Dam Failure

When buying or selling a home in California, consumers can learn if the property is in a zone marked for potential flooding due to dam failure. The disclosure companies will not say which dam is causing the risk, or if it’s several since often there will be more than one dam along the same creek or river, as is the case along the Los Gatos Creek.

Quick take: you can learn about the areas which may be impacted by flooding due to dam failure at this link: Dam Breach Inundation Map Web Publisher.

Dam failure versus flood plain

To clarify, the 100 year flood plains and 500 year flood plains are different from the flooding due to a breach in a dam or levee.  If someone buys a financed home in a 100 year flood plain, the lender will require flood insurance.

Also relevant to the requirement for flood insurance: the 100 and 500 year flood plain inundation risks are considered to be “natural hazards” and the flooding from dam breaches are not, since humans have created the dams (or levees).  Per FEMA: “a flood resulting from changes in river flows is a natural hazard, whereas flooding due to a dam failure is considered a manmade hazard, and therefore excluded from the National Risk Index.”

Approximately 25% of floods in the U.S. are outside of the flood plains. It may be advisable for those in other areas to consider buying flood insurance even if it is not required by the lender.

Map of flood risk from dam failure

Recently Clair sleuthed out the State of California’s Dam Breach Inundation Map Web Publisher. This interesting and helpful tool is an interactive map covering both most counties and most dams. I encourage our readers to check it out.


Dam failure flood zones in Santa Clara County

Click on the image to view the live Dam Breach Inundation Map to better understand the risk of dam failure

This Dam Breach website indicates which dams have their flood risk from failure marked in one of four colors to indicate the higher or lower risk levels. In Santa Clara County there are more than a dozen dams marked at the highest risk category, and some of them hold a large capacity of water. (Some of the dams appear to not be at lake-like reservoirs, but more like covered percolation ponds or holding tanks.)

The concern today, of course, is what happens if we experience a large earthquake and any of these dams fail.

Lake Anderson in South County is the largest reservoir in Santa Clara County. It is undergoing a 10 year seismic retrofit and until it’s completed is at only 3% capacity for safety reasons.  You can view a short video about the retrofit here. With that in mind, take a look at the image below, which showcases what areas could be inundated should the dam there fail.

Dam failure flood zone with failure of main dam at Lake Anderson

Click to view the live map

The map doesn’t show if the flooding goes beyond the county line to the south, and unfortunately not every county appears to be participating in this website and not every reservoir is covered. I was curious about the enormous San Luis Reservoir just beyond our county’s border, but there was no map for that one.