07/23/2001 Council Packet - Laserfiche WebLink

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• • • Jeff Smyser, City Planner July 11, 2001 Page 3 Item 12 — Physical Impacts on Water Resources There is some question of the rationale for proposing to fill a wetland so that there is better visibility from the freeway frontage. Is this a hardship? The avoidance argument (page 15) is not very compelling as there seems to be significant ability to develop the site to the west. Any wetland mitigation (page 16) within the regional park would have to be a part of the existing master plan or a master plan update that would require approval by the Council. There should be at least an equal amount of public parkland purchased to replace the areas being used for wetland mitigation for this private development. Funding should also be provided for on going maintenance and monitoring of the "created" wetlands. Item 14— Water - related Land Use Management District The flood plain of George Watch Lake in the Regional Park should be avoided instead of being filled. Item 17— Water Quality: Surface Water Runoff Existing site soil is of naturally high infiltration media, therefore the predevelopment 100 - yr flood event results in a total site runoff rate of only 1.54 cfs. The applicant proposes to maintain predevelopment runoff rates by addition of two stormwater ponds, a bio- retention basin, and bio - swales. The runoff from the site will be collected in a 13.5 ac -ft detention pond, which will outlet to George Watch Lake of Rice Creek Regional Park Reserve through a 2 -inch and an 8 -inch pipe. Nutrient load simulations indicate a predevelopment phosphorous load of 22 lbs. per year and a post - development load of 60 lbs. per year. The draft EAW states the following: "while runoff rates will be maintained at predevelopment rates, it is not known what, if any, impact will result from the increased volume of runoff into the wetlands in the regional park reserve and George Watch Lake". Council staff is concerned that the applicant has not determined the effects of projected increased volume of stormwater discharge on the Rice Creek Regional Park Reserve wetlands and George Watch Lake. Increased water levels in the park may significantly alter shoreline plant ecology and habitat. Due to the potential for significant alteration to plant communities in a regional park reserve, it is our opinion that an analysis of water level changes in the park reserve due to increased runoff from the site must be completed before an EIS determination can be made. Council staff commends the applicant for inclusion of bio - filtration basins and bio - swales in the plan. However due to the existing soil's high capacity for infiltration, much more can be done to reduce the volume of runoff from the site. Rain - gardens can be added to infiltrate much of the runoff from the buildings' roofs. Frequent placement of infiltration trenches in the parking lot will also reduce runoff volumes. We recommend the applicant try to reduce runoff volume, as well as runoff rate, to predevelopment conditions. As the applicant did not include a site stormwater system map in the Draft EAW, it is not possible for us to determine the placement of proposed bio- filtration basins and bio- swales. The applicant should include this map in the final EAW. Council staff is concerned about the potential for plugging of the two small diameter outlet pipes of the large detention basin. It will be essential to install skimmers or other trash collectors, and to frequently maintain the pipe inlets.