North Lake Dam

Challenges

North Lake Dam is a 70-foot-high, high-hazard, earthen embankment dam located about 35 miles west of Trinidad, Colorado.  Historic dam safety issues at the site included seepage problems, slope instability, outlet surging, collapse and degradation of principal spillway intake pipes, inadequate auxiliary spillway capacity, and structural deterioration of the reinforced concrete auxiliary spillway.

The City of Trinidad (City) did not have sufficient funds to implement a comprehensive rehabilitation program and the work was required to be performed in phases over a 20-year period.

Solution

In 1998, RJH staff began assisting the City with the development of concepts to address these dam safety issues.  RJH staff led the development and evaluation of comprehensive rehabilitation alternatives.  The alternatives evaluation also identified potential environmental permits that would be required to complete the repairs.  As part of this initial work, a qualitative risk assessment was performed to prioritize items for repair to accommodate the City’s budget constraints.  This was one of the first qualitative risk assessments performed for a high-hazard dam in Colorado.

 Over the next 20 years, rehabilitation of the dam was completed in several phases.  The City retained RJH to provide engineering services for alternatives evaluation, design, and construction management associated with each phase of the rehabilitation.  Rehabilitation components included installation of a seepage blanket and stability berm, installation of a new auxiliary spillway on the left abutment, adding a new valve operating system, replacing a portion of the outlet conduit, installing a stream release facility, and lining an inlet pipe.

 Portions of the construction were performed using a design-build approach, which significantly reduced the cost.  All construction was completed with a nearly full reservoir.