1
RAINFALL DATA
' In the Metropolitan Area, the average annual precipitation is
26 inches . About 4 inches of this falls as snow and the remaining
22 inches as rain . Average annual evaporation of small lakes is
30 inches.
' Rainfall occurs in varying intensities over varying periods
' of time , and no two rainfalls are exactly alike. A thunderstorm
will deposit a large amount of rain in a short time on a small
area. Long duration, low intensity storms deposit a large amount
of rainfall over a larger area. An analysis of rainfall data must
be made to predict runoff quantities for the design of a storm
' sewer system.
The U. S . Weather Service for years has recorded the precipi-
tation amounts from rainfalls at various measuring stations. They
prepared a graph from this data and presented it in the United
States Weather Service 's Technical Paper 40. (See Appendix. ) The
graph shows the average frequency for storms having a definite in-
tensity or rate of rainfall for a specific time period in the
' Metropolitan area. The curves shown are for maximum storm inten-
sities which may be anticipated to occur in 2 , 3, 5 , 10 , 25 and 50
' year frequencies .
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RAINFALL DESIGN
In storm sewer pipe design, capacities are provided to accom-
modate the expected runoff from storms having a certain antici-
patedfrequency of return . To design for a 25 year storm (a storm
with a 4% probability of occuring in any given year) would require
a greater pipe capacity than required for a 2 year storm (50% pro-
bability of occuring in any given year) . The storm frequency
selection used for storm sewer design is the result of a compari-
son of the probable flooding and nuisance damage versus the cost
of the system. The 5 year return frequency used for this report
1 is the generally accepted design storm for storm sewer pipe in
1 suburban areas . This rainfall has a 20% probability of occurrance
in any given year.
1 Ponding areas are designed to control runoff from a 100 year
storm. Hydrographs , as shown in the Appendix, were developed for
these ponds which show the peak. flow and runoff volume for the de-
sign storms . From this , the required storage volume and outlet
capacity is determined.
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STORM SEWER RUNOFF
Using Shorewood' s comprehensive zoning plan and assuming sat-
urated development, the runoff quantities were calculated using
' the Rational Formula, Q=CIA where :
Q = Peak runoff in cubic feet per second
C = Runoff coefficient
I = Rainfall intensity in inches per hour
A = Contributing drainage area in acres
The runoff coefficient relates a percentage of runoff antici-
pated from precipitation . This is dependent on development den-
sity, type of soil , vegetation cover, and ground slopes. The Soil
Survey Manual for Hennepin County, prepared by U. S. Department of
Agriculture Soil Conservation Service in cooperation with the
' Minnesota Agricultural Experiment Station and issued in April
1974 , was used to analyze the soil types in Shorewood. In gener-
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al, Shorewood has medium-textured and moderately fine textured
1 soils that developed in glacial till , and level organic soils.
These soils are moderately permeable.
Based on Shorewood's Zoning Ordinance and the soil types
found in the Soil Survey Manual , the following runoff coefficients
' were determined:
Residential Areas C
R-1 (1 acre lot, 3000 sq. ft. bldg. ) .24
' R-2 & R-3 (1/2 acre lot, 2000 sq. ft. bldg. ) .26
R-4 & R-5 (Multiple , 35% bldg. & driveways) . 40
' Commercial
C-1, C-2 & C-3 . 50
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' In addition, commercial areas were investigated individually
where the extent of impervious surface cover was extensive and
trunoff correspondingly larger.
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DRAINAGE DISTRICTS
Natural drainage basins divide Shorewood into sixteen dis-
' tricts . These boundaries and elevations were determined from top-
ographic maps with 2 foot contour intervals for Shorewood and
Tonka Bay, and 10 foot intervals in other areas. Although the
outlets for some districts enter another district, the storm sewer
design for each district is discussed independently. The design
' includes the required capacity for the outfall discharging from an
upstream district.
In the analysis for each drainage district, the existing
1 storm culverts in Shorewood were field checked to verify the
existing drainage pattern . This pattern is delineated on an indi-
vidual map for each district. Additional existing storm sewer in-
formation was obtained from the Minnesota State Highway Depart-
ment, Hennepin County , and adjacent communities.
11 A storm sewer system was designed for saturated development
using the streets and natural drainage ways as a collection sys-
tem. It was necessary to assume a street pattern in the unplatted
areas . Although changes in the exact location of these assumed
streets are likely, the general route, the size of pipe and the
quantity of storm runoff will generally remain the same. Trunk
storm sewers as shown are sized to collect runoff from areas with-
in 1000 feet from drainage boundaries . In some locations proposed
storm sewer is shown where there are existing natural drainage
channels , for purposes of cost estimates . These channels should
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be continued to be used unless maintenance require uire that a
problemsq
storm sewer be installed. Additional lateral storm sewer may be
required in some locations . This layout will serve as a guide for
' required storm sewers as development occurs .
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ADJACENT COMMUNITY PAiRTICIPATION
' The apportioning of costs between municipalities for a storm
sewer constructed as a joint venture requires negotiations between
these communities to determine the most equitable method of cost
sharing. The requirement for inter-community storm sewers are
discussed along with the individual drainage district affected.
One method commonly used is to apportion cost on the basis of
contributing area. That is , the cost of a storm sewer to a com-
munity is based on the proportion of that community' s area served,
to the total area served.
The estimated cost sharing for communities up-stream of
' Shorewood is included in the cost estimate summary.
' Locations where Shorewood storm sewer drainage discharges in-
to another community are indicated. No attempt was made to design
' the storm sewer for that community or determine the cost sharing
required by Shorewood.
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COST ESTIMATE SUMMARY
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The Total storm sewer construction cost estimates for each
drainage district are summarized in this section. These costs arc
Ibased on January 1975 prices with an Engineering News Record (ENR)
cost index of 2103. Future cost estimates must be determined by' up-
dating these cost figures with an appropriate cost index.
1 Costs included are construction costs for pipe installation and
appurtenant work. Cost estimates for land acquisition or easements
Iare included with indirect costs.
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CONSTRUCTION COST ESTIMATE
Outside
DISTRICT Shorewood Communities
IPurgatory Creek $ 94 ,000 $
Carson Bay 7, 000
*Lake William 111, 000 (Greenwood) 8 ,000
IISt. Alban's Bay 67 ,000
Christmas Lake 76 , 000
*Galpin Lake 164, 000 (Chanhassen) 88 ,000
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*Gideon Bay 213 , 000 (Tonka _Bay) 16 ,000
Lake Mary 81, 000
Central 72 ,000
II South Central 27 ,000
Lake Minnewashta 13,000
North-Central 286 ,000
II West-Central 644 ,000
Lake Virginia 91,000
West 82 ,000
47 ,000
Islands
1 $ 2 ,075 ,000 $ 112 ,000
+ 25% Indirect 519 ,000 28 ,000
Total $ 2 , 594 ,000 $ 140 ,000
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*Cost Sharing Estimate based on contributing acres
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' ASSESSMENT POLICY
General practice, in communities similar to Shorewood, is to
apportion the cost of a storm sewer on an area basis as well as on
' anticipated runoff. The assessment cost them, is based on a cost
per sq. ft. of land area. This area cost assessment varies with
' zoning, since land use affects the amount of runoff anticipated.
' Total area in Shorewood is approximately 3 , 845 acres. This is
comprised of 675 acres of lakes and wetlands leaving a gross land
' area of 3, 170 acres. Assuming 15% (470 acres) will be parks and
streets , there are 2 ,700 assessable acres. Land use distribution
includes 40 acres C-3, Commercial, 35 acres R-4 and R-5, multiple,
' with the remaining 2 , 625 acres residential.
The storm sewer cost of $2 , 594, 000 should be apportioned over
' the assessable area in proportion to runoff for the different zon-
ings.
' An assessment policy, based on these costs and assessable areas
' (using an average . 25 runoff coefficient for residential property)
would be as follows :
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Residential $0. 0216/sq. ft.
Multiple 0. 0345/sq/ft.
' Commercial 0. 0432/sq/ft/
A policy with a 1 mill levy on a 1974 assessed valuation of
' $16 , 000 ,000 over a 20 year period would yield $320 ,000 . The remain-
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ing construction cost of $2 ,274 ,000 assessed on the same basis as
' above would be as follows :
' Residential $0. 0190/sq. ft.
Multiple 0. 0304/sq. ft.
' Commercial 0. 0380/sq.ft.
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