Daniel G. Wren

The evolution of an oxbow lake in the Mississippi alluvial floodplain

The history of sediment accumulation in an oxbow lake located on the Mississippi alluvial floodplain was reconstructed based on sedimentation rates determined using 14C activities from bulk sediment fractions and from 210Pb and 137Cs measurements. Higher rates of sediment accumulation consistent with frequent flooding when first abandoned 3,800 to 5,000 years before the present were followed by slower sedimentation rates consistent with migration of the Mississippi river away from the oxbow and less frequent flooding. This low sedimentation rate persisted for several thousand years until the surrounding land was cleared for agricultural use in the late 19th century. A 50-fold increase in the rate of sediment accumulation has persisted from the time of land clearing to the present, doubling the total mass of accumulated sediment in a single century.

Analysis of Vegetation Effect on Waves Using a Vertical 2D RANS Model

ABSTRACT Wu, W.; Zhang, M.; Ozeren, Y., and Wren, D., 2013. Analysis of vegetation effect on waves using a vertical 2D RANS model. A vertical two-dimensional (2D) model has been applied in the simulation of wave propagation through vegetated waterbodies. The model is based on an existing model, SOLA-VOF, that solves the Reynolds-averaged Navier–Stokes (RANS) equations with the finite difference method on a staggered rectangular grid and uses the volume of fluid method to capture the free surface. The model is enhanced in this study by adding the drag and inertia forces in the momentum equations to account for the vegetation effects, implementing the subgrid-scale model for turbulence closure, and incorporating wave-maker, sponge layer, and bottom friction in boundary conditions. The model was first validated using measurement data collected from the literature and then applied to simulate wave propagation in flumes covered by rigid and flexible model and live vegetation. The considered live vegetation species are Spartina alterniflora (smooth cord grass) and Juncus roemerianus (needlegrass rush), which are commonly distributed on coastlines. The model is able to reproduce wave attenuation through the vegetation zone observed in the experiments. The drag coefficients are calibrated in the vertical 2D RANS model and the analytical model based on the wave energy equation and linear wave theory, and the calibrated values in the two models are reasonably close.

Distributions of velocity, turbulence, and suspended sediment over low-relief antidunes

Understanding the interactions between sediment transport and turbulence in a supercritical flow is useful in the study of river hydraulics and fluvial systems and the modeling of such flows in nature. Toward this end, 11 profiles of suspended-sediment concentration and fluid velocity were collected in supercritical conditions over low-relief antidunes in a recirculating laboratory flume. It was found that velocity profiles agreed well with the law of the wall, and that turbulence intensities were similar to those in clear-water flows. The classic Rouse equation was found to under-predict concentration values in the upper 60-80% of the flow depth and a slightly modified version was proposed and successfully tested against experimental data from the present study and others.

Measurements of coupled fluid and sediment motion over mobile sand dunes in a laboratory flume

The relationship between turbulent fluid motions and sediment particle motions over mobile sand dunes was investigated by using a laser Doppler velocimeter and an acoustic backscatter system in laboratory experiments performed at the USDA-ARS-National Sedimentation Laboratory. Profiles of acoustic backscatter from particles and at-a-point turbulence data were collected while translating both measurement devices downstream at the speed of mobile dune bedforms. The resulting data set was used to examine the frequency (recurrence frequency) at which the fluctuating backscatter and fluid velocity signals exceeded magnitude thresholds based on the standard deviation (σ) of the local velocity and the magnitude the acoustic signal resulting from backscatter from suspended particles. The slope of the downstream and vertical velocity recurrence frequencies generally indicated a gradually increasing recurrence time with increasing elevation. The recurrence frequency for acoustic backscatter data was not strongly variable with elevation. The closest correspondence between the recurrence frequencies of sediment backscatter and vertical velocities at the 1σ magnitude threshold was in a region defined by X/L<0.4 and y<6 cm. The downstream velocity was most closely related to backscatter in a small region at 0.4

Simultaneous determination of mercury and organic carbon in sediment and soils using a direct mercury analyzer based on thermal decomposition–atomic absorption spectrophotometry

The purpose of this work was to study the feasibility of using a direct mercury analyzer (DMA) to simultaneously determine mercury (Hg) and organic matter content in sediment and soils. Organic carbon was estimated by re-weighing the sample boats post analysis to obtain loss-on-ignition (LOI) data. The DMA-LOI results were statistically similar (p<0.05) to the conventional muffle furnace approach. A regression equation was developed to convert DMA-LOI data to total organic carbon (TOC), which varied between 0.2% and 13.0%. Thus, mercury analyzers based on combustion can provide accurate estimates of organic carbon content in non-calcareous sediment and soils; however, weight gain from moisture (post-analysis), measurement uncertainty, and sample representativeness should all be taken into account. Sediment cores from seasonal wetland and open water areas from six oxbow lakes in the Mississippi River alluvial flood plain were analyzed. Wetland sediments generally had higher levels of Hg than open water areas owing to a greater fraction of fine particles and higher levels of organic matter. Annual loading of Hg in open water areas was estimated at 4.3, 13.4, 19.2, 20.7, 129, and 135 ng cm(-2) yr(-1) for Beasley, Roundaway, Hampton, Washington, Wolf and Sky Lakes, respectively. Generally, the interval with the highest Hg flux was dated to the 1960s and 1970s.

Turbulent Flow and Sand Transport over a Cobble Bed in a Laboratory Flume

AbstractImproving 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...

Using lake sedimentation rates to quantify the effectiveness of erosion control in watersheds

The effectiveness of erosion control methods is difficult to measure, hampering the development of management practices and preventing accurate assessment of the value of erosion control structures over time. Surface erosion can vary widely over an area, particularly if gully erosion is present, and the use of sediments transported in streams for quantifying erosion is hindered by the highly variable nature of fluvial sediment loads. When a watershed drains into a lake, accumulated sediments have the potential to yield information about historic rates of sedimentation that can be used to evaluate the effectiveness of previous erosion control measures. In the present study, sediments from five natural oxbow cutoff lakes in the Mississippi River alluvial floodplain were dated using lead-210 decay rates and bomb-pulse derived cesium-137 with the goal of relating trends in sedimentation rate to reductions in erosion due to management practices. It was found that the radioisotope dating methods were best used in concert with known dates for implementation of management practices. Changes in sedimentation rate over time frames as short as 12 years were detectable. Larger lakes showed smaller changes in sedimentation rate relative to smaller lakes.

ACCURATE LACUSTRINE AND WETLAND SEDIMENT ACCUMULATION RATES DETERMINED FROM 14C ACTIVITY OF BULK SEDIMENT FRACTIONS

In the absence of identifiable macrofossils in lacustrine sediments, radiocarbon dating must rely on pollen or bulk sediment fractions. Bulk sediment fractions are not generally preferred because they contain an unknown mixture of organic material of variable age, they may contain dead carbon such as lignite that is difficult to eliminate, and material of aquatic origin may be subject to reservoir effects. If the various processes that contribute carbon to the system are relatively constant over time, however, changes in 14C activity with depth may be used to accurately estimate sediment accumulation rates even if the absolute ages are erroneous. In this study, fine-grained fractions (250710 m organic material, humic acids extracted from <250-m fraction, and untreated <250-m fraction combusted at low temperature) were analyzed and compared with terrestrial plant stems (twigs), charcoal, and wood fragments in sediments from an oxbow lake in Mississippi, USA. The 14C activities of the bulk fractions were highly linear with depth and produced consistent calculated sediment accumulation rates similar to, and perhaps more reliable than, rates determined using twigs or charcoal.

Effects of silt loading on turbulence and sand transport

Abstract The transport of bed material and fluid turbulence are affected by many factors, including the fine sediment load being carried in a channel. Current research has focused on sand-sized particles introduced to gravel beds, while the effect of silt load on sand transport has received less attention. Experiments on the effects of silt load, in concentrations 0-26,900 mg l −1 , on sand transport were performed with a recirculating laboratory flume using three different sand bed configurations: ripples ( Fr =0.24), dunes ( Fr =0.34), and dunes ( Fr =0.48). Three Acoustic Doppler Velocimeters were arranged to measure flow and turbulence quantities simultaneously in one vertical. Sand transport did not change in a consistent manner with increasing silt load, increasing up to 4,000 mg l −1 for dunes ( Fr =0.48) and up to 2,000 mg l −1 for dunes ( Fr =0.34) and then declining to near the clear water case with increasing silt concentrations. Silt addition for the ripple case caused a relatively small change in sand transport, decreasing with added silt up to approximately 2,000 mg l −1 and then increasing as silt went up to approximately 10,000 mg l −1 . Dunes ( Fr =0.48) decreased in length and height as silt increased, while dunes ( Fr =0.34) did not show a consistent trend. A clear trend of decreasing Reynolds stress with increasing silt concentration was observed in the ripple case, with a 33% reduction in near-bed Reynolds stress caused by an 8,900 mg l −1 concentration of silt.

Suspended-Sediment Measurements in Laboratory Flumes using Acoustic Techniques

Automated measurement of suspended sediments is crucial to the study of sediment transport. The short duration, high-intensity flows that are responsible for a large fraction of sediment movement are best observed by continuous monitoring systems. Acoustic systems are ideal for continuous monitoring and add the advantage of non-intrusively measuring through the water column. In the present work, a single-frequency acoustic system for measuring suspended-sediment concentration in fluvial environments is described. The equipment and procedures used in developing hardware and software for the acoustic technique in two sets of laboratory flume experiments will be presented. Both implicit and explicit methods were used to convert backscatter data into sediment concentrations. The implicit method yielded 39% error and the explicit, 20%.