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This research has been conducted to develop the use of passive acoustic monitoring (PAM) in rivers, a surrogate method for bedload monitoring. PAM consists in measuring the underwater noise naturally generated by bedload particles when impacting the river bed. Monitored bedload acoustic signals depend on bedload characteristics (e.g., grain size distribution, fluxes) but are also affected by the environment in which the acoustic waves are propagated. This study focuses on the determination of propagation effects in rivers. An experimental approach has been conducted in several streams to estimate acoustic propagation laws in field conditions. It is found that acoustic waves are differently propagated according to their frequency. As reported in other studies, acoustic waves are affected by the existence of a cutoff frequency in the kilohertz region. This cutoff frequency is inversely proportional to the water depth: larger water depth enables a better propagation of the acoustic waves at low frequency. Above the cutoff frequency, attenuation coefficients are found to increase linearly with frequency. The power of bedload sounds is more attenuated at higher frequencies than at low frequencies, which means that, above the cutoff frequency, sounds of big particles are better propagated than sounds of small particles. Finally, it is observed that attenuation coefficients are variable within 2 orders of magnitude from one river to another. Attenuation coefficients are compared to several characteristics of the river (e.g., bed slope, surface grain size). It is found that acoustic waves are better propagated in rivers characterized by smaller bed slopes. Bed roughness and the presence of air bubbles in the water column are suspected to constrain the attenuation of acoustic wave in rivers.

Many concepts have been proposed to explain hydrologic connectivity of hillslopes with streams. Hydrologic connectivity is most often defined by qualitative assessment of spatial patterns in perched water tables or soil moisture on hillslopes without a direct linkage to water flow from hillslopes to streams. This form of hydrologic connectivity may not explain the hydrologic response of catchments that have network(s) of preferential flow paths, e.g. soil pipes, which can provide intrinsic connectivity between hillslopes and streams. Duplex soils are known for developing perched water tables on hillslopes and fostering lateral flows, but the connectivity of localized perched water tables on hillslopes with soil pipes has not been fully established. The objectives of this study were to characterize pipeflow dynamics during storm events, the relationships between perched water tables on hillslopes and pipeflows, and their threshold behavior. Two well-characterized catchments in loess soil with a fragipan were selected for study because they contain multiple, laterally extensive (over 100 m) soil pipe networks. Hillslopes were instrumented with shallow wells adjacent to the soil pipes, and the wells and pipe collapse features were equipped with pressure transducers. Perched water tables developed on hillslopes during a wetting up period (October – December) and became well connected spatially across hillslope positions throughout the high flow period (January – March). The water table was not spatially connected on hillslopes during the drying out (April-June) and low flow (July-September) periods. Even when perched water tables were not well-connected, water flowing through soil pipes provided hydrologic connectivity between upper hillslopes and catchment outlets. Correlations between soil pipeflow and perched water tables depended on the size and location of soil pipes. The threshold relationship between available soil-moisture index plus storm precipitation (ASI + P) and pipeflow was dependent on the season and strongest during dry periods and not high flow seasons. This study demonstrated that soil pipes serve as a catchment backbone of preferential flow paths that provide intrinsic connectivity between upper hillslopes and streams.

Soil pipes are important subsurface flow pathways in many soil erosion phenomena. However, limited research has been performed on quantifying and characterizing their flow and transport characteristics. The objectives of this research were to determine the applicability of a streamflow model with transient storage in deriving flow and transport characteristics of soil pipes. Tracer data from pulse inputs were collected in four different soil pipes after a fluorescein dye was injected in the upstream end of each soil pipe network in three branches (west, middle, and east) of a main catchment and a back catchment in Goodwin Creek Experimental Watershed in Mississippi. Multiple sampling stations were positioned along each soil pipe network. The transient storage zone model OTIS-P was executed inversely to estimate transport parameters by soil pipe reach such as the soil pipe cross-sectional area (A), soil storage zone cross-sectional area (As), and exchange rate between the soil pipe and the soil storage zone (αs). Model convergence was achieved, and simulated breakthrough curves of the reaches were in good agreement with actual tracer data for eight of the nine reaches of the three branches of the Main Catchment and five of the seven reaches of the Back Catchment soil pipe. Simulation parameters for the soil pipe networks were similar to the range of values reported for flow and transport characteristics commonly observed in streams. Inversely, estimated soil pipe flow velocities were higher with increased tortuosity, which led to a smaller cross-sectional areas predicted for the soil pipe flowpaths, while other parameters were not sensitive to tortuosity. In general, application of One-Dimensional Transport with Inflow and Storage-P to this unique soil pipe condition suggested larger transient storage (As and αs) compared with most stream systems. This was hypothesized to be because of relatively higher ratio of the wetted perimeter to flow area in the soil pipe, the hydraulic roughness of the soil pipe, potential retention in collapsed portions of the pipe, and interaction with smaller preferential flow systems. Copyright © 2015 John Wiley & Sons, Ltd.

Little is known about the association of soil pipe collapse features with soil properties or land use history. Three loess covered catchments in northern Mississippi, USA were characterized to investigate these relationships. Soil pipe collapses were characterized for their size, type feature and spatial location along with soil properties across the three catchments. Although mapped as the same soil, one of the catchments did not contain pipe collapse features while the other two had 29.4 and 15.4 pipe collapses per hectare. These loess soils contained fragipan layers that are suspected of perching water, thereby initiating the piping processes. Pipe collapses associated with subsurface flow paths were not always consistent with surface topography. The surface layer tended to be non-erodible while layers below, even the upper fragipan layers, were susceptible to erosion by pipeflow. Soil properties of the lowest fragipan layer were highly variable but tended to prevent further downward erosion of soil pipes and thus formed a lower boundary for gullies. Middle to lower landscape positions in one of the piped catchments contained anthropic soils that were highly erodible. These anthropic soils were previously gullies that were filled-in in the 1950s when forested areas, assumed to have been established when land was previously converted from crop to forest land, were converted to pasture. Three decades after this land use change from forest to pasture, pipe collapses became evident. In contrast, the adjacent catchment that does not exhibit pipe collapse features experienced severe sheet and rill erosion prior to the 1930s while in cotton production. The surface horizons above the lower fragipan layer were completely removed during this period, thus the top-soil layer that tends to form a bridge above soil pipes in the more erodible subsoil layers was removed. This study showed that knowledge of soil characteristics or topography alone do not explain the distribution of soil pipe collapses as past land use can play a definitive role. Copyright © 2015 John Wiley & Sons, Ltd.

Soil erosion, in its myriad forms, devastates arable land and infrastructure. As an integral landscape feature, gully erosion is a complex system as its evolution is controlled by upstream migration of a gully head (headcut face), incision of the gully bed (plunge pool) and gravitational mass-movement (widening) on gully channel sidewalls. Bed incision is often limited by the presence of a less erodible soil layer. When erosion reaches such a layer, the gully typically widens, creating a wide shallow cross section. Once a gully is initiated, transport and deposition of the eroded soil and widening of the channel further govern its evolution. Our knowledge of these processes within agricultural regions, however, is still quite limited with theory largely borrowed from river hydraulics. Experiments were conducted to examine gully expansion (widening) as a function of channel slope and overland flow discharge. Packed soil beds were subjected to simulated rainstorms followed by clear-water overland flow. As bed slope and discharge increased, channel widening increased. An empirical equation, based upon measured channel width from constant discharge experiments, was developed in terms of slope and discharge. By further quantifying key aspects of gully erosion processes, results from this experimental campaign will significantly improve soil erosion prediction technology for use in agricultural regions.

Experiments were conducted in a laboratory flume channel to evaluate the effects of increasing amounts of sand on its transport over and through an immobile coarse gravel bed. Detailed measurements of sand transport rate, bed texture, and bed topography were collected for four different discharges at approximately the same flow depth of 0.2 m for 11 different elevations of sand in the gravel bed. Sand transport was measured using both physical samples and a density cell. For a given flow rate, increases in the elevation of sand relative to gravel resulted in decreases of bed shear stress from 32–44% and increases in sand transport by three orders of magnitude. For the highest two discharges, the sand merged into a small number of long and low bed forms that translated through and over the gravel bed. A collapse of the transport data was accomplished by relating the sand transport rate to the bed shear stress scaled by the cumulative probability distribution function of the gravel surface evaluated...

Shortly after peak stage of the 2011 Mississippi River flood, water samples were collected from the river, from sand boils near the toe of the levee, and from actively flowing relief wells over a 55 km stretch north of Vicksburg, MS. Two distinct pathways for seepage under the levee were identified based on the elemental composition of water samples. Sand boil discharge was similar to water from relief wells only at a location where the levee sits on ancient channel fill deposits. Seepage at this site is forced along a deeper pathway beneath the fine-grained channel fill. Where the levee sits on sandy point bar deposits, shallow flow beneath the levee is unimpeded. The chemical composition of discharge from sand boils at these sites was clearly not just river water, nor a simple mixture of river and groundwater, but appears to reflect unique weathering or redox interactions occurring within the upper portion of the alluvial aquifer. Distinguishing shallow and deep seepage pathways may prove useful for evaluating site specific risk of levee failure.

The long-term success and sustainability of agriculture in the Lower Mississippi River Basin will depend largely on water resources. Aquifer decline in the region has been documented since the 1980s and continues today. Artificial recharge is one possible tool that could help alleviate this decline. This study describes a novel test of artificial recharge through direct well injection at a site in western Poinsett County, Arkansas. Water was taken from a source well 800 m (0.5 mi) from the injection well at a nearly uninterrupted rate of 565 gallons per minute for four days. A maximum rise in water level of 11.3 m (37.2 ft) at the injection well and 0.2 m (0.7 ft) at a well 100 m (330 ft) away from the injection well was recorded. Air entrainment was not extensive, particularly given that the recovery water-level data resulted in hydraulic conductivity values that were as large or larger than other values measured for the alluvial aquifer in the area. The success of artificial recharge hinges on the water quality of the source water. Water quality was collected monthly at five local reservoirs and the primary ditch used to fill each reservoir from January through April 2014. A reduction in turbidity was measured in the reservoirs compared to ditches. Water quality conditions in February were most conducive among the four months studied (January to April), and suggest this period as the most effective period to attempt artificial recharge in this area. However, more data are required to sufficiently characterize and strengthen these initial conclusions.

Improving the prediction of sand transport downstream of dams requires characterization of the interaction between turbulent flow and near-surface interstitial sands. The advanced age and impending decommissioning of many dams have brought increased attention to the fate of sediments stored in reservoirs. Sands can be reintroduced to coarse substrates that have available pore space resulting from periods of sediment-starved flow. The roughness and porosity of the coarse substrate are both affected by sand elevation relative to the coarse substrate; therefore, the turbulence characteristics and sediment transport over and through these beds are significantly altered after sediment is reintroduced. Past work by the writers on flow over sand-filled gravel beds revealed that the fine-sediment level controls the volume of material available for transport and the area of sediment exposed to the flow. The present work expands on the gravel-bed experiments by conducting similar measurements of turbulent f...