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253 254 255 256 257 258 259 260 261 262 263 264 265 the filling of wetlands, resulted in an under-sized storm sewer system for today's conditions. The following areas of concern were identified in the analysis: The intersection of Draper Avenue and Midland Hills Road The wetland located between 2235 and 2211 Rosewood Lane North ("Rosewood Pond"). The side yard at 2241 Rosewood Lane South This pipe configuration at Draper Avenue and Midland Hills Road creates a bottleneck on the system, which causes localized street flooding in 100, 10, and 5-year events. Rosewood Pond and the side yard at 2241 Rosewood Lane South were identified as flood prone in the 100- and 10- year events. The following table shows the results of the modeling for the flood prone areas. 2241 Rosewood Rosewood Pond Low buildin elevation 937.60 940.30 100-Year Hi h Water Elevation 939.57 941.42 10- Year Hi h Water Elevation 937.48 940.35 Approximate additional storage 0.75 ac-ft 1.1 ac-ft needed to accommodate 100-Year event 266 The streets in this neighborhood are in good condition; upsizing the entire storm sewer 267 system would not be a cost-effective manner in which to improve the drainage 268 conditions, as it would require significant pavement removal and excavation. In addition 269 to costs, upsizing the pipe would increase flow rates into Walsh Lake. Since the outlet of 270 Walsh Lake is controlled by a lift station, increased flow to Walsh Lake could cause 271 significant problems downstream. 272 Creating additional storage possibilities throughout the existing storm sewer system, such 273 as wetland enhancement/pond excavation and rain garden construction, can alleviate the 274 stress on the existing system; reduce the threat of flooding, while also improving water 275 quality. By creating additional storage to reduce the risk of flooding, it is anticipated that 276 the street flooding will also be reduced. Creating upstream storage will reduce the amount 277 of water that reaches the dual pipe, and the timing of the water reaching the dual pipe will 278 also be more staggered, alleviating street flooding even further. 279 280 281 282 283 284 Action to Date: This subwatershed was added as a problem area in 2007. Recommended 2009 action: In order to create the additional storage recommended in the report, a final proj ect needs to be designed. We have hired WSB and Associates to complete the following work 285 • Additional study. The pond that was constructed as a part of the Midland Hills 286 Condominium development, which may have additional capacity than is currently 287 being used. Analysis would be completed to determine how much storm water could 288 be redirected to the pond and what improvements would be needed to do so. City of Roseville 2008 Storm Water Annual Report Page 8 of 14