Outfalls, Right and Wrong
Dec. 16, 2019
When it rains, water falls from the sky and runs off our roofs, yards, patios, parking lots, sidewalks, roads, and driveways into storm drains, through pipes or ditches and into our bayous and creeks. The end pipe that drops that collected rainwater into our streams is called an outfall. The receiving stream, usually part of our natural drainage system, carries the rainwater away to the sea.
Buffalo Bayou, which begins far out on the Katy Prairie and runs all the way to the San Jacinto River and Galveston Bay, is our central drainage waterway flowing through the center of Houston, with many tributary creeks and streams and storm pipes emptying into it.
In many cities, to avoid overwhelming the drainage system with too much water all at once, causing flooding, residents are encouraged, even required, to disconnect their downspouts or drainpipes from the city stormwater or sewer system and let the rainwater spread out and flow slowly over yards and gravel, etc. But that’s a different story.
The outfall that releases all this collected rain runoff into our bayous and creeks can be big or small, concrete or metal, round or square. But there are wrong ways and right ways to install them in the banks of our streams.
Installing them the wrong way—pointing across the stream, for example—can block the flow like a dam during storms, even causing the water to flow back upstream and out of the banks.
Outfalls installed the wrong way can also cause erosion of the opposite bank, as well as around the pipe itself.
Taxpayers end up paying to repair the damage done by improperly installed outfalls. Property owners who flood or have their banks eroded away also pay.
In our stormy Bayou City where engineers have been dealing with drainage and flooding problems for a long time, one might think that we would always get it right. But somehow we are burdened with numerous outfalls, old and new, installed in ways that block the flow during big storms, and damage the banks.
Are these outfalls in violation of local flood control standards? Are they in violation of generally accepted best practice?
Take for example the massive concrete outfall on the south bank of Buffalo Bayou in Buffalo Bayou Park just downstream from the Memorial Drive Bridge. Long ago, before the Corps of Engineers stripped and channelized this section of the bayou in the 1950s, before the construction of Allen Parkway (Buffalo Drive) in 1925 connecting downtown and River Oaks, this was a small tributary stream flowing through a deep ravine into the bayou. The tributary, like many, was enclosed and buried decades ago, and the current concrete-walled outfall was installed during renovation of the park around 2014.
The outfall, located just upstream of a bend, faces towards the opposite bank. Since the outfall was constructed, the section of asphalt sidewalk on that opposite bank, installed around the same time, has washed away, along with much of the bank, erosion that began even before Harvey in August 2017.
Upstream v. Downstream
Obstacles to the flow in Buffalo Bayou during storms has been a heightened point of contention since Harvey. Residents living near a straightened stretch of the bayou upstream of Beltway 8 in west Houston believed that bends in the naturally meandering river downstream had contributed to the massive flooding many suffered when the Corp of Engineers was forced to open the floodgates on the two federal flood control dams during that record storm. The dams, Addicks and Barker, hold back the flow in Buffalo Bayou and several tributary streams when there is a potential for flooding downstream.
Directly below the dams the bayou was stripped and straightened by the Corps in the 1950s for about six miles to what is now Beltway 8. The land adjoining this straightened stretch is now Terry Hershey Park and owned by the Harris County Flood Control District.
Prompted by residents on that straightened stretch upstream, the Flood Control District recently completed a $350,000 study of Buffalo Bayou, looking at whether meanders or low bridges were blocking the flow.
Now it also happens that there are at least six major stormwater outfalls of varying sizes aimed directly at the opposite bank on this straightened stretch of the river, seemingly in violation of Flood Control standards, universal best practice, and common sense.
Outfalls Not Part of the Study
The Flood Control District study found that the meanders downstream of Beltway 8 were not blocking the flow during storms, as the flooding bayou naturally flows over and cuts across the meanders when necessary. The study also concluded that existing bridges over the bayou were not blocking the flow in any significant way, and in any case raising them would create only a minor, localized reduction in water levels.
But the study, conducted by the engineering firm Huitt-Zollars, did not officially look at the impact of the stormwater outfalls on the flow in the bayou during storms, Michael Tehrani, vice-president of Huitt-Zollars, said in a recent telephone interview.
Tehrani, who is nevertheless personally familiar with the outfalls as a frequent bike rider in Terry Hershey Park, said that he thought “one or two” of the outfalls might make a “small difference” in the flow when discharging during a storm. The angle of the outfalls had a “potential for erosion on the other side” but it was “not a big factor,” he said.
Asked if he thought the outfalls were in violation of Flood Control standards, he responded, “Who am I to say the outfalls are in violation?”
Are the Outfalls in Violation of Flood Control Standards?
The Harris County Flood Control District establishes the criteria used in approving stormwater outfalls for both the City of Houston and Harris County.
The current criteria, as described in the latest version of the District’s Policy, Criteria, and Procedure Manual, states that the “alignment of channel confluences and large pipe or box outfalls is critical with regard to channel erosion (scour) and energy losses caused by turbulence and eddies.” (HCFCD Interim PCPM 2019, p. 136)
The manual recommends a “small angle of intersection between the side and main channel to minimize erosion potential and energy loss,” i.e. turbulence and blocking the flow. Angles between 30 and 60 degrees are “generally satisfactory.” Angles between 60 and 90 degrees are “discouraged” but “permissible” if the velocity in both channels is less than 4 feet per second during a 100-year storm. Angles greater than 90 degrees “can cause severe hydraulic and erosion problems and are therefore not permissible.”
How Fast Does the Water Flow?
Buffalo Bayou is normally a slow moving stream. But how slow is it during a heavy rain?
According to engineer Tehrani, during a 100-year storm (on Buffalo Bayou currently defined as 16-17 inches of rainfall in 24 hours or 1.25 inches in 5 minutes) the velocity is 2 to 2.5 feet per second. He said he based that velocity figure on models from the Federal Emergency Management Agency (FEMA).
In the straightened, narrower section upstream of Beltway 8 the flow would be faster, closer to 2.5 feet per second, Tehrani said, and downstream the meandering flow would be slower, at the lower end of 2 feet per second.
But is that computer-generated model a true reflection of how fast the water really flows in Buffalo Bayou? Our river experts were skeptical. Their experience of the rain-swollen river is different.
Actual figures on velocity in our streams are difficult to come by, points out Jeff East, water surface specialist with the US Geological Service (USGS) Houston office. The USGS has long maintained gages (USGS spelling) measuring the depth and volume of flow on Buffalo Bayou and other Texas rivers and streams. But it was only after Harvey in 2017 that the agency installed a gage to measure the speed of the flow in Buffalo Bayou.
That new velocity gage is on Buffalo Bayou at Piney Point Road in west Houston. Gages at Piney Point (so named for the tall pines that grew there) are used by the Corps of Engineers to monitor the flow and water level downstream from the federal dams.
So How Fast Did the Bayou Flow During Imelda?
The velocity gage was in place during Tropical Storm Imelda on September 17-19, 2019. Overall that storm was classified as a 100-year storm, having dropped 21.1 inches during 24 hours on Harris County. Imelda was the third wettest storm in Harris County behind Harvey (28.6 inches in 24 hours) and Allison in 2001 (28.5 inches in 24 hours) since the district began keeping detailed rainfall records in the 1980s.
But Imelda dropped different amounts of rain on different parts of the county. Less rain fell on and around Buffalo Bayou during Imelda, resulting in a much lower, more frequent storm classification, ranging between a 5-year and a 10-year storm. At West Beltway 8 the 24-hour rainfall was 7.4 inches and at Piney Point it was 8.4 inches. Technically on this part of upper Buffalo Bayou, Imelda was a mere 5-year storm. (p. 16)
And what was the measured velocity in Buffalo Bayou at Piney Point during this relatively low-level rainfall?
Double the Speed with Half the Rainfall
During Imelda, on Sept. 19, 2019, the volume of water flowing past the Piney Point gage measured a fairly high 8,000 cubic feet per second (cfs). (Base flow during dry weather is about 150 cfs.) At that time the gage measured the velocity at 4.4 feet per second—nearly twice the speed estimated by engineering firm Huitt-Zollars for twice as much rainfall. (A 100-year storm is defined as 16.7 inches in 24 hours for this part of town.)
So it would seem that the velocity in Buffalo Bayou during a 100-year storm would greatly exceed the limit for a “permissible” outfall installed at 90 degrees or perpendicular to the bank. Or even at 60 degrees.
It should be noted that City of Houston stormwater pipes are required to carry rain runoff at a minimum velocity of 3 feet per second with the pipe flowing full, during a much smaller 2-year storm. (p. 135)
What’s the Impact?
Outfalls like this can back up the flow for as much as a quarter mile, reports geologist and river guide Tom Helm, also a Save Buffalo Bayou board member. Helm has paddled the bayou during storms (not recommended for the average person). Specifically Helm paddled through Terry Hershey Park all the way to downtown Houston during a heavy downpour that began April 27, 2009. During that unnamed storm, which fell heavily on the west side of town, the volume in Buffalo Bayou at Piney Point also reached nearly 8,000 cubic feet per second, a record at that time. It was flowing at around 4,000-5,000 cfs when Helm had his adventure. “Trying not to die,” as he put it.
“My observation has been that during flooding rainfall events, the stormwater outfalls above Beltway 8 act as hydraulic dams and back up the flow of Buffalo Bayou as much as a quarter mile upstream,” wrote Helm in an email. “This same phenomenon happens at the confluence of White Oak and Buffalo bayous.”
Bruce Bodson, a fisheries biologist, environmental lawyer, river runner, and founder of Lower Brazos Riverwatch (and also a board member of SBB) reports the same thing. “In my experience, when perpendicular storm drains are dumping heavily, they create a localized situation much like a rising tide. The water flows create a ‘water dam.’”
What Should Be the Standards?
Here are a few basic recommendations for outfalls as described by the Scottish Environmental Protection Agency:
- Outfalls should be placed on straight sections of rivers where there is less risk of erosion. (p. 25)
- The discharge should be in line with the flow of the river to reduce turbulence and erosion. Ideally the outfall pipe should be angled at 45° to the direction of flow. (p. 30)
- No part of the outfall structure should protrude beyond the line of the bank. (p. 30)
- The height between the outfall pipe and the riverbed should be minimized to help reduce erosion. (p. 30)
What Can Be Done?
After the Flood Control District presented the results of the meander bypasses and bridges study on Oct. 17, we contacted Alan Black, director of operations for the district, about the outfalls blocking the flow on Buffalo Bayou in Terry Hershey Park. Unaware of the right-angle pipes, he asked for photographs and we sent them.
Perhaps in the future, stormwater outfalls will be built differently.