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City of Arden Hills - Indian Oaks Area Downstream Drainage Review <br />March 12, 2018 <br />Page 3 <br /> <br /> <br />Building a legacy – your legacy. <br />Equal Opportunity Employer | wsbeng.com <br /> <br />Figure 2. Tributary Drainage Area <br /> <br /> <br /> <br /> <br />An XP-SWMM model of the downstream drainage system and entire tributary drainage area was <br />compiled to investigate the effects of increasing discharge rates from the IOC Wetland. The model <br />indicated that downstream highwater levels adjacent to residential properties may be affected by <br />increasing rates. 15”, 18”, and 21”/24” combination discharge pipe sizes were used to evaluate increasing <br />peak discharge rates from the IOC Wetland. Analysis of larger diameter pipes was determined to <br />increase downstream discharge rates too substantially to the downstream system to be considered for a <br />design option. <br /> <br />Through evaluation of the downstream flow path, we determined that increased discharge rates could be <br />mitigated in the wetland located just north of Colleen Avenue, here-in referred to as “Colleen Wetland”. <br />The Colleen Wetland sits substantially lower than the adjacent properties. A bituminous trail from Colleen <br />Avenue and Arden View Drive traverses the downstream flow path with an 18” culvert as the outlet. The <br />existing condition models indicate that stormwater overtops the trail during the 10-year and 100-year <br />storm event. The Colleen Wetland and trail crossing provide an opportunity for potential improvements to <br />temporarily detain stormwater to reduce discharge rates. See Figure 3 for the location of the Colleen <br />Wetland and trail crossing. HydroCAD was utilized to analyze this area and upstream drainage. <br />