Fox Hill Drive Response to Comments(1)
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Fox Hill Drive
Memorandum
RESPONSE TO COMMENTS
To: Scott Sobiech PE, Barr Engineering and Terry Jeffrey RPBCWD District Administrator
From: Seth Loken PE
CC: Denali Homes, Andy and Maria Awes
Date: August 15th, 2022
Subject: Response to Riley Purgatory Bluff Creek Watershed District Comments
This memo provides a response to watershed district comments. Since the last submittal, a
geotechnical of draft borings has been provided. Due to the heavily wooded nature of the site and
city application for subdivision, removal of significant trees is prohibited at this time which would
be necessary to conduct infiltration tests. Additional updates include responses to Rule B and G
comments
IC1. Because the project involves land-disturbance below the 100-year floodplain of Wetland
1 (949.25 ft) and Lotus Lake (897.46 ft), the project must conform to all the criteria in
RPBCWD Rule B. Some of the needed information includes computations documenting
the proposed project will provide full compensatory storage all fill below the 100-year
flood level, in accordance with RPBCWD Rule B.3.2 and the project must demonstrate no
adverse impact (Rule B, Subsection 3.3). The installation of storm sewer to wetland 1 has
been removed at the request of the city to preserve as much existing canopy as possible.
The installation of storm sewer and rip rap to Lake Lotus has been updated to discharge
at water’s edge. This will involve cut only. Typical detail for rip rap installation includes
overcutting of diameter of rip rap and installation with fabric and rock.
IC2. Because the project proposes a new outfall to a public water (Lotus Lake), the project
must conform to all applicable criteria in RPBCWD Rule G-Waterbody Crossings and
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Structures, including but not limited to subsection 3.1, 3.3, and 3.7. Noted. See further
responses below.
IC3. Please provide detailed plan view and scaled profile of the proposed outfall to Lotus
Lake and Wetland 1, including riprap, slope grading, etc. Detailed view of storm sewer to
Lotus Lake provided.
IC4. There does not appear to be soil borings or infiltrometer tests provided in the submittal.
Please provide site specific soil borings and infiltration testing at the proposed stormwater
management facilities. The soil borings and in-situ infiltration testing must be completed
by a state licensed soil scientist, engineer, or geologist in accordance with RPBCWD
Rule J.5.4. Soil borings have been added.
IC5. Please provide a detailed design of the proposed surface and underground infiltration
facilities, including the outlet structure, cross sections with elevations and materials listed,
and pretreatment (per Rule J, Subsection 5.4.d) Engineer design of systems included.
Full system design to be completed by others when contractor is chosen. Final details will
be provided to the city.
Because the project appears to involve work below the 100-year flood elevation of two waterbodies
(Wetland 1 and Lotus Lake), the project must conform to the requirements in the RPBCWD Floodplain
Management and Drainage Alterations rule (Rule B, Subsection 2.1). To conform to the RPBCWD Rule B the
following revisions are needed:
B1. The project proposes to install storm sewer discharging into wetland 1. Because the plans to not
appear to include the profile for the FES300 to FES302, additional information is needed to
demonstrate either no fill will occur, or that full compensatory storage is provided for this work in
accordance with RPBCWD Rule B.3.2. Please provide documentation that sufficient storage will be
provided adjacent to the same waterbody to compensate for the loss of floodplain storage from
this wetland. Alternatively, please demonstrate that no fill will be provided for the placement of the
storm sewer and riprap. No longer discharges below 100 year HWL of wetland per city request.
B2. Project proposes a new outfall into Lotus Lake (FES 500). While this work is above the 100-year
elevation of Lotus Lake (897.46 ft), the grading for the installation of riprap (shown on the plan view)
has the potential to result in fill within the 100-year floodplain of Lotus Lake. Additional information
is needed to demonstrate either no fill will occur, or that full compensatory storage is provided for
this work in accordance with RPBCWD Rule B.3.2. Please provide documentation that sufficient
storage will be provided adjacent to the Lotus Lake to compensate for the loss of floodplain
storage. Alternatively, please demonstrate that no fill will be provided for the placement of the
storm sewer and riprap. See standard city detail required for rip rap installation. Requires a low
point creation as part of the rip rap design and subcut for installation of fabric and granular
material.
B3. Please provide documentation and modeling to verify the grading conducted below the 100-year
floodplain of the wetland 1 and Lotus Lake will not have adverse offsite impacts and will not
adversely affect flood risk, water quality, and channel stability in accordance with RPBCWD Rule
B.3.3. Existing and proposed modeling of the project meets the required rate control requirements
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per district. 100 year increase in volume is shown from existing to proposed as part of modeling
however the project proposes to treat a portion of Fox Hill Drive (2400 SF impervious) and ROW that
was previously routed untreated to Lotus Lake. See included plans for flared end update from city.
B4. Add a note to the drawings requiring construction be conducted so as to minimize the
potential transfer of aquatic invasive species (e.g., zebra mussels, Eurasian watermilfoil,
etc.) to the maximum extent possible. Noted.
Because the project will involve the alteration of 50 cubic yards or more of earth and involves more than
5,000 square feet of land-disturbing activity, the project must conform to the requirements set forth by the
RPBCWD Rule C - Erosion Prevention and Sediment Control. To conform to the RPBCWD Rule C, the
following revisions are needed before the application will be considered complete:
C1. FES500 appears to discharge runoff above the normal water level of Lotus Lake. While riprap is
proposed at the FES, there appears to be the potential for erosion to occur on the Lotus Lake
shoreline. Please provide detailed, scaled profile of the proposed outfall to Lotus Lake, including
riprap, slope grading, etc. and computations to confirming that any surface flow on the bank will
not lead to erosion. Per this comment the flared end section was extended to Lotus Lake and
outlets at the surveyed water level. Rip rap will extend below the water level.
C2. Name, address and phone number of the individual who will remain liable to the District for
performance under this rule and maintenance of erosion and sediment-control measures from the
time the permitted activities commence until vegetative cover is established (per Rule C,
Subsection 4.3a). This information does not need to be provided prior to making a
recommendation to the RPBCWD Board but will be needed before the permit is released. Will
provide when NPDES permit is applied for.
Because runoff from the site is tributary to Wetland 1, the project must conform to the requirements set
forth by the RPBCWD Rule D – Wetland and Creek Buffers. To conform to RPBCWD Rule D, the
following revisions are needed before the application will be considered complete:
D1. RPBCWD’s review of the MNRAM provided results in the wetland being considered medium value,
thus requiring a 40-foot average, 20-foot minimum buffer. Computations have been updated based
on this information.
D2. There appears to be a retaining wall proposed within the Wetland 1 buffer. Rule D, subsection 3.3d
does not allow structures within the buffer area. Please revise the buffer plan or design to remove
the retaining wall from the buffer. Retaining wall has been removed.
D3. While the buffer area is clearly shown on sheet 13, the buffer monument locations appear to be
missing on the plan view. Please update the plan to show the monument locations. Location(s) for
markers, at a minimum along each lot line, with additional markers at an interval of no more than
200 feet and, for subdivisions, on each lot of record to be created (Rule D, Subsection 3.4) Buffer
monuments have been added.
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D4. Please update the plans to require that all disturbed areas within the buffer be restored with native
vegetation (Rule D, Subsection 3.3). Note added to wetland plan.
D5. Before any work subject to District permit requirements commences, buffer areas and maintenance
requirements must be documented in a declaration recorded after review and approval by RPBCWD
in accordance with Rule D, Subsection 3.4.This can be provided after the RPBCWD Board considers
the permit application and will be included as a recommended condition for Board consideration.
Noted.
Because the applicant proposes to place a new outfall Lotus Lake, a public waterbody, the project must
conform to RPBCWD’s Waterbody Crossings and Structures Rule (Rule G). To conform to the RPBCWD Rule
G, the following revisions are needed before being considered complete:
G1. Have you confirmed with the city that there is not an existing storm sewer and outfall to Lotus Lake
that the project could connect to rather than proposing a new outfall to Lotus Lake. See plansheet
from city. Flared end referenced is 300’ away and would require significant disturbance in order to
connect into.
G2. Please provide information demonstrating that the proposed outfall provides a public benefit (Rule
G, subsection 3.1). Currently there is no secondary emergency access to Big Woods Blvd resulting
in a 1200 LF dead end street which exceeds city standards. In order to treat the proposed stub road
and the fact that any existing outfall to Lotus Lake is not in close enough proximity to connect to a
new outfall is being proposed. The outfall is being proposed in a manner to minimize disturbance to
Lotus Lake and existing shoreline. As part of the work for the development the property owner will
be restoring the shoreline which has been neglected further providing a benefit to Lotus Lake.
G3. The following information and analysis must be provided to demonstrate the construction of the
proposed outfall:
a. FES500 appears to discharge above the normal water level of Lotus Lake. This
has the potential to result in significant bank erosion and does not appear
consistent with Rule G, subsection 3.3a which requires minimization of
disturbance and erosion of natural shoreline and bed resulting from peak flows;
See previous comment response.
b. when feasible, utilize discharge to stormwater treatment ponds, artificial stilling or
sedimentation basins, or other devices for entrapment of floating trash and litter,
sand, silt, debris, and organic matter prior to discharge to public waters; and
c. use natural or artificial ponding areas to provide water retention and storage for
the reduction of peak flows into waterbodies to the greatest extent possible.
There is a natural berm near the Lake according to topography taken via survey.
The flared end discharges at this point to allow water retention and storage to
maximum extent possible.
G4. Please provide detailed plan view and scaled profile of the proposed outfall to Lotus
Lake, including riprap, slope grading, etc. Provided.
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G5. The plan set needs to include notes to address criteria 3.7a, 3.7b, 3.7c. Added.
G6. Subsection 3.7 requires compliance with the applicable criteria in Rule F subsection 3.3.
Please provide information demonstrating that the criteria in Rule F subsection 3.3 are
achieved. City detail is specified on plans which conforms to these rules.
G7. Permit applicant must provide a maintenance and inspection declaration for the outfall. A
maintenance declaration template is available on the permits page of the RPBCWD
website (http://www.rpbcwd.org/permits/). A draft declaration must be provided for
District review prior to recording. This information does not need to be provided prior to
making a recommendation to the RPBCWD Board Noted.
Because the project will involve more than 5,000 square feet of land-disturbing activity, the project must
conform to RPBCWD’s stormwater management rule as described in Rule J, Subsection 3. Because the
proposed activity will increase the site imperviousness by more than 50%, the criteria of Section 3 will apply
to the entire project site, subsection 2.3. Under paragraph 2.5 of Rule J, Common Scheme of Development,
activities subject to Rule J on a parcel or adjacent parcels under common or related ownership will be
considered in the aggregate, and the requirements applicable to the activity under this rule will be
determined with respect to all development that has occurred on the site or on adjacent sites under
common or related ownership since the date this rule took effect (January 1, 2015). Because a permit is
being considered at the July 13, 2022 meeting for the shoreline restoration work (permit 2022-043), the
current activities proposed must be considered in aggregate with the activities proposed under this
application, Permit 2022-053.To conform to the RPBCWD Rule J, the following revisions are needed before
being considered complete:
J1. The proposed conditions drainage map suggests that the driveway work within the ROW will
not be routed to a BMP for abstraction, water quality, or rate control. Please update all of
the stormwater computations to account for all the existing and proposed impervious
surfaces on the site. This must include the imperviousness associated with the work in the
right of way and associated with permit being considered at the July 13 meeting for the
shoreline restoration work (i.e. permit 2022-043). Imperviousness has been updated. 2400
SF of previously untreated existing impervious is now treated as part of this project.
J2. Please provide additional information to clarify how the driveways for Lots 1 & 2 of Block 2
are captured and routed to the underground stormwater facility on Lot 1. The grading
appears to suggest that runoff from the driveways and potentially a portion of the roofs
would flow directly north to Fox hill drive. Please update the drawing/design to ensure the
contractor understand these areas must be routed to the BMP. If the design is relying on
gutter flow along the south side of Fox Hill Drive to turn the corner and be captured in the
proposed CB, please demonstrate that flows up through the 100-year event will take the
corner rather than overflowing to the east. The existing “curb” on fox hill drive is a bit roll
curb. Upon installation of curb cut for the road stub the water will be directed towards the
low point in the stub road.
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J3. The drainage arrow for Lot 1 Block 1 also indicates runoff from the driveway will flow north to
Fox Hill drive. Please clarify on the drawings to ensure this runoff is routed to the stormwater
facility. Driveway graded to create low point in driveway where flow will cross driveway to be
routed to the stormwater treatment facility.
J4. The drainage arrow for Lot 3 Block 2 also indicates runoff from the driveway will flow north to
Fox Hill drive. Please clarify on the drawings to ensure this runoff is routed to the stormwater
facility. As much drainage as possible is routed to stormwater facilities as physically possible
without further tree removal.
J5. The plans will need to ensure erosion prevention between the FES500 outfall and Lotus
Lake. The current outfall design as the potential to cause soil erosion downstream of the
proposed riprap and at the shoreline. Please provide computations documenting that
erosion will not occur or consider alternatives to convey the runoff to Lotus Lake without
exacerbating erosion potential. Please revise design to account for erosion prevention
modifications. Stilling basin and silt fence are included as part of storm installation.
J6. Comments related to HydroCAD modeling:
a) The rainfall depths used in the 24-hour rainfall simulations (2-year, 10-year, and 100-
year) must be revised to 2.87”, 4.27” and 7.41”, which are the point estimates for the
RPBCWD centroid. Adjusted.
b) The current snowmelt modeling appears to only be generating 6.95 inches of runoff.
Please update the snowmelt modeling to produce 7.2 inches of runoff. Adjusted.
J7. Comments related to MIDS modeling:
a) The MIDS modeling only include 2.303 acres of the 2.465 acres site boundary. Please
confirm the only area exclude is the Wetland 1 buffer areas? Correct. Only areas
excluded will be wetland buffer or areas the city would require as conservation area.
b) MIDS modeling results for abstraction and water quality are dependent on the soil
conditions below the proposed BMPs. Without soil boring information it is not possible
to confirm the abstraction and water quality criteria are achieved. Please complete soil
borings at each BMP location.Noted.
J8. There does not appear to be soil borings or infiltrometer tests provided in the submittal.
Please provide site specific soil borings and infiltration testing at the proposed stormwater
management facilities. The soil borings and in-situ infiltration testing must be completed by a
state licensed soil scientist, engineer, or geologist in accordance with RPBCWD Rule J.5.4.
Noted and included.
J9. Please provide a detailed design of the proposed surface and underground infiltration
facilities, including the outlet structure details, cross sections with elevations and materials
listed, and pretreatment identified (per Rule J, Subsection 5.4.d) Noted and included. One
note, these are engineer designed but not designed yet by manufacturer. Final manufacturer
designed plans will be included in as built to city and watershed.
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J10. In accordance with RPBCWD Rule J.3.6, no structure may be constructed or
reconstructed such that its lowest floor elevation is less than 2 feet above the 100-year
event flood elevation in accordance with RPBCWD Rule J.3.6. Please provide a summary
showing that all proposed low floor elevations for proposed and existing lots adjacent to
wetland 1, proposed surface and underground stormwater facilities, meet the rules.
J11. Provide computations demonstrating the drawdown of the required abstraction volume
will be drawn down within the required 48 hours.
J12. Permit applicant must provide a maintenance and inspection declaration. A maintenance
declaration template is available on the permits page of the RPBCWD website
(http://www.rpbcwd.org/permits/). A draft declaration must be provided for District review
prior to recording. This information does not need to be provided prior to making a
recommendation to the RPBCWD Board.
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