C. G. Wilson

Conservation practice effects on sediment load in the Goodwin Creek Experimental Watershed

Water quality and aquatic habitat due to unstable stream channels and high sediment concentrations during storm runoff events are major environmental concerns on the 2,132 ha (5,266 ac) Goodwin Creek Experimental Watershed in north Mississippi. Effects of enrolling erodible lands in the Conservation Reserve Program (CRP) and instream grade stabilization structures were evaluated using measured rainfall, runoff, and sediment concentration data and model simulations. Signatures of naturally occurring radionuclides indicated that 78% of the total sediment load originated from channel sources. The change of land to a CRP-like state (reducing cultivated land from 26% to 8%) reduced erosion and runoff from fields and thus decreased total sediment concentration by 63% between 1982 to 1990. Simulations using the Fluvial Routing Analysis and Modeling Environment model indicated that mean sediment yields would increase from 15% to over 200%, depending upon location in the watershed, if in-channel structures were not present. The combined effect of the grade control structures and the change of lands to a CRP-state was to reduce sediment yields by 78% near the outlet of the watershed.

Stability Analysis of Semicohesive Streambanks with CONCEPTS: Coupling Field and Laboratory Investigations to Quantify the Onset of Fluvial Erosion and Mass Failure

AbstractThe overarching goal of this study is to perform a comprehensive bank stability analysis that is phenomenologically sound by considering both mass failure and fluvial erosion. The nature of this study is twofold. First, field and experimental analyses are conducted to generate data for channel cross-section properties, soil index properties, and mechanical and erosional strengths at two sites in a representative, midsize, midwestern stream in southeastern Iowa that is subjected to frequent flash floods and characterized by active fluvial erosion and cantilever failure. Second, the channel surveys and data obtained from the field and laboratory analyses are used as input parameters for an established one-dimensional, channel evolution model, namely, the conservational channel evolution and pollutant transport system (CONCEPTS, version 2.0, Langendoen and Alonso 2008), to estimate the factor of safety for mass failure (FSm) and fluvial erosion (FSf) and simulate the bank retreat as a result of eithe...

Quantifying Sediment Sources to the Suspended Load of an Agricultural Stream Using Radioisotopes

The goal of this study was to understand better the delivery of sediment to streams in small, intensively agricultural watersheds of the U.S. Midwest by determining the amount of sediment coming from the fields and stream banks during three consecutive runoff events. The natural activities of Beryllium-7 and Lead-210 in different source soils were compared with the corresponding activities of the suspended sediment collected in the stream during these events. Both a simple two end-member mixing model and a Bayesian model were used to determine the relative contributions from the source areas to the suspended load of each event. The two end-member approach suggested that ~60% of the sediment carried in the stream during the first event was eroded upland soils and was attributed to a “first flush” of readily available material from past events. For the second and third events, the amounts of eroded upland soils were ~34% and ~26%, respectively, because less material was readily available for mobilization. The two end-member model results compared favorably with the Bayesian model, which also incorporated Cesium-137 as a third tracer. Additionally, these results were confirmed with the clockwise hysteresis observed in the different events. During the third event, a flash flood, stream bank collapse was observed and bank retreat estimates from multiple methods compared favorably with the partitioning results. Quantifying sediment sources in watersheds will allow land managers to target more accurately areas where Best Management Practices (BMPs) are most needed to control sediment-related problems. INTRODUCTION Quantifying erosion and sediment delivery at the watershed scale has proven difficult due to high temporal and spatial variability of the different erosion processes occurring over a landscape (Church 2006), despite recent progress in understanding the mechanisms of erosion (e.g., Romkens et al. 2002; Govers et al. 2007). This problem is further exacerbated in agricultural areas where anthropogenic activities, including tillage (e.g., Van Oost et al. 2006) and channel straightening (e.g., Urban and Rhoads 2003) are added stressors. The need still exists to identify and quantify 1243 World Environmental and Water Resources Congress 2014: Water without Borders

Using tracers to derive sediment provenance after the occurrence of a 500-year flood in a Midwestern stream

In June 2008, catastrophic flooding of the Cedar River inundated a large portion of downtown Cedar Rapids, Iowa. As a result, floodwaters deposited a large amount of sediment throughout the city. The sediment origin is of intense scientific and public interest due to the quantity of deposited sediment and the potential presence of persistent organic pollutants (PCBs, chlordanes, and synthetic fragrances) attached to the sediment. The primary objective of this study is to determine the temporal and spatial patterns of deposited sediment. Two potential sediment sources have been identified: the bed of the Cedar River and the channel banks. We hypothesize that sediment from the two sources was distributed heterogeneously throughout the city. Therefore, samples were gathered from the two source areas, as well as from terrestrial areas within and adjacent to the flooded region. We will compare the deposited sediment gathered from the terrestrial areas with the sediment obtained from the source areas using natural biogeochemical tracing techniques. The tracing techniques will include an analysis of radionuclides (e.g., 7 Be, 210 Pb, 137 Cs), stable isotopes ratios (e.g., δ 15 N and δ 13 C), and elemental ratios (C/N). This comparison will allow us to determine the provenance of the sediment particles, which will assist us in developing a sediment budget for the segment of the Cedar River that flows through downtown Cedar Rapids. Further, we will identify the key processes affecting sediment delivery and redistribution that occurred during the flood. Sediment dynamics during floods is poorly described in the existing scientific literature. The methods established in this study will provide for engineers and researchers the ability to track the transport of sediments in similar floods.

A Comparison of Watershed Models in the Clear Creek, IA, Watershed

The Clear Creek, IA watershed (CCW), which drains to the Iowa River, experiences severe surface erosion due to a combination of high slopes, erodible soils, and extensive agriculture. Several computer models are currently available that can evaluate the magnitude of erosion occurring in the CCW at the watershed scale. Each model is inherently different resulting from unique combinations of possible assumptions and algorithms; moreover, each model is bounded by its own limitations regarding spatial scaling. Two public domain models were used in this study to evaluate the water and sediment fluxes from a sub-watershed in the headwaters of the CCW. The Water Erosion Prediction Project (WEPP) is a dynamic, process-based, continuous simulation, erosion prediction model that is most applicable to hillslope erosion processes, namely sheet and rill erosion. Simulation of the hydrologic and erosion processes in WEPP is best performed in small watersheds up to 2.6 km 2 . The ANNualized AGricultural Non-Point Source Pollution Modeling System (AnnAGNPS) is designed to evaluate hydrology, soil erosion, and nutrient pollution at the watershed scale. AnnAGNPS will process data on a single cell scale (0.16 – 1 km 2 ) but will integrate the information from these cells into sub-watersheds ranging from 200 to 1650 km 2 . The runoff, gross upland erosion, and the fluxes of water and sediment from the CCW sub-watershed were compared for 2-, 5-, and 10-year simulations assuming a single soil type and land-use throughout the catchment. These simulations were also compared to more detailed simulations from AnnAGNPS that implemented the various heterogeneities of soil type and land-use in the CCW. Predicted runoff was similar for both models; however, for the homogeneous simulations, WEPP produced higher upland erosion rates and sediment fluxes. The heterogeneous simulations for AnnAGNPS produced sediment fluxes similar to estimated fluxes from the watershed.

Goodwin Creek Experimental Watershed – Effect of Conservation Practices on Sediment Load

The Goodwin Creek Experimental Watershed, a benchmark watershed in the USDA-ARS Conservation Effects Assessment Project (CEAP), drains 2132 ha in the north central part of the state of Mississippi, USA. The watershed is characterized as having high sediment yield (13.2 t/ha/yr) and unstable channel substrate and banks. The effectiveness of management practices applied to the watershed will be evaluated as part of CEAP, and new practices and strategies for continued reduction in sediment loading will be explored using watershed computational models. Land use on the watershed has changed from 26 to 6 percent cultivated with corresponding increases in timber (26-38%) and pasture (48-55%) lands over the period of record. Annual concentrations of sediment have decreased from about 5000 ppmw in 1982 to about 2000 ppmw at the present. Sediment source tracking using naturally occurring radionuclides has indicated that channel processes are one of the main sources of sediment to the streams of the watershed. In addition to the reduction in sediment, a significant reduction has occurred in the relation between runoff and precipitation in the first part (April-July) of the land use year. Simulations using AnnAGNPS have been shown to favorably compare to the relative trends of the measured rates of runoff and sediment concentration except for periods of cultivation on agricultural lands. Enhancements or applications with advanced channel erosion models are needed to better reflect ephemeral gully and channel erosion.

Goodwin Creek Experimental Watershed - Assessment of Conservation and Environmental Effects

Goodwin Creek, a benchmark watershed of the Conservation Effects and Assessment Project (CEAP), drains 2132 ha in the north central part of the state of Mississippi. Drainage is westerly as part of the Yazoo River Basin, a tributary of the Mississippi River. Sediment yield rates (14.5 t/ha/yr) in the region are among the highest in the nation. Phosphorus and fecal coliform levels also exceed water quality standards. The effect of land use and management practices on erosion and transport of sediment and contaminants has been the major thrust of research conducted on Goodwin Creek and is an important component of the CEAP project. Analyses are in progress to evaluate the effects of conservation practices associated with channel and watershed management on sediment loadings throughout the watershed. Specific management practices which will be evaluated include channel bank vegetation, stream habitat improvement and management, grade and channel stabilization structures, and drop pipes. Studies to identify sediment sources are also in progress and will be coupled with measured flow and sediment data for comparison to simulations using a combination of the AnnAGNPS watershed model and the CONCEPTS channel-evolution model. Several management/climatic scenarios will be assessed using these two models to identify the most cost-effective suite of management practices to safeguard downstream water quality.

Alternative Tile Intake Design for Tile Drainage: A Case Study

The overall goal of this study is to demonstrate the effectiveness of an alternative tile intake (ATI) at reducing runoff, sediment, and nutrient loads from agricultural fields during extreme storm events through an ongoing demonstration project in the Clear Creek, IA watershed. An ATI is a best management practice (BMP) consisting of a modified gravel intake atop a layer of wood chips. Gravel intakes have been shown to be highly efficient at trapping sediment and sediment-bound particulates, like phosphorus, by enhancing settling through ponding and filtration, while the addition of the wood chips is to facilitate denitrification, similar to a bioreactor. Additionally, these intakes can help attenuate runoff relative to conventional intakes by reducing the flow rate of runoff into the subsurface. In this study, which will ultimately utilize a mixture of numerical, analytical, laboratory, and field methods, a three-part control volume is conceptualized consisting of the contributing hillslope, the ATI, and the subsurface pipe below the ATI. To date, the Water Erosion Prediction Project (WEPP) has been used to simulate the field conditions at the demonstration site for calculating runoff volumes and sediment fluxes to the ATI for different magnitude events. A physical model of the installed ATIs at the demonstration site is currently being used in the laboratory to quantify the saturated hydraulic conductivity and filter efficiency of different combinations of pea gravel and wood chips. These laboratory experiments will also be complemented with analytical exercises and infield monitoring of the installed ATIs. Preliminary measurements suggest that ATIs have a filtering efficiency of about 80%.

Bank Stability Analysis for Fluvial Erosion and Mass Failure

The central objective of this study was to highlight the differences in magnitude between mechanical and fluvial streambank erosional strength with the purpose of developing a more comprehensive bank stability analysis. Mechanical erosion and ultimately failure signifies the general movement or collapse of large soil blocks due to geotechnical instability and is the upper limit of streambank erosion. Conversely, fluvial erosion is the detachment of individual particles or aggregates due to the shearing action of flow and is the lower limit of streambank erosion. A total of 24 streambank samples from a representative stream in the U.S. Midwest (i.e., Clear Creek, IA) with semi-cohesive soils were analyzed in terms of both mechanical and fluvial erosional strength. Mechanical strength was measured using a direct shear device and ranged from 400 to 6,600 Pa. Fluvial erosional strength was measured using a conduit flume, which applied a shearing force to the sample and had values between 1.28 and 2.37 Pa. Thus, mechanical strength was two to three orders of magnitude larger than fluvial erosional strength, which suggests that identifying the different modes of streambank erosion (e.g., mechanical or fluvial) during a hydrograph is needed to provide better design specifications for bank stabilization practices.

Examining seasonal trends in sediment source contributions in an intensely cultivated Midwestern sub-watershed using Bayesian un-mixing

This study aims to shed some light on the dynamics of sediment source contributions in the South Amana Sub-Watershed (SASW) in Iowa, USA, at a seasonal scale. Field studies were performed periodically from April to August 2007, in which sediment source and eroded samples were collected from the uplands and the stream network. The δ 13 C and δ 15 N signatures of the samples were obtained via mass spectrometry and used as “fingerprints” to identify the different contributing sources. For sources that were not sampled, literature values of δ 13 C and δ 15 N from similar systems in the region were adopted. Un-mixing of the eroded samples using the tracer signatures was achieved with an enhanced version of an existing Bayesian, Markov Chain Monte Carlo (MCMC) model, structured to well represent the intensely cultivated SASW system. Balance in sediment source contributions shifted between upland and in-streams sources over the season. Preliminary results from the study confirm observations from other studies that hydrological factors and sediment availability significantly affect the relative contributions of upland and in-stream sources. In addition, the study shows that canopy cover vs. bare soil plays an important role and should be taken into consideration when performing un-mixing studies designed to identify sediment sources.