Erosion and Sediment Control
Gabion Structures for Sediment Control
Changes in land use and climate have resulted in increased runoff and surface erosion from the upper areas of watersheds, directly affecting the water quality. Some of the elements that greatly affect the water quality include sediment and other pollutants transported to the water bodies through natural erosion over an exposed ground surface, and stormwater runoff from the active construction site as well as manmade disturbance to the stream bank. Erosion is a natural process that contributes to sediment production and transport. Erosion can take place within the water path system itself and outside that such as new development and construction sites. The most effective sediment control is accomplished by addressing the root causes of the erosion at the source. However, this may not always be technically and economically feasible, and sediments are transported to the waterways. Transported sediments can significantly reduce the storage capacity of water reservoirs and negatively impact the water quality. Many research studies in river basin reported that sediment pollutant is one of the leading causes of impaired water quality in rivers and lakes nationally. It is recommended to mitigate sediment transportation into water bodies by constructing one or several traverse structures to retain the sediments before they reach the receiving water body. Gabion sediment control structures are among the most common measures used for sediment control application. These structures are constructed across manmade channels or natural stream beds to flatten the gradient of the channel thereby reducing water flow velocity and to allow sediment accumulation behind the gabion structure. Gabions are double twisted wire mesh container of variable sizes, uniformly partitioned into internal cells by diaphragms positioned approximately 3 ft centers, interconnected with other similar units and filled with stone at the project site to form flexible, permeable, monolithic structures such as check dams, forebay walls, and drop structures for sediment control applications. The use of gabions enhances the efficiency of sediment retention and filters water through the 30% voids inside. The specific trapping efficiency varies based on the surface area, depth of dead storage, and the particle size distribution and concentration of sediment entering the device. In order to improve sediment trapping efficiency, filter fabric or graded stone backfill can be placed on the upstream of the gabion. This paper highlights the design and construction aspects of gabion sediment control structures - check dams, forebay walls, and drop structures, and discuss their contribution towards water quality improvement.