Buffalo Bayou is one of Houston's primary stormwater conveyance waterways, extending from the Barkers Reservoir in west Houston, through the downtown region, to the Houston Ship Channel. The bayou is historically susceptible to ongoing natural channel erosion. Where the bayou flows under the Sam Houston Tollway, which was constructed in the mid-1980s, this continuing erosion process began to jeopardize the integrity of this essential roadway system.
The channel erosion is exacerbated by a combination of disturbed sandy soil strata and turbulent flow conditions created by the confluences of three major conveyance systems directly beneath the tollway — the meandering Buffalo Bayou; a major tributary to the bayou, Rummel Creek; and a 10-foot by 10-foot storm sewer outfall. In addition, the wide main lane bridge structure prevents sunlight from promoting the growth of a vegetative root mass that could help stabilize the channel banks. Further compounding the erosion issues are several locations where surface runoff erodes the vegetated and non-vegetated channel side slopes.
Moreover, the area around and near the bridge crossing serves the Terry Hershey Park, a popular park operated and maintained by Harris County Precinct 3. The park has one of the most heavily utilized trail systems in Harris County, encompassing more than 500 acres of park land and more than 12 miles of hike-and-bike trails. While the park was easily accessible to the people north of the bayou, potential park users south of the bayou had to traverse unsafe terrain with dangerously steep side slopes and an unprotected path along and across the busy tollway frontage road to access the park's trail system. This project was successful in combining the needed erosion protection measures with trail improvements that provided safe access to Terry Hershey Park for residents south of the Bayou.
Concerned at the rate of erosion near the supporting foundations to the tollway, the Harris County Toll Road Authority (HCTRA), in February 2008, commissioned Lockwood, Andrews & Newnam Inc. (LAN), a planning, engineering, and program management firm headquartered in Houston, and its sub-consultants to develop an environmentally compatible and aesthetically pleasing erosion control plan that collectively addresses and corrects each of the individual erosion problem areas, preserves and promotes existing natural habitats, and provides improved access for park visitors.
"The goal was to implement an all-inclusive plan that would protect the bridge and embankment foundations, prevent future erosion, and improve connectivity to the park for the surrounding neighborhoods," said Derek St. John, P.E., CFM, LAN's associate and team leader.
To stop further erosion and repair existing erosion damage, the LAN project team first set out to determine the different causes of erosion. The project team collaborated with the HCTRA, Harris County Flood Control District (HCFCD), the city of Houston, Harris County Precinct 3, and the U.S. Army Corps of Engineers - Galveston.
Data collected from multiple site visits, as-built drawings, aerial photos, digital terrain models and planimetric mapping, computer models, flood insurance studies, and Tropical Storm Allison Recovery Project documentation was evaluated both during and after storm events. The team also witnessed and evaluated the aftermath of a record-setting event on Buffalo Bayou in April 2009, when water surface elevations exceeded the Federal Emergency Management Agency's 100-year limits.
LAN engineers also analyzed a detailed velocity distribution in the channel and overbank areas to identify critical project areas that required more robust protection measures. After an exhaustive review, the following four causes of erosion were found:
- surface runoff erosion,
- high-velocity channel erosion,
- high-frequency event toe erosion, and
- erosion due to turbulent conditions
Based on the analysis, LAN developed an integrated erosion control plan that would address all four causes of erosion with complementing protection measures. Natural and sustainable improvement measures were thoroughly reviewed but were determined inadequate given the limited right-of-way and the susceptibility to extreme erosion conditions. Highly durable erosion control measures were selected to counteract the harsh conditions and to promote the longevity of the project area. Each erosion control measure was designed and implemented to correct and/or prevent a specific erosion failure mechanism, as well as to build layers of protection against the destructive forces of nature, providing insurance against the potential for costly bridge and roadway embankment repairs. The various erosion control measures implemented include:
Gabion mattresses — A 12-inch-thick gabion mattress was constructed at key locations along the bayou at a minimum slope of 3H:1V. The gabion mattress acts as one continuous structure with sufficient weight to adequately protect against the high-velocity turbulence created by the flows in Rummel Creek and the storm sewer outfall.
Gravity gabion walls — In areas with steep side slopes and limited ability to lessen the slopes, gravity gabion walls were built. Constructed at a 1H:1V slope with approximate width varying from 13.5 feet to 16.5 feet, the gabion walls extend from 6 feet below the channel flow line to the top of the bank.
Cantilever retaining walls with drilled secant shaft — Constructed adjacent to a critical exit ramp and an extremely steep channel side slope section, this measure serves two functions: protecting the existing tollway ramp structure against potential future slope failure and preventing channel bank surface erosion by supporting a multi-use trail that captures and safely discharges surface runoff. With the secant wall shafts extending well beyond the flow line of the channel, the wall was designed as a secondary line of defense to support the load of the exit ramp in the event of a side slope failure along an armored side slope.
Boulder rip-rap — This measure was implemented to prevent loss of soil to channel bottom and side slopes in high-velocity zones. The riprap, varying in thickness from 24 to 72 inches, also protects against high-frequency event toe erosion.
Mortar-embedded rip-rap — This measure prevents erosion from concentrated sheet flow runoff falling from slotted rails on the edge of the elevated tollway structure. Constructed at a 3H:1V slope, the grouted 18-inch concrete riprap stretches from the bank toe along the edge of the bridge to the 100-year water surface elevation.
Articulated concrete blocks (ACB) — Designed for park areas to be aesthetically pleasing, this measure was implemented at an existing channel access point to Buffalo Bayou along the north bank. Constructed from the bank toe to the edge of the existing park trail edge at a minimum slope of 4H:1V, ACBs prevent soil loss in high-velocity zones and provide a smooth surface for launching boating or water equipment.
Energy-dissipating drop structures and baffles — Constructed at the end of the existing 10-foot by 10-foot storm sewer outfall, this measure minimizes channel turbulence by reducing the discharge velocity at the outfall to Buffalo Bayou.
Vegetation — In addition to these man-made improvements, another critical erosion control measure was maintaining and preserving the natural vegetation and landscaping on existing side slopes. Extensive efforts were undertaken to preserve the maximum possible amount of natural vegetation. The repaired areas were replanted with native trees and grasses. Additionally, areas beneath the wide main lane bridge structure were vegetated with shade-tolerant plants.
Another key objective of this project was incorporating betterments for the Terry Hershey Park system that complement the erosion control improvements. Connectivity to a previously constructed pedestrian/bike bridge was provided for people from the south side of the bayou to the park system, as well as for maintenance and security access. A safe trail system was constructed with guard rails to protect pedestrians from steep drop-offs. Additionally, the site was improved aesthetically through the use of integrated and complementary material textures and extensive landscaping, much of which was designed to serve as an erosion control measure through root stabilization.
"After safely providing access to the park for the citizens south of the bayou, there has been a significant increase in the number of people who use this wonderful park facility" said St. John.
Aside from the complexity of designing and tying together multiple erosion control measures to provide a cohesive level of protection, challenges associated with this project included overcoming extreme constructability issues and improving the overall stream habitat. Constructability of the prescribed erosion control measures was taken into account early in the project process and was a consistent theme throughout the design. Erosion control measures were selected — and in some cases eliminated — based on their ability to be constructed cost effectively and their potential impact on the environment.
Of particular importance was the constructability of the drilled shaft secant wall. Access and sufficient working space for the necessary construction equipment was reviewed in detail, resulting in key modifications to the preliminary design. This improved the constructability, minimized the use of expensive and non-standard construction equipment, and minimized the destruction of existing natural vegetation. Due to these design reviews in the preliminary stages of the design process, the project was built with a small number of change orders that deviated from the design.
Another challenge encountered by the team was the sustained release of water from Addicks and Barker reservoirs upstream of the project site. To maintain safe working conditions, the project team worked with the U.S. Army Corps of Engineers to coordinate releases. When discharges from the reservoirs were not being released, a series of cofferdams were used to direct water away from the construction site, allowing workers to work safely with heavy equipment.
Lastly, LAN developed a construction strategy that preserved the natural habitat while improving overall water quality. Maintaining habitat connectivity upstream and downstream of the project was a critical environmental consideration, as was the construction of deep pools and riffs that simulate natural stream conditions. The use of boulder-style rip-rap to protect the channel bottom allows for rapid siltation and habitat restoration. Water quality was improved by reducing soil displacement and sediment transport from project site.
The $3.48 million Buffalo Bayou erosion control project, completed in September 2011, prolonged the life expectancy of one of the most traveled tollways in Harris County, preserved the existing habitat, and improved pedestrian usability, all while maintaining minimum disruption to the surrounding area. The project, which reflects many of the up-to-date methods of erosion control, is an excellent model for future erosion control projects.
Jay Srinivasan and Stephanie T. Martin work for LAN, as technical writer and marketing specialist, respectively. This article was written in collaboration with Derek St. John, P.E, CFM, associate, team leader for LAN; and Brian Whitney, P.E., CFM, project manager for LAN.