2B, Valentine Park Drainage Analysis Review

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City of Arden Hills November 2,2010 Page 2 Finally, the hydrologic model that has been created for this project is a powerful tool that the City can and must utilize to efficiently evaluate the downstream effects of various BMPs installed in the upper sub- watersheds. The goals of this analysis would be reduced downstream flooding in Valentine Park,and increased water quality for stormwater flowing to Valentine Lake. DRAINAGE ANALYSIS The conclusions drawn in the drainage analysis appear appropriate given the input parameters,storm events modeled,and the results of the model. During our site visit we observed lake water approximately 15-in to 20-in deep in the Snelling Avenue 36"RCP culvert extending into the park,adjacent to the parking area,and backing up the park outlet ditch approximately 25-ft. This water level roughly corresponds to a lake elevation of 877.0. This elevation is approximately 1.5-ft higher than the stated Normal Water Level(NWL). This elevated lake water level creates a"tail water"condition at the culvert that increases the localized flooding within Valentine Park- One discrepancy we identified during the site visit may be related to the condition of the outlet for Valentine Lake. The report states that a recent lake level of 879.0,recorded on 10/14/08,is higher than the stated NWL of 875.5 likely due to beaver dams,within the lake's outlet channel,which have since been removed. We understand that the City and Ramsey County currently monitor and maintain the lake's outlet channel condition. We recommend that effort continue in order to keep the lake's outlet channel clear of debris and invasive species and control the Ordinary High Water Level(OHWL). It is our understanding that the 875.5 elevation was obtained from the Rice Creek Watershed District. However,the watershed district now recommends that Valentine Lake water level data be obtained from the Minnesota Department of Natural Resources(Mn/DNR). Our review of current Mn/DNR lake data conflicts with the elevations listed in Figures 3 and 4 of the drainage analysis and presumably the values entered into the hydrologic model (see Figure 1). According to Mn/DNR the OHWL of Valentine Lake is 878.7, the highest recorded water level is 881.1 taken on 07/03/78,and the latest reading taken on 10/01/10 was 879.58. The Mn/DNR defines the OHWL as: ...an elevation delineating the highest water level that has been maintained for a sufficient period of time to leave evidence upon the landscape,commonly the point where the natural vegetation changes from predominantly aquatic to predominantly terrestrial." The Mn/DNR makes no mention of a NWL of 875.5,and in general does not use that term. In addition, the lowest elevation recorded for the lake during the past 10 years was approximately 876.7 during the summer of 2002. We have included the Mn/DNR water level graph showing Lake Valentine water Ievels from the year 2000 to 2010(see Figure 1), which identify the elevation of 878.7 as the overall average elevation of the lake. We recommend that the lake OHWL and the 100-yr high water(881.1)level,as recorded by the Mn/DNR, be utilized to calibrate the XPSWMM model as a verification of the model's accuracy_ FA-CivillARDRU16120645\T16.120645 Report RevieW_FINAL.dorx