Memo - Nine Mile Creek Bank Stabilization and Habitat EnhancementMemorandum
Public Works Dlent
To: Honorable Mayor and Members of the City Council
CC: Rick Getschow, City Manager
From: John R. Bradford, City Engineer
Date: 08/08/2008
Re: Nine Mile Creek Bank Stabilization and Habitat Enhancement
This work session item relates to the final feasibility report prepared by WSB &
Associates and Barr Engineering for the above mentioned project. The Nine Mile
Creek Watershed District will present the feasibility report with the proposed
improvements.
The project is estimated to cost approximately $4.05 million and includes
improvements along the creek from Excelsior Boulevard to the Hwy 169 crossing.
Part of the work is located in the City of Minnetonka for storm water quality
treatment.
All costs are covered by the Watershed District except for Easements (75% City),
trail construction (50% NMCWD /50% others including the City and Three Rivers
Park District) and Roof Drain connection at 10801 Excelsior (50% NMCWD/ 50%
others).
The project will be up for Watershed District Approval in early September, which
will open the way for construction beginning in November 2008. This project will
be completed in two winter construction seasons.
1
1.0 Introduction
This report summarizes the proposed actions for improving the stability and water quality of the
North Fork of Nine Mile Creek (North Fork), located in the City of Hopkins. It is prepared in
accordance with Section 103D.71 1 of the Minnesota Watershed Act under the direction of the Board
of Managers of the Nine Mile Creek Watershed District.
1.1 Background
The Nine Mile Creek Watershed District (District) was established by the Minnesota Water
Resources Board in 1959. Stormwater management within the urbanizing Nine Mile Creek watershed
was guided initially by the District's Overall Plan dated March 1961. That plan was revised by the
District in April 1973, as prescribed by the Minnesota Water Board. The 1973 revised Overall Plan
guided development in the District until it was further revised in May 1996 . and again in 2006 (Water
Management Plan), in accordance with the Metropolitan Surface Water Management Act and
Watershed Law: Minnesota Statutes Chapters 103B and 103D, respectively. The City of Hopkins
submitted a petition to the District in 2006 for the Nine Mile Creek Stabilization Project
(Appendix A). The stream stability and water quality improvements recommended in this Engineer's
Report area to be undertaken as a Basic Water Management Project, as described in the District's
2006 Water Management Plan.
1.2 Project Location
The proposed improvement projects are primarily located in the City of Hopkins, Minnesota (see
Drawing G -01, Appendix B). The North Fork begins in Hopkins, south of Excelsior Boulevard
(County Road 3) and west of 11 Avenue, where stormwater from a drainage area of approximately
560 acres is discharged via a 12 -foot by 7 -foot concrete box culvert storm sewer. The study area
includes an 8,500 foot reach of the North Fork, beginning at Excelsior Boulevard and extending in a
southeastern direction to the Highway 169 culvert crossing. This portion of the North Fork is
primarily located in Hopkins, with a small portion (600 feet) located in Minnetonka, just west of
Highway 169. The study area also includes the sites of several proposed water quality ponds located
in or near the creek floodplain.
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1.3 Summary of Problems
1.3.1 Stream Stability
Channels in urban areas are often lacking in many of the features that define stable channels, such as
a defined floodplain and a stable, meandering pattern. Such channels have often been straightened
over the years, and their floodplains filled, resulting in higher velocities and shear stress during high
flows. These channels tend to be eroded by fluvial bank erosion, but the stresses are greater due to
the confined nature of the channel, and the subsequent erosion can be much more severe.
Similar to many other urban stream systems, the North Fork of Nine Mile Creek suffers from
streambank erosion. Based on a detailed site evaluation, the major erosion problems occurring in this
portion of creek may be attributed to the historical realignment of the channel from a meandering
shape to a straightened channel, combined with increased runoff due to urbanization. Erosion issues
develop as the creek attempts to return to a more stable, meandering pattern. This evolution is
occurring via bank slumping, formation of central bars which direct flow to one bank or the other,
and channel widening. In some areas, the resulting cut banks are very high, likely reflecting some
degree of downcutting (channel incision).
1.3.2 Untreated Stormwater Runoff
The watershed that drains to the North Fork of Nine Mile Creek was primarily developed prior to the
formation of the District and before the onset of stormwater quality treatment requirements.
Consequently, most of the stormwater runoff from the watershed is discharged to the creek with little
or no treatment, resulting in significant loading of sediment and nutrients, such as phosphorus, to the
creek system.
1.4 Project Objectives
The objective of the project is to provide a stable creek system and provide the treatment of
stormwater runoff where practicable and feasible.
1.5 Public Participation
The Nine Mile Creek Watershed District has identified the importance of public involvement and
seeks opportunities to incorporate public involvement into District projects. The District has worked
in conjunction with the following stakeholders during the preliminary phases of the Nine Mile Creek
Stabilization Project in Hopkins: City of Hopkins staff and their representatives, City of Minnetonka,
Three Rivers Park District, Board of Soil and Water Resources, and local residents. The numerous
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public and stakeholder involvement opportunities that have occurred throughout this project are
identified below:
• Numerous meetings with City of Hopkins staff and their representatives
• Workshop with Hopkins City Council and Nine Mile Creek Watershed District Board of
Managers on October 9, 2007
• Two citizen open house meetings on December 5 and 6, 2007
• Meeting with Hopkins city staff and residents of residential development (Nine Mile Cove)
adjacent to proposed project site on January 10, 2008.
• Meeting with the District administrator and engineer, Hopkins city staff, and Three Rivers
Park District on February 22, 2008.
• Meeting with Board of Water and Soil Resources (BWSR) on April 9, 2008 regarding
potential wetland impacts.
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4.0 Proposed Improvements
4.1 Stream Stabilization
Similar to many other urban stream systems, the North Fork of Nine Mile Creek suffers from
streambank erosion. Based on a detailed site evaluation, the major erosion problems occurring in this
portion of creek may be attributed to the historical realignment of the channel from a meandering
shape to a straightened channel, combined with increased runoff due to urbanization. Erosion issues
develop as the creek attempts to return to a more stable, meandering pattern. This evolution is
occurring via bank slumping, formation of central bars which direct flow to one bank or the other,
and channel widening. In some areas, the resulting cut banks are very high, likely reflecting some
degree of downcutting (channel incision).
The proposed stream stabilization improvements for segments of the North Fork between Excelsior
Boulevard and 9` Avenue South are summarized in Figure 4 -1 and segments between 9` Avenue
South and Highway 169 are summarized in Figure 4 -2. Realignment of the channel to a more stable,
meandering form is proposed for three reaches of the study area. In the remaining reaches, where
area does not permit channel realignment, reshaping of the banks to shallower slopes and bank
revegetation are recommended. Stabilization of the channel toe and grade control will be necessary in
some locations. Schematics of these stream stabilization practices are shown in Figures 4 -3 through
4 -9. The recommended improvements for each segment are summarized below.
4.1.1 Segment A
4.1.1.1 Existing Conditions
Segment A extends approximately 575 feet downstream from the south (downstream) side of
Excelsior Boulevard to the Hennepin County Regional Railroad Authority (HCRRA) trail (see
Figure 4 -1). The existing typical side slopes of the creek embankment are approximately 2:1 on the
west bank and 3:1 on the east bank. The bottom width of the channel varies within the segment, but
is typically 15 -feet wide. The current roof drain configuration from 10801 Excelsior Boulevard (car
wash building) is resulting in erosion of the west creek embankment from overland flow and seepage
issues. Erosion of the east creek embankment is being caused by runoff from 10751 Excelsior
Boulevard (Scherer Brothers Lumber Company).
4.1.1.2 Recommendations
Recommendations for erosion problems in Segment A include the reshaping of banks, stabilization of
lower stream banks using limestone and fieldstone riprap, and revegetation of the upper stream banks
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(see Drawing C -15, Appendix B). To remedy the erosion being caused by the roof drains of 10801
Excelsior Boulevard, consolidation of the roof drains and installation of a single piped outlet to the
creek is recommended. In addition, construction of an 8 -foot gravel trail along the east side of the
creek is proposed.
4.1.2 Segment B
4.1.2.1 Existing Conditions
Segment B extends approximately 550 feet downstream from the Hennepin County Regional
Railroad Authority trail to the 5` Street South crossing (see Figure 4 -1). The existing typical side
slopes of the creek embankment vary significantly, with typical slopes of 3:1 on the west and east
banks. The bottom width of the channel varies within the segment, but is typically 15 -feet wide.
4.1.2.2 Recommendations
Recommendations for erosion problems in Segment B include the reshaping of banks, stabilization of
lower stream banks using natural fieldstone riprap, and revegetation of the upper stream banks (see
Drawings C -13 and C -14, Appendix B).
4.1.3 Segment C
4.1.3.1 Existing Conditions
Segment C extends approximately 250 feet downstream from the 5 Street South crossing to the
SOO Line railroad (see Figure 4 -1). The existing typical side slopes of the creek embankment are
approximately 3:1 on the west bank and 1:1 on the east bank. Existing erosion control features in
Segment C of the North Fork include gabion mats on the east and west embankments downstream of
the 5 Street South crossing and rip rap throughout the segment. At approximately 150 feet south of
5 Street, there is severe erosion on the east embankment. Sediment deposition has built up in the
creek near this location.
4.1.3.2 Recommendations
Recommendations for erosion problems in Segment C include stabilizing approximately 100 feet of
east streambank with a sheetpile wall and natural fieldstone riprap to remedy the existing erosion
problem and installing fieldstone riprap toe protection on the west side of the creek (see
Drawing C -13, Appendix B). It is also recommended that the sediment deposition located
approximately 200 feet downstream from the 5` Street South crossing be removed from the creek
bed, and the existing gabions be covered with topsoil and vegetation will be restored.
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In addition, the two existing 90 -inch corrugated metal pipe (CMP) culverts will be replaced with an
equivalent reinforced concrete box culvert or arch pipe. Design details will be provided upon receipt
of information from WSB.
4.1.4 Segment E
4.1.4.1 Existing Conditions
Segment E extends 450 feet downstream from the 11 Avenue South crossing to 7 Street South (see
Figure 4 -1). The existing typical side slopes of the creek embankment vary significantly, with typical
slopes of 3:1 on the west and east banks. The bottom width of the channel also varies, but is typically
20 -feet wide in this segment.
4.1.4.2 Recommendations
Recommendations for erosion problems in Segment E include the reshaping of banks, stabilization of
lower stream banks using fieldstone riprap, and revegetation of the stream banks with native shrubs
and trees (see Drawings C-11, Appendix B). The area northeast of the creek, adjacent to the existing
alley, should be regraded to provide a more gradual slope to the creek. The creek will be shifted to
the southwest, as needed, to accommodate the bank stabilization on the northeast side, and the
southwest streambank will be regarded and revegetated, accordingly. The installation of drain tile
between the existing alley and creek is recommended to improve drainage in the alley.
4.1.5 Segment F
4.1.5.1 Existing Conditions
Segment F extends approximately 820 feet downstream
from the 7 Street South crossing to the 9 Avenue
South crossing (see Figure 4 -2). The existing typical
side slopes of the creek embankment vary significantly,
ranging from 10:1 slopes to 1:1 slopes throughout this
section. The bottom width of the channel also varies,
but is typically 10 -feet wide. An existing 54 -inch
corrugated metal pipe (CMP) located on the west side
of the creek, approximately 230 feet downstream of the
7 Street South culvert, comes into the creek at an
elevation above the creek bed. The stream bank is
eroding around the culvert.
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4.1.5.2 Recommendations
Recommendations for erosion problems in Segment F include the reshaping of banks, stabilization of
lower stream banks using fieldstone riprap, and revegetation of the stream banks with native shrubs,
trees, and grasses (see Drawings C -9 and C -10, Appendix B). Recommended stabilization techniques
include installation of a rock vane approximately 350 feet upstream of the 9` Avenue South crossing,
to divert channel flow toward the center of the creek and away from the bank. See Figure 4 -3 for a
more detailed description of how rock vanes provide bank protection. The invert of the existing
54 -inch CMP should be lowered to the normal flow level of the creek. In addition, construction of a
shared -use paved trail is proposed on the northeast side of the creek.
4.1.6 Segment G
4.1.6.1 Existing Conditions
Segment G extends 750 feet downstream from the east
(downstream) side of 9th Avenue South crossing (see
Figure 4 -2). Valley Park occupies the east bank of the
channel in this reach. Westbrooke Way parallels the
channel within 50 feet of the west bank for most of
Segment G. Prior to 1950, the channel demonstrated a
gradual meandering pattern at this location; however,
the channel has since been straightened. Although the
stream alignment in Segment G is relatively straight,
the effective wetted area is slowly evolving toward a
slightly meandering pattern. This is happening
primarily via channel widening, and through bank downstream
slumping and creation of central bars that direct the
flow into one of the banks. Two storm sewers discharge into the creek within this reach with both
discharge points located perpendicular to the natural flow of the creek, contributing to bank erosion.
Central bars
and channel widening in
from 9 Avenue South.
Segment G, looking
4.1.6.2 Recommendations
The proposed solution to erosion problems in Segment G includes the realignment of the channel to a
meandering pattern and establishment of deep- rooted vegetation. The channel will be realigned to the
east of the current channel (see Drawing C -08 and C -09, Appendix B). This location reduces the risk
of bank undercutting along Westbrooke Way. The proposed stream alignment is based on the
meander observed in this location prior to straightening, based on historic aerial photos. Vegetation
management, described in Figure 4 -7, will be critical for stabilizing this segment. The realigned
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channel will be graded and vegetated with deep- rooted vegetation to provide bank stability. Erosion
control blanket is proposed to protect the plantings until they are established. Because the new
alignment will lie entirely to the east of the current channel, vegetation in the new channel should be
established prior to redirecting the flow from the old channel to the new channel. This will require at
least one growing season. The existing channel will be filled with the material excavated from the
new channel after flow is introduced into the new channel.
The storm sewer discharge located near the beginning of Segment G will be redirected to enter the
creek discharging in the downstream direction. The second storm sewer discharge (30 -inch RCP) will
be extended to the new channel location and redirected to enter the creek in the downstream
direction. The existing pedestrian bridge in this segment will be removed and may be reused near the
proposed water quality pond. A new pedestrian bridge will be constructed across the new channel.
4.1.7 Segment H
4.1.7.1 Existing Conditions
Segment H includes a 600 -foot section of the North Fork downstream of Reach G (see Figure 4 -2).
To the northeast of the channel is the city park and community garden. Westbrooke Way lies very
close to the southwest side of the channel for the entire length of Segment H. The characteristics of
this reach are similar to those of Segment G, although bank slumping is more extensive in this reach
and the channel is becoming wider. The banks are higher in this reach, indicating greater
downcutting than in Segment G. Aerial photography confirms that since 1950 the channel has moved
southwest, encroaching on the present -day location of Westbrooke Way. The erosion is most severe
along the southwest bank with bank undercutting already compromising the road embankment of
Westbrooke Way.
4.1.7.2 Recommendations
Existing land use prevents the realignment of Nine Mile Creek within this reach. Proposed solutions
to erosion problems in Segment H include the reshaping of banks, establishment of deep- rooted
vegetation, and the placement of stabilization works, including bank stabilization and grade
stabilization. To prevent additional erosion along the road embankment of Westbrooke Way,
installation of a sheet pile wall is proposed, to be located 2 feet east of the existing sidewalk (see
Drawing C -07 and C -08, Appendix B). Stone toe protection (Figure 4 -4) and biolog toe protection
are proposed to stabilize the bank toes. Constructed riffles (Figure 4 -9) are proposed to provide grade
control and prevent downcutting in the stream. The stream banks will be graded and revegetated with
native vegetation. A 50 -foot vegetated buffer is proposed for a 100 -foot portion of this reach.
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Right bank erosion in Segment H, looking towards the right bank and Westbrooke Way
4.1.8 Segment
4.1.8.1 Existing Conditions
Segment I extends 490 feet downstream from Segment H, to the inflow of three tributary channels
entering from the north, south, and west (see Figure 4 -2). A community garden occupies the area
north of the channel. A low -lying wetland area lies west of the channel. As with Segment G,
downstream of 9 Avenue, the channel appears to have been straightened in the past, but is evolving
toward a slightly meandering pattern. The meandering is developing through bank slumping,
resulting in the creation of central bars that direct the flow into either bank. Slumping is significant
on both sides of the channel. The banks are not as high as the most severe sections of Segment H, but
are higher than in Segment G. Channel widening is more significant in this reach than in Segment G
or Segment H. A 12 -inch pipe drains runoff from the community garden into the creek at the
midpoint of this reach. The outlet of the pipe is set back about 5 feet from the creek, resulting in
localized bank erosion.
4.1.8.2 Recommendations
Recommendations for Segment I include realignment of the channel to a more stable meandering
pattern. The realigned channel would be located in the wetland area southwest of the present channel
(Figure 4 -2) and will not infringe on the community garden. Historical data does not extend far
enough to determine the natural stream alignment in this reach prior to straightening. Therefore, the
stream alignment geometry will be based approximately on the historical meander observed in
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Segment G. The new channel will be graded and vegetated with deep- rooted plants to provide bank
stability. Erosion control blanket will be used to protect the plantings until they are established. The
existing channel will be filled with the material excavated from the new channel after the vegetation
is established. The new alignment will lie entirely to the west of the current channel, allowing
vegetation in the new channel to be established prior to redirecting the flow. The storm sewer pipe
draining the community garden will be extended to the new channel. Disturbance of the existing trees
at the south end of the segment will be minimized.
4.1.9 Segment J
4.1.9.1 Existing Conditions
Segment J includes a 510 -foot section of the North Fork beginning at the junction of the main
channel with two tributary channels entering from the north and west (Figure 4 -2). A medium - density
residential development (Nine Mile Cove) exists on the north floodplain. A combination of wetland
and upland areas exist to the south of the channel. Review of historic aerial photos indicates that this
reach was straightened in the past but has evolved back to a meandering pattern between 1969 and
1979. High cut banks occur within this reach near the confluence of the main channel and side
channels. Slumping occurs on both sides of the channel but is most significant along the cut banks.
Towards the downstream end of Segment J, the cut bank occurs on the north side of the channel and
is encroaching on the walking path and residential areas.
An existing 21 -inch RCP culvert enters the north side of the creek from the adjacent Nine Mile Cove
residential development. The outlet of the pipe is set back about 6 to 8 feet from the creek, resulting
in localized bank erosion, and the discharge point is perpendicular to the natural flow of the creek.
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Right (south) cut bank and bank slumping in Segment J, looking downstream
Left (north) bank slumping near residential area in Segment J
4.1.9.2 Recommendations
Existing land use and tributary channels prevent significant realignment of Nine Mile Creek within
this stream segment. Proposed solutions to erosion problems in Segment J include the reshaping of
banks to a milder slope and adding stone toe protection (Figure 4 -4), biolog toe protection and
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constructed riffles (Figure 4 -9). The banks will be vegetated with deep- rooted plants to add stability.
Buckthorn removal will be pursued to reduce shading of the stabilized banks. The plantings will be
protected with erosion control fabric and until they are established. The existing 21 -inch culvert will
be modified by installing a drop manhole and extending the culvert to the creek, with the outlet
installed with a 45- degree bend. The proposed modifications will help to dissipate energy prior to
discharge of stormwater into the creek and minimize streambank erosion.
4.1.10 Segment K
4.1.10.1 Existing Conditions
Segment K extends 2,100 feet downstream from Segment J to the west side of Highway 169
(Figure 4 -2). Based on review of historic aerial photographs, the creek was a straight channel running
southeast across the present -day wetland south of the sedimentation pond from approximately 1950
to 1969. The creek was realigned between 1969 and 1979 to flow east through a sedimentation pond
before flowing south along Highway 169. The cut banks and slumping evident in Segment J
continue to the existing sedimentation pond. Sandy bars at the south end of the pond suggest the flow
short circuits the pond. There are no additional discharges into the pond under existing conditions.
Sediment deposition near inflow to pond in Segment K
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Downstream of the sedimentation pond, the main channel travels east towards Highway 169, where
it is joined by a drainage ditch from the north. The channel then turns south and flows along
Highway 169. The stream alignment in Segment K along Highway 169 is relatively straight. The
effective wetted area is slowly evolving toward a slightly meandering pattern as described for
upstream reaches. This is occurring primarily via channel widening, and through bank slumping and
creation of central bars that direct the flow into one of the banks only.
Bank slumping in Segment K below pond, looking downstream along Highway 169
4.1.10.2 Recommendations
Recommendations for Segment K include realignment of the channel to a more stable meandering
pattern. The realigned channel would be located in the wetland area southwest of the present channel
(Figure 4 -2). The first approximately 500 feet of the channel will meander along the historic channel
location, before turning south towards the Highway 169 crossing. The proposed meander pattern is
based on the natural meander of a more stable reach of Nine Mile Creek located approximately one
mile downstream of the project site. The new alignment will lie entirely to the south of the current
channel, with vegetation in the new channel to be established prior to redirecting the flow. The new
alignment will also result in a smoother connection with the culvert crossing of Highway 169.
The new channel will not flow through the existing sedimentation pond. Disconnecting the creek
from the pond is anticipated to have a positive effect on transport of bedload sediment in the creek;
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as the pond currently trapping bedload sediment, therefore increasing the likelihood of bed
degradation (channel downcutting) in the reach downstream of the sedimentation pond. The existing
channel leading to the sedimentation pond can be filled with material excavated from the new
channel after vegetation is established (see Drawing C -05, Appendix B). Low flows from the
drainage ditch parallel to Highway 169 will be rerouted through a new water quality pond and then
to the existing pond to provide water quality treatment.
4.2 Improved Stormwater Treatment
The watershed that drains to the North Fork of Nine Mile Creek was primarily developed before the
onset of stormwater quality treatment requirements. Consequently, most of the stormwater runoff
from the watershed is discharged to the creek with little or no treatment, resulting in significant
loading of sediment and nutrients, such as phosphorus, to the creek system. To improve the quality of
stormwater being discharged to the North Fork of Nine Mile Creek, several water quality
improvement projects are being recommended as part of this overall project. These improvements are
described below in further detail.
4.2.1 Construction of Opportunity Court Stormwater Pond
4.2.1.1 Existing Conditions
The storm sewer system along Opportunity Court in Minnetonka currently discharges untreated
stormwater from a 21 -acre drainage area directly into the large wetland complex located west of
TH 169 (see Figure 4 -10). The City of Minnetonka requested that the NMCWD include a proposed
stormwater treatment pond for the Opportunity Court drainage area in the Nine Mile Creek
Stabilization Project. The City of Hopkins has since amended their formal project petition to include
the request (see Appendix A).
4.2.1.2 Recommendations
Construction of a small water quality pond is proposed directly north of 5401 Opportunity Court
(Figure 4 -10). The water quality pond was designed to avoid impacts to the existing wetland. As a
result, the pond is sized to only treat stormwater from low -flow conditions. Stormwater from the
existing 30 -inch RCP trunk system will be diverted to the proposed pond via installation of a 12 -inch
pipe (see Drawing C -18, Appendix B). The 12 -inch pipe will convey stormwater from the smaller
precipitation events to the pond, with the large events bypassing the pond.
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4.2.2 Treatment of Stormwater Discharge from TH 169 Storm Sewer System
4.2.2.1 Existing Conditions
The Trunk Highway 169 ( TH 169) storm sewer system collects stormwater from a drainage area of
approximately 307 acres, as shown in Figure 4 -11. The stormwater runoff is conveyed southward
along TH 169 and discharged into the existing ditch that runs parallel to TH 169 via an 84 -inch
RCP. Runoff from approximately 238 acres of the 307 -acre drainage area (77 %) receives little or no
stormwater treatment prior to discharge into the existing ditch, and ultimately the creek system.
The existing ditch system parallel to TH 169 also receives flow from the 77 -acre Park Valley
drainage area, which is discharged to the ditch just north of the existing service road via a culvert
beneath 2nd Avenue. From this location, stormwater runoff is conveyed eastward through the ditch
and combines with the drainage ditch parallel to TH 169. The stormwater from this drainage area
receives some treatment in the wetland area upstream of the culvert under Nine Mile Cove.
4.2.2.2 Recommendations
The area between the TH 169 storm sewer outlet and the creek is predominantly wetland, so there is
very little opportunity for construction of stormwater treatment Best Management Practices (BMPs)
in the upland areas. As a result, the proposed stormwater BMPs were designed to provide treatment
while minimizing negative impacts to the existing wetland system. The proposed stormwater
treatment consists of rerouting the flows from the TH 169 and Park Valley drainage areas through a
series of water quality ponds, including the existing sedimentation basin (South Park Pond) that the
North Fork currently flows through. The first pond in the treatment system is to be constructed in the
existing wet meadow area west of TH 169 (see Drawing C -16, Appendix B). The second pond in the
treatment system is the existing South Park Pond, which will be dredged to increase the dead storage
volume available for water quality treatment. The designs of the proposed ponds are described in
additional detail below.
The permanent pool (dead storage) volume of a water quality pond has a significant impact on the
effectiveness of treatment. The proposed system was designed to minimize wetland impacts and
avoid mitigation requirements. Due to the related design constraints in pond footprint and depth, the
proposed permanent pool volume (5.3 acre -feet) falls below the NURP recommended permanent pool
volume for the untreated TH 169 drainage area (9.4 acre - feet). A P8 model was used to assess the
pollutant removal efficiency of the proposed treatment system. Based on modeling results from 1949
to 2000, the proposed treatment system achieves (on average) approximately 42% and 75% removal
of total phosphorus (TP) and total suspended solids (TSS), respectively. Although the removal
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efficiencies are less than what would be expected if the NURP- recommended dead storage volume
were provided, the pollutant removal represents a significant overall reduction in phosphorus and
sediment reaching the creek system (average annual reduction of 116 lbs TP and 60,000 lbs TSS).
Construction of Water Quality Pond
The first pond in the treatment system is to be constructed in the existing wet meadow area west of
TH 169. Flow through the existing ditch parallel to TH 169 will be re- routed into the proposed pond
through a new open channel. A control weir will be constructed in the existing ditch to redirect flows
into the pond under low -flow conditions, and allow flow southward through the existing ditch under
high -flow conditions (see Drawing C -16, Appendix B).
The pond has been designed to enhance the functions and values of the existing wet meadow, which
is currently a low quality wetland dominated by reed canary grass. The pond design incorporates a
meandering water edge, with points and bays to improve wildlife habitat, and side slopes of 5:1 to
promote aquatic vegetation. Excavation of the proposed pond has been limited to depths less than
6.6 feet (2 meters). No fill is proposed. A vegetation management plan for the proposed pond will be
completed as part of the final design and approval process, to ensure that the area surrounding the
pond blends in with the adjacent natural landscape.
Dredging of Existing South Park Pond
The second pond in the proposed treatment system is the existing South Park Pond. The North Fork
currently flows through this sedimentation pond, but will be re- routed to the south under the
recommended stream realignment (Segment K, Section 4.1.2.3). Flow from the proposed water
quality pond will be routed into the existing South Park Pond through an open ditch that meanders
through the existing wetland area.
Dredging of the existing South Park Pond is recommended to increase the dead storage volume
available for water quality treatment. The proposed excavation volume is approximately 4,000 cubic
yards, with dredging depths not to exceed an elevation of 867 ft MSL (estimated original pond
bottom based on historic pond clean out information). The normal water level of South Park Pond
will be controlled at 870.0 ft MSL by a boulder control outlet located just upstream of the existing
timber bridge (see Drawing C -17, Appendix B). The proposed dead storage volume in South Park
Pond is approximately 3.7 acre -feet.
P: \Mpls \23 MN\27 \2327G48 9 -Mile Hopkins \WorkFiles\Reports _Memos \Engineers Report \Engineers_Report_Final Jul- 08.doc 29
4.3 Trail Construction
Construction of a multi -use trail is proposed as part of the overall project. The proposed trail begins
approximately one block south of Excelsior Boulevard and extends in a southeasterly direction
running generally parallel to the North Fork, to the crossing of the North Fork and TH 169 (see
Drawing C -21, Appendix B). The proposed trail will be constructed by the Nine Mile Creek
Watershed District, in partnership with the Three Rivers Park District and the City of Hopkins. The
proposed trail is ultimately intended to be part of the Three Rivers Park District regional trail system
that will run through Hopkins, Edina and Richfield to Cedar Avenue in Richfield.
4.4 Fish Habitat
While lack of flow during summer months is the primary limiting factor for fish populations in this
reach of Nine Mile Creek, the longitudinal and lateral channel variability in stream depth, current
velocity, and bottom substrates resulting from the stream restoration work will help improve the fish
community. The creation of a meandering channel shape, riffle and pool structure, and the addition of
root wads at outer banks will increase the variability and complexity of the fish habitat available and
provide greater water depths for refuge areas during periods of low flows. Stabilization and
revegetation of the banks and riparian corridor will reduce water temperatures by providing improved
overhead cover.
Little fish community data has been collected for this reach of Nine Mile Creek, but an extrapolation
of the fish community data from the closest downstream location can be used to generalize potential
benefits. The pool and riffle complexes and fieldstone toe protection, constructed with larger
substrate sizes, can benefit bigmouth shiner (Notropis dorsalis), brook stickleback (Culaea
inconstans), creek chub (Semotilus atromaculatus), eastern blacknose dace (Rhinichthys stratulus),
fathead minnows (Pimephales promelas), Johnny darters (Etheostoma nigrum), and white suckers
(Capastomus commersoni). The increased water depths resulting from the creation of pools behind
the riffles and adjacent to root wads can benefit the white suckers (C. commersoni). The variability of
current velocity will benefit most of the species, but particularly the eastern blacknose dace
(R. stratulus) and white suckers (C. commersoni) will benefit form the increased velocities found in
the riffle sections. The lower velocities found in the pools will especially benefit brook stickleback
(C. inconstans) young -of -year, creek chub (S. atromaculatus) juveniles, and Johnny darter
(E. nigrum) young -of -year. The reduced sedimentation should also benefit reproduction by all
species.
P: \Mpls \23 MN \27 \2327G48 9 -Mile Hopkins \WorkFiles\Reports_Memos \Engineers Report \Engineers_Report_Final Jul- 08.doc 30
5.0 Cost Estimate
Tables 5 -1 and 5 -2 detail the estimated costs for the proposed improvements between Excelsior
Boulevard and 9` Avenue South and between 9 Avenue South and Highway 169, respectively. The
estimated total cost of the recommended improvements described on this Engineer's Report is
$4,048,800.
P: \Mpls \23 MN\27\2327G48 9 -Mile Hopkins \WorkFiles \Reports_Memos \Engineers Report \Engineers_Report_Final Jul- 08.doc 42
DESCRIPTION
UNIT
EST. QTY.
UNIT PRICE
EXTENSION
Mobilization /Demobilization
L.S.
1
$50,000.00
$50,000.00
Rock Erosion Control Const. Entrance
Each
3
$1,000.00
$3,000.00
Site Restoration (Seed and Mulch)
Acre
3.7
$3,500.00
$13,000.00
Tree Clearing and Brush Removal (< 9" Dia.)
Acre
1
$10,000.00
$10,000.00
Remove Selected Tree (> 6" Dia.)
Each
25
$900.00
$22,500.00
Gravel Path, 8' Wide
S.Y.
580
$25.00
$14,500.00
Steel Sheet Piling Wall
S.F.
2800
$45.00
$126,000.00
Roof Drain Collection System
L.S.
1
$3,000.00
$3,000.00
Existing Culvert Modifications and Repairs
L.S.
1
$6,000.00
$6,000.00
Silt Fence
L.F.
300
$2.00
$600.00
Riprap (Mn /DOT CL III)
Ton
2030
$75.00
$152,300.00
Granular Filter Rock
Ton
1700
$45.00
$76,500.00
Rock Boulder Vane
Each
1
$2,000.00
$2,000.00
Rock Filter Dike
Ton
40
$45.00
$1,800.00
Two Rail, Red Cedar Split Rail Fence
L.F.
600
$10.00
$6,000.00
Imported Topsoil
C.Y.
700
$22.00
$15,400.00
Landscaping and Live Plants
L.S.
1
$15,000.00
$15,000.00
Replace 2- Ex. 72" CM PA Culverts under 5th St
L.S.
1
$400,000.00
$400,000.00
Traffic Control
L.S.
1
$10,000.00
$10,000.00
Pipe Removal
L.F.
200
$15.00
$3,000.00
Wood Fiber Blanket
S.Y.
3700
$3.00
$11,000.00
Sod
S.Y.
7780
$4.50
$35,000.00
Bituminous Trail
S.Y.
1335
$35.00
$46,700.00
Easements
L.S.
1
$47,000.00
$47,000.00
Special Excavation
C.Y.
1233
$15.00
$18,500.00
Miscellaneous Storm Sewer
L.S.
1
$35,000.00
$35,000.00
Notes:
Unless noted, the Nine Mile Creek Watershed District (NMCWD) will cover 100% of approved project costs
50% of costs related to roof drain corrections at 10801 Excelsior Boulevard will be covered by NMCWD
50% of costs related to trail construction will be covered by NMCWD
Land acquisition costs will be split between NMCWD and City of Hopkins (25% NMCWD / 75% City)
Table 5 -1. Engineer's Opinion of Probable Cost
Excelsior Blvd. to 9th Ave. S.
Nine Mile Creek Stabilization Project- Hopkins, MN
Unit Price Subtotal
15% for Contingencies
Engineering, Legal and Administrative (30 %)
July 17, 2008
$1,123,800.00
$169,000.00
$388,000.00
Subtotal $1,680,800.00
P: \Mpls \23 MN \2712327G48 9 -Mile Hopkins \WorkFiles \Engineers Opinion of Probable Cost_1.xls Rev Eng Report- north of 9th av 43
DESCRIPTION
UNIT
EST. QTY.
UNIT PRICE
EXTENSION
Mobilization /Demobilization
L.S.
1
$75,000.00
$75,000.00
Storm Sewer Drop Manhole
Each
1
$1,500.00
$1,500.00
45- Degree Bend
Each
1
$700.00
$700.00
Rock Erosion Control Const. Entrance
Each
3
$1,000.00
$3,000.00
Earthwork/Compacted Common Fill (CV)
C.Y.
17200
$7.00
$120,400.00
Earthwork/Common Excavation (CV)
C.Y.
15000
$10.00
$150,000.00
Excavation, Water Quality Pond and Channels (CV)
C.Y.
7350
$10.00
$73,500.00
Excavation, South Park Pond (CV)
C.Y.
4000
$35.00
$140,000.00
Excavation, Opportunity Court Water Quality Pond (CV)
C.Y.
1330
$10.00
$13,300.00
Water Level Control Weir
L.S.
1
$15,000.00
$15,000.00
Boulder Control Outlet
L.S.
1
$7,500.00
$7,500.00
Site Restoration (Seed and Mulch)
Acre
2
$10,000.00
$20,000.00
Tree Clearing and Brush Removal (< 6" Dia.)
Acre
1
$10,000.00
$10,000.00
Remove Selected Tree (> 6" Dia.)
Each
25
$900.00
$22,500.00
Remove Existing Gravel Trail w /restoration
S.Y.
625
$5.00
$3,100.00
Install Root Wad w/ Footer Log
Each
6
$1,200.00
$7,200.00
Fiber Log (Bio Log)
L.F.
1250
$8.00
$10,000.00
Steel Sheet Piling Wall
S.F.
6750
$45.00
$303,800.00
10' x 60' Prefabricated Steel Bridge
Each
3
$22,000.00
$66,000.00
Bridge Abutments
Each
3
$25,000.00
$75,000.00
Silt Fence
L.F.
2500
$2.00
$5,000.00
Rock Boulder Riffle
Each
14
$2,600.00
$36,400.00
Rock Boulder Vane
Each
3
$1,600.00
$4,800.00
Riprap (Mn /DOT CL III)
Ton
380
$60.00
$22,800.00
Granular Filter Rock
Ton
190
$35.00
$6,650.00
Rock Filter Dike
Ton
70
$40.00
$2,800.00
Two Rail, Red Cedar Split Rail Fence
L.F.
600
$10.00
$6,000.00
Imported Topsoil (CV)
800
$15.00
$12,000.00
Storm Pipe Extension (12" to 24 ")
_C.Y.
L.F.
85
$30.00
$2,550.00
Storm Pipe Extension (30" to 48 ")
L.F.
88
$70.00
$6,160.00
Storm Diversion Pipe (12 ")
L.F.
58
$50.00
$2,900.00
12" Pipe Apron
Each
1
$600.00
$600.00
Furnish & Install 60" Diameter Manhole
Each
1
$5,000.00
$5,000.00
Landscaping and Live Plants
L.S.
1
$30,000.00
$30,000.00
Strip, Salvage, and Replace Topsoil
C.Y.
1000
$3.00
$3,000.00
Regional Shared -Use Paved Trail
S.Y.
3000
$30.00
$90,000.00
Kiosk- Education /Outreach
L.S.
1
$10,000.00
$10,000.00
Wetland Mitigation'
S.F.
110,000
$2.00_ $220,000.00
1 Wetland mitigation requirements pending official determination by the Army Corps of Engineers
Notes:
Unless noted, the Nine Mile Creek Watershed District (NMCWD) will cover 100% of approved project costs
50% of costs related to amenity pedestrian bridges (Segment G) will be covered by NMCWD
50% of costs related to trail construction will be covered by NMCWD
Land acquisition costs will be split between NMCWD and City of Hopkins (25% NMCWD / 75% City)
Table 5 -2. Engineer's Opinion of Probable Cost
9th Ave. S. to Highway 169
Nine Mile Creek Stabilization Project
July 17, 2008
Unit Price Subtotal
15% for Contingencies
Engineering, Legal and Administrative (30 %)
$1,584,000.00
$237,600.00
$546,400.00
Subtotal $2,368,000.00
P: \Mpls \23 MN \27 \2327G48 9 -Mile Hopkins \WorkFiles \Engineers Opinion of Probable Cost_1.xls REV Eng Report- South of 9th Av 44
Segment A
- Shape eroded stream banks
- Riprap lower stream banks
- Construct 8 -foot gravel trail
- Revegetate upper stream banks
- Consolidate roof drains at 10801
Excelsior Blvd, pipe to creek
Segment C
- Stabilize 100 feet of east streambank
with sheetpile and riprap
- Replace two 72" CMPA
with a single equivalent culvert
Segment D
- Stable reach; no action required
Segment B
- Shape eroded stream banks
- Riprap lower stream banks
- Revegetate upper stream banks
Segment E
- Grade northeast area adjacent to alley
to provide more gradual slope to creek
- Shape eroded stream banks
- Riprap lower stream banks
- Install drain tile between alley and creek
- Plant native trees and shrubs along
stream corridor
- Add trail to connect to south
- Place fieldstone around culverts
- Shift creek to southwest as needed
to accommodate bank stabilization
- Regrade and revegetate southwest
streambank
Segment F
- Shape eroded stream banks
- Fieldstone riprap on lower stream banks
- Stabalize west bank using rock vane
- Plant native shrubs, trees and grasses
- Add 8 -foot bituminous trail on east side
- Lower storm sewer to normal flow level
of creek
Connection to Storm Sewer
--- Existing Ditch
- Existing Channel
New Channel
Filled Channel
New Pipe
Sheet Pile
M Pedestrian Bridges
e
Feet
0 150 300 600
FIGURE 4 -1
PROPOSED STREAM IMPROVEMENTS
EXCELSIOR BOULEVARD TO 9th AVE SOUTH
North Fork of Nine Mile Creek
Hopkins, MN
E
Segment F
- Shape eroded stream banks
- Fieldstone riprap on lower stream banks
- Stabalize west bank using rock vane
- Plant native shrubs, trees and grasses
`i - Add 8 -foot bituminous trail on east side
- Lower storm sewer to normal flow level
of creek
:X.;
Segment G
- Create new, meandering channel
- Reuse one existing pedestrian bridge
- Install one new pedestrian bridge
- Avoid park trees
- Redirect /extend two stormsewer pipes
Segment H
- Install sheet pile in embankment
next to Westbrooke Way
- Construct riffles for grade control
- Stabilize banks with rock vanes,
c l root wads and stone toe protection
Segment J
- Reshape eroding banks
- Construct riffles for grade control
- Stabilize banks with rock veins,
root wads and stone toe protection
Segment K
- Create new meandering channel
- Separate stream from pond
3 Relocated Pedestrian Bridge
- Divert ditch flow to proposed pond via open channel Pl.
- Construct water quality pond
- Maintain channel adjacent to Highway 169
for drainage
Segment 1
- Create new, meandering channel
- Extend stormsewer from community garden
- Avoid trees at south end
Connection to Storm Sewer
Existing Ditch
- Existing Channel
New Channel
W
N
Filled Channel
New Pipe
Sheet Pile
Pedestrian Bridges
■ Diversion Weir
e
Feet
0 150 300 600
FIGURE 4 -2
PROPOSED STREAM IMPROVEMENTS
9th AVE SOUTH to HIGHWAY 169
North Fork of Nine Mile Creek
Hopkins, MN