Channel response and sediment yields in Brookeen Creek, Central Texas

摘要

Land and water management activities such as agriculture, soil conservation, and reservoirs for water supply and flood control, have caused channel degradation in many first and second order ephemeral streams that drain nearly 70 percent of the cohesive environments of the Blackland Prairie. Resurveyed historic cross-sections on Brookeen Creek, a 16 square mile agricultural watershed in Hill County, Central Texas, revealed a maximum channel incision of 7 feet during a 33 year period (1958-1991). Degradation induced channel bank failures are evident in the form of slumps and block failures. Channel incision has exposed the Taylor Marl bedrock along the entire creek. Comparisons of measured storm sediment with sediment yields calculated using the Modified Universal Soil Loss Equation (MUSLE) indicate that nearly 50 percent of the total watershed sediment yield may be contributed by channel loss. The average measured storm sediment yield was 4.4 tons/acre. The U.S. Army Corps of Engineers models HEC-1 and HEC-2 were used to quantify the hydrologic and hydraulic parameters to describe channel development. Bankfull discharges nearly doubled with the onset of agriculture in this region. Land area under soil conservation increased by 14 percent from 1943 to 1991, but did not alter peak discharges significantly. Modeled channel velocities for bankfull flows are erosive and in the range of 6 to 11 feet per second. According to the widely accepted conceptual model of channel process-response by Harvey and Watson (1986), Brookeen Creek has progressed from a Type I degrading stream in 1958 to a Type II degrading stream in 1991 and has entered stage III-IV. The study reach is currently aggrading, banks are still failing, and at least 1.5 feet of aggradation is required to establish an equilibrium slope and stable bed and bank conditions. Either grade control to reduce bank height and transport capacity or toe protection to reduce bank angles are needed. A combination of both treatments is recommended.