Location: Watershed Physical Processes Research2012 Annual Report
1a. Objectives (from AD-416):
The objective of this effort is to assess the processes associated with ephemeral gully development through the study of: ephemeral gully evolution pertaining to ephemeral gully widths and networks; soil resistance to gully erosion; and the effect of agricultural practices on gully erosion across a range of temporal and spatial scales including the influence of above and below ground biomass. These studies will lead to new or enhanced algorithms for use in ephemeral gully erosion models. Ephemeral gully erosion represents an important and often dominant sediment source within watersheds in the U.S. and worldwide that is often overlooked when evaluating the effect of conservation practices in controlling erosion.
1b. Approach (from AD-416):
Develop and execute novel experimental programs to understand the processes leading to the initial growth and development of gullies under varying hydrologic and topographic conditions. Define the key pedologic, hydrologic, and hydrodynamic parameters that control the magnitude, morphology, and rate of soil loss, gully erosion, and landscape degradation due to gully development. Develop theory and equations to predict soil loss and gully erosion on hillslopes and agricultural fields under different management practices and integrate these into USDA watershed and soil erosion models.
3. Progress Report:
The Big Sioux River, SD, has relatively high suspended-sediment loads, and these loads can exceed the state’s mandated maximum values for suspended sediment. To reduce these loads in the immediate future, engineered logjams will be placed along the streambanks where excessive erosion is occurring. It is hypothesized that by stabilizing key streambank locations, a measureable reduction in suspended sediment efflux would occur, thereby bringing the stream channel into compliance for longer periods of time. To address this issue, a physical model has been constructed at the University at Buffalo specifically to simulate flow within the Big Sioux River. Engineered log jams of various types and spacing will be placed into the channel, and the turbulent flow around these structures, and the forces acting on the structures, will be quantified in both clear-water, fixed-bed and mobile-bed conditions. These results will be shared with the USDA to validate and verify numerical models also to be used to facilitate the emplacement of jams along the Big Sioux River to maximize their effects. It is envisioned that the experimental results will be used to develop design criteria for the use of engineered log jams for bank protection and stream restoration. Soil erosion remains the primary cause of soil degradation worldwide, and concentrated channels such as rills and gullies are the principle causes for this erosion. Yet little information currently exists to predict the location, magnitude, and spatial evolution of rill networks on hillslopes and agricultural regions. Data from previously conducted experiments using a soil-mantled flume and subjected to simulated rain and downstream baselevel lowering were used to quantify the growth, development, and spatiotemporal evolution of rills and rill networks. Results show that: (1) headcuts formed by baselevel lowering were the primary drivers of rill incision and network development, and the communication of this wave of degradation occurred very quickly and efficiently throughout the landscape; (2) rill networks extended upstream by headcut erosion, where channels bifurcated and filled the available space; (3) rill incision, channel development, and peaks in sediment efflux occurred episodically, linked directly to the downstream baselevel adjustments; (4) flows were supply-limited and most of the sediment efflux was genetically linked to headcut development and migration; (5) hydraulic geometry relationships varied over time and between experimental runs but agreed well with previous findings; and (6) rates of headcut migration were well correlated to rates of overland flow. These findings have important implications for the prediction of soil loss, rill network development, and landscape evolution where headcut erosion can occur.