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Houston's Flooding Issue


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I spoke with a consultant from San Antonio yesterday. He does quite a bit of work with waterways and watersheds, etc. Anyway, I talked with him about some issues in southern Brazoria County and flooding and he told me that no matter what goes on down there--even if there was no development, the area would still flood like crazy (which it already does).

He then mentioned a study that Harris County did with the floodplains and said that they had someone map the floodplains without any development at all. The conclusion, there would only be a 20% change in the likelihood of flooding in Houston. So the difference between the concreting of Houston and absolutely no development is 20%?

The reason, apparently, is the region's soil types. Supposedly when they get wet, they become almost like concrete, taking forever to absorb water.

Interesting findings.

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The study seems to overlook the effects of vegetation in slowing the runoff of rainwater.

While it's true that the soil here is less permeable than in other places, the presence of grass, trees, etc. provides a huge amount of surface area to which water can cling. Rather than running off all at once, water is slowly released to either trickle to the ground or evaporate. In addition, the presence of plants slows the run-off of rainwater to bayous.

The absorbency of local soils tells only part of the story. Flood prevention is as much about the controlled release of water as it is about total rainfall.

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I agree that vegetation provides surfaces for water to "cling" to, but I think that vegetation's biggest benefit is the filtering of pollutants.

Nonetheless, a classmate of mine made it plain and simple--"once a swamp, always a swamp". And swamps are covered in thick, lush vegetation--and water.

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As a hydrologic and hydraulic engineer here in houston, I deal with flooding issues and developing new subdivisions to prevent further flooding. There are a lot of things we can do to help in flooding, but it is inevitable.

The 20% difference seems quite small. It is closer to 35%.

The most you can do is just be prepared for flooding.

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  • 2 weeks later...
As a hydrologic and hydraulic engineer here in houston, I deal with flooding issues and developing new subdivisions to prevent further flooding.  There are a lot of things we can do to help in flooding, but it is inevitable.

The 20% difference seems quite small.  It is closer to 35%.

The most you can do is just be prepared for flooding.

Not to diminish the differenece between 20% and 35%, but when you think of the amount of land area here that is covered, it's still amazing that the difference between no development at all and current, sprawled development is one-third.

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Yes, it is bigger. The main reason houston has flooding issues is before 1984 the concept of reducing floods was just ennlarge a channel and the worst case pave the channel to speed the water up. By 1984, the issue of runoff volume became the issue to address besides speeding the flow in the channels.

The only problem, there wasn't any volume to store this water. This began Harris County Flood Control Districts' (HCFCD) detention policy. Just recently it has been revised again.

HCFCD has a standardized detention policy set while Houston had a very relaxed policy. Recently Mayor White has pushed through a new policy which primarily follows HCFCD's revised policy for most developments except for really small developments (under 1 acre). This important step may be too little to late.

HCFCD also has a long range plan to build more regional detention facilities and wetlands mitigation banks throughout the county. The addition of these will slowly assist in reducing flooding.

Another issue that is a much more complicated issue to address is in older areas where little to no storm sewers exist or they are completely under-designed by todays standards.

This is about to be addressed in a section of the heights and other projects are underway in design throughout the city.

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  • 3 weeks later...

Yeah, flash flooding is evident but not as serious because it doensnt stick around nearly as long as a steady rain flood. The ground becomes too saturated, so it can not hold any more water, which means the run off becomes stronger and the water doesnt go very far because Houston is so flat.

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The fact that more water and faster water runoff occurs from development is the purpose behind detention facilities. The slow the entry of the water into the channel. The volume is still more, but the detention facilities just slowly release it.

Over the next 10 to 15 years, many projecst are on tap to reduce the risk of flooding.

Another important thing to remember. Street flooding is a common occurence in Houston because it is planned that way for large storms. The concept of many neighborhoods is to use the streets as channels to convey the water to a main channel or detention pond. This concept means that storm sewers don't have to be too large which saves on money for construction. I many subdivision, home prices could easily double if storm sewers were sized to handle 100 year storm events.

At my office, we played with the figures from a few subdivions to see what a 100 year storm sewer would cost. The developer would have to pass on the cost to the buyers.

I think having some water in the street is worth it to have a house that is a cheaper cost.

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