Most Houston streets are designed to be part of the city’s drainage system. During a typical rainfall, water flows through storm sewers under the streets or in roadside ditches to a drainage channel such as a bayou. When the capacity of the storm sewers or ditches is exceeded, the streets themselves will hold the water—they will flood—until the storm sewer or ditch has additional room to drain the water.

Mitigating Flooding

Enhancements to streets and waterways have done a good job of managing normal rainfall. But the aging drainage infrastructure was designed for heavy rains, not 100-year storms. Under such conditions, when rainfall exceeds several inches per hour for several hours, many areas flood.

Heavy rainfall drains through the streets, storm sewers, creeks, and bayous to larger common waterways, such as Brays Bayou. In Harris County, these waterways all flow southwest and eventually empty into Galveston Bay. Areas that drain into common waterways are called watersheds. Harris County has 22, each with its own flooding problems and unique challenges.

Willow Waterhole Bayou lies in the Brays Bayou Watershed. Rainfall within the watershed’s 129 square miles flows into Keegan’s Bayou, Willow Waterhole Bayou, and other waterways and eventually drains into Brays Bayou. It then flows eastward into the Houston Ship Channel and out into Galveston Bay.

Houstonians who live in the Brays Bayou Watershed have been plagued by flooding since the area was developed. The U.S. Army Corps of Engineers undertook serious flood mitigation efforts beginning in the 1950s, straightening and lining many bayous with concrete, including Brays. Then in 1988, the Corps and Harris County Flood Control District established the Brays Bayou Flood Damage Reduction Project, or "Project Brays."

Project Brays
Houstonians who live in the Brays Bayou Watershed have been plagued by flooding since the area was developed. The U.S. Army Corps of Engineers undertook serious flood mitigation efforts beginning in the 1950s, straightening and lining many bayous with concrete, including Brays. Then in 1988, the Corps and Harris County Flood Control District established the Brays Bayou Flood Damage Reduction Project, or "Project Brays."

Project Brays has involved 75 separate projects, including widening and deepening 21 miles of the bayou so that it now moves 30% more water, modifying 32 bridges, and creating four major detention basins to receive stormwater spillover.

Brays Bayou Watershed

Detention Basins

Detention basins are large excavated areas connected to a bayou or channel. They are designed to store excess stormwater until the threat of flooding has passed and the water can safely drain back into the waterway. Project Brays created four basins that span 900 acres and can hold approximately 3.5 billion gallons of stormwater:

• Arthur Storey Park Stormwater Detention Basin (211 acres, 1.15 billion gallons)

• Mike Driscoll Park Stormwater Detention Basin (47 acres, 200 million gallons)

• Eldridge Stormwater Detention Basin (337 acres, 1.5 billion gallons)

• Willow Waterhole Stormwater Detention Basin (292 acres, 600 million gallons)

Willow Waterhole Detention Basin
The Willow Waterhole detention basin complex can detain up to 600 million gallons (1,865 acre-feet) of stormwater that would otherwise flood homes and businesses. Begun in 2004 and completed in 2018, the six interconnected wet-bottom lakes range in size from 10 to 110 acres and spread out over 300 acres. They were excavated deep enough that the lakes are the same elevation as Willow Waterhole Bayou and contain water year round. The lakes in the detention basin are also deep enough to sustain life, making them attractive for wildlife. Fish, birds, and many other small animals have been spotted along the trails.

Willow Waterhole

The six wet-bottom lakes spread out over 300 acres and hold up to 600 million gallons of water.

Two of the six compartments—Scout Lake and Heron Lake—are separated from Willow Waterhole Bayou by earth berms and concrete weirs. During high rainfall events, water from the bayou encounters control structure in the bayou. The control structure in the large concrete “bridge” downstream of Heron Lake has an opening to allow normal water flow to pass through. This structure causes stormwater to slow down and back up in the bayou. When the bayou reaches an elevation of 50 feet, the stormwater flows into the basin compartments over the weirs, preventing the water from continuing down the bayou.

The basins work passively; that is, floodwater moves in and out as a result of elevation differences, gravity flow, and a system of concrete weirs and culverts. The pipes between the basins and the bayou have flap gates on them that do not allow water to flow into the bayou until the bayou’s water has fallen. The weirs and control structures are concrete because strong currents would erode dirt and grass structures.

The Scout Lake weir, a slope in the concrete bank, separates the bayou and the lake. When storm water reaches a certain level in the bayou, gravity causes it to flow from the bayou into the basin. A similar weir is located in Heron Lake.

Willow Waterhole Bayou’s bottom of stream, its bed, at the Landsdowne bridge, is 43.70 feet above sea level. (Houston—one of the flattest cities in the U.S.—has an elevation of just under 50 feet above sea level.) The ground elevation at the Landsdowne gauge is 2 feet higher than at the Scout Lake and Heron Lake weirs. During a high water event when the gauge reads approximately 52 feet, this means that water from the bayou has begun going over the weirs into the lakes. (See the chart on the Harris County Flood Control District page.) The overflow weirs can handle rainfall rates of about 13 inches in 24 hours. More rain than that, such as during Hurricane Harvey in 2017, will cause the bayou water to flood the surrounding neighborhoods.

After a storm, as bayou water recedes, the compartments begin to empty slowly through culverts with flap gates that prevent water from flowing from the bayou into the lakes. The water flows out from all the lakes, which are connected by large box culverts under the roads, into Scout Lake and Heron Lake and then drains back into the bayou. The small size of the culverts with flap gates allows the water in the basin to drain slowly as the level in the bayou recedes.

Outflow culverts in Scout Lake and Heron Lake are designed to direct basin water back into the bayou as water in the bayou recedes.