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Ms. Katie Larsen <br />04/04/2025 <br />Page 2 <br />• Stormwater Management <br />Stormwater management for the Natures Refuge North Addition consists of storm sewer <br />draining streets and yards, two sedimentation basins, and a large basin excavated for fill <br />material. The stormwater systems shall meet the City's and Rice Creek Watershed District's <br />(RCWD) Stormwater Management requirements. All proposed future public streets must <br />meet City design standards. Existing and proposed discharge rates are summarized below. <br />Pre- and Post- Development Discharge Rates (cfs) <br />Condition <br />2-Year <br />10-Year <br />100-Year <br />10-Day Snowmelt <br />Needed if basins <br />Existing <br />0.27 <br />1.25 <br />9.48 <br />landlocked <br />Needed if basins <br />Proposed <br />0.68 <br />2.85 <br />12.81 <br />landlocked <br />Comments: <br />1. Outflow rates increase for all modeled storm events beyond what can be attributed to <br />model accuracy. Further attempts at providing rate control must be made. For <br />example, ditching flows from back yards to the inlets routing to BMPs, lowering the <br />FES north of Wetland 50 to better route flows into Pond 500, or small rate -control <br />basins at areas that cannot reasonably be routed into a BMP. <br />a. If increases cannot be avoided, it must be demonstrated that they will not <br />cause an adverse downstream impact. <br />Flows have been adjusted and plans modified. <br />2. The SWMP water quality calculations show 0.90 acres of existing impervious, <br />however the HydroCAD existing modeling has 3.57 acres of unconnected roof land <br />cover. Please clarify these differences. <br />Existing impervious adjusted within water quality calculations. <br />3. Please revise the Water Quality calculations in the SWMP as they do not match <br />HydroCAD modeling. For instance, HydroCAD modeling shows 16.86 total acres and <br />7.52 acres of impervious routed to Pond 600, but the SWMP has a total area of 9.77 <br />acres and 0.196 acres of impervious. <br />Water quality calculation updated. <br />4. NURP criteria specify a maximum depth of 10'. Pond 500 has a depth of 11', raising <br />the bottom elevation to 10' will bring it into NURP compliance and still keep the <br />majority of its dead volume. <br />Pond 500 has been revised to a depth of 10' <br />5. Per new RCWD standards, any new impervious flowing to a pond designed in <br />accordance with NURP criteria counts as full water quality treatment, therefore the <br />full volume credit can be claimed for new impervious flowing to Pond 500 and 600 if <br />properly designed. <br />Water quality revised. <br />6. Pond 300 should be renamed to Lake 300 as its depth is well in excess of the 10' <br />max depth outlined in NURP criteria. Additionally, it cannot be counted asproviding <br />water quality treatment. <br />Note has been revised. <br />7. Please clarify why drainage area 26S is modeled. It is offsite and has no changefrom <br />existing to proposed conditions. <br />26S was modeled assuming the Street B connection to Andall may occur. We are <br />assuming the new street will have ribbon curb and there will be no drainage increase <br />to 26 (for now) <br />8. The 2.5" event should be modeled in HydroCAD to determine runoff volumes needed <br />for pond dead storage. <br />Nature Refuge North Addition Engineering Review <br />