Watershed Texas

What about those magic wetlands?

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Development in League City threatens wetlands

Hurricane Harvey was tough on wetlands, and even tougher on wetland scientists like me. I have spent a career focusing on wetlands along the Upper Texas Gulf Coast. Like the Lorax, I speak for the wetlands (and the prairies and forests and all the rest of it!). I have devoted considerable energy to expounding all of the virtues of wetlands, including their ability to decrease downstream flooding. So much so that at least one top flood official accused me believing in “magic wetlands”. He didn’t think wetlands did all that much in terms of stormwater detention (or anything else apparently).

As with many other aspects of our lives, Harvey changed everything. It modified my views on upstream wetlands and flood control. Harvey totally overwhelmed every single prairie-pothole wetland on the Katy Prairie, and on every other prairie in our area for that matter. All of these wetlands filled with water very quickly very early in the storm and all of the soils in or out of the wetlands became saturated very early as well, such that virtually all the rain falling anywhere, even on the sandiest of soils, ran off the landscape and into our creeks and bayous. And then the water in the bayous rose and the rest is burned into our memories.

That wetlands detain stormwater and reduce downstream flooding is an axiom of watershed science. Depressional wetlands like our prairie potholes do have a volume, and they do detain that volume of water during a storm. This volume is sufficient to make a very large difference for moderate storms, say up to 8-10 inches. But large storms such as a Tax Day or a Memorial Day storm – not so much. For a whopper of a storm like Harvey—the difference upstream wetlands make starts to become negligible.

An average rainfall of 35 inches over all of Harris County (Harvey) is just over 1 trillion gallons.  At most, there are about 50 billion gallons of stormwater detention capacity in Harris County wetlands (no one has measured this—I had to make some very broad assumptions). So that means that the wetlands at best could handle about 5% of the total volume of Harvey rainfall. In the large scheme of things, it’s not much. And the scheme of things in Harvey is indeed very large.

So much for the magic wetlands. But what about our engineered drainage system?  I calculate a somewhat larger detention capacity—between our large US Army Corps of Engineers Katy Prairie reservoirs (~400,000 acre-ft) and Harris County Flood Control District detention (about 34,000 acre-feet), we have about 130 billion gallons of detention volume.  More than what we have for wetlands, but still only about 14% of Harvey. As we painfully saw, also overwhelmed.

And what of green stormwater infrastructure—rain gardens, bioswales, green roofs, etc.? We don’t have any good numbers here, but you can be sure that even if these practices were widespread, the volume would be very small relative to wetlands and detention basins. These practices are designed to capture at best a 2 inch storm.

The foregoing figures are pretty rough estimates—but they give us an “order of magnitude” estimate. And that estimate says Harvey trumps it all.

The fundamental question before us, then, is what is our standard of protection? If it is a storm of 10 -12 inches or less, then lets continue with the 100-yr floodplain. We must also include our prairie pothole wetlands in this standard.

If it is Harvey, these features won’t save us—only the floodplains will.

And the wetlands?  I still speak for them, Harvey or no Harvey. Wetlands provide a number of benefits—floodwater detention at small to medium storms of course. But they provide many more benefits, water quality chief among them.  Wetlands cleanse water in ways that detention basins cannot. Every wetland we fill or destroy puts another nail into the coffin for Galveston Bay. It is just that direct of a connection. We are to the point now where each individual wetland lost makes a difference. Fifty years ago, the loss of one wetland was not such a big deal. Wetlands are getting scarcer, an unconscionable amount lost to development.

Green stormwater infrastructure?  These practices mainly affect water quality. And believe me, we need to positively effect water quality! So we need all the bioswales and green roofs we can get. But let’s not delude ourselves that these features can do anything against a Harvey.

And what of sprawl?  Yes—the impervious surfaces that come with sprawl do increase downstream flooding. But at the scale of Harvey, this doesn’t seem to matter so much.

Because if we are going to plan like another Harvey is coming our way, then floodplains are the only green infrastructure that counts. Reclaiming our floodplains, which could absorb a Harvey, becomes our number one task. Yes buyouts are difficult and painful, but Rome wasn’t built in a day, and neither will the emerald necklaces of wide, unoccupied floodplains that eventually will run through our city, if we choose to reclaim the floodplains.  Make room for the river, say the Dutch. Let’s say it too. We have to get development out of the floodplains to make room for the bayous.

And about those magic wetlands. What is magic but that which we don’t understand but marvel at? There is so much more about wetlands than we understand—more even than we can understand. If you know even a little about wetlands then you will understand the magic. We can make Houston magic by saving the wetlands in our prairies and forests, and by making room for the bayous – genuine green infrastructure.

And why not build a magical city served by beautiful and functional green infrastructure? We are at the juncture where we have to decide which path to follow.  Do we make room for the bayous and enjoy the green space, or do we continue to fight with nature, trying to diminish flooding instead of reducing impacts?

 

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