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United States Department of Agriculture

Agricultural Research Service

Title: Modification and Performance of the Coshocton Wheel with the Modified Drop-Box Weir

item Bonta, James

Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 7, 2002
Publication Date: June 3, 2002

Interpretive Summary: Often it is necessary to measure runoff in stream channels that are steep or not straight for recommended lengths upstream from a flow-measuring device. At the same time, composite runoff-water samples are often required. The performance of a flow-measuring device for nonstandard channel conditions (drop-box weir) used in conjunction with a water sampler (Coshocton wheel) was evaluated. The drop-box weir has the capability to pass large sediment concentrations that would plug up commonly-used flow-measuring devices. It has been found in previous studies that the Coshocton wheel must be reevaluated when upstream approach conditions to it are different from standard conditions. It was found that the original shielding on the sampling slot of the Coshocton wheel, that minimizes splash into the slot, had to be augmented with a second splash shield for proper performance. The modified sampler performed well under steady flows and when flow rates were constantly changing (such as would occur during runoff). Data allowed the determination of the upper limit (flow rate) of performance and guidelines for sizing a modified drop-box weir based on sampler performance. It was found that the upper limit for sizing a drop-box weir was the same as that found independently in another study when the drop-box weir was evaluated alone. The results extend the utility of the modified drop-box weir and Coshocton wheel to field conditions (steep and skewed channel slopes) and high sediment concentrations that would preclude the use of commonly used devices. Federal, state and university researchers, state agencies and consulting firms will benefit from the results of this study.

Technical Abstract: Water-chemistry/sediment and runoff data from erosion plots and small watersheds are often needed for erosion and water quality studies where sediment concentrations can be large, or where channel conditions are not appropriate for conventional measuring devices, such as steep and skewed channels. The performance of a flow-measuring and sampling system was evaluated comprising a modified drop-box weir as the approach to the Coshocton wheel proportional sampler. Three splash-shield configurations on the sampling slot of the Coshocton wheel were investigated. It was found that the dual-splash shield was required to insure proper proportional sampling of large flows and that the Coshocton wheel worked well with the weir as the sampler approach under steady and unsteady flow conditions. The average proportional catch for the dual shield under steady flows was 0.0126. For unsteady flows the average sampler fraction was 0.0120. The difference in sampler fraction was not statistically significant. It was found that there was no difference in a parameter needed for sizing a drop-box weir based on sampler performance, compared with the parameter determined by performance of the drop-box weir alone. This study extends the utility of the drop-box weir and Coshocton wheel under extreme field conditions such as steep and skewed channels.

Last Modified: 4/17/2015
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