2B, Valentine Park Drainage Analysis Review

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City of Arden Hills November 2,2010 Page 4 When designing a stormwater treatment train,the most efficient use of space and money dictates that the rate control pond be the first BMP in the train. It has the capacity to absorb the incoming peak flows and discharge water in a more controlled manner. All of the BMPs downstream of the rate control pond can then be designed based on the discharge rate of the pond and not the incoming peak flow rates,thereby increasing efficiency and decreasing required space. The recommendations included adding fill material to Valentine Park to bring the mean elevation up from approximately 880.0 to 882.0. According to the XPSWMM model,those two feet of fill would protect the park amenities from flooding up to the 100-yr storm event. One alternative that was not evaluated in the drainage analysis was adding less fill to the park to protect it from a smaller storm, i.e. a 10-, 20-, or 50-yr storm and accept more frequent localized flooding as a result of construction cost savings. We do not believe that additional fill within Valentine Park will ultimately affect the OHW or the HWL of Valentine Lake. The area of the park proposed to be raised is very small when compared to the overall surface area of Valentine Lake. This can be evaluated within the XPSWMM model to provide a reasonable prediction of the change,if any. In addition the most current FEMA Flood Insurance Study (.tune 2010)lists a 500-yr high water elevation for Valentine Lake of 881.3. A second possible alternative would include modifying the PMP to incorporate volume control and water quality at the source,as described above, and then address the remaining flooding problem,if any, at Valentine Park through future hydrologic model evaluations. COST TO VALUE RELATIONSI41P VALUE OF THE IMPROVEMENTS The OPTION#1 stormwater management treatment train appears to have been appropriately sized,the proposed park elevation corresponds to the modeled 100-yr storm event,and the cost estimate included in Appendix C appears reasonable for the identified improvements. As mentioned above, there may be a more cost effective solution to the Iocalized flooding experienced at Valentine Park. The solution will require additional time to be realized as the PMP improvements are - constructed. It is very likely the most cost-effective solution will require a combination of integrating volume control and water quality BMPs into the PMP in the upper Valentine Park sub-watersheds. Based on the results of the XPSWMM model, implementing source control BMPs into the Glenview Avenue and Crystal Avenue sub-watersheds may have the greatest impact in reduced localized street flooding and downstream flooding within Valentine Park. Once the volume of stormwater flowing from the upper sub-watersheds to Valentine Park is reduced,the Valentine Lake outlet continues to be monitored and maintained by the City and Ramsey County,and the OHWL of the lake is verified at the appropriate elevation,the hydrologic model should be modified to reflect these improvements. With a revised and updated model the City can verify the need,if any,of additional park improvements to alleviate localized flooding. CONCLUSIONS There may be additional cost savings available to the City to address the localized flooding at Valentine Park. Portions of the approach will require additional time to evaluate,model, plan,and implement,but a _ number of the possibilities are straight-forward and simple, i.e. monitor the condition of the outlet of F:1_CiviIV"ffiT1 6 1 2 05451T16.120645 Report Rcview_F1NAL.docx