By Kathe Malouf
Special to the Sun
Shortly after two major earthquakes rocked buildings and rattled nerves in the Kern River Valley earlier this month, U.S. Army Corps of Engineer officials reported that it was “business as usual” at Isabella Dam, stating that the 6.4 magnitude earthquake on July 4 and 7.1 magnitude earthquake on July 5 did not cause any structural damage or safety concerns.
Safety inspections on Isabella Dam were conducted immediately following the two earthquakes that were both centered outside of Ridgecrest with the use of both digital technology and on-the-ground inspections.
Due to the ongoing construction that has been underway for the past three years at Isabella, Corps officials have been monitoring both Main and Auxiliary dams for seismic activity on a regular basis.
When an earthquake is detected – whether it is first felt on the ground, or an alert is received from the USGS – Corps personnel immediately initiate their safety inspections, said Calvin Foster, Chief of the Corps of Engineers’ Southern Operations Branch, Sacramento District.
At Isabella Dam, Foster said the Corps relies on a digital device called an accelerograph to verify seismic activity. And beginning on the morning of July 4, there was plenty of seismic movement to verify.
Foster explained that because accelerographs detect ground motion as well as the number of seismic events, they are used to confirm whether or not there has been seismic activity in the area.
“If the accelerographs confirm seismic activity, we will continue with our inspection protocol, which is a visual inspection of both dams,” Foster said.
At Isabella, accelerographs are placed along the Main and the Auxiliary dams and are typically read on a weekly basis. But following the seismic activities of the July 4 and July 5 earthquakes and aftershocks, the accelerographs were read more frequently, as part of the Corps’ immediate post seismic event inspection.
While it is not up to the Corps to determine the magnitude of an earthquake or aftershock, the accelerograph is used only to verify whether or not there has been seismic activity. The Corps relies on USGS to determine the magnitude of the earthquake.
“Upon verification of seismic activity, inspectors physically walk the dams looking for settlement, cracking, sloughing, loss of grade and/or movement at the dams,” Foster said, adding that inspectors will look for movement at the toe and abutments of both dams.
“Inspectors also look for changes in water color or quantity of water flow at the outlet works, as well as look for cracks in the control towers and the spillway,” Foster said.
Monitoring wells are also checked and the readings from those devices are noted, Foster said. The gates and other equipment are also checked to ensure operation capabilities.
Fortunately, no problems were found at Isabella Dam following the earthquakes.
“Inspections following the most recent earthquakes have not revealed any signs of distress in the main or auxiliary dams, nor did they cause any change to normal operation of the dams,” Foster said. “It is business as usual at Isabella Dam, which includes continued interim risk reduction measures and daily safety checks already built into the project plan.”
Even when there has been no seismic activity, safety inspections are performed on a daily basis as part of the on-going Dam Safety Modification Project construction at Isabella Dam.
“We currently do daily visual checks on the dam during construction,” Foster said. “We check the toe for seepage and we check for changes on the site so we can be familiar with routes of approach and egress during an emergency response. Both USACE Operations personnel and contractor personnel conduct regular inspections.”
Foster said that the Corps is prepared for natural disasters, such as earthquakes and flooding events and that preparation centers on safety.
“Safety remains the number one priority for the Corps of Engineers, safety of people and property. The Corps included interim risk reduction measures and daily safety checks in the project plan from the start,” Foster said.
In addition to the operations team that is permanently assigned at Isabella, the Corps’ Dam Safety team also maintains an office on site during construction and they often respond concurrently with the Operations Team. Foster noted that as a separate entity, the Dam Safety team follows up with their own inspections following a seismic event.
Both the Main and Auxiliary dams are earthen dams, completed in 1953. Concern for Isabella reservoir’s safety was first identified in 2005, when the Corps determined that the dams and spillway posed an unacceptable risk to public safety. The Corps launched a dam safety study and based on risk assessment during that study, the Corp determined Isabella dams to be unsafe under extreme flooding events. In 2012, the Corps released the Isabella Lake Dam Safety Modification project report, recommending remediation measures that would address overtopping, seismic and seepage issues identified with Isabella Lake’s main and auxiliary dams to reduce the likelihood of dam failure.
While the aftershocks have decreased in and around the Kern River Valley, inspections at Isabella Dam will continue until the completion of the Isabella Dam Safety Modification Project, which is expected sometime in 2022.
“Our goal is that when completed, the project will reduce the risk of dam failure to the downstream communities in the Kern River Valley and Bakersfield,” he said.