reThought Flood

Will California Flood in 2024?

Quick answer – YES! At least somewhere, it will. Storms have already battered the southern coast, so yes, California will flood.  

However, as we enter springtime, recall the atmospheric river events and record-heavy snowpack of 2023. Flooding happened over most of the state. Tulare Lake became a lake again, after being dry since a brief spell in 1983, and fifty years dry prior to that. A levee breach in Pajaro led to the tragic loss of many homes and businesses. Agriculture took the hardest hit throughout the state. However, major metropolitan areas such as Sacramento were spared.  

Emergency managers (and insurers) were on alert in anticipation with the huge snowpacks in the Sierras, threatening the filling reservoirs. The 2018 emergency repairs to Oroville Reservoir helped focus national attention on the reservoirs and their significant role in flood control, along with providing water supplies. If the reservoir capacity became overwhelmed, the water would have to go somewhere, and would have had catastrophic effects.  

However, the California Department of Water Resources (Cal DWR) did a great job managing discharges, and, while local impacts occurred, overall the state was spared the severe flooding that would have occurred if reservoir capacity was exceeded. 

Consider their complex challenge: 

  • if the snowpack melts quickly, the reservoirs must be able to handle the volume, so Cal DWR may discharge water quickly and allow some flooding. But…
  • if the snowpack melts slowly, the drawn-down reservoirs may not meet summer water demand. 
The Central Sierra Snow Lab

Other factors such as environmental and natural needs and phenomena (fish spawning, stream temperatures, etc.) must also be weighed. We’re glad scientists with models are in place to make these choices, although always someone loses (such as Pajaro).

Where are we this year? Several atmospheric rivers have already put record snowfalls into the mountains. Let’s look where we are now, compared to last year (courtesy of Cal DWR).  

First, the major reservoirs. They can be grouped into the interior reservoirs, which receive the runoff from the mountains directly (Shasta, Oroville, New Melones, Trinity, and Don Pedro), and others. The others either have less volume, or are coastal. The latter act more as like transfer points to meet continuous demand, and are always near full.

Overall, the reservoirs are above historical averages. This might be a concern, but recall that more years than not, California is in drought conditions, and the reservoirs might be lower.  Where were they a year ago?

California reservoir levels at 30 March 2024 and 2023

Side-by-side comparison may be more helpful. The table shows the fill level of the main California reservoirs on March 30, 2024 and March 30, 2023, both as a percentage of each reservoir’s total capacity and as a percentage of its average historical level at roughly the same date.

For reference, 1000 acre-feet of water = almost 326M gallons; in 2022, San Jose used on average 97M gallons per day. Half of that demand came from reservoirs and the Sacramento Delta.

Not much comfort there! Current water levels are higher than last year at four of five reservoirs, and all are above their historical average. So, given adverse conditions, we may see another year that suffers fluvial (river) flooding, like last year. What constitutes adverse conditions?

  • newly forming atmospheric rivers
  • intense weather fronts
  • extra-tropical storms (hurricanes)
    • reservoir upsets (failures, mismanaged outflows)
A drone view of water levels at the Bidwell Bar Bridge located at Lake Oroville in Butte County, California. Photo taken January 30, 2024.

How likely are those conditions? That’s the million-dollar question – although the first three all tend to be more likely when atmospheric temperatures are higher.

To prevent them overtopping, reservoir levels have to anticipate the melt off of the snowpack. Cal DWR reports that the snow water equivalent is way less than last year, down by more than half, which is one bit of good news. Also on the good news front, after a wet weekend NOAA’s WeatherView currently reports no new atmospheric rivers, no hard, heavy weather fronts, and no tropical storms.  

Nothing is more reliable than changes in the weather, but for now, Cal DWR should be able to shepherd the state through the upcoming season, at least in terms of river flooding , which suggests that barring major climatological events (which are a genuine possibility), Californians could enjoy a flood-free spring.

https://cdec.water.ca.gov/reportapp/javareports?name=DLYSWEQ

Check out https://www.nnvl.noaa.gov/weatherview/index.html for the animated view of weather flows.

Year over year snow pack

Ed Haas

Ed Haas joined reThought Insurance in October 2019 as property risk and CAT modeling consultant. Ed was formerly with Marsh Risk Consulting for over twenty years as Senior VP, with a focus on natural hazard modeling for flood, wind, earthquake, and other perils. Work included managing data for modeiing, interpreting results, and applying them to insurance and risk management programs. The work included site assessments at a wide variety of unique properties throughout the US and internationally. Ed was the Risk Consulting leader in the Marsh Real Estate Practice and served as lead risk advisor to the largest clients. Responsibilities included insurance program design in regard to selection of limits and deductibles, as well as designing and managing appropriate risk control programs. Specification for new construction with regard to CAT perils was a key contribution to these clients, Ed’s career started with FM Global, and has held P&C broker’s and professional engineer licenses.

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