Christopher S. Thaxton

Path sampling method for modeling overland water flow, sediment transport, and short term terrain evolution in Open Source GIS

A path sampling method is proposed for solving the continuity equations describing mass flows over complex landscape surfaces. The modeled quantities are represented by an ensemble of sampling points which are evolved according to the corresponding Green function. The method enables incorporation of multi-scale/multi-process treatments. It has been used to develop simulation tools for overland shallow water flow and for sediment transport. The spatial pattern of sediment flow and net erosion/deposition is modeled using the closure relationship between sediment transport capacity and detachment developed for the USDA Water Erosion Prediction Project. The tools were recently implemented as modules in Open Source GRASS GIS. Their application is illustrated by the study of impact of land use and topography change on overland flow and sediment transport at North Carolina State University campus.

Hydrodynamic assessment of various types of baffles in a sediment retention pond

We assessed the relative improvement to the sediment trapping effectiveness of a permanent-pool sediment retention pond due to the installation of baffles composed of different materials commonly used on construction sites. A suite of experiments was performed at the Sediment and Erosion Control Research and Education Facility (SECREF) at North Carolina State University in which an acoustic Doppler velocimeter was used to record steady-state flow velocity and signal-to- noise ratio data. The data was gathered at 25 grid points at two depths within the pond for three different fixed input flow rates. The free flow maximum mean velocity in the pond, averaged over all input flow rates, was reduced by roughly 75% due to the presence of baffles composed of jute germination biotextile backed by coir fiber. Baffles made from a standard tree- protection fence, folded and tied together into three layers to reduce pore size, reduced the free flow maximum mean velocity by 65%, while baffles made of standard silt fabric fence reduced the free flow maximum mean velocity by 55%. A similar trend in the reduction of the signal-to-noise ratio along the length of the pond confirmed that the jute/coir baffles most effectively reduced the concentration of turbulent density fronts over that of the tree-protection fence or silt fence baffles, or free flow. In addition, analysis of the transverse velocity variance and vertical velocity gradients for each experiment further demonstrated that the jute/coir baffles most effectively diffused inflow momentum along the width and depth of the pond. The results of our analysis were used to calculate a sediment trapping efficiency based on Stokes settling. The minimum grain size captured would range from 30 to 42 microns with jute/coir baffles, compared to 68 to 86 microns for free flow.

Sediment capture effectiveness of various baffle types in a sediment retention pond

The relative sediment trapping effectiveness of a permanent-pool sediment retention pond was assessed due to the installation of baffles composed of different materials commonly used on construction sites. A suite of experiments was performed at the Sediment and Erosion Control Research and Education Facility (SECREF) at North Carolina State University in which an acoustic Doppler velocimeter was used to record steady-state flow velocity data at 50 grid points within the pond at three steady input flow rates. Hydrodynamic data were taken for free flow and for three different baffle materials: jute germination blanket backed by coir fiber, standard tree protection fence, folded and tied together into three layers to reduce pore size, and standard silt fabric with weirs. The experiments were conducted with a characterized soil injected upstream at a fixed rate with sampling at the outlet. At the completion of each baffle experiment, particle size distribution was determined for sediment deposits at fixed points in the pond bed. Analysis of the hydrodynamic data suggests that all baffles greatly reduced and diffused flow compared to an open pond. The jute/coir baffle outperformed a standard silt fence with weirs and a triple layer of tree protection fence. Results from soil composition analysis and exit turbidity measurements per baffle configuration confirmed that the jute/coir baffle was the most effective in improving sediment retention in the pond.

Hydrodynamic and Sediment Capture Assessment of Various Baffles in a Sediment Retention Pond

We assessed the relative improvement to the sediment trapping effectiveness of a permanent-pool sediment retention pond due to the installation of baffles composed of different materials commonly used on construction sites. A suite of experiments was performed at the Sediment and Erosion Control Research and Education Facility (SECREF) at North Carolina State University in which an acoustic Doppler velocimeter was used to record steady-state flow velocity data at 50 grid points within the pond at three steady input flow rates. Hydrodynamic data was taken for three different baffle materials and for free flow. The experiments were repeated with a characterized soil injected upstream at a fixed rate. At the completion of each baffle experiment, particle size distribution was determined for sediment deposits at fixed points in the pond bed. Analysis of the hydrodynamic data suggests that all baffles greatly reduced and diffused flow compared to an open pond. The jute/coir baffle outperformed a standard silt fence with weirs and a tree protection fence (tripled over to reduce porosity). Results from soil composition analysis, exit turbidity measurements, and estimates of the captured volume per baffle configuration confirm that the jute/coir baffle is the most effective in improving sediment retention in the pond.

VERTICAL SORTING AND PREFERENTIAL TRANSPORT IN SHEET FLOW WITH BIMODAL SIZE DISTRIBUTIONS OF SEDIMENT

The vertical sorting of grains that occurs during the process of sheet flow largely has been ignored in parameterized models for sediment transport. Here we performed computer simulations of sheet flow transport with bimodal distributions of coarse grains under forcing from idealized nearshore waves. The results demonstrate that the mobile bed rapidly experiences vertically segregation of grains by size, where the larger grains migrate to the top of the mobile bed, while the smaller grains percolate to the bottom. Consequently, the larger grains transport at a rate disproportionately higher than their mass fraction compared to the smaller grains.

SIMULATIONS OF DISTRIBUTED WATERSHED EROSION, DEPOSITION, AND TERRAIN EVOLUTION USING A PATH SAMPLING MONTE CARLO METHOD

We present a new GRASS GIS module r.terradyn that evolves a given terrain using sediment flux information provided by the SIMWE (SImulated Water Erosion) GRASS GIS modules r.sim.water and r.sim.sediment originally developed by Mitas and Mitasova (1998). SIMWE is a distributed, bivariate, steady-state watershed scale sediment erosion, transport, and deposition model that employs a path sampling Monte Carlo method in which erosion, transport, and deposition conditions are treated as a continuous field, resulting in fully distributed erosion/deposition patterns.. Module r.terradyn modifies the original digital elevation model (DEM) over time steps, each corresponding to a single rainfall event, which is then used as the input DEM for subsequent SIMWE and r.terradyn iterations. New techniques were derived that include the application of a gravitational diffusion term, an approximate Neumann boundary condition routine for use with GRASS GIS module r.slope.aspect, a comparative band-pass filter for numerical stability of the iterative feedback system, and a simple rainfall excess calculation methodology derived from accumulated runoff curve number tables that enables spatially distributed infiltration. Application of r.terradyn to a sample watershed demonstrates results for distributed land cover and infiltration and for various soil types. Terrain change impact from disturbed areas is also presented. Preliminary comparisons to field observations and total discharge data are currently being used to calibrate model parameters. Verification of the model is still ongoing as data becomes available. The influence of grain size dependent transport mechanisms on short-term and long-term topological changes induced by human impact, such as mining and construction, may lead to the determination of the optimum location, size, and frequency of control measures to more cost effectively meet emerging TMDL requirements.

Integrated nearshore wave amplitude model for use on portable devices

We present the IN-WAM system which provides the user, in real time and in remote locations, predictions of the wave heights and wave shapes of ocean waves that are incident on a given coastline. The system is executed by a user in the field on a handheld computer which provides a simple GUI for model setup, execution control, and results display. IN-WAM utilizes a proven monochromatic wave model REFDIF-I. Once set up with coastline bathymetry data, IN-WAM automatically obtains real time deep water buoy wave data and coastal wind data for model support. Preliminary comparison of predictions to observations show that wave heights at breaking are overpredicted by an average of 20%, but are overpredicted consistently across all test sites. Wave breaking type (plunging or spilling) are almost always predicted correctly. The successful design and implementation of IN-WAM demonstrates that the technology exists to provide support for recreational, scientific, and military decision making in remote locales where conventional access to such services is unavailable.