Nationwide, 694 Corps-owned dams deliver multiple benefits to the nation, including flood risk management, recreation opportunities, navigation and hydropower. However, should a dam fail, it could be devastating downstream.
In 2005, the Corps overhauled how it looks at dams from a solely standards-based approach to a dam safety portfolio risk management approach, according to Suzi Hess-Brittelle, geologist and the District’s dam safety program manager.
From 2005 to 2009, a national cadre of experts performed an initial risk screening of all Corps’ dams, including those in the District. The screening gave each dam a relative risk rating to prioritize dams with highest risk first.
The new approach also expanded the routine inspection program to include periodic assessments every 10 years, in addition to the periodic inspections already performed on each dam every five years. These periodic inspections are the backbone of the dam safety program, Hess-Brittelle said. The assessments will help dam safety experts look at the bigger picture in terms of the probability, for example, of a reservoir reaching water surface elevations that could initiate a dam failure.
The District’s first periodic assessment was conducted this summer on Trinidad Dam, near Trinidad, Colorado. Besides District personnel from Engineering and Operations and Hydrology and Hydraulics sections, experts from the Sacramento and Seattle districts and the Risk Management Center participated, offering an “unbiased set of eyes,” which Hess-Brittelle said “makes for a better assessment.”
The assessment process consisted of an inspection, a Potential Failure Mode Analysis (PFMA) and a Qualitative Risk Assessment (QRA). To save money and resources, the assessment was done in conjunction with a periodic inspection. The assessment also included a review of historic dam data, as the assessment team had gathered any historic reports available at the Trinidad and District offices and scanned and archived them.
During the PFMA, the team brainstormed all the ways Trinidad Dam could fail without breaking the laws of physics. Then the credible failure modes were developed and assessed using a QRA. In the draft assessment report, the team has preliminarily concluded that Trinidad Dam is not as vulnerable as the initial risk screening suggests. Much of the risk is driven by the fact that the City of Trinidad is located only two miles downstream.
Among the report’s other recommendations are improved risk communication with the surrounding communities, including partnering exercises using the Emergency Action Plan with local, state and federal interests; closer monitoring of seepage; installation of inclinometers to monitor embankment movement; replacing or rehabilitating piezometers, as needed; and increasing surveillance and monitoring for reservoir pools above 6,220 feet.
Future periodic assessments are planned at a rate of one dam a year, with Abiquiu Dam next.