Restoration of an incised channel in southeastern Nebraska

摘要

South Table Creek is a small perennial, sand-bed stream in southeastern Nebraska, and is a tributary to the Missouri River. Its physical characteristics are typical of thousands of miles of small streams in the upper Midwest. These streams, and their watersheds, have been manipulated to the point that they have become unstable, downcutting into highly erodible loess deposits. The specific project reach, located on the Arbor Day Farm near Nebraska City, Nebraska, was typical of an entrenched system that lacked a functional floodplain. Channelization of the project reach in the early 1970's reduced its length by roughly 33 percent, and resulted in a second episode of incision and channel destabilization. This recent phase of incision created a deep, narrow channel with vertical banks that exceeded the critical height for soil stability. The restoration effort included reestablishment of a meandering planform and bounding floodplain based on historic aerial photography and current valley topography. Planform design was predicated on the need to establish a stable channel slope and appropriate hydraulic geometry. In absence of flow data, a locally derived design discharge was validated by comparing sediment transport rates for three cross sections that represented successive stages of incised channel evolution. Channel geometry was then developed using the design discharge to meet target sediment transport conditions. Streambank design and channel stabilization measures incorporated numerous bioengineering techniques. Materials used for bank toe protection included straw bales, wood logs, stone, root wads, and various combinations of these materials. Upper bank bioengineering included coir fabric wrapped soil lifts, and live willow fascines, posts, and poles planted in a number of different configurations. The presence of loess material, shale bedrock, and perched water table conditions made channel reconstruction especially challenging. The anticipated effects of this restoration project are increased channel stability, improved water quality, and improved terrestrial and aquatic habitat due to a restored connection between the channel and its floodplain.