Location and Description
Whittier Narrows Dam is a flood risk management and water conservation project constructed in 1957 and operated by the U.S. Army Corps of Engineers, Los Angeles District. The project is located, as its name implies, at the "Whittier Narrows,” a natural gap in the hills that form the southern boundary of the San Gabriel Valley. The Rio Hondo and the San Gabriel rivers flow through this gap and are impounded by the reservoir. The communities of Montebello and Pico Rivera are located immediately downstream.
Whittier Narrows Dam, a typically dry flood risk management structure located 11 miles east of downtown Los Angeles, has been reclassified from Dam Safety Action Classification 2 to DSAC 1.
The DSAC 1 rating indicates that the U.S. Army Corps of Engineers considers the incremental risk – the combination of life or economic consequences with the likelihood of failure – to be very high. The reclassification as DSAC 1 identifies the dam as one of the highest priority dam safety projects in the Corps’ portfolio of dams.
In a May 25, 2016, memorandum to Col. Kirk Gibbs, commander of the Corps’ Los Angeles District, Mr. James Dalton, chief of Engineering and Construction at Corps headquarters, emphasized that new findings with respect to the anticipated performance of the spillway gates drove the reclassification.
The Los Angeles District is currently working with a nationwide team of experts to develop a plan to reduce the risk associated with the spillway. The Corps anticipates that some of the potential solutions will be in operation prior to the 2016-2017 winter rains; other measures will likely be installed before the end of 2017.
The reclassification of Whittier Narrows Dam resulted from a Dam Safety Modification Study that the Corps is conducting on the structure. Initiated in 2012, the DSMS is a comprehensive evaluation of the risks posed by the structure on the community. The study identifies the hazards that can cause harm, determines who or what can be harmed, estimates the probability of the harm occurring, and recommends solutions (risk reduction measures and plans) that can be implemented to reduce the identified risk considered to be unsafe (above the Corps’ tolerance level).
The study has identified the following “risk-driving” failure modes:
Premature opening of the automatic spillway gates. As discussed above, this failure mode led to the DSAC reclassification. In order to reduce the probability that the dam could be overtopped by an extreme storm, the spillway gates are currently designed to begin opening automatically when the pool exceeds elevation 228.5 feet, though the opportunity for a malfunction begins as the pool reaches elevation 224 feet. (Note: In the dam’s nearly 60 year history, the maximum water elevation has been 216.6 feet in the San Gabriel River basin and 213.5 feet in the Rio Hondo River basin.) The ingenious existing design, while requiring no external power or human intervention, requires many moving parts, each of which has some likelihood of malfunctioning during operation. Due to these issues, similar systems have been replaced in some high hazard dams.
“Backward erosion piping” of the foundation. This occurs when the seepage pressures from a large pool cause erosion of the sand underlying the dam. Due to the short-term nature of the anticipated floods, the dam was built on the sandy soil foundation without many of the basic “belt and suspender” features traditionally included on dams, such as cutoff walls (to prevent seepage from going under the dam) or downstream filters (to prevent erosion should such high seepage pressures occur). Scholarly research conducted in recent years has shown that dams with foundations such as Whittier Narrows, even with relatively short-duration reservoirs, have a higher likelihood of failure than originally thought. In February 1983, such erosion did initiate in one location near the Rio Hondo outlet, but the pool was drawn down before it became a significant problem. The dam has since been modified in the area of the 1983 event to prevent a recurrence, but other areas of the dam still need to be modified.
Overtopping. Earthen dams will typically fail catastrophically if flood waters exceed the crest elevation and flow over the top. A team of experts is still examining this possibility; however, at this point it is known that it is extremely unlikely to occur, but the consequences would be if it did. This portion of the assessment will be completed in July 2016. The risk is exacerbated by local subsidence, where the area around and west of Rosemead Boulevard has subsided nearly two feet, likely as a result of mineral extraction.
Measures Being Taken
Measures to permanently address the “risk-driving” failure modes are currently being developed and evaluated. The Corps estimates that the draft Environment Impact Statement will be available for public review in early summer of 2017.
Construction is tentatively projected to begin in 2020.
An additional concern is whether the dam will actually perform as designed during rare floods, when the automatic spillway becomes operational.
Balancing the concerns and impacts of the downstream and upstream communities is challenging. Montebello and Pico Rivera communities desired to minimize flood damages from either downstream flooding on the San Gabriel, Rio Hondo, and Los Angeles rivers or upstream flooding from a larger reservoir. The local partners, Los Angeles County Flood Control District (the predecessor of the Los Angeles County Department of Public Works) and the Corps will work with the surrounding communities to choose a plan that would keep the dam safe in extreme storm events; however, there may still be significant downstream flooding.
Under certain conditions, the spillway on the San Gabriel River can release more than 20 times what the downstream channel can safely contain within its levees. Depending on the size of the discharge (flows coming out of the dam), flooding could extend from Pico Rivera (immediately downstream of the dam) to Long Beach. A critical need is for the communities in the downstream flood plain to have developed and practiced emergency action plans. Based on DSMS work with the local communities, the Corps has learned that none of the affected communities have plans in place which adequately address this potential risk. Through its Silver Jackets program, the Corps and the State of California are working to support the City of Pico Rivera’s efforts to develop a flood-specific emergency action plan.
Public Safety is Priority One
The U.S. Army Corps of Engineers operates over 700 dams that serve a variety of purposes, including navigation, flood damage reduction, water supply, irrigation, hydropower, recreation, environmental enhancement and combinations of these purposes. As part of our responsibility in managing these dams, the Corps has a comprehensive Dam Safety Program that has public safety as its primary objective. Corps dams are routinely inspected and continually evaluated for safety in accordance with the Federal Guidelines for Dam Safety issued in 1979.