Roger A. Kuhnle

Quantifying relative contributions from sediment sources in Conservation Effects Assessment Project watersheds

A technique using the relationship between the naturally occurring radionuclide tracers, 7Be and 210Pbxs, was used to differentiate eroded surface soils and channel-derived sediments in the fine suspended sediment loads of runoff events in five Conservation Effects Assessment Project watersheds. A simple two end-member mixing model was used to determine the relative contribution from each source. Results suggest that eroded surface soils were more prevalent in the suspended load early in a runoff event, but channel contributions dominated the suspended load at later stages. The method proved useful for multiple sites due to a constant proportion of the atmospheric deliveries of the two radionuclides globally. Use of only two radionuclide tracers simplifies the differentiation of sediment sources within a watershed but limits precision.

Effect of land use changes on sediment transport in Goodwin Creek

The Goodwin Creek Research Watershed (21.3 km2) is located in the north central part of Mississippi in the bluff hills just east of the Mississippi River floodplain. Land use on the watershed has been surveyed annually and the percentage of cultivated land has decreased from 26% in 1982 to 12% in 1990. During this 9-year period the concentration of fines ( 2.0 mm) have decreased by 39%. The decrease in the percentage of cultivated land affects the sediment budget of the watershed in two ways. A source of readily eroded sediment is removed, and the energy of the flowing water available to erode and transport sediment is reduced. The reduced flow in the channels from the decrease in cultivated land in the watershed was probably the main cause for the lower transport rates of sand and gravel.

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.

Fluvial transport of sand and gravel mixtures with bimodal size distributions

Abstract Initiation of motion and fractional transport rates of sediments with bimodal size distributions were measured in a laboratory channel and in a natural channel. Transport experiments were conducted with unimodal 100% sand and 100% gravel bed sediments and three bimodal mixtures of the two sediments: 10% gravel—90% sand, 25% gravel—75% sand, and 45% gravel—55% sand. In the bimodal sediment beds the critical shear stress for the initiation of motion showed very little change with grain size for the sand sizes, but increased with grain size for the gravel sizes. For low flow strengths, only the sand sizes were in transport, while for the highest flow strengths the bed load sediment size distribution approached that of the bed material.

Test of a Method to Calculate Near-Bank Velocity and Boundary Shear Stress

Detailed knowledge of the flow and boundary shear stress fields near the banks of natural channels is essential for making accurate calculations of rates of near-bank sediment transport and geomorphic adjustment. This paper presents a high-resolution laboratory data set of velocity and boundary shear stress measurements and uses it to test a relatively simple, fully predictive, numerical method for determining these distributions across the cross-section of a straight channel. The measurements are made in a flume with a fairly complex cross-section that includes a simulated, cobble-roughened floodplain. The method tested is that reported by Kean and Smith in Riparian Vegetation and Fluvial Geomorphology in 2004, which is modified here to include the effects of drag on clasts located in the channel. The calculated patterns of velocity and boundary shear stress are shown to be in reasonable agreement with the measurements. The principal differences between the measured and calculated fields are the result of secondary circulations, which are not included in the calculation. Better agreement with the structure of the measured streamwise velocity field is obtained by distorting the calculated flow field with the measured secondary flow. Calculations for a variety of narrower and wider configurations of the original flume geometry are used to show how the width-to-depth ratio affects the distribution of velocity and boundary shear stress across the channel.

Effect of Large Woody Debris Structures on Stream Hydraulics

Large woody debris structures hold promise as cost -effective stream corridor rehabilitation measures. Effectiveness of these structures placed in incised, rapidly eroding, sand -bed channels is governed by the effects of the structures on the velocity field at the toe of eroding banks. Effects of recently placed large woody debris structures were examined using dep th and depthaveraged velocity data collected using self -contained acoustic Doppler/pressure transducer data loggers. Six of these instruments were placed along two cross -sectional transects at the apex of a sharp meander bend in Little Topashaw Creek, Mississippi. Simultaneous records of runoff events before and after placement of large woody debris structures along the outside of the bend showed that the debris structures were effective in reducing mean storm event velocities from about 150% of channel centerline velocities to levels between 3% and 45% of centerline velocities. The region protected by the structure experienced moderate deposition (<0.4 m) during the first high flow season following construction. A series of laboratory flume tests were run to assess accuracy of the acoustic Doppler/pressure transducer instruments, which were found to produce depth records within 5% of manual measurements and velocity measurements within 5% of depth-averaged velocities determined by numerically integrating point velocities measured using a laboratory acoustic Doppler velocimeter.

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

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...

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.