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March 3, 2024<\/p>\n\n\n\n
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For years the Harris County Flood Control District has been giving public presentations that include a slide<\/a> purporting to explain \u201cWhy is Harris County Flood Prone.\u201d<\/p>\n\n\n\n According to Flood Control, the reasons Harris County floods are:<\/p>\n\n\n\n Strangely missing from this list is arguably the number one cause of flooding in our city: it\u2019s covered in concrete, asphalt, and roofs, otherwise known as impervious surface. (p. 182-183<\/a>) That means artificial hard stuff that doesn\u2019t allow the rain to soak into the ground but instead increases runoff, sending stormwater really fast into our streets and overwhelming our drainage systems, natural and built. The built city also absorbs a lot of heat, influencing rain patterns<\/a>, among other problems. (See \u201cCities Depaving for a Cooler Future<\/a>.\u201d)<\/p>\n\n\n\n Here are a few further references for the role of impervious surface in Houston\u2019s flooding:<\/p>\n\n\n\n Buildings Partly to Blame<\/a><\/em><\/p>\n\n\n\n More Pavement, More Problems<\/a><\/em><\/p>\n\n\n\n Houston’s Urban Sprawl Increased Rainfall, Flooding During Hurricane Harvey<\/a><\/em><\/p>\n\n\n\n Houston\u2019s Flood is a Design Problem<\/a><\/em><\/p>\n\n\n\n What Causes Floods<\/a><\/em><\/p>\n\n\n\n The Rapid Urbanization of Houston: How It Happened and Why It Matters<\/a><\/em><\/p>\n\n\n\n <\/p>\n\n\n\n Why Not Mention Impervious Surface?<\/strong><\/p>\n\n\n\n We asked Flood Control why their explanation doesn\u2019t include impervious surface. A representative graciously responded:<\/p>\n\n\n\n \u201cYou are correct that impervious surfaces are a cause of flooding in our area. However, in our \u2018Why is Harris County Flood Prone\u2019 slide we are focusing primarily on the sources of flooding that our capital and maintenance projects can directly address. As you know the Flood Control District does not have regulatory authority over development and the amount of impervious surface added each year.\u201d<\/p>\n\n\n\n Well, true. But the slide is about the reasons Harris County floods, not what Flood Control can do about it. And the flood control district doesn\u2019t have authority over rainfall or the topography or soil type either.<\/p>\n\n\n\n But what about Flood Control\u2019s claim that the landscape is flat and covered in \u201cclay soils that do not soak up excess rainfall quickly\u201d? Actually since most of it is covered in impervious surface, these points would almost seem irrelevant. But they are also not exactly true.<\/p>\n\n\n\n Is the Houston Region Really Flat? Not So Much<\/strong><\/p>\n\n\n\n A 2022 study from the University of Texas<\/a> confirmed what anyone who grew up around here already knows: the seemingly flat landscape is in fact sculpted with numerous sloping hills carved out by the many branching channels, otherwise known as gullies or creeks, that feed into our major streams, such as Buffalo Bayou.<\/p>\n\n\n\n <\/p>\n\n\n <\/p>\n\n\n\n Unfortunately, many of these gullies have been filled in and houses, streets, or other structures built on top. Bad idea. Streams, like rivers, have a memory, and water will flow that direction anyway. A big problem if your house is built on top of an old gully.<\/p>\n\n\n\n Some interesting historic gullies that have been filled or developed include Spring Gully underneath what was then called Carolina Street in downtown Houston. Draining into Buffalo Bayou, Spring Gully reportedly was used as an escape route for people fleeing slavery. (p. 19<\/a>) Harris Gully is another major gully<\/a> that ran along what is now Kirby Drive, through Rice University<\/a>, through what is now the Texas Medical Center (formerly part of Hermann Park), and into Brays Bayou.<\/p>\n\n\n\n <\/p>\n\n\n <\/p>\n\n\n\n But What About the Soil?<\/strong><\/p>\n\n\n\n Flood Control has long maintained that Houston\u2019s natural landscape is basically like a concrete parking lot anyway: not only flat but basically impermeable. But this is wrong, as we pointed out years ago<\/a>. Most of the soil in Harris County is moderately highly absorbent to very highly absorbent.<\/p>\n\n\n\n And it\u2019s even more absorbent if we stop mowing. Plant roots facilitate the infiltration of rain where the soil is less absorbent. But mowing causes plants to devote their energy into growing taller rather than extending roots. So better to let the grass grow higher<\/a> and plant native plants<\/a>.<\/p>\n\n\n\n So What Does Flood Control Have Authority Over<\/strong>?<\/strong><\/p>\n\n\n\n Basically Flood Control\u2019s capital and maintenance projects<\/a> are devoted to handing out millions of dollars in contracts to private companies to mow the grass on the banks of our channelized streams. Plus millions more to dredge and dredge and dredge again, removing sediment and valuable sand that the stream on its own would shift gradually towards to the gulf and onto the beach anyway, naturally rebuilding banks along the way, all for free. Plus millions more to widen, deepen, and damage our streams, compacting the soil and beds with heavy equipment, destroying the stream\u2019s natural ecosystem and ability to cleanse and absorb the water. Plus cutting down forests and digging up prairies to build stormwater detention basins.<\/p>\n\n\n\n The flood control district was created by the Texas Legislature in 1937<\/a> to be the local partner with the US Army Corps of Engineers in its flood control projects. The program was to strip, straighten, and cover in concrete as many streams as possible, which is why so many Houstonians think a bayou is an artificial ditch. (Largely overlooked in the district’s enabling law is the obligation to conserve forests. p. 1<\/a>) In 1979 Harris County Commissioners Court, which oversees the district, authorized Flood Control to widen its focus and build detention basins to hold stormwater<\/a>. Since then the district has been criticized for focusing primarily on large, regional detention basins that require rain runoff to get there somehow. However, the district has limited authority for drainage, and it is local authorities within the county such as the City of Houston that are responsible for stormwater drainage into the streams managed by Flood Control. <\/p>\n\n\n\n Channelizing streams, collecting and moving as much stormwater as possible is an outdated, counterproductive practice<\/a>, not just because of its damaging impact to the environment (and exorbitant maintenance costs) but also because it actually reduces capacity and increases flooding and erosion<\/a>. The Corps of Engineers has moved away from some of its destructive practices (pp. 15-16<\/a>) and local agencies across the country have adopted policies focusing on slowing, stopping, and absorbing rain before it enters our steams. (See here<\/a> and pp. 32-33<\/a> and p. 78<\/a> and also China<\/a>.) The City of Houston in recent years has created some tax incentives for private developers and homeowners<\/a> to install Green Stormwater Infrastructure<\/a> to help slow, absorb, and cleanse stormwater runoff. This includes a property tax reduction for reduced amounts of impervious surface.<\/a><\/p>\n\n\n\n\n
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