Sims Bayou: Articulating Concrete Blocks Enhance Erosion Control Installations

摘要

Houston's Sims Bayou had a history of flooding the neighboring community every 5 years. To combat the flooding, the Bayou needed to be deepened, widened, and stabilized, but options were limited. The improvements would need to prevent disturbance of highly erodible clays and silty sands during storm events. Due to the Bayou's urban setting, the channel width was restricted by right-of-way limitations making an earthen channel impracticable. Initial design concepts in 1986 focused on reinforced, cast-in-place channel lining and driven sheet pile drop structures for tributary protection along major portions of the 30.9 kilometer flood control project. In 1991, Bayou neighbors formed the Sims Bayou Coalition and voiced concerns regarding issues of aesthetics and habitat. The emerging concerns for the environment and the aesthetics of the Bayou were powerful enough that the Army Corps of Engineer Corps and the Harris County Flood Control District, in coordination with community stakeholders, developed a channel modification plan that allows for channel stabilization, erosion control, and a softer, more natural environment using articulating concrete blocks (ACBs). ACBs are preformed blocks that are placed individually or in cable-tied mats along the bed and banks of a channel. The blocks stabilize the channel but have open areas that quickly vegetate, forming a green blanket and providing more agreeable habitat for the local wildlife. The blocks proved to be an equitable solution for the main channel but an additional problem was created when the channel was deepened. The many lateral streams that feed the main channel were now much higher than the channel bed and they threatened to degrade, which would cause erosion of the upstream channels. The standard method for mitigating these threats involved driving steel sheet pile grade control structures into the lateral streambeds to stabilize the upstream channel and then paving the downstream area with concrete to handle the hydraulic forces created by the resulting waterfall. Ayres Associates was hired to design sloping drop structures for the lateral streams out of ACBs. Over the five drop structures, over $1 million was saved with the revised design. The designers incorporated a complex geometry that not only meets the functional requirements, but eliminates the brash contrasts and ecological deficiencies of the steel and concrete design. The gradual slopes create much less turbulence downstream of the structure, making the pad of concrete at the confluence of the lateral stream and the main channel unnecessary and the open cells of the ACB allowed for establishment of vegetation on the slopes.