How sprawl makes flooding worse

This month, residents along Stockton’s Smith Canal face a tough choice: pay extra for a canal head gate to protect against rising water or continue purchasing costly flood insurance. According to FEMA, these residents live in a flood zone (despite the fact that Smith Canal has never flooded) and need to protect themselves. Because we are so flood prone in the Central Valley (though, thankfully, not recently), there is always a healthy debate on the best way to protect ourselves. Unfortunately, this debate is always centered on levees and insurance, ignoring that fact that the valley’s rapid urbanization contributes to increased water runoff. In short: Sprawl makes flooding worse, but is usually passed over when discussing flood preparedness.

Obviously, sprawl on its own does not create excessive amounts of rain or cause levees to collapse. However, sprawl does exacerbate these events. By replacing soil with asphalt and concrete, rainwater that would otherwise be absorbed by the ground is instead funneled into the Delta through storm drains, creating more work for overtaxed levees during extended periods of rainfall. When deciding on the region’s growth, we need to understand that the way we build our communities can either alleviate the effects of flooding or amplify them.

Sprawl and floods don't mix

Sprawl and floods don’t mix

Here’s how it happens; if rain falls on farmland or in a park, storm water is absorbed by soil and vegetation, eventually replenishing the water table, where it waits to be used by farmers and/or city residents. In contrast, water falling on your typical suburban street/rooftop or strip mall cannot be absorbed by solid surfaces and is instead diverted to a storm drain and rushed into the nearest waterway. Rain falling in Oak Park stays in the ground while rain at a shopping center is sent to the Delta. On a large scale, all of these rain drops falling on impervious surfaces add up to higher water levels.

While the scenarios I just described are abstract, there has been plenty of quality research done on this topic. As Kaid Benfield notes in his excellent blog, the EPA estimates that “denser development will produce less than a third of the total runoff that the large-lot development will.” In small areas, this may not seem like much. But over an entire city, county and region, excessive amounts of asphalt and concrete may transport more storm water into a stream or river than it can handle, flooding areas that may not have flooded otherwise.

EPA’s findings are corroborated by other studies. In Michigan, researchers found that urbanization is the biggest contributor to increased runoff totals in regional watersheds. In Sao Paulo, the proliferation of impervious surfaces is blamed for an increasing frequency of flooding.

To be fair, developers are usually required to mitigate storm water runoff, which I believe is one of the reasons why Spanos and Grupe developments feature man-made lakes (I am not completely certain on this. If someone knows more, please share). However, the easiest way to mitigate storm water runoff is to simply limit the city’s asphalt footprint. There are plenty of opportunities to develop in areas of Stockton that already have roads and buildings. The effect of infill development on storm water is essentially zero as these areas are already paved over.

While flooding occurs naturally, the way we decide to grow can either help mitigate against flooding or intensify its effects.  If Central Valley cities continue their outward growth, the spread of impervious surfaces will gradually strain levees, making floods more commonplace. Instead, reigning in outward development can keep communities safer from heavy rains, hopefully keeping other neighborhoods from having to pony up for costly flood insurance.

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Categories: Smart Growth

Author:David A. Garcia

David A. Garcia created SCL in March of 2012. Garcia is a Stockton native with a background in urban policy and planning, holding a Bachelor's Degree from UCLA as well as a Master's Degree in Public Policy from the Johns Hopkins Institute for Policy Studies. He currently serves as the Policy Director at the UC Berkeley Terner Center for Housing Innovation. David was also COO at Ten Space, a real estate development firm focused exclusively on Downtown Stockton, and continues to advise on their projects. Prior to that, he worked three years as a researcher/analyst for a Congressional research agency in Washington, DC. The views expressed on this site are entirely of the author's

One Comment on “How sprawl makes flooding worse”

  1. Jon Seisa
    April 30, 2013 at 12:55 pm #

    Well, this research and the bizarre FEMA/insurer lender’s connection is rather extremely highly suspicious. Particularly in lieu of the horror tales we keep reading of FEMA’s relentless agenda to force entire communities against their will across the nation to buy additive unaffordable and extremely expensive flood insurance under the new flood zone designations (property nullification and takeovers), or force upon targeted residences the second impossible option to foot the entire bill with an expense engineering solution (a loaded and stacked option), though the cited supposed flood hazard in many cases has never happened, historically, or in recent memory; and this second option is a funded cost that the government, itself, should pay out of the mega-tonnages of taxes citizens have ALREADY paid via property taxes to support these agencies and their pet projects [schemes]. It is clear that both schemes are designed to cause and force residences to choose neither solution and instead to ultimately vacate whole regions of the U.S. designated as flood prone zones, not so much in the vein or interest for the safety of the populace, but to force them to accept alternative living conditions that FEMA deems suitable; this is covert “social engineering” to shepherd population into isolated highly dense locations. Additionally, the bureaucratic process to counter the flood designation of one’s property requiring many outlandish expenses and fees to defend a homeowner’s case against a flood zone designation is purposely made financially draining, so that FEMA’s sublime agenda wins.

    Case in point of the irrationality of their claims, the ENTIRE Los Angeles Basin of megalopolis-SPRAWL including the Los Angeles River, and other rivers like the San Gabriel River and Santa Ana River, ALL completely concreted over, in an area of the world considered a desert, an arid Mediterranean climate, prone to winter flash flooding and high rain precipitation, has for decades literally NO flooding issues (except in the high desert regions that are NOT concreted over), because the entire L.A. Basin runoff drainage system with backup basins and reservoirs is completed concreted over, which swiftly drains the runoff waters into the channels, funneling the excessive waters into conduits that deliver it to the ocean, preventing flooding in the urban megalopolis-sprawl. The entire system is well designed and engineered and was paid by tax revenues and via property taxes, NOT directly out of the pockets in a lump sum from targeted regional residences in specific areas claimed as prone to flooding. So this appears to contradict the FEMA/insure lender’s rationale that is being paraded out and foisted on communities for dubious reasons under the guise of their voiced façade.

    And then there is this little detail that FEMA conveniently overlooks, the increasing U.S. DROUGHT, where there is NO rain; and thus NO FLOODING. Last I heard (last week) the Colorado River is becoming extinct and the Mississippi River is at alarming low elevations never before seen. FEMA is really barking up the wrong tree and this actually reveals their true agenda even more so.


    “FEMA, lenders wrongly charge Oregon homeowners flood insurance”:

    “The FEMA Scheme – Calaveras County Taxpayers Association”:'s%20Flood%20of%20Falsehoods.pdf

    “Dramatic flood insurance increases could force residents to flee the coast, restoration officials say”:

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