reThought Flood

San Diego Flooding and the 1,000-Year Myth

San Diego was hit hard by a deluge rain event in late January, before this month’s atmospheric river events. Cars were swept away, people waded in hip-deep water, and streets, homes, and businesses were – quite literally – under water. Unsurprisingly, a lot of people were heard saying “I never saw this before!” In various news releases and press reports, officials described the event using phrases such as “extraordinarily rare and powerful.” One city official said: “This is what we’d classify as a thousand-year storm.”

Hyperbole aside, most of the areas affected were already known to be vulnerable to flood. Much of the real estate affected was along and to the west of Cholla Creek, in defined flood zones found easily on Federal Emergency Management Agency (FEMA) maps. But a flood is always a surprise if it never happened to you before, or it’s happening in a new location.

That’s no excuse to be unprepared. Good risk management anticipates what might occur, and estimates the likelihood of it happening. We shouldn’t assess future risks based on what has not happened, and typically we do not, but – for one reason or another – flood is often treated this way.

Odds on

Reliable reports (not emotional exclamations made in the moment) have identified the rainfall on January 22, which caused the flooding, at an intensity of about three inches in 12 hours. That’s enough to overflow most creeks and drainage systems, especially in urbanized areas. But it turns out that according to our current understanding of rainfall intensities, a downpour of that strength is likely to occur relatively often in the precise areas of San Diego that flooded last month.

This frequency estimate provided by NOAA – the National Oceanographic and Atmospheric Administration – is a probability of between 1% and 2% (or every 100 to 50 years). That, at least, is the number in its Precipitation-Frequency Atlas of the United States, updated in 2014. That ten-year-old data may seem a bit like old news, but the work involved in compiling the Atlas is extraordinary, so NOAA data is an excellent starting point to determine current flood risk.

An updated probability will take into account the increased pressure added by climate change, and the weather isn’t the only factor driving up flood risk. An even greater contributor to the incidence of local flooding is increased development. More paved surfaces + more rooftops = more floods. The flood odds deteriorate even further when the maintenance of flood channels and floodways has been neglected, and when obstructions have been allowed to accumulate at culverts and bridges.

Each of these problems leads to greater flood vulnerability in areas that may not have a recent history of inundation. They may not be on the flood maps. That can lead people to judge the future based on what hasn’t happened – and therefore to float to incorrect conclusions.

But look back further, and one can usually find some flood history, even in San Diego. The 48 federal disasters declarations affecting San Diego County since 1965 include nine floods, five severe storms, one hurricane, and one severe coastal storm. These weather catastrophes did not all smash into the same neighborhood, but the experience of 16 serious precipitation events in in 60 years (not one thousand) has left lessons to be learned.

San Diego has been paying attention, and has begun to take action. In 2018 the City audited its stormwater systems, and found that its storm-drain pipes have been failing with a worrying increased frequency over the previous couple of years. The investigation prompted three major calls to action:

A graph of a storm drain pipe failure

1:  Expedite the replacement of deficient piping

2:  Pursue a long-term funding strategy

3:  Improve enforcement

The need for this type of flood-infrastructure maintenance spending may not be music to the ears of taxpayers. Nor may flood defenses seem like a priority when so many necessary projects demand a share of strained budgets. But the alternative was demonstrated in January, when the rains came. Such rains will keep coming, too: I can promise, with 100% confidence, that we won’t have to wait 999 years. Indeed, a bit to the north, we waited only one week!

Where do we go from here? Cutting-edge risk assessment technology, the likes of which we use at reThought Flood, takes all these factors and many others into account when assessing the flood risk not just of individual buildings, but even of different areas within a large building. That allows us to produce the best-possible understanding of the flood risk right now at a given location, and also its likely risk in the future.

As long as we build more and maintain less, and climate change (whatever its cause) drives up the intensity of rain events, flooding will happen where it is expected – and occasionally where it’s not. That means flood insurance is essential for every property, everywhere.

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.