Federal Project in Late Forties Reduced Bayou Capacity Upstream
October 29, 2018
By the early morning of Aug. 28, 2017, after about thirty hours of record heavy rains from Hurricane Harvey, storm runoff was rising so fast in the two federal reservoirs in west Houston that the Corps of Engineers feared the water would overflow the aging earthen dams. They made the unprecedented decision to open the dam floodgates, slowly at first and then, in the following days, much wider, as stormwater did indeed begin to spill around the northern end of Addicks Dam north of Interstate 10.
For most people living and working on Buffalo Bayou, the peak of the flooding had already passed on Sunday, Aug. 27, when the floodgates were still closed. That Sunday storm runoff draining from the paved and built city into the bayou below the dams sent anywhere from two feet to eight feet or more into homes and other structures built on or near the banks.
But when the dam floodgates were opened in the first hours of Monday, roaring floodwaters pouring out of the dams spread out and inundated neighborhoods built on the bayou’s floodplain below the dams for more than six miles to just below Beltway 8 (West Belt). Many of these homes had not flooded earlier.
The horrific flow through the open floodgates eventually reached more than 16,000 cubic feet per second (cfs). For much of Buffalo Bayou in west Houston flood stage is around 6,000-7,500 cfs.
As a result, many more homes and other structures flooded on the upstream stretch of Buffalo Bayou between the federal dams and Beltway 8 than downstream of the beltway. (Homes built in the reservoir flood pools behind the dams also flooded, but that’s another story.)
Blaming the Bayou. “Kinks” and “Bottlenecks” Downstream
This six-mile stretch of the bayou upstream of the beltway was straightened and shortened by the Corps of Engineers more than seventy years ago. The bayou downstream of the beltway remains a meandering stream, largely unchannelized, twisting and turning its way nearly to Shepherd Drive. Below Shepherd the bayou was stripped and channelized in Buffalo Bayou Park by the Corps in the Fifties. From there it flows along a heavily altered route through downtown, eventually becoming the Houston Ship Channel.
In the wake of this disastrous flooding upstream of the beltway, distraught and shell-shocked property owners, many of them without flood insurance and dealing with severe losses, have been blaming the “kinks” and “bottlenecks” in the meandering bayou below Beltway 8 for flooding their homes. The belief is that the meandering bayou constricted the flow and caused water to backup and flood property upstream. The common complaint is that the bayou downstream is too narrow and is “like a hose” that gets a “kink” in it, causing the water to stop and back up.
Many, including developers and other business interests in west Houston, are calling for the meandering, relatively natural bayou downstream to be deepened and widened to accommodate double or triple the volume of flowing water—something close to the 16,000-plus cfs that occurred during Harvey.
Neighborhood activists upstream have also proposed a plan for large concrete culverts to be dug into the banks, presumably under streets and homes, in order to bypass the meanders downstream. In response to community pressure, the recent Harris County flood bond election included a proposal to spend $500,000 to study these “high flow bypasses,” “channel conveyance capacity” around Beltway 8, as well as bridges over Buffalo Bayou that may be obstructing floodwaters.
There are even calls for the trees to be cut down in Terry Hershey Park along the straightened channel upstream, as well as inside the flood-control reservoirs, Addicks and Barker, which are vast public parks. This theory is that trees take up storage space and block the flow.
Why Did More Structures Flood Upstream?
One only has to look at a map of house flooding as reported to the Federal Emergency Management Agency to determine that many more structures were flooded between the dams and the beltway than along the meandering bayou downstream.
But what was the reason for that?
A major cause is the straightening and channelizing of the upper bayou as well as a tributary creek done decades ago by the Corps of Engineers, according to a study done for Save Buffalo Bayou by hydrologist Matthew Berg. Of course, houses and other structures that flooded immediately below the dams weren’t there when the Corps straightened the bayou, shortening it and altering its course.
The Corps project cut the upstream capacity of the bayou in half, says Berg.
In addition, the floodplain surrounding the bayou upstream—the area onto which the excess water overflows—is much wider because it is lower in elevation, a result of long-term natural subsidence due to faults in the region, notes William Dupré, professor emeritus of geology in the Department of Earth and Atmospheric Sciences at the University of Houston.
See this study by Houston geologist Tom Helm, a board member of Save Buffalo Bayou, of the elevations and floodplain of Buffalo Bayou between Eldridge and Beltway 8.
“Downstream meanders are not causing the flooding upstream,” says Dupré.
Some seventy years ago the Corps of Engineers built two dams on the prairie west of Houston, then mostly farm and ranchland. The dams were to hold back floodwaters of then-forested streams that flow into upper Buffalo Bayou. Damming these tributaries and the bayou itself during heavy rains was supposed to protect the city of Houston downstream from flooding. The 18,000-year-old bayou, the city’s main waterway, runs for some fifty miles from its headwaters near Katy through Barker Reservoir and the center of the city, emptying into the San Jacinto River and Galveston Bay.
The Corps also stripped, straightened and shortened the bayou for more than six miles below the dams. They also stripped and straightened Langham Creek where it flows into Buffalo Bayou from Addicks Reservoir north of Interstate 10.
The idea was to make the water flow faster through the channelized creek and bayou so that the dam reservoirs could be emptied more quickly. The expansive reservoirs of the two federal dams were designed to be dry, remaining prairie and riparian woodlands. The creek and bayou flow freely through the dams, and the floodgates are closed only during storms. Emptying the reservoirs as quickly as possible is necessary to prepare for more rainfall and prevent the earthen dams from overflowing, a common cause of dam failure. The dams were not designed to hold water for long periods of time.
After the dams were built, the forested floodplains that remained alongside the bayou were thought to be protected from flooding. These natural floodplains, often sloping, traversed by small creeks and ravines, and shaded by tall oaks in the midst of the flat, open prairie, were developed into desirable residential neighborhoods, offices, hotels, apartments, businesses and parking lots, including behind the channelized stretch of the bayou, now Terry Hershey Park, where trees have since regrown.
Protecting the Meandering Bayou
But the Corps wasn’t finished. The plan was to do to the rest of Buffalo Bayou downstream what the Army engineers had done in the mid-Fifties and early Sixties to Brays and White Oak bayous: strip, straighten, and line it with concrete. Opposition from influential property owners and environmentalists stopped that project. Straightening and concreting natural waterways has since been mostly abandoned as a flood management practice because it increases flooding and water pollution and because of the high and continuing cost, both financially and ecologically. The focus of flood risk reduction now is on slowing the flow, managing flooding in place, making room for the river, and working with, rather than against, the natural functioning of a stream. In other words, stopping stormwater before it floods a stream.
Buffalo Bayou below what is now Beltway 8 remained a meandering stream lined with tall trees, most of it privately owned.
The Corps Project Reduced by Half the Capacity of the Upper Bayou
But in straightening that upper six-mile section of the bayou, the Corps reduced by at least half the storage volume of the stream in that stretch. Removing the meanders shortened the stream. Shorter streams carry less water.
“The upper section in the old days held more than twice as much as today,” notes hydrologist Berg.
In addition, by straightening Langham Creek and aiming it almost directly into the channelized bayou, the Corps inadvertently made flooding worse for the future homes that would be built near the intersection of those two streams.
“Where two streams come together is the worst place to develop,” says Berg, referring to the point where the two streams meet north of the bayou and west of Eldridge Drive. “The two streams block each other at high flows.”
“This would be an awesome place for buyouts.”
Meandering Bayou is Deeper, Wider, and Steeper
The straightened section of the bayou in Terry Hershey Park is not just shorter than it used to be. It’s also shallower and narrower than the meandering bayou below the beltway. Despite the apparently popular belief that the winding bayou downstream is narrower and constricting the flow, the fact is that the bayou below the beltway is much wider than upstream both at the channel bottom and from bank to bank across the top, according to Berg’s research.
The bayou also has a steeper slope downstream which causes the water to flow faster, says Berg.
“The Corps exchanged greater volume [in the shorter, straightened stretch] for shorter distance and steeper slope,” says Berg.
While some would argue that the increase in velocity in a straighter, shorter stream can make up for the loss of volume of a longer, winding stream, Berg notes that this tradeoff is not always so simple. “You really need to look at how a stream works along its entire length, not focus just on any one spot.”
Bayou Not Like A Hose. Meanders Not Causing Flooding Upstream
Meanders don’t cause the water to backup upstream, points out Dupré. Flooding tends to increase in the place where the meanders are, where the water slows, he explains.
“Straightening a channel to increase its velocity and lower its stage typically results in increased flooding downstream of the straightened channel, as that is where the water slows and stage rises.”
The bayou is not like a hose or pipe. It doesn’t have a top on it. Where it meets obstructions (“kinks”), it slows, rises to overflow the banks, and spreads out.
But “we don’t see this [downstream] because the channel gets deeper,” explains Dupré. And wider, says Berg. In fact, the bayou channel gets deeper and wider the farther downstream you go.
However, in general, straightening, shortening, increasing the velocity does tend to cause flooding downstream, confirms Dupré, as well as increasing erosion up and down the stream. And while the bayou downstream is deeper and wider, not all areas downstream were spared the full force of the stormwater shooting out of the straightened stretch upstream.
Neighborhoods just below Beltway 8, in particular the residences on Legend Lane, built on a large point forming the first large meander below the beltway, suffered a near direct hit from the floodwater rushing down the chute from Terry Hershey Park, as did the Briargrove Park neighborhood directly across the way. Subsidence in these neighborhoods, more than a foot between 1997 and 2017, has increased vulnerability to flooding here too, as it has upstream and elsewhere in the county, particularly the northwest.
Neighborhood activists downstream have suggested properties in these neighborhoods, particularly Legend Lane, are candidates for buyouts also.
Seeking Its Natural Channel
Another problem is that the altered bayou, like all flowing rivers and streams, tends to seek out its natural channel. (See also this study of the straightened sections of Buffalo Bayou by geologist Helm.) And in this case residences have been built on top of or very close to the historic path of the straightened bayou, directly in the path of the flooding river. Subdivisions have also been built on top of buried or filled creeks and ravines. Recognition that opening up these buried waterways can reduce flooding, among other benefits, has led to a global movement to “daylight” buried urban streams.
Dredging, Deepening, Widening. Making the Bayou More than Twice as Wide and Deep
Dredging, deepening and widening the bayou doesn’t work. Dredging makes flooding worse, and also causes the banks to collapse and the channel to fill with sediment as the bayou, like any stream, attempts to restore its dynamic equilibrium, a balance of flow, shape, slope, and movement of sediment. This constant natural adjustment by the bayou can lead to increased and continuing costly maintenance, and continuing ecological damage, when authorities continue to fight the river over its depth and width, and straightness.
But let’s imagine how deep and wide Buffalo Bayou would have to be in order to accommodate a flow of 16,000 cfs plus without overflowing its banks.
The bayou’s peak at Beltway 8 during the dam releases from Harvey was 71.22 feet on August 31, 2017, a flow of some 22,900 cfs. Let’s pretend that’s not going to happen again. We’ll deal only with official flood stages. Flood stage on Buffalo Bayou at Beltway 8 is officially 62.2 feet, or a flow of some 8,000 cfs. But that’s flooding. We want to prevent flooding. Therefore, we would have to keep the flow below the so-called “action” stage: below 59 feet or below a flow of 6,400 cfs.
So in order to accommodate a flow of 16,000 cfs without the bayou overflowing its banks upstream and around Beltway 8, the bayou would have to be at least 2.5 times wider and deeper.
What to Do?
Well, making Buffalo Bayou more than twice as deep and wide is a ridiculously costly, physically impossible, and pointless idea. Among other issues, above about halfway up the bank, the land along Buffalo Bayou is mostly privately owned. Not going to work and not going to solve the problem anyway. So what is the solution?
Bypassing the meanders downstream with huge concrete culverts somehow buried under streets and houses is also wildly expensive and ecologically damaging. That’s not going to solve any problems and likely would create more problems. Among other issues, Berg notes that the bayou bottom and portions of the banks upstream of West Loop 610 are largely sand and highly erodible, and that the particular soils in this area are highly corrosive to both concrete and steel and would soon have to be replaced at great cost.
As for removing trees in an effort to speed up the flow, Berg points out that doing so would make the banks unstable as the roots of trees and other vegetation help hold the banks together, protecting against erosion and absorbing water.
However, redirecting several large stormwater outfalls that point directly across the stream, blocking the flow, might help. One example is the massive outfall at Beltway 8. Outfall pipes and boxes set at right angles to the channel are a violation of the flood control district’s design guidelines. (p. 111)
Making Room for the River
The solution is, in a way, the opposite of what some are calling for. Rather than dredging and channelizing the bayou downstream, the more practical and effective response is to allow the channelized stretch upstream to return to its meandering course.
Berg, Dupré, and many others (see here too) suggest the obvious: buying out properties in harm’s way, removing structures hopelessly deep in floodplains. Make room for the river. Let the bayou naturally restore its meanders, naturally lengthening the channel, creating greater capacity for the flow, slowing the flow to prevent flooding downstream while creating more room for the bayou to overflow.
“Any place you can put the bends back in is a good idea,” says Berg.
“I tend to be of the philosophy that we need to give room for the river,” says Dupré.
Addicks and Development of the Katy Prairie. Detention Requirements Not Working
Concerned residents also might want to focus on the source of all that water flooding their homes.
Both Berg and Dupré, along with numerous other experts, point out that the real problem is too much rain runoff flowing into Addicks Reservoir, and that continued development of the Katy Prairie, including the land around Cypress Creek north of the reservoir, is making the problem worse. The overflow from a flooding Cypress Creek ends up in an already overburdened Addicks Reservoir.
New subdivisions, malls, and roads on the prairie are increasing the flow of rainwater into the reservoir. Requirements that developers detain or hold back the runoff are not working, stated Christof Spieler, project manager for the Greater Houston Flood Mitigation Consortium, at a recent symposium on flooding. The consortium, in its April 2018 report, called for “[i]mprovements to Addicks and Barker, preservation and restoration of native prairie, and appropriate restrictions on new development” to address flooding. (p. 64)
Experts have for years been drawing attention to this problem. See this 2011 paper by Ronald Sass, fellow in global climate change at the Baker Institute and Wiess Professor of Natural Sciences emeritus at Rice University. Also in 2011 the Houston Sierra Club filed suit in federal court over the alleged failure of the Corps to analyze the flood-related impact on Addicks and Barker reservoirs of the Grand Parkway (Highway 99) and subsequent development of the Katy Prairie.
And yet, as Houston faces increasing and more frequent rains, county and city officials weekly continue to approve plats for new development on land that should be preserved as open space for stormwater detention. (See also here.)
“Development in west Houston is really a problem,” says Dupré, who suggests strengthening and deepening the reservoir to handle increased runoff, already “more water than [the dam] was planned for.” Also critical is addressing the overflow from Cypress Creek, he says.
“We need a more effective Addicks. It’s really a national disgrace,” says Dupré, pointing out that because of the consequence of failure to such a populous area, in 2010 Addicks and Barker dams were both listed among the six most dangerous federal dams in the United States.
“By the time water gets to the reservoirs, it’s already too late,” says Berg.
“We have to look at causes of the problems, not the symptoms.”
Green and Nature-Based Solutions Work Best
Fortunately, looking at the causes of and potential solutions to this flooding catastrophe seems to be happening.
Earlier in October Harris County Commissioners Court approved participation by the Harris County Flood Control District in a three-year study of Addicks and Barker Reservoirs and surrounding watersheds, including Buffalo Bayou and its tributaries. The Corps of Engineers will lead the study, and the $6 million cost will be paid with federal funds.
The Corps now has a mandate to “engineer with nature.” And the fact is that in the long run non-structural and nature-based solutions to flood risk—preserving and restoring the Katy Prairie, for instance—are more efficient and sustainable than “traditional structural measures” like dams, levees, and digging up bayous, as confirmed by a 2014 report from the American Society of Civil Engineers. (p. 27)
If we’re going to save the city by discouraging more development in west Houston, urban flooding experts like Sam Brody suggest that we will have to incentivize development elsewhere. If we can offer hundreds of millions in cash and tax breaks to Amazon, maybe we can find a way to make it attractive for developers to redevelop areas closer to town.