Lambertson Lakes No. 3 Dam

Challenges

The City of Thornton (Thornton) obtained Lambertson Lakes No. 3 dam as part of a nearby residential development.  The dam was presumably constructed during the 1930s for agricultural purposes although no records existed regarding its design or construction.  The existing high-hazard dam had numerous identified safety issues, including uncontrolled embankment and foundation seepage, embankment stability, undersized spillway, lack of low-level outlet, and overgrowth of trees and shrubs.

The dam is located in a city park and surrounded by residences.  Structural modifications would need to be aesthetically pleasing and blend in with the surrounding features of the park.

Solution

The City retained RJH to evaluate the existing dam and develop a final rehabilitation design based on a preferred alternative concept developed previously by others.  While performing preliminary design work, RJH recognized that the breach estimates used to determine the hazard classification were extremely conservative.  RJH reassessed the breach estimates using collected geotechnical data and, in close coordination with the Colorado Office of the State Engineer (SEO), determined that the rehabilitated facility would meet the classifications of a low-hazard dam.  The reclassification resulted in cost savings of 40 percent to Thornton and simplified SEO final design requirements.

Final design consisted of a new culvert spillway with a partial-ogee “bath tub” control structure and concrete-lined spillway chute, spillway energy dissipation stilling basin, new outlet works, and embankment modifications.  RJH worked extensively with landscape architects and the community to maintain the aesthetics of the park and lessen the visual impacts of the dam improvements, and also collaborated with Colorado Parks and Wildlife for wetland protection and to relocate fish prior to construction.