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RPBCWD Preliminary Plan Review Comments - 8-6-19ice R I L E T f 18681 Lake Drive East PURGATORY Chanhassen,MN 55317 BLUFF CREEK 952-807-6885 WATERSHED DISTRICT www.rpbcwd.org Memorandum To: Bob Generous, Senior Planner From: Terry Jeffery, Watershed Planning Manager Subject: Proposed Redevelopment at 590 W. 79th St. Preliminary Plan Review by RPBCWD Date: August b, 2019 c: Claire Bleser; Administrator Riley-Purgatory-Bluff Creek Watershed District Scott Sobiech, District Engineer Riley-Purgatory-Bluff Crook 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 portion of the project but offers the following summary of preliminary RPBCWD review comments for the proposed W. 79th Street Re-Development in Chanhassen, MN. Because the parking lot across W. 70tt' Street and this activity are intended to meet the landowner's objective, this will be reviewed as a common scheme of development. Additional comments will likely arise once an official application is submitted for RPBCWD review. We encourage the developer to discuss the project with the RPBCWD prior to applying. Further, we suggest that the application be submitted concurrently with the City 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. This memorandum is not intended to be an exhaustive look at the potential regulatory implications as the reviewed materials do not constitute a full plan. This project will require permit approvals from the RPBCWD Board of Managers. Rule B: Floodplain Management and Drainage Alterations The District model indicates that the 1% storm event flood elevation is 949.49 feet. This elevation encroaches onto the subject property. If any fill is proposed below this elevation, the criteria set forth in Rule B, Section 3 will apply. It currently applies to the proposed parking area. This area is known to have historic flooding, particularly at the intersection of W. 79th and Great Plains Blvd.The city is encouraged to consider the implications of increased impervious surface and the further restrictions the proposed modifications will impose upon creating additional flood volume storage. C pr otect. manage- restore. 2 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 be altered (Rule C, Section 2.1). Please see Rule C, Section 3 for the applicable criteria and notes that must be included with the erosion prevention and sediment 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 No wetlands are present on site. Therefore, this Rule is not germane. 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. Review of information did not provide enough detail to conclude if, in total, the new parking area and the additional patio areas will exceed 50% of the total site impervious surface. If it does not, then only the new and fully reconstructed impervious surfaces must be treated. If new and fully reconstructed impervious surfaces does exceed 50% of existing impervious surface then the applicant will be required to treat the entire site including the new parking area. 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. 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). HydroCAD or another acceptable H&H model must be provided. In addition to the stormwater summary, the actual electronic model must be provided. Provide for the abstraction onsite of 1.1 inches of runoff from the new or fully reconstructed impervious surface of the parcel; 3 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 In situ infiltration tests will be required to verify model assumptions. The applicant is strongly encouraged to get these tests prior to submittal of the application materials. o In the event that the applicant seeks to have the site viewed as restricted, they must provide adequate documentation to demonstrate the restricted nature of the site. This must consider efforts to remove obstacles to achieving the abstraction goal such as relocation of project elements to address varying soil conditions or requesting variances from local zoning ordinances. o Infiltration is not the only method of abstraction; the applicant must consider other methods of abstraction and provide details as to why they are or are not considered practicable. Some examples may include tree canopy interception over impervious surfaces. See Center for Watershed Prtection work on this practice. 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. 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 5 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 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.