A Note on Impervious Surfaces

Between 1925 and 1955, Houston made two changes that massively increased the amount of stormwater runoff generated by development.

(1) We switched our standard residential street from a 16 foot shell-surfaced lane with ditch drainage to a 27 foot concrete slab with curb and gutter.

(2) We switched from raised floor house construction to concrete slab construction.

In the wake of last year’s floods, the tendency among media types has been to castigate developers. Developers are bad because they build new developments on the periphery that increase runoff downstream, they’re bad because they don’t construct additional detention when redeveloping infill sites that are already 100% imperv, and they’re bad for having built apartment complexes in the floodway 40 years ago.

However, on this issue, perhaps we shouldn’t be looking to developers for change. Perhaps we should be looking to the city.

Consider roadways. Using TxDOT estimates, a section of standard slab street should run about $45,000 per 100′ of pavement, of which $28,000 is street and sidewalk and $17,000 is storm drainage. By contrast, a Heights-style swale street built to the same pavement section would run only $17,000 – $12k for the street and $5k for culverts.

Narrower streets would not only reduce runoff, they’d also make housing cheaper. With 50′ lots, this would cut at least $7,000 off the price of each home – probably closer to $10,000 once intersections and collector streets are factored in.

Raised floor construction, by contrast, costs a bit more than slab, but it has the huge advantage of opening up land under the house for stormwater infiltration. And while slab currently predominates, there are industry groups who would love to explain the benefits of raised floors to potential homebuyers.

Houston’s detention requirements are based on a specific method of stormwater analysis (the Rational Method), for which specific runoff coefficients are proscribed for different land uses. Houston also allows engineers to calculate their own runoff coefficient, if the total perv/imperv areas are known.

If we want to encourage raised-floor construction, the fastest way to do so would be to add language exempting raised floor structures from impervious cover calculations. Alternately, some sort of reduction factor could be used – perhaps 1,000 SF of raised floor only counts for 250 SF of slab.

This would immediately push local builders towards raised floor, because the additional cost per house would be offset by the additional lots that could be developed on land that would otherwise go to stormwater detention. National developers would keep building slab for awhile, since their home designs are standardized across regions. But eventually they’d pick it up too.

One thought on “A Note on Impervious Surfaces”

  1. Interesting thoughts, but obviously you have not enjoyed the horrors of ditch drainage in front of your home. I lived in the Heights for 15 years. Trash cans in the ditch weekly (Thanks COH), fighting trying to mow the ditch, cars getting stuck in it periodically. It was a nightmare. I can only imagine making it gravel/swale and having the joys of dust.

    By contrast, I live in SE Houston now in a 1960’s development called Meadowcreek Village. It’s practically paradise. Wide concrete streets in reasonably good condition. Excellent drainage. Curb and gutter that is easy to live with. Plenty of parking. Trash cans never wind up in a 3 foot hole that I have to climb down and drag back out of. We have larger lots with big yards here, which gives water somewhere to soak in. Newer developments are practically building townhomes.

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