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Referral Review Response from RPBCWD 05-08-2014To: Sharmeen AI -Jaff, Senior Planner, City of Chanhassen From: Scott Sobiech, PE Subject: Next Steps Learning Center Expansion Development Review by Riley- Purgatory-Bluff Creek Watershed District Date: May 8, 2014 Review Comments The City of Chanhassen requested that the RPBCWD review the proposed development at the Next Steps Learning Center based on the draft RPBCWD rules. Table 1 summarizes the RPBCWD rules that would likely apply to the project based on the information submitted. Table 1: RPBCWD Rule Summary RPBCWD Rules Applicable to Project Rule B: Flood lain Rule C: Erosion and Sediment Control X Rule D: Wetland, Lake and Creek Buffers X Rule E: Dredging and Sediment Removal Rule F: Shoreline and Streambank Stabilization Rule G: Waterbody Crossings and Structures Rule H: Appropriation of Public Surface Water Rule I: Appropriation of Groundwater Rule J: Stormwater Management X The following sections provide specific review comments related to the rules that would apply to this project. Rule C: Erosion and Sediment Control Since he project involves disturbance of approximately 0.5 acres, it would require a permit based on the draft rules. In the draft rules, Rule C is applicable when a project disturbs greater than 5,000 square feet. The draft rules require that an erosion and sediment control plan be prepared. The requirements of the erosion and sediment control plan are listed in Rule C, Section 3.1 of the draft RPBCWD rules. The submitted design plans include temporary silt fence, but do not show any other erosion and sediment control practices. A rock construction entrance shall be used at the access point to the site. A sediment control measure must be used to prevent sediment laden discharge from entering the creek during construction. This includes the discharge leaving the site through the spillway to the creek. The erosion and sediment control plan should also include provisions to minimize the disturbance intensity and duration by phasing disturbance where possible. In the draft rules, slopes of 3:1 (H:V), such as on the outlet berm from the dry pond, shall be adequately stabilized. Stormwater facilities shall be protected during construction, and all construction site waste shall be properly managed including concrete truck washout. The draft rules require that all temporary erosion and sediment control BMPs be maintained until the completion of construction and vegetation is established sufficiently, and then must be removed. Soil surfaces disturbed or compacted during construction and remaining pervious must be decompacted to a To: Shormeen AI -Jaff, Senior Planner, City of Chanhassen From: Scott Sobiech, PE Subject: Next Steps Learning Center Expansion Development Review by Riley- Purgatory-Bluff Creek Watershed District Date: May 8, 2014 Pace: 3 depth of 18 inches prior to final revegetation. All disturbed areas must be stabilized within 7 calendar days. The draft rules require that the permit holder inspect and maintain all erosion and sediment control facilities, features and techniques until final site stabilization. A log must be kept of the inspections and be available to the District upon request. Rule C, Section 4 of the draft rules details the required information and exhibits that would need to accompany an Erosion and Sediment Control Permit Application. Rule D: Wetland, Lake and Creek Buffers Because the project disturbs greater than 5,000 square feet and will therefore require a permit through Rule J: Stormwater Management of the draft rules, the project would need to be considered for a permit through Rule D of the draft rules. The project is adjacent to Riley Creek, which is a public water. Therefore, an average buffer width of 50 feet and a minimum buffer width of 30 feet along Riley Creek would be required through the draft rules. The wooded area currently present along Riley Creek likely meets the buffer requirements of the draft rules. However, the draft rules require additional information and documentation be provided. The buffer area would need to be documented, marked in the field, and recorded as such in accordance with Rule D, Section 3.2. The buffer alterations made by constructing the riprap spillway from the dry pond outlet to Riley Creek would need to be constructed in accordance with Rule D, Section 4 of the draft rules. This includes additional documentation in the erosion and sediment control plan, minimizing disturbed area in the buffer, and native restoration of disturbed areas. Rule D, Section 7 outlines the required information and exhibits that would be required through the draft rules. Rule J: Stormwater Management Because the proposed project disturbs more than 5,000 square feet, the project would require a permit with an approved stormwater management plan through the draft rule. The stormwater management rule includes criteria for 1) peak runoff rates, 2) volume abstraction, and 3) water quality. Depending on the area of disturbance, these criteria would apply to different portions of the site. If the project disturbs more than 50 percent of the existing impervious surface or increases the impervious area by more than 50 percent, the criteria would apply to the entire project parcel. Otherwise, the criteria would only apply to the disturbed areas and additional impervious surface on the project parcel. The project does not increase the impervious area by more than 50 percent. The provided project documentation does not quantify the amount of impervious area that will be disturbed. The listed area of total disturbance is 0.5 acres. For the purposes of this review, it is assumed that the proposed project disturbs less than 50 percent of the To: Shormeen AI -Jaff, Senior Planner, City of Chanhassen From: Scott Sobiech, PE Subject: Next Steps Learning Center Expansion Development Review by Riley- Purgatory-Bluff Creek Watershed District Date: May 8, 2014 existing impervious, and the criteria outlined in the draft rules would be applied to disturbed areas and new impervious area. The HydroCAD model and the drainage maps do not match for areas 51.2 and 51.3. The time of concentration used in the HydroCAD model may not be appropriate based on the size and ground cover in each area. Please verify these inputs and adjust the design if necessary. There is a discrepancy in the berm elevation between the HydroCAD model and the design plans. The HydroCAD model shows the overflow at 950.4, and the plans show the top of berm at 950.8. Please verify the overflow elevation and adjust the design accordingly. Overflows from the dry pond during the 100 - year event should be minimized. The slope of the berm must be stabilized in such a manner that it provides sufficient erosion protection during the 100 -year event. The plans show a riprap slope being used at the primary outlet from the dry pond. Please provide additional information including the extents of the riprap slope in plan view and the downstream limits of the riprap slope. Please provide computations verifying that Class II riprap provides adequate erosion protection from the discharge from the dry pond and that the spillway is designed to contain flows from the 100 -year event. Please also verify that the sod shown on the slope above the riprap slope will be stable when the dry pond overflows. The 4" PVC outlet from the dry pond has a very high risk of plugging, increasing the risk of the pond overtopping the berm. We would recommend reconfiguring the inlet to incorporate skimming to reduce the risk of plugging. A preferred alternative would be to utilize a sump structure at the inlet, increase the diameter of the inlet pipe, and use an elbow section to provide skimming. In accordance with the draft rules, the project would be required to use Atlas 14 rainfall data in the implementation of its stormwater management plan. The plan would be required to limit peak runoff rates for the 2 -, 10 -, and 100 -year frequency storm events using a nested 24 -hour rainfall distribution and a 100 -year frequency, 10 -day snowmelt event. The stormwater management plan for the project does address peak runoff rates for the 2 -, 10-, and 100 -year frequency storm events; however, the events were modeled using TP -40 rainfall data and the SCS Type II distribution. This rainfall data needs to be updated to Atlas 14 information and the 100 -year, 10 -day snowmelt event needs to be considered to be in compliance with the draft rules. The updated hydrology must be performed to verify the project meets rate control requirements. The draft rules require that the project provide for the volume abstraction onsite of 1.1 inches of runoff from all disturbed areas and all new impervious areas. Pretreatment must be provided for infiltration or filtration practices. The proposed plans include a dry pond; however, the dry pond has no features that To: Shormeen AI -Joff, Senior Planner, City of Chanhassen From: Scott Sobiech, PE Subject: Next Steps Learning Center Expansion Development Review by Riley- Purgatory-Bluff Creek Watershed District Date: May S, 2014 would provide for the abstraction of runoff. The dry pond would need to be modified or additional BMPs added to the site. The draft rules also require that the project provide for at least 75% annual removal efficiency for total phosphorus (TP), and at least 90% annual removal efficiency for total suspended solids (TSS) from runoff from all disturbed areas and all new impervious areas. The project would need to submit verification that its design meets these requirements. Rule J, Section 3.4 outlines the maintenance requirements for all stormwater management structures. All stormwater management features must be properly maintained in perpetuity to ensure that they continue to function as designed. A maintenance plan would need to accompany the permit application. Rule 1, Section 4 outlines the required exhibits that would need to accompany the permit application. In addition to information that was provided as part of the current submittal, a permit application would need to include a geotechnical analysis including soil borings at proposed stormwater management facilities, and computations completed to design the retention BMPs, and a narrative addressing incorporation of retention BMPs. Summary Based on the information provided to date, the proposed design does not meet the permit requirements of the RPBCWD draft rules. The draft rules would require the project to obtain Erosion and Sediment Control; Wetland, Lake, and Creek Buffer; and Stormwater Management permits. To be in compliance with the draft rules, the project would need to submit an erosion and sediment control plan showing control measures to protect Riley Creek during and after construction. These erosion and sediment control measures would need to be in addition to the silt fence currently shown on the design plans. To meet the draft buffer requirements, the project would need to designate, mark, and record the wooded buffer between Riley Creek and the development and preserve and maintain the area in accordance with the buffer requirements. Additional information and revised computations are needed to meet the draft Stormwater Management rules. Design computations must use Atlas 14 reference data and nested 24 -hour rainfall data and must include the 100 -year, 10 -day snowmelt event. For all disturbed areas and all new impervious areas, the project must provide rate control at all discharge points for the 2 -, 10-, 100 -year rainfall events and the 100 -year, 10 -day snowmelt event. The project must also provide volume abstraction of 1.1 inches of runoff, 75% annual removal efficiency of TP, and 90% annual removal efficiency of TSS from all disturbed To: Shormeen AI -Jaff, Senior Planner, City of Chanhassen From: Scott Sobiech, PE Subject: Next Steps Learning Center Expansion Development Review by Riley- Purgatory-Bluff Creek Watershed District Date: May 8, 2014 Page: 6 areas and new impervious area. Additionally, the inputs to the HydroCAD model need to be verified for appropriate times of concentration and overflow elevations. More information is needed on the design of the riprap slope and its extents. The outlet design from the dry pond should be reviewed for additional options to provide improved skimming to minimize plugging and maintain BMP function.