Roger A. Kuhnle

Bio: Roger A. Kuhnle is an academic researcher from Agricultural Research Service. The author has contributed to research in topics: Sediment & Sediment transport. The author has an hindex of 25, co-authored 90 publications receiving 1991 citations. Previous affiliations of Roger A. Kuhnle include United States Department of Agriculture & Michigan Technological University.

Field Techniques for Suspended-Sediment Measurement

Abstract: The measurement of suspended sediment, particularly in field settings, is important in the documentation of sediment transport and deposition. Many measurement techniques have been used with varying degrees of success. The techniques, including their operating principles, advantages, and disadvantages are discussed. The techniques discussed include acoustic, bottle, pump, focused beam reflectance, laser diffraction, nuclear, optical backscatter, optical transmission, and spectral reflectance. Emphasis is placed on instrumentation techniques, as this is the area of suspended-sediment measurement that has the greatest potential for improving sediment data. Acoustic technology (if further developed) emerges as a promising technology because of its ability to measure the concentration profile without intruding into the flow. This technology-transfer information will be valuable to practitioners and researchers needing to choose a means of measuring suspended sediment. The choice of a measurement technique has...

Local Scour Associated with Angled Spur Dikes

Abstract: A series of experiments were conducted in which the volume of the scour hole associated with model spur dikes was measured in a laboratory flume under clear-water overtopping flows. Spur dike models were angled at 45, 90, and 135° to the downstream channel sidewall with contraction ratios of 0.125 and 0.250. The main goals of the experiments were to evaluate the effect of the three angles on the volume of scour and potential aquatic habitat and on minimizing erosion adjacent to the streambanks. The experiments showed that of the three angles tested, the least erosion of the bed in the near bank region was associated with the spur dikes with 90° angles, while the greatest volume of the scour hole was associated with the 135° spur dikes. It was concluded that spur dikes with 135$ angles showed the best potential for providing improved aquatic habitats while minimizing the potential for erosion of the channel bank.

Bed load transport fluctuations in a gravel bed laboratory channel

Abstract: Flume experiments were conducted to investigate the mechanisms of transport of a gravel-sand mixture by shallow unidirectional flows. Two water recirculating sediment feed flumes were used: one with a 6 m long and 0.15 m wide channel and other with an 11 m long channel with widths of 0.74 m and 0.53 m. The sediment, poorly sorted gravel with a mean size of 3 mm, was fed at the upstream end of the channel at steady rates from 0.03 kg s−1 m−1 to 1.0 kg s−1 m−1. Sediment transport rate out of the channel varied in all runs, at approximate periods of 3 min in the runs with high transport rates to 14 min in the runs with low transport rates. The runs with low transport rates also showed fluctuations in total transport rate at periods of about 25 min. Transport rates of each size fraction varied with time in a distinctive pattern in all runs. The time variations were caused by the migration of very long and low bed load sheets in the runs with low to moderate transport rates and dunelike bed forms in the run with the highest transport rate. The bed surface in all runs was coarser in size than the original sediment mix except that with the highest transport rate (run H5) in which the size distribution was nearly the same as the original.

Incipient Motion of Sand‐Gravel Sediment Mixtures

Abstract: Experiments on the incipient motion of gravel and sand mixtures of 0:100, 10:90, 25:75, 45:55, and 100:0 percent ratios of gravel and sand, respectively, were made in a laboratory flume. A series of transport experiments was made for each of the five sediment beds and the critical shear stress for motion was calculated for 12 size ranges by interpolating or extrapolating the bed shear stress for a very small transport rate. The sand in each of the five bed mixtures began to move at nearly the same bed shear stress. All gravel sizes began to move at nearly the same bed shear stress in the 100% gravel experiments, but in the sand-gravel experiments the gravel showed an increase in critical shear stress with increasing size. Reasons for this change in the initiation of motion of the gravel from the 100% gravel to the sand-gravel mixtures may result from the abundant sand-sized sediment inhibiting the formation of a coarse bed surface layer in the sand-gravel experiments.

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

Abstract: 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.

A systematic analysis of eight decades of incipient motion studies, with special reference to gravel-bedded rivers

Abstract: Data compiled from eight decades of incipient motion studies were used to calculate dimensionless critical shear stress values of the median grain size, t* c 50 . Calculated t* c 50 values were stratified by initial motion definition, median grain size type (surface, subsurface, or laboratory mixture), relative roughness, and flow regime. A traditional Shields plot constructed from data that represent initial motion of the bed surface material reveals systematic methodological biases of incipient motion definition; t* c 50 values determined from reference bed load transport rates and from visual observation of grain motion define subparallel Shields curves, with the latter generally underlying the former; values derived from competence functions define a separate but poorly developed field, while theoretical values predict a wide range of generally higher stresses that likely represent instantaneous, rather than time-averaged, critical shear stresses. The available data indicate that for high critical boundary Reynolds numbers and low relative roughnesses typical of gravel-bedded rivers, reference-based and visually based studies have t* c50 ranges of 0.052-0.086 and 0.030-0.073, respectively. The apparent lack of a universal t*50 for gravel-bedded rivers warrants great care in choosing defendable t* c50 values for particular applications.

Surface-based Transport Model for Mixed-Size Sediment

Abstract: We present a transport model for mixed sand/gravel sediments. Fractional transport rates are referenced to the size distribution of the bed surface, rather than subsurface, making the model completely explicit and capable of predicting transient conditions. The model is developed using a new data set of 48 coupled observations of flow, transport, and bed surface grain size using five different sediments. The model incorporates a hiding function that resolves discrepancies observed among earlier hiding functions. The model uses the full size distribution of the bed surface, including sand, and incorporates a nonlinear effect of sand content on gravel transport rate not included in previous models. The model shares some common elements with two previous surface-based transport models, but differs in using the full surface size distribution and in that it is directly developed from a relatively comprehensive data set with unambiguous measurement of surface grain size over a range of flow, transport rate, and sediments.

Surface-based bedload transport relation for gravel rivers

Abstract: Bedload transport in gravel-bed rivers is accomplished by means of the mobilization of grains exposed on the bed surface. This mobilization is due to the action of fluid forces on the exposed grains. Substrate particles can participate in the bedload only to the extent that local or global scour results in their exposure on the surface. It follows that a calculation of the bedload transport rate of mixtures should be based on the availability of each size range in the surface layer. Herein an existing empirical substrate-based bedload relation for gravel mixtures, developed solely with reference to field data, is transformed into a surface-based relation.