May 25, 2018

Growing Pains

A Texas university turns a storm drainage problem into an amenity

Erosion control managed a storm water drainage problem
The restored stable stream included an armored channel, pool and riffle grade control, and gentle side slopes.

Texas A&M University is experiencing a growth spurt. The university, founded in 1871 in what is now the city of College Station, Texas, has witnessed a 30% increase in gross square footage of development during the past 15 years alone.

This especially is true with the west campus-the formerly agricultural pastures and fields west of Wellborn Road. This area is drained, almost entirely, by White Creek, running largely through steep-sided, heavily wooded ravines. The increased runoff from the rapid development has taken a toll on this natural corridor. The stream bed has cut deep below the natural flood plain, and there are bank stability concerns all along the stream. In some places, bank failures have damaged utilities and are threatening the foundations of structures.

Bracing for Impact

As pasture and cropland was converted to buildings and parking lots, the rate of water flow in the watershed increased significantly, and streets and storm sewers concentrated the flow. The impact to White Creek was greatest at storm sewer outlets, where erosion has cut the stream bed down 10 ft or more below its pre-development grade.

The stream erosion and resulting bank instability has impacted campus infrastructure:

  • Numerous water lines, sanitary sewers and storm sewers have broken or are at risk of breaking in places where they cross, connect to or run beside the stream.
  • In some places, the unstable banks threaten buildings, bridges, parking areas, streets and trails.
  • The banks of the stream behind the George Bush Library have been repaired twice and are in need of repair again, as the hillside between the stream and Barbara Bush Drive continues to slide. The library pond also is at risk as the erosion cuts into its embankment.
  • The containment of a former solid waste landfill was threatened when the stream meandered into the side wall. Emergency reconstruction and armoring have been followed with permanent slope stabilization and stream modifications.
  • The bank supporting propane tanks at the Brayton Fire School was eroded to a shear vertical escarpment. Stream adjustments have led to deposits and aggradation at the base of the cliff, which will be followed with structural measures to divert the flow away from this outside bend.
  • The abutment of the bridge to the wastewater treatment plant is at risk of becoming flanked by the stream, requiring further bank protection.

Increased flows in White Creek impact more than the built environment. The university designated a green belt area along the stream as an irreplaceable remnant of the natural, forested bottom land, with flora and fauna that should be preserved. The existing vegetation helps stream stability, but as the flows increase and the stream cuts deeper below the natural floodplain, the erosive energy increases, taking down trees and structures as banks fail and the channel deepens.

Storm water drainage problem in Texas

White Creek bend at a former landfill site suffered from inscreased local development.

Smart Solutions

The Texas A&M Utilities and Energy Services Department is taking corrective measures to restore and retain White Creek’s natural state and protect the infrastructure from further damage. The measures include stream and bank repairs and stabilization using natural restoration principles and regional detention. The goal is to preserve and enhance the natural resource by working with nature rather than trying to fight it.

The university began addressing the White Creek erosion in 2014 with a project to stabilize the stream where it passes through a new campus attraction called The Gardens at Texas A&M University. White Creek is an integral feature of this 40-acre teaching garden, running for several thousand feet among the gardens, amphitheater and wetlands. The stream restoration involves engineered rock riffles, flow diversion weirs, bank stabilization, and erosion-prevention features using stone and vegetation. A water control structure at the downstream end of the gardens attenuates high flows, spreading the stream into the reconnected floodplain.

Rather than containing the creek in an underground storm sewer, the creek is being restored along its remaining length to its natural condition, with pools and riffles to stop head-cutting, flow-training weirs to stop bank erosion, re-sloping the stream banks to restore the natural flood plain and provide more conveyance for high flows, and regional detention to attenuate extreme storm events.

The drainage plan, designed to accommodate the anticipated growth in the west campus, centers around the construction of four detention basins. These regional detention basins eliminate the need for smaller basins at each new building. White Creek #1 and White Creek #2, because they are limited in volume, serve mainly to reduce stream velocities and absorb energy to prevent further damage. Research Park and White Creek #3 have enough volume to provide future growth without increasing runoff. White Creek detention basin #3, on the west side of the Harvey Mitchell Parkway, is being designed to meet planned development needs.

The water control structures for detention basins White Creek #1 and White Creek #2, located on one of the most severely eroded branches, provide grade control, as well as detention. Grade control structures are being designed to stabilize the stream bed, stepping the stream down the steep gorge in a series of armored riffles and pools. A rock-hardened riffle will drop the water from the upper pool, to the lower pool and from the lower pool to the existing channel. At the downstream end of each riffle, a stilling pool will absorb the energy from the waterfall.

The Research Park detention basin will detain storm water before it enters the creek. The 80-acre-ft reservoir will accept the runoff from much of the planned development in the western area of the west campus, and will allow up to 134 acres of additional development without increasing the peak discharge from a 25-year, 24-hour storm.

Bank stabilization measures involve cutting the banks back to gentler slopes and planting grasses, forbs, shrubs and trees to hold the soil. Areas that will experience high velocity are armored with large stones or riprap. Bend way weirs are constructed from riprap or large stones to divert the flow away from eroding outside bends. The drainage control measures at Texas A&M are a good start to better manage a valuable irreplaceable resource. 

About the author

Dan Miller, P.E., is senior project manager and principal water resources engineer for Stanley Consultants. Bob Henry, P.E., is manager for technical services, utilities and energy services department, for Texas A&M University. Miller can be reached at 563.264.6304, and Henry can be reached at 979.862.4604.