Part One in a Three-Part Series on HARC’s program of work and research utilizing LiDAR in the Houston-Galveston region.
From Data to Images
As we have witnessed in the past month, Texas weather can change from extreme drought to torrential rain in the blink of an eye. The vast amount of water that landed on southeast Texas seemed to have almost nowhere to go. In spite of the lessons learned from Tropical Storm Allison in 2001, Houston still experienced floods, especially near our urban Bayous. Why? Because that’s what bayous are supposed to do! Why was there so much damage? Because we drastically altered the natural landscape next to our Bayous over time. Once upon a time, there was natural vegetation and wetlands surrounding all of our Bayous and river beds. The Houston-Galveston region (30% of Harris County) is now covered with impervious surfaces (like roads, parking lots and roofs). The rain that falls on those impervious surfaces can’t seep into the ground, so it runs into our storm drains, drainage ditches and makes its way into our bayous and eventually Galveston Bay.
Ideally, there should be many more acres of wetlands to help absorb these flood waters. But wetlands, once numerous in the greater-Houston area, have largely disappeared due to development and subsidence. Between 1953 and 1989, 23,000 acres of freshwater wetland were lost in the Houston-Galveston Bay region. Since 1996, another 19,000 acres of freshwater wetlands have been lost. That’s a total of over 65 square miles of freshwater wetlands that no longer exist. Wetland alteration is the most important indicator of flood damage in the built environment. Property damage due to rain events in Harris, Galveston and Brazoria counties have increased by $95,000-$495,000 (1.5-100%) as the number of permits to build on or alter wetlands has doubled (Brody et al 2007).
Aren’t all wetlands protected?
Since 1989, the federal government has implemented a policy of “no net loss” of wetlands. However, the types of wetlands that are protected under the federal government’s jurisdiction (alteration of protected wetlands requires a federal permit and typically an associated wetland restoration or protection activity) have been a subject of debate for decades. Not all wetlands are protected under the government’s jurisdiction. Most protected wetlands area adjacent to rivers, lakes, bayous, and the bay and lie in the 100-year flood plain. Freshwater wetlands (such as those found in remaining coastal prairies) that lie outside of this area are not protected by the federal permitting system. Recently, there has been very good news for coastal prairie wetlands that once dominated the Houston-Galveston Bay region. In May 2015, the EPA announced a new Clean Water Act rule that specifically protects Texas coastal prairie wetlands.
What’s the difference? Doesn’t one wetland equal another?
No, not really. Aside from obvious differences in plant species and amount of water, the location of wetlands is also very important if they are to mitigate flooding. One of the operating principles of “no net loss” is that wetlands can be “replaced” by creating another wetland elsewhere. Unfortunately, that might not help any future flooding scenarios at the original site. Also, not every location can become a wetland. Wetland soils and plants are unique – they are meant to be flooded, so they can tolerate higher than normal flows without being smothered by excess water.
How do wetlands impact flooding?
Wetlands provide vital services not only as habitat, but also for improving water quality and flood mitigation. Wetlands slow, contain, store and filter flood waters. When floodplain wetlands are allowed to function properly (are not drained or impounded), floodwaters can spread out over wetlands, reducing the velocity of flowing water and allowing time for water to slowly seep into the ground or be filtered by vegetation and sediment before flowing into rivers and bayous. Wetlands can hold water for days or even weeks, unlike paved surfaces and yards which release water in a matter of minutes or hours.
But changing wetlands can make them better, can’t it?
We used to think that we could improve wetlands by channeling or impounding them, but that didn’t really work. We dug channels to “help” wetlands drain. Artificially channeled wetlands drain faster than natural wetlands, reducing their effectiveness and increasing erosion. Impounding is when wetlands are engineered to partially fill. Impoundments that stop or reduce water flow can also end up destroying wetlands. Impoundments also increase mosquito production by creating pools of standing water. Wetlands have had a misplaced reputation for being breeding grounds for mosquitoes. In fact, natural wetlands have fewer mosquitoes because they can support fish, amphibians and other predators that eat mosquito larvae. Temporary puddles and pools of water in birdbaths and clogged gutters at homes and businesses are much more likely habitats for mosquitoes.
What can we do?
Can you spot a wetland? Knowing how valuable wetlands are, hopefully you can appreciate them a bit more. Wetlands can be extremely beautiful, especially when they are home to migratory wildlife. They offer opportunities for recreational boating, bird watching, and fishing.
Destruction or modification of most wetlands requires a permit from the U.S. Army Corps of Engineers. As citizens, we can participate during the public comment periods for many permits, but the first step is to ask questions and work with developers so that hopefully, our activities don’t destroy, fill, or drain wetlands. If existing wetlands are filled, neighboring properties could be at increased risk for flooding. Wetlands can even serve as natural amenities for nearby neighborhoods.
Regional and local conservation groups will sometimes open wetland restoration to public volunteers. Conservation groups like the Galveston Bay Foundation, Ducks Unlimited, National Audubon Society, The Nature Conservancy and others have been working hard to purchase and conserve wetland areas in our region. Supporting wetland conservation is a great way to protect wetlands so that they can flood the way that they are supposed to.