March 2023 Atmospheric Rivers in California

CoreLogic® estimates 5,895 single- and multifamily homes at risk behind breached levee in Pajaro, California

Heavy and continuous rainfall from another round of atmospheric rivers caused riverine and flash flooding across areas of northern and southern California. The most recent round of storms began on Friday, March 10, with a second deluge on Monday, March 13, following a brief pause in the rainfall. Additional heavy rainfall is forecast across California as another atmospheric river is expected to bring more moisture to the West Coast.

In early 2023, a series of atmospheric rivers caused widespread flooding in California.

Pajaro River Levee Breach

Multiple homes and farmlands in Pajaro and Watsonville, California, flooded on Friday, March 10, when the Pajaro River in Central California overflowed its levee. Pajaro and Watsonville are located on the border of Monterey and Santa Cruz Counties and are separated by the Pajaro River, which is leveed to protect the homes and farmlands behind. On Friday, March 10, the water level in the Pajaro River surpassed the levee, leading to a breach and a partial collapse. The result was the flooding of homes and agricultural lands. Additional flooding was observed in Watsonville when the Salsipuedes Creek and Corralitos Creek overflowed.

CoreLogic® estimates that 5,895 single- and multifamily residential properties - with a combined reconstruction value (RCV) of $2.88 billion - are at risk of flooding due to this event (Table 1).

The estimate above accounts for all at-risk homes and does not represent the number of homes that were actually flooded. Only a subset of the 5,895 at-risk structures in Pajaro and Watsonville flooded. The RCV figures represent the cost to rebuild a structure after its complete destruction, and this number accounts for the materials, equipment and labor required to fully restore a property. These figures do not include the value of the land. Structures damaged by floodwaters may not have suffered 100% loss up to the full RCV.

This estimate includes all of Pajaro and portions of Watsonville with a risk score of at least 30 (Moderate to Extreme Flood Risk), according to the CoreLogic Flood Risk Score (Figure 1). Homes with a Moderate Flood Risk Score were considered when accounting for any additional flooding since more rainfall is expected.

Table 1: Count and Total Reconstruction Value (RCV) of Single- and Multifamily Residential Structures At Risk Due To Pajaro River Levee Breach

Single-family homes include standalone residential structures of less than four stories, mobile homes, duplexes, manufactured homes and cabins. Multifamily homes include apartments, condominiums and multi-unit dwellings with more than four units.

The elevated risk scores in Pajaro as well as southern and eastern Watsonville are due to the flat landscape of the floodplain surrounding the Pajaro River. Pajaro and Watsonville are protected by earthen levees on the left and right banks of the Pajaro River, which was constructed in 1949, according to the U.S. Army Corp of Engineers National Levee Database. The levees have been tested on several occasions since construction, having only failed once due to flooding in 1995. However, during each test, significant bank erosion occurred. In 1995, floodwaters seeped through the levee, causing erosion of the sides and eventually flowing over the levee.

For more information on the CoreLogic Flood Risk Score, visit https://www.corelogic.com/insurance/hazard-risk-solutions/.

Figure 1: CoreLogic Flood Risk Scores in Pajaro and Watsonville, California

Other Hazards Across the State

According to the National Oceanic and Atmospheric Administration (NOAA), seven-day rainfall totals from March 8 through March 15 exceeded 20 inches in certain areas along the coast and in the Sierra Nevada Mountains (Figure 1). Nearly 10 inches of rain have fallen near San Luis Obispo and Santa Cruz, California. Approximately five inches of rain have fallen in San Francisco and Los Angeles. Heavy rainfall in urban environments often leads to flash flooding when wastewater management systems are overwhelmed.

Figure 2: Observed 7-Day Precipitation Totals in California and Nevada from March 8 - 15. Recent Burn Scars Denoted by Dark Red (2022), Light Red (2021) and Yellow (2020) Polygons

Continuous rainfall has saturated soils across the state, increasing the risk of landslides. Heavy and continuous rainfall in the more mountainous regions of California is especially dangerous due to recent wildfire activity (Figure 2) since wildfire burn scars strip the plant matter that provides soil cohesion. When the soil in burn scars is saturated, the risk of landslides increases. Residents living downslope of recent wildfire burn scars should be on alert for potential landslides.

The CoreLogic Event Response Team is monitoring the situation in California. Updated impact summaries will be posted to http://hazardhq.com if more information becomes available.