Raising Bridges, Building Meander Bypasses on Buffalo Bayou Doesn’t Reduce Flooding, Study Says

New Idea is to Strip and Widen Bayou Channel in Terry Hershey Park Above Beltway 8

New Problem: Increased Flooding Downstream

 

Oct. 20. 2019

A study commissioned by the Harris County Flood Control District has found negligible flood-reduction benefits to building bypass channels through meanders or raising bridges on Buffalo Bayou.

But the District is instead contemplating stripping the trees and vegetation and digging out the engineered banks of Terry Hershey Park to widen the channel by fifty feet on both sides for some six miles below the dams in west Houston. The District is still in the process of scraping out new overflow basins on the south bank in the park.

These findings were presented at a packed public meeting Thursday, Oct. 17, in the Memorial area of west Houston. The study, the result of public pressure from property owners who flooded in neighborhoods adjacent to Terry Hershey Park, was conducted by the engineering firm Huitt-Zollars and funded with $350,000 in Harris County flood bond funds.

Buffalo Bayou originates in the prairie near Katy, Texas, and flows for some fifty-three miles through the center of Houston, emptying into the San Jacinto River and Galveston Bay. It is the main river flowing through the city and a central part of the region’s natural drainage system. Below Beltway 8 to Shepherd Drive just east of downtown, Buffalo Bayou remains a mostly winding, wooded stream, and it was these many meanders that some residents upstream of the beltway blamed for flooding their homes when the floodgates on the dams were opened during Harvey.

 

 

The Meeting and The Findings: Is Nature a Better Engineer?

Alan Black, director of operations for the Flood Control District, opened the meeting by explaining what the district does and why the region floods (reasons which did not include the fact that so much of the city is covered in impervious surface). The first priority for the district is still “deepening and widening bayous,” said Black. Unfortunately that is a futile, never-ending pursuit, like building bigger freeways. It also leads to bank collapse, lifeless streams filling with silt and polluted water, a barren landscape, and continuous costly maintenance.

Modern flood management, rather than collecting and moving as much water as fast as possible, focuses on stopping stormwater before it floods a stream and on managing flooding in place. For instance, “lag time” is the amount of time it takes for rain to fall on the ground and enter a stream. The shorter (faster) the lag time, the higher the peak flow (flooding) in the stream. Trees, deep-rooted vegetation, detention basins, rain gardens and more can help increase the lag time and reduce flooding. There are many ways that neighborhoods and individuals can take responsibility for slowing, spreading out, and soaking in stormwater. (See also here.)

The district, which has limited legal tools dating from 1937 when it was founded, does focus also on building detention basins to temporarily hold stormwater, as well as on moving people out of harm’s way through buyouts.

 

Bridges

Michael Tehrani, vice-president of Huitt-Zollars, explained the study findings, which were “not that promising,” he told the large crowd, which included politicians and representatives of the Corps of Engineers. Buffalo Bayou is relatively flat, a “lower velocity channel” with a lot of trees; the channel itself “controlling the water and determining the Water Surface Elevation, not the bridges.”

Tehrani said that their hydraulic study looked at raising 32 bridges on Buffalo Bayou between Highway 6 below Barker Dam and downtown Houston. The minimal result would be to reduce the Water Surface Elevation by only a couple of inches during a rainfall of 19 inches in 24 hours (currently in Harris County known as a 500-year flood). And the reduction would only be just upstream of the bridge.

 

Meander Bypasses: Nature Already Created Them

Tehrani noted that meanders create a longer path for the river. (And therefore carries a greater volume of water, a good thing, though he didn’t say that.) The engineering firm analyzed 18 meanders between Beltway 8 and Shepherd Drive and found that “nature has already created a quasi-semi bypass” through them.

As a result, digging out artificial bypass channels would have minimal impact on the Water Surface Elevation during a “500-year flood” – less than an inch for 78 percent of the meanders, and that only for just upstream of the meanders.

 

 

Combining Raising Some Bridges, Building Some Bypasses or Widening the Bayou

The engineering firm then considered raising some bridges, digging out some bypasses, and creating some detention somewhere. (p. 22)  Result: negligible, complicated, and expensive.

The firm then turned to the option of widening the channel in the previously straightened (and narrowed, reduced capacity) bayou in Terry Hershey Park above Beltway 8. The Flood Control District owns the land in this stretch of the bayou. The idea (not yet firm or funded) would be to dig out the lower banks, creating a wide flood shelf, on both sides of the bayou, eventually replacing the sidewalk and possibly restoring some trees.

Note how the blue 500-year floodplain in the district’s graphic of the proposed channel widening corresponds to the original meandering path of the bayou and Turkey and Rummel creeks.

 

 

 

Reducing Flooding Upstream, Increasing Flooding Downstream

The channel-widening option would reduce the Water Surface Elevation above Beltway 8 by more than four feet and potentially remove 240 structures from the 100-year floodplain and 877 structures from the 500-year floodplain, according to the study.

The estimated cost would be $216 million, including $118 million to purchase the land to create the detention needed near Beltway 8 to keep the greater volume stormwater flowing through the park from flooding people downstream.

Black emphasized that the channel-widening plan was just an idea and that as yet there was no money to fund it.

Here is where you can watch the Facebook video of the presentation.

Here are the slides from the presentation.

The Flood Control District is accepting comments on the study through Oct. 31. Here is where you can comment.

For more information about Houston flooding and flood-reduction strategies, here is a link to the complete list of research and studies from the Greater Houston Flood Mitigation Consortium.