UCI Podcast: Evaluating California’s Flood Risk
March 1, 2023 - Brett Sanders, UCI professor of civil and environmental engineering, has been focusing his research efforts on understanding flood risk to Southern California’s communities. Over the years he has studied the impact of climate change-caused sea level rise along California’s coast. Lately he has been examining flood risk to regions further inland, in many cases affecting people of lower socioeconomic status. In this episode of the UCI Podcast, Sanders talks about his research and its relevance in this season of substantial rainfall in the U.S. West.
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So today we’re obviously going to talk about flood risk, particularly in California, Southern California. You’ve been doing a lot of research lately into this topic, and tell me, I’ve seen some of your research prior to this, looking at coastal flooding, but a lot of your later work seems to be focused on more what, what you would call urban flooding. Is that right?
Right. The urban flooding is really the intersection of flood water in the built environment, like cities, not unlike what we live in around Southern California. A lot of people, or some people will consider urban flooding across California to also be a coastal flooding issue because our cities are along the coast of the United States. So there’s previous work we did that was focused on, specifically on sea level rise and flooding risk right along the coast where we had coastal communities like Newport Beach and Tijuana and the Tijuana River Valley, where we had done some research on how coastal communities could better plan and prepare for future flooding with the aid of simulation tools. We sort of over time broadened the footprint of our work to go inland, even into like inland areas like Riverside.
One of the things that came across in the study that came out in Nature Sustainability last fall was that a lot of this type of flooding, this sort of inland flooding impacts communities that are not the same type of communities that you see in Newport Beach, in some of these really close-to-the-coast communities. Can maybe explain a little bit about what you discovered about these communities inland that are being impacted now?
Yeah, that’s a great question, Brian. Our work looks specifically at the differences in the flood exposure in Los Angeles County to coastal flood risk, which we defined as flooding from a high tide, with a storm surge along the California coast compared to flooding along rivers, which, rivers of Los Angeles County, which are like the San Gabriel River, Los Angeles River, Compton Creek, Dominguez Channel. Those main stem drainage systems, which cut through the urban core of Los Angeles. And, we also compared that to what we call pluvial flooding, which is flooding caused by rainfall that hits the street and runs down through neighborhoods and collects, collects in lowland areas and, and creates challenges there as well. And, what perhaps is, is no surprise to many in California is that the coastal flood risk is a, is a risk that, impacts a more affluent communities than disproportionately white communities along the coast.
And we found that our river flood risk was disproportionately impacting communities with a large black population and also communities that had large Hispanic population fractions. So it helped us appreciate that. Depending on how you think about flooding, are you thinking about flooding as an inland risk associated with flood channels that might be undersized for a changing climate? Are you thinking about coastal communities that might be exposed to rising seas? When we think about those two different risks, we realize that the impacted populations are quite different. You know, a more affluent white community along the coast and, in inland areas, the impacted communities tend to be black and Hispanic populations and populations that are less affluent. When we looked at the pluvial flood risk, which is the flood risk from rainfall, that tended to be something that impacted, populations somewhat evenly.
So we didn’t see strong inequalities relative to people’s exposure to, rainfall flood risks. I think that in some level, that makes sense in the sense, because we all get hit by rain. You know the rain falls on everyone and streets aren’t that different from community to community. But there are differences in whether communities sit near rivers, whether those communities in lower topography or whether those communities are along the coast. And that leads to what you might call inequities in flood risks that we uncovered in our study.
One of the things that was mentioned in that Nature Sustainability study last fall was a risk that came from what you called atmospheric rivers. And I’ve read in the news over the last few months that that has been something that we have experienced here in Southern California, large rainfall events, brought on by atmospheric rivers. Was it a surprise to you to see these atmospheric rivers hit California so hard in these last couple of months?
Yeah, it wasn’t a surprise. We know that California’s flood risk is linked to these strong atmospheric rivers that bring bands of moisture into the state for a day at a time. And, and in fact, there were sort of a sequence of atmospheric rivers that impacted us in January. And in fact, we know that the worst floods in the history of California have occurred with the sequencing of multiple atmospheric rivers. So, the largest floods in the past have occurred when you see one after another of these wet storms roll in back-to-back. And so just about every day there’s rainfall for weeks, literally weeks on end. And when that happens, the land surface is become saturated with water, and there’s just no additional storage available for rainwater to collect.
And so you reach a point where rainwater strikes the land surface, it runs straight off, tries to find its pathway downhill towards the ocean. And that leads to the really the greatest flood risks the state can face. And we saw evidence of that this past January. We saw places like Central Coast, from Ventura, Santa Barbara got hit storm after storm, they got saturated soaking wet, and then they got hit with another storm. And then even more rainfall came down. Areas like Salinas had levy’s break. And after a levy’s break, highways were flooded, communities were cut off, lost their connection to critical infrastructure like hospitals. And so there was a crisis in northern California with communities in Monterey County that couldn’t evacuate because there was no pathway to evacuate.
Up in the Central Valley, we saw rivers that just crested above levees and caused levees to break in large swaths of land. In the Central Valley, flooded Highway 99 was cut off this atmospheric river dynamic that has now sort of entered into the lexicon of our conversation about the weather is something that’s happened for a long period of time. But only more recently have atmospheric scientists come to appreciate the way that these atmospheric rivers bring moisture out of the Pacific Ocean in the form of these large bands of water vapor, transporting across the atmosphere and to try to build appreciation for the power of these wet storms, they came up with a name and gave it a rating index and tried to build awareness of the threat because the threat of these atmospheric rivers is comparable to the threat brought on by large tropical storms that impact our Gulf Coast and our East Coast.
Now what happens is we just don’t see them coming back to back to back that frequently. We’ve been mired in year after year of drought, and in the drought years, we just see one storm at a time, typically, and so we don’t see those big floods, and we don’t see the reservoirs fill up across the state. And we’ve had a smaller snow pack. But this past year we saw evidence of what a more normal winter looks like in California, based on the historical record. When you see a storm after storm after storm roll in, bring a lot of moisture, build the snow pack, saturate the land surface, and we saw that the floods that come with that, those storms that come back to back. So in fact, when we were planning the L.A. flood risk study, we had in mind the reality that there could be a future storm similar to one that occurred in 1862, which bankrupted the state and caused basically widespread damage that bankrupted state and forced the government to move out of Sacramento into San Francisco.
Los Angeles had been hit by big floods in the 1920s and the 1930s. And the cities had a history of big floods, but over time they built up infrastructure that contained a lot of those floods. And over time we’ve had more and more droughts, which led a lot of people to even remember that we faced a flood risk. So that was a motivation in our preparation of that study. That’s reason why we did this study because we knew there’s a risk in Los Angeles. We knew that awareness was low, and, but we weren’t sure exactly the scale of the exposure, how many people would be exposed to a flood, and we didn’t know to what extent the flood might or might not be an equitable impact. And so that was kind of the rationale behind that study.
Another rationale behind that study was that we’ve seen across the United States that more vulnerable communities are unable to recover from floods, and that sends communities sort of spiraling downwards, less investment, people leaving the area. And we wanted to know, is that part of California’s future? Do we have vulnerable communities focused, where, where there’ll be hit by a flood and, and they’ll also suffer, long-term challenges in the recovery. So trying to build awareness about our, our risks as a state, communities that are especially vulnerable was part of our motivation.
Do you think the heavy rainfall this year, the 2022-2023, rainy season in California, and some of the results of that are causing people, policy makers, civil engineers to think about new civil infrastructure approaches to dealing with these issues?
Absolutely. We, we’ve seen two major messages come from this event in our understanding of it. One message is that we are vulnerable. We’re seeing levies break, we’re seeing roads cut off. This was not the equivalent to the great floods we’ve had in the past. I don’t know the exact return period, but we’re going to be facing floods larger than one we faced this past year. So the first message was that we have vulnerabilities, we have infrastructure that’s aging and needs attention. The second message is that we don’t have a good flood control infrastructure for the purpose of water, long-term water conservation and water security. And we saw, calls left and right for more capacity to capture water, retain it, store it, and use it to address the challenges we have with the drought.
That sounds, that always sounds to me as though it’s something easier said than done. And you know, you always see people on social media saying, why can’t we capture all this water? Well, it’s all just running out to the ocean. Why can’t we, you know?
Yeah. And part of the reason it’s so hard to do is because we didn’t design our infrastructure to do that. So most of our coastal water infrastructure, our flood channels were designed for two things and two things only. One was to protect people, for the safety of people that they don’t get hurt during a flood. And the second was for economic, development and growth. So we developed our flood channel infrastructure to, to sweep water to the ocean as quickly as possible to keep people safe and to protect, economic development, buildings and housing and industries because the state had faced a number of devastating floods and, and knew if they got hit by, if the flood spread out and flowed through developed areas, it was extremely costly and extremely deadly.
Today we are living with infrastructure that was designed for, for sweeping water quickly to the ocean. So the challenge is, if we as a nation are reinvesting in our infrastructure, can we reinvest in a way that we achieve a different set of functions from our rivers? Can we redesign the river to slow down the water, to spread it out to, to recharge groundwater basins, to process nutrients? Can we redesign our rivers so that they’re more integrated with our ecosystems? Can we design our rivers so that they create, green spaces with trees that offer shade during heat waves? These are the things we should be asking as we have an enormous opportunity with the infrastructure funding that’s being provided by the federal government, enormous opportunity to rebuild, reinvest our infrastructure. We can’t follow the same objectives that we used a hundred years ago. We need a new set of objectives. And I think that’s really the huge opportunity we face today to simultaneously address flood risk and also meet these water sustainability challenges. And that that is not easy. You’re absolutely right. There’s going to be some tough choices to do that. We’ll need more space and land is expensive and not easily acquired, but that, that is an opportunity we face today.
What are you working on now and the next 12 months, research-wise, anything interesting that you’d like to share?
Yeah, so two things. One is that we now know that there are the really much greater flood exposure in Los Angeles County than anyone realized based on existing flood maps. We don’t know what the actual exposure is for other parts of California. So, one of the things we’re working on is to expand our modeling tool beyond Los Angeles County. We’ve got parts of Orange County covered, but we’re partnering with San Diego County, with Riverside County, with San Bernardino County, with Ventura County. And so one of the things we’re going to do is, is expand our model to cover all of Southern California and look at the exposure facing roughly half of the state’s population, and roughly half of the state GDP that follows within that area and get a sense of what is the real risk we’re facing today.
And that’s that’s extremely important for risk awareness. Just raising awareness of what’s at risk can be very influential in helping communities, cities to update, building codes, land use, zoning, that sort of thing. And that can go a long way towards managing flood risk.
The other thing we’re working on is to chart future risks, considering two things. One is a warmer climate with more intense rainfall, more frequent wildfires, higher sea levels along the coast, perhaps changes in wave energy. So we need to think systematically about the way that the hazards are changing from even starting in the mountains with mountain runoff and mud and debris flows, inland areas, more intense rainfall and runoff in coastal areas with higher sea levels, and perhaps changes in the wave climate. And then we also want to look into the future and ask how could we adapt our infrastructure, perhaps widening channels, raising levies, putting in dams, putting in water capture infrastructure, and we can then use our modeling tools to test out which of those would be most effective, which of those would be most equitable, who would benefit from those adaptation measures? How do we pay for them? And we can really lay out a set of options for the state of California to think about how it simultaneously manages flood risk and works towards its water sustainability needs and does so in an equitable way.
– Brian Bell / UCI