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.