San Luis Obispo County Flood Control District in California is launching a five-year programme to strengthen the 50.5m high Lopez Lake dam, in response to recent findings and several studies that say the dam could fail during an earthquake. The cost of repairs to the water supply dam are expected to be paid by the water users in the County. Lopez Lake has a capacity of nearly 52,000 acre-feet.

  California state dam safety division initially required the County to maintain the lake at 60% of capacity, due to safety concerns. In 1993, it raised the allowed limit to 72%, and more recently that limit was raised to 83% capacity. The order to keep the lake below maximum capacity is meant to minimise the risk to downstream residents due to a possible dam failure.

The state dam safety division has determined that the alluvial foundation below the dam’s downstream embankment slope could lose strength under severe seismic loading, and could cause uncontrolled releases of the reservoir contents. A fault line runs a half-mile away from Lopez and is considered inactive, though experts studied what might happen in the event of a 7.5 magnitude earthquake on that fault. An earthquake that measures 7.5 on the Richter scale has four times more ground motion than the 1989 Loma Prieta earthquake — measuring 7.1 on the scale — which led to the collapse of a section of the Bay Bridge and killed more than 60 people. The 1906 San Francisco earthquake measured 8.3.

Another round of studies were expected to begin in July to help choose the best alternative way to retrofit the dam and make it earthquake-proof. The studies will last approximately six months and cost US$300,000, which will be paid out of county Flood Control and Water Conservation District reserves. Dam renovations are expected to begin in the fall of 2000 and end in mid-2002.

According to State of California Dam Safety Division, at least 94 of the more than 1200 dams in the state have undergone seismic renovations since 1971.