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Attachment 8. Review from Riley-Purgatory-Bluff Creek Watershed District – May 1, 2017resourceful. naturally. BARRengineeringandenvironmentalconsultants -_ Memorandum To: Kate Aanenson, Community Development Director From: Scott Sobiech, PE Subject: Proposed Avienda Preliminary Plan Review by Riley-Purgatory-Bluff Creek Watershed District Date: May 1, 2017 c: Claire Bleser; Administrator Riley-Purgatory-Bluff Creek Watershed District This proposed redevelopment will need a Riley-Purgatory-Bluff Creek Watershed District(RPBCWD) permit prior to beginning construction activities. We appreciate the opportunity to provide preliminary comments on the project. The RPBCWD has not received a permit application for this project but offers the following summary of preliminary RPBCWD review comments for the proposed Avienda Development in Chanhassen, MN. Additional comments may arise once an official application is submitted for RPBCWD review. We encourage the developer to discuss the project with the RPBCWD prior to submitting an application. The RPBCWD permit application, rules and rule guidance are available for download on the RPBCWD website: http://www.rpbcwd.org/permits/. The following comments are based on the rules that would likely apply to this project and highlight the areas where revisions or additional information would be needed to meet the RPBCWD rules.These review comments do not constitute approval, a variance, or exemption from the rules.Therefore this project will require permit approvals from the RPBCWD Board of Managers. Rule B: Floodplain Management and Drainage Alterations Rule B: Floodplain Management and Drainage Alterations applies if a project alters or fills land below the 100-year flood elevation of a waterbody in the watershed. Based on the submitted plans, it is unclear if the project will fill below the 100-year flood elevation of the wetland on the site.In this case Rule B would apply,and the comments below should be reflected in the plans. Compensatory storage at the same elevation (+/- 1 foot)and within the floodplain of the same waterbody must be provided for any fill below the 100-year flood elevation. Rule C: Erosion and Sediment Control Erosion and Sediment Control Permit (Rule C) is required from the RPBCWD because more than 50 cubic yards of earth will be placed,altered, or removed and more than 5,000 square feet of land-surface area will altered (Rule C, Section 2.1). Please see Rule C, Section 3 for the applicable criteria and notes that Barr Engineering Co. 4300 MarketPointe Drive,Suite 200,Minneapolis, MN 55435 952.832.2600 www.barr.com To: Kate Aanenson,Community Development Director From: Scott Sobiech,PE Subject: Proposed Avienda Preliminary Plan Review by Riley-Purgatory-Bluff Creek Watershed District Date: May 1,2017 Page: 2 must be included with the erosion control aspects of the proposed project. The plans provided do not include the necessary erosion control notes or show all needed erosion control features.The needed erosion control notes for inclusion on the plan sheets are available for download under supporting documents on the RPBCWD website: http://www.rpbcwd.org/permits/. Rule D: Wetland and Creek Buffer A Wetland and Creek Buffer Permit (Rule D) is required from the RPBCWD because the proposed activities trigger RPBCWD Rule J (Rule D, Section 2)and there are several wetlands on the project parcel downgradient of the proposed work based on the submitted site survey, National Wetland Inventory mapping, and indicated in the project narrative. Efforts should be taken to avoid and minimize wetland impacts where possible in a manner consistent with the MN WCA and with consideration given to the comments of the Technical Evaluation Panel where the District initially commented. Comments provided below are preliminary in nature and the applicant should review the RPBCWD's Wetland and Creek Buffer Rule for full details on submittal requirements.The following criteria would apply. A copy of the wetland delineation report and MnRAM assessment must be provided with the application package to confirm the wetland value in accordance with RPBCWD Appendix Dl.The RPBCWD value may differ from the value assigned by the city of Chanhassen. Buffer must be indicated by permanent,free-standing markers at the buffer's upland edge, in material conformity with a design and text provided by the District.A marker must be placed along each lot line,with additional markers at an interval of no more than 200 feet or where the buffer changes direction.The location of the markers and a detail for the markers must be provided. A note must be included on the plans indicating:The potential transfer of aquatic invasive species e.g.,zebra mussels, Eurasian watermilfoil, etc.) must be minimized to the maximum extent possible. Before any work subject to District permit requirements commences, buffer areas and maintenance requirements must be documented in a declaration and recorded in the office of the county recorder or registrar. Rule J: Stormwater Management A Stormwater Management Permit (Rule J) is required for this project because more than 50 cubic yards of earth will be placed, altered, or removed and more than 5,000 square feet of land-surface area will be altered (Rule J, Section 2.1) based on the information provided by the City. Based on the information provided, it appears the project is a new development and the criteria in Rule J, Section 3 would apply to the entire parcel. Comments provided below are preliminary and general in nature and the applicant should review the RPBCWD's Stormwater Management Rule for full details on submittal requirements. The following criteria would apply. To: Kate Aanenson,Community Development Director From: Scott Sobiech, PE Subject: Proposed Avienda Preliminary Plan Review by Riley-Purgatory-Bluff Creek Watershed District Date: May 1,2017 Page: 3 An applicant for a permit under this rule must demonstrate, using a model utilizing the most recent applicable National Weather Service reference data (e.g.,Atlas 14),that the implementation of its stormwater management plan will: o Limit peak runoff flow rates to that from existing conditions for the two-, 10- and 100- year frequency storm events using a nested 24-hour rainfall distribution,and a 100-year frequency, 10-day snowmelt event, for all points where stormwater discharge leaves the site; A hydrologic model to simulate runoff rates for pre-and post-development conditions for the 2-, 10-, 100-year,and 10-day snowmelt frequency storm events using Atlas 14 precipitation depths with a nested rainfall distribution must be provided. Nested storm distributions are available for download at the RPBCWD website(www.rpbcwd.org/permits).An electronic copy of the model must be provided for review. The HydroCAD assumes pervious areas are hydrologic soil group B and C but the project narrative indicates the site is all clay(HSG D). Please revise and clarify. The narrative and table 7 indicate rate control is not achieve for the SW tributary area. Rate control must be provided at this location and all locations where discharges leaves the site. Provide for the abstraction onsite of 1.1 inches of runoff from the new or fully reconstructed impervious surface of the parcel; o Soil borings must be provided to support the infiltration rates used in the design and confirm that the ordinary high groundwater level is at least three feet below the bottom of any proposed filtration or infiltration features. o While reuse systems are proposed it does not appear that the proposed system will achieve the required 1.1 inch of abstraction. The presence of clay soils alone does not allow use of the restricted site criteria in Rule J, Subsection 3.3.In order to be considered for restricted site, a detailed sequencing analysis of all site restrictions is required. Soil borings 20, 21 and 26 indicate the presence of silt sand which suggest infiltration through abstraction methods might be doable. Relocation of project elements to address varying soil conditions must be considered. Provide for at least sixty percent(60%)annual removal efficiency for total phosphorus, and at least ninety percent(90%)annual removal efficiency for total suspended solids from site runoff. The onsite abstraction of runoff may be included in demonstrating compliance with the total suspended solids and total phosphorus removal requirements. o The MIDS calculator assumes pervious areas are hydrologic soil group B and C but the project narrative indicates the site is all clay(HSG D). Please revise and clarify. o An electronic copy of the MIDS model must be provided for review. To: Kate Aanenson,Community Development Director From: Scott Sobiech,PE Subject: Proposed Avienda Preliminary Plan Review by Riley-Purgatory-Bluff Creek Watershed District Date: May 1,2017 Page: 4 o Documentation justifying the removal efficiencies directly entered into the MIDS calculator for the wet ponds must be provided. Alternatively,the ponds could be simulated in P8. o The P8 model used for justifying loading to MNDOT wetlands must be submitted for review. 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 and no stormwater management system may be constructed or reconstructed in a manner that brings the low floor elevation of an adjacent structure into noncompliance with this standard. General Comments: o All stormwater management structures and facilities must be designed for maintenance access and properly maintained in perpetuity.A maintenance and inspection plan must be provided.The maintenance and inspection plan must include specific maintenance requirements for the proprietary BMPs installed. o See Rule J, Section 4 for permit exhibit requirements. Summary Based on the information provided to date,the proposed design does not meet the RPBCWD rules and a permit from the RPBCWD will be required prior to construction to likely cover the following: Erosion and Sediment Control,Wetland and Creek Buffer,and Stormwater Management.The online permit application, rules, maintenance declaration template,and financial assurance templates are available for download on the RPBCWD website:http://www.rpbcwd.org/permits/. Please contact us with any questions.