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.
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.