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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.