Des Moines

Detention

Basin Retrofit

Study

To qualify for the SRF Sponsored Projects Program, the stormwater practices must be designed following guidance in the Iowa Stormwater Management Manual (ISWMM) and need to manage the runoff generated by a 1.25” rainfall event. Over 90% of rainfall events in Central Iowa have been recorded as 1.25” or less in total volume. Geographic Information Systems (GIS) tools are applied to map the watershed areas that drain to each of the basins included in this study. Factors such as watershed area, soil conditions and the amount of impervious cover are evaluated for each site. The area, depth and outlet conditions of each basin are analyzed and a series of metrics are calculated indicate which basins are the best candidates to meet SRF program criteria. The results of these screening metrics are reviewed with City staff. Based on those factors, five site locations are selected for more detailed study. Given the size of watershed draining to each site, it was determined that stormwater wetlands would be the most cost-effective approach to meet the SRF program's requirements. Grading plans were developed for each site, following ISWMM's guidelines. These designs create a “stormwater maze” which forces water to flow through a long, meandering path across the wetland complex. Water quality is improved through biological uptake, evapotranspiration and settling. When complete, these stormwater wetlands are large areas filled with a blend of shallow and deeper water zones, divided by areas of dry land. RDG recommended that native Iowa vegetation be established within the wetland complex. Paths initially used for maintenance vehicles and equipment could be improved to allow public access around and through these natural areas. The trails could also integrate other features such as educational signage, bird blinds, fishing piers, boardwalks, bridges or stepstone crossings The prepared concept plans demonstrate that it is feasible to meet the requirements of the SRF program at each of the five sites. There are benefits and challenges for each retrofit opportunity. Aesthetics, visibility and public access may all be considered when deciding which sites are most desirable for retrofit. Some sites will manage larger volumes of water and may be more cost-efficient. The final report provided to the City of Des Moines summarizes these considerations and provides recommendations for the prioritization of future improvements throughout the city.