Eng. Comments - 22-10 Fox Hill Drive Subdivision
Memorandum
To: Sharmeen Al-Jaff, Senior Planner
From: Erik Henricksen, Project Engineer
Joe Seidl, Water Resources Engineer
CC: Charles Howley, Public Works Director/City Engineer
George Bender, Assistant City Engineer
Jill Sinclair, Environmental Resource Specialist
Charlie Burke, Public Works Operations Manager
Date: 9/13/2022
Re: 581 Fox Hill Drive (Fox Hill Drive) Preliminary Plat Subdivision
Review – Planning Case No. 2022-10
DRAINAGE AND UTILITY EASEMENTS AND RIGHT-OF-WAY
The preliminary plat provided illustrates mostly typical public drainage and utility easements
(DUE) along the proposed subdivision’s lot lines with five foot DUE along the side and rear lot
lines and a 10 foot DUE along the front lot lines. Additional DUE is being proposed along Block
1 Lot 1 along the shoreline of Lotus Lake and Block 2 Lot 1, Lot 2 and Lot 3 which encompass a
wetland along with private stormwater BMPs. Private stormwater BMPS to serve the needs of
the development must be wholly located outside of all public easements upon submittal of the
final plat. This can be accomplished by either relocating the BMPs or adjusting the public DUE.
The applicant is proposing to dedicate 10 feet of right-of-way to provide for a 50-foot right-of-
way width along Fox Hill Drive, 30 feet of right-of-of way to provide for a 50-foot width along
Carver Beach Road, as well as a 50-foot right-of-way bisecting Block 1 Lot 1 and Block 2 Lot 1
to provide connectivity to the south for when the area further subdivides. The newly dedicated
50-foot right-of-way intersecting with Fox Hill Drive will eventually provide a needed secondary
access to the area and will connect Fox Hill Drive to Big Woods Drive; the Lotus Woods
subdivision to the south, recorded in 2020, provided the right-of-way connection off Big Woods
Drive. While these widths are not the city’s current standard for right-of-way widths (60 feet),
the dedications are consistent with the surrounding neighborhood.
EXISTING CONDITIONS SURVEY
The applicant provided an existing condition survey dated July 16, 2021. The existing condition
survey describes an easement granted to the Northern States Power Company from July 22,
1941. Based on the location of the overhead power from the survey it would appear the
easement may be located within right-of-way to be dedicated with the final plat. Any underlying
easements within public easements, including public right-of-way, must be vacated or released
prior to recording of the final plat.
WETLANDS
The proposed plans show one wetland onsite that was delineated by Kjolhaug Environmental
Services on November 23, 2020. The report included a MNRAM assessment. The wetland was
classified as a high value wetland (manage type 1) with a minimum buffer width requirement of
20 feet and an average width of 40 feet as outlined by Riley Purgatory Bluff Creek Watershed
District (RPBCWD) rules. City Ordinance requires a 25 foot buffer and 30 feet principle
structure setback from wetland buffers. The construction plans dated September 1, 2022 show a
wetland buffers and setbacks that is consistent with Article VI, Chapter 20 of City Ordinance -
Wetland Protection and appears to meet watershed buffer requirements.
GRADING
The site’s existing and proposed grades generally slope down both east and west from a high
point located near the center of the property. The developer is proposing to mass grade a
majority of the site in order to construct the public street, house pads and stormwater
management areas. The preliminary grading plans dated September 1, 2022 submitted for the
preliminary plat review appear to be in compliance with Section 7-19 and 18-40 of City
Ordinance; requirements of these sections include separation from low floor elevations of the
proposed homes to the highest known groundwater elevations, ensuring lowest building openings
have enough freeboard from any emergency overflows adjacent to the homes, ensuring driveway
grades are within limits set by Ordinance (0.5%-10%), ensuring drainage will be routed away
from building pads, illustrating adequate erosion control measures are planned for, etc.
DRAINAGE
In the existing condition there is a large drainage break along a hill located near the center of the
project area which directs water either east or west. Stormwater runoff flows to one of three
discharge points, east to Lotus Lake, northwest to Carver Beach Road or to the onsite wetland on
the west side of the site. Offsite drainage from the private parcel south of the development also
drains to the wetland onsite and was included in the hydraulic analysis. The wetland onsite does
not have a structural outlet in the existing condition but overtops a natural spillway outlet and
drains northwest to Carver Beach Road when inundated. Drainage to Carver Beach Road is
picked up by catch basins in the northwest corner of the intersection of Carver Beach Road and
Fox Hill Drive and drains to existing stormwater pond Lotus Lake 1-2-1.
The proposed condition largely maintains the existing drainage patterns, continuing to drain to
the same three discharge points. The proposed design uses surface grading to convey runoff to
one of four privately owned best management practices (BMPs) which treat the stormwater
before it discharges to one of the three discharge points. The proposed BMPS include four
underground infiltration systems on the four separate lots. Drainage from the proposed future
road which separates Blocks 1 and 2 is proposed to be routed to the underground infiltration
system on Block 1 Lot 1 for treatment. An outlet for the wetland is proposed to be directed to the
underground infiltration system on Block 2 Lot 1, with a natural spillway acting as the
emergency overflow and continuing to drain northwest to Carver Beach Road. The proposed
design shows the private system outlet directly behind the back of curb on public drainage and
utility easement located at the southeast corner of Block 2, Lot 3. Stormwater from this outlet
would discharge across the roadway where it would be eventually picked up by public
infrastructure located on the west side of Carver Beach Road. This configuration creates safety
and city maintenance concerns, and as such shall be redesigned. The applicant shall extend the
public storm sewer system across Carver Beach Road to connect the private system. Piped
outlets for the proposed BMPs are directed to one of the three discharge points. The final design
will likely require connections to public stormwater infrastructure along Carver Beach Road. The
proposed design appears to mix private and future public stormwater associated with the future
public road. The mixing of public and private stormwater systems creates difficulties with
operation and maintenance. As such the system shall be reconfigured to separate the public and
private stormwater BMPs. The public BMP must be sized to treat all stormwater form the future
roadway. As such the applicant shall provide updated drainage computations and figures as
required. Furthermore, because the drainage system on the east side of the development takes on
public stormwater the downstream conveyance system must be made public to allow the city to
maintain its stormwater infrastructure. The final design shall be updated to show the public
portions of the stormwater system is located within drainage and utility easement.
The bounce in the onsite wetland is unchanged or slightly decreases for the 2-, 10- and 100-year
storm events from existing to proposed conditions. Because this wetland extends onto adjacent
properties the 100-year HWL must be maintained which has been confirmed by the modeling.
EROSION PREVENTION AND SEDIMENT CONTROL
The proposed development will exceed one (1) acre of disturbance and will, therefore, be subject
to the General Permit Authorization to Discharge Stormwater Associated with Construction
Activity Under the National Pollution Discharge Elimination/State Disposal System (NPDES
Construction Permit). The applicant has prepared and submitted a Surface Water Pollution
Prevention Plan (SWPPP) to the City for review. The SWPPP is a required submittal element
for preliminary plat review along with the Erosion and Sediment Control Plan in accordance with
Section 19-145 of City Ordinance. No earth disturbing activities may occur until an approved
SWPPP is developed. This SWPPP shall be a standalone document consistent with the NPDES
Construction Permit and shall contain all required elements the permit. The SWPPP will need to
be updated as the plans are finalized, when the contractor and their sub-contractors are identified
and as other conditions change. All erosion control shall be installed and inspected prior to
initiation of site grading activities.
There is an area on the northeast portion of Block 1 Lot 1 that is being proposed for an outfall
pipe to Lotus Lake which will requiring trenching. The ESCP and tree preservation plans must
address this excavation and it’s impacts upon submittal of final plans for review and approval.
Enhanced erosion control measures are required when excavating near bodies of waters and must
be in accordance with the NPDES General Construction Permit.
STREETS
The proposed subdivision abuts Fox Hill Drive to the north and Carver Beach Road to the west.
Access to Block 1 Lot 1 and Block 2 Lots 1-3 of the proposed subdivision are proposed to be had
from Fox Hill Drive. As discussed previously under “Drainage and Utility Easements and
Public Right-of-Way” of this report, right-of-way is being dedicated to facilitate the connection
of a secondary access for the neighborhood which will extend from Fox Hill Drive to Big Woods
Drive. The applicant is proposing to construct the street to the maximum extent practicable
within the newly dedicated right-of-way. Construction of a future street at this time is preferred
when feasible rather than accepting an escrow for its future construction due to the difficulty and
variability in projecting the appropriate amount to escrow as it is typically unknown when future
street connections will be constructed.
In order to ensure that the alignment and grades of the proposed street will be conducive to the
future connection to Big Woods Drive the applicant shall provide exhibits illustrating the
alignment, profile, and grades upon submittal of final construction plans and must approved prior
to recording of final plat.
SANITARY SEWER AND WATERMAIN
The newly proposed subdivision will have access to adequate public sanitary sewer and water
facilities within Fox Hill Drive right-of-way. The plans illustrate tapping two new service lines
for both sanitary and water and connecting to two existing services for both sanitary and water
that were installed in 1975. Any existing services proposed to be connected to shall be verified
prior to connection of their serviceability such as televising the sanitary stubs to the public main;
any existing services not used shall be removed and abandoned accordingly.
The applicant is proposing an approximately 120-foot extension of a municipal water main along
the alignment of the future street which will eventually connect Fox Hill Drive and Big Woods
Drive. This extension is necessary to create a water main loop to the area which will provide
better serviceability, water quality, and available flows during firefighting operations. The
proposed alignment of the extended municipal main is in general conformance with City
Standards and Specifications. All newly installed public utilities will be required to adhere to the
City of Chanhassen’s Standard Specifications and Detail Plates, which will be the governing
specifications for said improvements. Upon acceptance by City Council the water mains will
become publicly owned and maintained. A municipal sanitary sewer extension is not required.
STORM WATER MANAGEMENT
Article VII, Chapter 19 of City code describes the required storm water management
development standards. Section 19-141 states that “these development standards shall be
reflected in plans prepared by developers and/or project proposers in the design and layout of site
plans, subdivisions and water management features.” These standards include water quality
treatment resulting in the removal of 90% total suspended solids (TSS) and 60% total
phosphorous (TP), and runoff rate control for the 2, 10, and 100-year storm events. The proposed
development is located within the RPBCWD and is therefore subject to the watershed’s rules and
regulations. A Stormwater Management Report dated August 15, 2021 (but assumed to be for
August 15, 2022 based on date submitted) was submitted by the applicant to the City and
provided to the watershed district concurrently as part of the preliminary plat review. Approval
from the RPBCWD has not been received for the site.
As outlined in the City’s Surface Water Management Plan adopted in December 2018, the City
requests regional ponding areas as opposed to individual on-site BMPs. Regional systems are
more efficient and are easier to maintain. Due to the large hill onsite it does not seem feasible to
direct drainage from the entire site to one regional BMP. However, it does seem feasible to
combine requirements and reduce the number of onsite BMPs to one BMP east of the hill (near
Lotus Lake) and one west of the hill (near the onsite wetland). The City explored whether the
existing Lotus Lake 1-2-1 stormwater pond could be used to provide water quality treatment for
the portion of the site which drains northwest to Carver Beach Road. The site is within
RPBCWD and is required to meet all applicable watershed rules which includes volume
abstraction of 1.1 inches of runoff from all new or disturbed impervious for redevelopment
where over 50% of the project area is being disturbed. RPBCWD does allow for the use of
existing regional stormwater management BMPs to meet its rules however Lotus Lake 1-2-1
does not provide the RPBCWD required volume abstraction and couldn’t be used alone to meet
RPBCWD’s rules. The applicant shall investigate the potential of a more regional BMP design
and submit justification for the final design configuration as part of the final plat submittal.
With the currently proposed infiltration BMPs the water quality rules appear to be met since the
volume abstraction requirement is met. A draft geotechnical report was included in the most
recent submittal however soil boring logs were only included for three out of the four soil
borings taken. The soil boring log for SB-4 should be submitted for review, and the final version
of the geotechnical report should be submitted when completed. Based on the three soil borings
provided the onsite soils are mostly clay with a sand/silt layer 5-10 feet below the surface across
the site. The applicant is proposing to excavate to the sand/silt layer and backfill with free
draining media under the proposed underground infiltration BMPs in order to get better
infiltration. The applicant shall include justification (geotechnical recommendation, infiltration
test results, etc.) for using design infiltration rates which differ from the MCPA’s standard design
infiltration rates based on the soil types shown in the soil boring logs. Once infiltration rates are
confirmed, water quality modeling will need to be updated to match the determined infiltration
rates, and water quality requirements should be confirmed to still be met. The applicant shall
resubmit updated models in their native form prior to recording the final plat to confirm the
City’s water quality rule is met.
The Stormwater Management Report and supporting Hydrologic and Hydraulic HydroCAD
models were reviewed. Modeling updates are required so the models can be used for rate control
and water quality analysis. The proposed design shows areas where stormwater from the
development is routed to BMPs, however in several instances the surface routing in unclear.
Further work is needed to update the grading plan and/or model so they better match the
proposed conditions. Furthermore, the design appears to modify the upstream drainage patterns
of the existing lot just south of Block 1, Lot 1. Additional analysis is required to show the
proposed design will not adversely change drainage of adjacent properties. The modeling
demonstrates that rate control can be met to all three discharge events for all storm event
modeled. HydroCAD modeling will need to be updated to better match the proposed conditions,
and all required updates from City and watershed comments. If the infiltration rates for the
different BMPs are modified to confirm rate control requirements are still being met prior to
recording the final plat.
As noted previously, RPBCWD rules require 1.1 inches of stormwater abstraction for all
impervious area (new and existing) for the entire project area when over 50% of the project area
is being disturbed. The proposed condition has an impervious area of 0.50 acres requiring
approximately 2,200 CF of abstraction. The proposed BMPs have a combined volume provided
of 2,331 CF exceeding the RPBCWD required volume. As noted previously, soil borings for SB-
1 through SB-3 (SB-4 not submitted) showed the onsite soils are mostly clay with a sand/silt
layer 5-10 feet below the surface across the site. To confirm the BMP design an updated
geotechnical report is required to show SB-4 and provide justification for infiltration rates at the
new location of the infiltration BMP located on Block 2, Lot 2.The applicant is proposing to
excavate to the sand/silt layer and backfill with free draining media under the proposed
underground infiltration BMPs in order to get better infiltration, however justification for the
design infiltration rates is required prior to confirm RPBCWD volume control requirements are
being met. As such the applicant shall receive conditional approval from the watershed district
prior to final plat submittal to the City.
As outlined in the City’s Surface Water Management Plan adopted in December 2018, the City
requires at least 3 feet of freeboard between a building elevation and adjacent ponding features.
The high water levels of all proposed BMPs should be noted on the plans, however based on the
HydroCAD modeling results it appears all proposed buildings are meeting freeboard
requirements with respect to the proposed BMPs. Freeboard requirements were confirmed to be
met with respect to the onsite wetland for all existing and proposed adjacent buildings.
All work proposed appears to be outside of the FEMA floodplain and above the Ordinary High
Water Level of Lotus Lake, so additional permitting with FEMA and/or the Minnesota
Department of Natural Resources is not anticipated to be required.
The infiltration BMPs located on each of four lots are to be privately owned and therefore will
require an Operations and Maintenance (O&M) Agreement and associated plan. The plan
identifies the maintenance schedule, responsible party, and should include information on how
the system will be cleaned out including the underground infiltration chambers proposed in the
construction plans. The applicant must provide a signed O&M Agreement prior to the initiation
of construction activities onsite. It is recommended that an HOA is created to streamline the
operation and maintenance of the separate BMP systems to reduce risk and responsibility for
future homeowners.
STORM WATER UTILITY CONNECTION CHARGES
City Ordinance sets out the fees associated with surface water management. A water quality and
water quantity fee are collected with a subdivision. These fees are based on land use type and
are intended to reflect the fact that the more intense the development type, the greater the
degradation of surface water.
This fee will be applied to the new lots of record being created. It is assessed at the rate in effect
at that time; the 2022 rate for low-density residential is $8,830.00 per acre of developable land.
ASSESSMENTS
Water and sewer partial hookups are due at the time of final plat. The partial hookup fees will be
assessed at the rate in effect at that time; 2022 rates for partial hookup fees are $691.00 per unit
for sanitary sewer and $2,562.00 per unit for water. The remaining partial hookups fees are due
with the building permit.
FEES
Based on the proposal the following fees would be collected with the development contract:
a) Administration Fee: if the improvement costs are less than $500,000, 3% of the
improvement costs.
b) Surface water management fee: $8,830.00/acre of developable land.
c) A portion of the water hook-up charge @ $2,562.00/unit.
d) A portion of the sanitary sewer hook-up charge @ $691.00/unit.
e) Park dedication fee @ $5,800.00/unit.
f) GIS fees @ $100 for the plat plus $30 per parcel: $160.00
g) Final plat process (review and recording of Plat and DC): $450.00
ENGINEERING RECOMMENDATIONS:
1. Any underlying easements within public easements, including public right-of-way, must
be vacated or released prior to recording of the final plat.
2. The applicant shall provide exhibits illustrating the alignment, profile, and grades of the
proposed street design from Fox Hill Drive to the future connection at Big Woods Drive
upon submittal of final plat.
3. The applicant and their Engineer shall work with City staff in amending the construction
plans, dated September 1, 2022 prepared by Seth Loken, PE with Alliant Engineering to
fully satisfy construction plan comments and concerns. Final construction plans will be
subject to review and approval by staff prior to commencement of any construction
activities.
4. The applicant shall enter into a Development Contract and pay all applicable fees and
securities prior to recording of final plat.
WATER RESOURCES RECOMMENDATIONS:
1. Private stormwater BMPS to serve the needs of the development must be wholly located
outside of all public easements upon submittal of the final plat.
2. Public stormwater infrastructure must be located wholly within drainage and utility
easements or public right of way upon submittal of the final plat.
3. The applicant shall provide a copy of conditional approval from the RPBCWD as part of
the final plat submittal.
4. The applicant shall provide an updated copy of the geotechnical report and infiltration
test results as part of the final plat submittal.
5. The applicant must update the Hydrologic and Hydraulic models per City and watershed
district comments and submit updated computations and models in their native forms
with the final plat and final construction plans.
6. The applicant must demonstrate that the proposed project will be in compliance with the
3 foot freeboard requirement to ponding features as part of the final plat submittal.
7. The applicant must confirm the stormwater routing of the site and include, underground
infiltration details, and connections to public infrastructure as part of the final plat
submittal.
8. The applicant must work with staff to improve the BMP configuration and stormwater
design to the maximum extent given the restrictions of the site and RPBCWD rules.
9. The applicant shall enter into an Operations and Maintenance Agreement for any
proposed privately owned stormwater facilities which shall be recorded concurrently with
the final plat.