1 million homes have long-term flash flood risk

February’s soaking from twin atmospheric rivers should serve as a reminder that despite Southern California’s legendary great weather, roughly one in five local houses have long-term risks of flash flood damage.

My trusty spreadsheet reviewed real estate risk measurements by Corelogic, which tells us that 1.1 million residential structures — single-family or multifamily — in the sixcounty region have at least a moderate risk of suffering damage from flash floods. That hazard is defined as “when precipitation rates are greater than the speed at which water drains into the ground surface.”

That means flooding dangers are faced by 21% of all local housing, according to this math.

Flash flooding is a particularly urban risk because abnormally large rainfalls often have nowhere to go in heavily concreted landscapes. Streets can overflow with rain, creating damage much like a creek could. Plus, the region’s hilly terrains create additional flood risks.

The weather gurus say such storms will be more frequent as we grapple with climate change. Property owners also should be aware that it’s likely the typical home insurance policy does not cover this kind of damage. A separate flood policy is required.

Please note that the average local residence at risk would cost $454,000 per home to rebuild — assuming a flooded home was a complete loss. That adds up to a possible $487 billion replacement risk across the region.

So consider the odds: Corelogic estimates that 305,224 of the Southern California homes facing flash flood hazards have elevated risk of damage — that’s 28%.

Ponder the flood danger by county, ranked by the number of homes with some level of danger …

LOS ANGELES >> 420,585 residences are at risk (21% of all homes) with 27% of those properties in higher danger. Replacement costs of the homes at risk average $474,000 per home, or $199 billion countywide.

RIVERSIDE >> 211,418 at risk (29% of homes) with 39% at higher danger. Replacement $452,000 per home or $96 billion.

SAN BERNARDINO >> 156,987 at risk (27% of homes) with 31% at higher danger. Replacement $394,000 per home or $62 billion.

ORANGE >> 122,372 at risk (16% of all homes) with 21% at higher danger. Replacement $487,000 per home or $60 billion.

SAN DIEGO >> 120,482 at risk (15% of all homes) with 22% at higher danger. Replacement $422,000 per home or $51 billion.

VENTURA >> 41,771 at risk (19% of all homes) with 21% at higher danger. Replacement $486,000 per home or $20 billion.

DAVID CRANE — STAFF PHOTOGRAPHER The remains of a home on Caribou Lane off of Beverly Glen lies in ruins Wednesday. A mudslide during heavy rain on Monday destroyed what was a two-story home.