Semester

Spring

Date of Graduation

2024

Document Type

Thesis

Degree Type

MS

College

Eberly College of Arts and Sciences

Department

Geology and Geography

Committee Chair

Charles Shobe

Committee Member

Brenden McNeil

Committee Member

Jeffrey Skousen

Abstract

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, Deckers Creek, indicate little to no observable trend of the stream returning to a meandering planform over 50 years after channelization. To determine the resisting forces that have maintained its stream pattern, I surveyed and sampled cross-sections of the stream for shear strength and particle size analysis. I 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. I then chose a meandering, less-modified analog stream (Laurel Run) to survey and sample for the same parameters as the channelized stream. This data provides constraints on some of the driving and resisting forces that have allowed the channelized stream to maintain its straightened pattern and the meandering stream to maintain sinuosity over time. The results of research indicated high shear strength and more fine-grained material along the banks of Deckers Creek. The data suggests that the straightened stream was dredged enough to lower the water table and desiccate the cohesive banks, which promotes higher shear strength and resistance to natural stream recovery. The streambank profile of the wider, shallower, meandering Laurel Run is much closer to the water table, providing wetter, sandier bank material, which lends itself to channel adjustment over time. The shear stress modeling suggests that the channelized stream has a higher shear stress potential than the meandering stream in a two-year flood, yet it still hasn’t begun recovery in over 50 years. To model how the channelized stream would respond to a flood event pre-channelization, I manipulated the channelized 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. In a two-year flood, the manipulated, meandering DEM produced much lower shear stress estimates at, and downstream of, tight meanders than the present-day channelized stream. It appears that the channelized stream has entered a regime in which, though dredging and straightening have increased shear stresses exerted on the channel boundary, simultaneous decreases in channel boundary erodibility due to dredging may offset the increased stress and prevent the channel from adjusting towards its pre-disturbance form. My 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 in this case.

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