This column discusses publicly available information resulting from the 2017 Oroville Dam spillway failure and emergency public evacuations downstream of Oroville, north of Sacramento. Former Gov. Brown’s orders caused the Div. of Safety of Dams (DSOD) to do two things: order inundation mapping for dams in California, and order DSOD staff to perform more extensive visual dam inspections. 135 dams whose risks were categorized as either “extremely high” (62), “high” (65), or “significant” (8) are currently posted on their website. Extremely high risk means “…considerable loss of human life or would result in an inundation area with a population of 1,000 or more.”. High risk means “…loss of at least one human life”.
In my Div. 3 area, three dams were required to submit “sunny day” risk assessment inundation maps: Lake Hodges, “extremely high” risk, and San Dieguito reservoir and Fairbanks Ranch Lake, both rated “high” risk. (Note, in San Diego County, 14 dams have submitted maps prepared by certified engineering firms: 12 were “extremely high risk” (9 of which are owned by the City of San Diego) and two were “high risk”, San Dieguito reservoir and Fairbanks Ranch Lake. Inundation mapping is used to develop emergency action plans. Mappings specifically note locations of schools, critical road points, law enforcement facilities, licensed healthcare facilities and CalFire facilities.
Viewing inundation mapping is very educational. “Sunny day” failure scenarios are superimposed over satellite photos of homes, etc. Values are calculated for elapsed time for wave action to hit particular locations, depth of inundation at particular locations, and velocity of stream flow.
Click here to see the full DSOS inundation maps. Click “View approved Inundation Maps”. Click “enter”. Scroll down to read disclaimer. Click “ok”. Once a window displays, click the second bullet point, Scenario 1. Type Hodges, Lake, in the search engine. (Hodges has two scenarios: spillway failure and dam failure.) Type San Dieguito, San Diego or Fairbanks, San Diego in the search engine. Again, please keep in mind inundation mapping assesses downstream hazard classification for each dam based solely on the size of the dam’s reservoir and population that would be impacted by a dam failure: it does not reflect the condition of the dam or its structures.
Integrity Assessment of Dams and Spillways
As stated in the introductory paragraph, the Oroville Dam Spillway failure also caused the DSOD to undertake more extensive visual inspections of California dams. As a result of these inspections, the City of San Diego, owner of Lake Hodges, is now required by the DSOD to reduce the volume of water stored in Lake Hodges by approximately 60% no later than August 1, 2019. Prior to these enhanced inspections, Lake Hodges was rated for 30,000 AF of storage, with a top elevation of 315 feet. As of 08/01/19, Hodges is permitted to hold no more than 13,000 AF of water, with a top elevation of 295 feet. The DSOD’s edict was a result of concern about hydrostatic pressure against the aging concrete dam wall. Decreasing the volume of water decreases the height of water against the concrete dam, reducing hydrostatic pressure, thereby significantly reducing risk to public safety.
The City of San Diego will be developing interim reservoir management plans to submit to DSOD by August 1, 2019. Financial impacts will be studied and presented Fall 2019. A possible plan would be to build a new dam face slightly downstream from the current dam face. The City of San Diego has been transferring large volumes out of Hodges to other reservoirs. The August 1st volume reduction deadline is expected to be met.
How does this affect YOU?
1.) DSOD action assessed public safety, and addressed the risk assessment of the integrity of the Lake Hodges dam. 2.) Your water rates are a combination of lower costs due to SFID’s rights to Lake Hodges water. Those rights have now been reduced 60%. However, depending on how Hodges water could be moved around to other reservoirs, or potentially banked, our volume of local water could be increased. However, moving around water entails increased power/transportation costs, to say nothing of potential storage costs. SFID management is committed to doing it’s very best to provide as much cost-effective local water as possible to our customers. Stay tuned.
Marlene King, S.F.I.D. Director, Div. 3
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