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.
Whittier Narrows Dam is the central element of the Los Angeles County Drainage Area flood control system and collects runoff from the uncontrolled drainage areas upstream along with releases into the San Gabriel River from Santa Fe Dam. If inflow to the reservoir exceeds the groundwater recharge capacity of the spreading grounds along the Rio Hondo or the bed of the San Gabriel River downstream, this water is stored temporarily in a water conservation pool. The Rio Hondo and San Gabriel sides of the reservoir each have their own water conservation pools. If the water conservation pool on the Rio Hondo side is exceeded, flows are released into the Rio Hondo at a rate which will not exceed the downstream channel capacity of either the Rio Hondo or the Los Angeles River. If the water conservation pool on either side of the reservoir is exceeded, a release of approximately 5000 cubic feet per second can be made into the San Gabriel River. If the pool in the reservoir exceeds flood control storage, the gates on the San Gabriel River outlet begin to open automatically and emergency releases are made into the San Gabriel River.
Dam Safety Issues
The primary objective of our Dam Safety Program is to maintain public safety by making sure the dams owned and operated by the Corps are safe, and risks to the public are minimized. An integral part of the program is the risk-informed screening process. Dams are classified based upon confirmed or unconfirmed dam safety issues, the combination of life or economic consequences should failure occur and the probability of failure. This process enables us to prioritize dam safety actions to correct deficiencies, which include interim risk reduction measures to be undertaken while further investigations are conducted and remedial actions are implemented.
Current Status
Whittier Narrows Dam received a Dam Safety Action Class II, or DSAC II, rating in December 2008 based on a Screen Portfolio Risk Analysis, or SPRA, conducted in May 2006. A DSAC II rating is given to dams where failure could begin during normal operations or be initiated as the consequence of an event. The likelihood of failure from one of these occurrences, prior to remediation, is too high to assure public safety; or the combination of life or economic consequences with probability of failure is very high.
Whittier Narrows Dam received a DSAC II rating because of the potential for:
- Overtopping of dam due to inoperable gates after major seismic event
- Loss of pool due to seepage and piping along the utility conduits and control facilities
- Loss of pool due to erosion and head-cutting of the embankment during a Project Maximum Flood (PMF)
- Loss of pool due to seepage and piping of foundation during normal to high pool levels
As a result of Whittier Narrows Dam’s DSAC II rating, the Corps has implemented the following Interim Risk Reduction Measures, or IRRMs:
- Remote monitoring
- Inspection and monitoring
- Flood mapping
- Update the Emergency Action Plan annually
- Inspect toe drain and gallery
- The Corps completed an Issue Evaluation Study (IES) that confirmed the Carbon Canyon Dam DSAC Rating and recommended initiating a Dam Safety Modification Study.
What’s next?
- Conducting a tabletop exercise (2013)
- Rehabilitate the relief well (2014)
- Repair toe drain and conduit (2014)
- Pre-positioning of materials (2014)
- Modifications are needed to address potential failure modes at Whittier Narrows Dam. The Corps will begin a Dam Safety Modification Study to be completed in July 2014. In the study, engineers will develop and evaluate scenarios to modify the dam to withstand failure during rare events. A Dam Safety Modification Study is similar to a feasibility study, with a simmialr projected durationof two to three years. The reslult of the study will be a recommended mitigation plan that will undergo peer review, go through an approval process and, depending on availability of funds, ultimately be designend and constructed, .
Public Safety is Number One Priority
Public safety is our number one priority. While we cannot completely eliminate risk, we can reduce it. Our screening and classification of Whittier Narrows Dam does not mean that failure is taking place. It means we have identified dam safety issues that don’t meet industry standards and the risk to public safety is unacceptable. Routine inspections and operation of the dam will continue and emergency action plans have been developed in coordination with local emergency management officials. Currently, there is no evidence to suggest an emergency situation exists or is about to occur.
Background
The U.S. Army Corps of Engineers owns and operates 610 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.