South Boulder Creek Regional Detention Facility

Challenges

During large storm events, South Boulder Creek overtops U.S. Route 36 (US 36), flooding downstream residential neighborhoods and disrupting a major regional transportation corridor.  To address this risk, the City of Boulder (City) completed a master planning effort that identified a regional detention facility immediately upstream of US 36 as the preferred flood mitigation alternative.  The proposed facility is located within a uniquely constrained site that includes the US 36 transportation corridor and Colorado Department of Transportation (CDOT) right-of-way, federally protected Ute ladies’-tresses orchid and Preble’s meadow jumping mouse habitat, City Open Space and Mountain Parks property, active floodplain and wetland resources, heavily used regional bicycle and pedestrian trails, and existing utilities.

Solution

RJH was retained by the City to plan, permit, and design a regional flood detention facility at the site.  RJH worked with the City and other stakeholders to develop a design concept.  The evaluation of the conceptual designs included populating an evaluation criteria matrix, which was informed by public input from a community open house and an associated questionnaire.

Features of the selected alternative include a 35-foot-high earthen embankment, a nearly ½-mile-long reinforced concrete spillway, an outlet tunnel under US 36, detention excavation with soil-bentonite barrier wall, landscaping, and environmental restoration.  The facility is classified as a high-hazard jurisdictional dam and regulated by the Colorado Office of the State Engineer (SEO).

RJH led the work from initial evaluation and assessment of the existing dam through geotechnical data collection, alternatives development, permitting, final design, and construction engineering. Permits were obtained from the SEO, U.S. Army Corps of Engineers, City, and CDOT. RJH is currently providing construction and resident engineering services.