Deadly Texas flood: Could California face a similar disaster?

San Francisco Chronicle​

19 hours ago

The catastrophic weekend flash flooding in Central Texas has killed nearly 80 people, with dozens missing and a death toll that's likely to rise. Despite occurring in a region known as ' Flash Flood Alley,' the sheer volume of rain — 6 to 10 inches in just three hours — combined with an unprecedented surge of the Guadalupe River, proved devastating. The river rose more than 20 feet within hours, transforming from a tranquil waterway into a torrent with flow rates greater than Niagara Falls.
It's a scene that may feel unthinkable in California, but is it? Extreme flash floods happen less often here than in Texas, but they do occur and could become more common as the climate warms. A national flood risk analysis highlights California's coastal mountain basins and Sierra Nevada foothills among America's flash flood 'hot spots,' alongside the Texas Hill Country where this tragedy unfolded.
The common ingredients are steep terrain, narrow canyons, hard soils that resist absorbing water, bursts of torrential rain and communities built directly in harm's way. But California adds another layer of risk: wildfire burn scars. After a fire, slopes lose their vegetation and can shed a deadly wave of mud, rocks, trees and water in what's known as a debris flow. Flash floods are fast-moving water surges that rapidly raise river level, while debris flows are thicker, slower, and often triggered by smaller storms hitting burned, unstable slopes. Both strike with little warning.
Southern California's San Gabriel and San Bernardino mountains are especially prone to this kind of disaster. On Christmas Day 2003, heavy rain on fire-scarred slopes triggered debris flows that tore through campgrounds and homes. At Waterman Canyon near San Bernardino, five adults and nine children were killed when mud and debris swept through a church camp, an event later linked in part to the design of a nearby Caltrans road.
Parts of the coastal range face similar dangers. In January 2018, an intense thunderstorm over the Santa Ynez Mountains dropped more than half an inch of rain in just 15 minutes. The storm triggered a deadly debris flow from the Thomas Fire burn scar that tore through Montecito (Santa Barbara County), killing 23 people.
Most of these California disasters have happened during the wet winter season, when large-scale storms fueled by atmospheric rivers are typically forecast days in advance. The Texas floods were so deadly in part because of their tropical moisture source and extreme rainfall rates. Totals nearing 2 feet required an air mass loaded with moisture and a slow-moving system that allowed storms to train repeatedly over the same area. In that case, the flooding was driven by the remnants of Tropical Storm Barry.
California is less likely to tap into that kind of tropical moisture, but it can happen. Monsoonal flow in July and August sometimes brings tropical air masses northward, triggering strong thunderstorms with heavy rain, especially in the Sierra Nevada. The east slopes of the Sierra, including Alpine and Mono counties, have seen monsoonal storms wash out roads and campgrounds with sudden flash floods. The risk is even higher in the state's high deserts, where fast-developing storms can drop torrential rain that the hard ground simply can't absorb fast enough.
While California may be less prone to tropical deluges like Texas saw, the ingredients for sudden, deadly floods are here, especially on steep, fire-scarred slopes.