A New Way to Map Flood Risks in Los Angeles

Last winter’s big storms caused enough wind and water damage that they won’t soon be forgotten, and neighbors are still talking to various city officials about possible ways to prevent or at least reduce potential hazards for this year’s storm season.

So when we heard about a recent effort by scientists at UC Irvine to create a new, more accurate map of potential flood risks across LA County, we were eager to pass it along.  The interactive map shows where and how deeply locations throughout the county (down to specific spots on specific blocks) could flood in a “100-year” rain event – the kind of rainstorm so severe it would occur only once in 100 years, or – put another way – of which there’s a 1% chance each year.

Jochen Schubert and Brett Sanders, two of the UCI scientists who worked on the study, wrote in a summary of the project that “severe flooding represented by a 100-year event is, by definition, very unlikely to occur in any given year, but it becomes more likely over longer and longer periods. For example, the probability of flooding increases to 10, 26 and 40% after 10, 30 and 50 years, respectively. This means that communities and infrastructure within the 100-year flood zone will eventually experience flooding, and by understanding the areas at risk, steps can be taken to save lives, reduce losses and enhance the ability of impacted communities to recover. It was undertaken to both better understand the city’s flooding risks, and to better understand which communities and groups of people would be most severely affected.”

The researchers also said in another story about the project that flood risk maps used by FEMA and insurance companies are often inaccurate because they look at a limited range of flooding factors.

According to that story, in a blog from ESRI, the company that makes the geographic mapping software used for the new project, “The maps that the Federal Emergency Management Agency (FEMA) uses for the [National Flood Insurance] program—and that many rely on to determine risks—aren’t required to include rainfall hazards, or the often-immense impervious surfaces of cities where rainfall runoff can’t be contained by street gutters.”

“While the maps cover coastal and river flooding, Sanders said in that story, “they don’t ask, ‘Well, if it rains really hard and the water hits the ground and runs into the streets and it can’t drain fast enough, where’s it going to pond, and who’s it going to flood?’””

So the UCI study took a much more detailed approach, looking at “three distinct types of flood hazards: rainfall hazards that create street flooding due to dense development with hardened surfaces across the coastal plain, streamflow hazards from runoff, mud and debris flowing into and overtopping drainage channels, and coastal hazards from high tides and waves.”

Schubert and Sanders said the project was designed to show both where flooding may occur and which groups of people would be most severely affected. Traditionally across the U.S. (and most notably in major disasters such as Hurricane Katrina) they say, “Poor and non-white populations have been disproportionately affected. Additionally, these same communities have been supported less with flood risk reduction and disaster recovery by governments, compared to more affluent communities. This has led to prolonged and incomplete recovery among socially marginalized, low-wealth, and vulnerable communities.”

And according to their maps, that division holds true in Los Angeles, too.

But according to the articles, the maps also presented some more general surprises.  In fact, says the ESRI story, the new maps show, with “unprecedented accuracy” that flooding potential “is far greater than suggested by federally-defined flood zones, and disproportionately impacts communities of color.”

In addition, “the population exposed to dangerous flooding, or a flooding event with 1 percent annual probability, is between 10 and 40 times greater than US government maps show.”

For example, here are some detailed sections from the interactive map, showing various parts of our Greater Wilshire area, and where flood risks could be the greatest in a 100-year rain event.  Many of these areas follow the watersheds of the natural streams that run through this part of the city, but the potential flood areas are not limited to those spaces.

According to the map key, dark blue areas are where waters could rise to “knee to waist level,” and the bright purple areas could be “greater than head” deep.

According to the researchers, the new maps can potentially be used to guide many kinds of disaster preparedness, environmental policy, and land use decisions. As Sanders told ECRI, “We can run different scenarios like, Well, what if we widen the river? What if we raise levies? What if we invest in parks and green spaces? … Can we slow down the water before it gets to the river and capture it and store it? I think a lot of people would like to do that.”

Also, he said, “you want to have something that’s good for water conservation and good for ecosystems and good for flood risk management and good for social equity.”

In fact, according to the ESRI story, the LA County Board of Supervisors has already “passed a motion directing the city’s public works department to develop a plan to address flood risks and inequities, while improving the city’s water conservation and drought measures.” And that, said Sanders, “is an impact you could only dream of.”