(NON-)RECOVERY OF AN AGRICULTURAL STREAM FROM STRAIGHTENING AND DREDGING

In recent history, natural, meandering streams have been straightened and dredged to reduce flooding. While this practice can be effective in reducing flooding locally, it often results in the degradation of stream water quality and aquatic ecosystems. A straighter channel inherently increases the stream gradient, which could increase flow velocity, shear stress, and potentially downstream sediment yield. Studies have shown that straightened, channelized streams often begin to return to a meandering pattern 35-50 years post-channelization. Yet cross-sectional surveys and air photo analysis of the stream reach in this study indicate little to no observable trend of the stream returning to a meandering planform over 50 years after channelization. We used the Hydrologic Engineering Center’s River Analysis System (HEC-RAS) 2-D surface flow modeling software to simulate flood events over the stream reach’s Digital Elevation Model (DEM) to explore the potential shear stress experienced by the stream and a shear vane to measure the shear strength of the stream banks. This data provides constraints on the driving and resisting forces that have allowed the stream to maintain its straightened pattern over time. To model how the stream would respond to the same flood event pre-channelization, we manipulated the stream reach’s DEM to take a more meandering path based on meander scars embedded in the DEM terrain and repeated the same HEC-RAS flood event simulation used in the present-day stream simulation. With a 25 m³/s discharge, we found that the shear stress was as high as 18 Pascals in the present-day straightened stream simulation and the highest shear stress in the meandering sections of the manipulated DEM simulation was 3.6 Pascals, likely because of reduced channel-floodplain connectivity in the modern straightened stream. Generally, shear strength decreases from ~60 kPa upslope to ~10 kPa downslope on either side of the streambank in the present-day stream. Ongoing work involves comparing shear strength, substrate particle size, and simulated shear stresses between the straightened stream and a nearby unmodified analog stream. Our analysis suggests that commonly estimated timescales (35-50 years) for stream recovery from hydrologic modifications might not apply to all landscapes and might be overly optimistic.