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United States Department of Agriculture

Agricultural Research Service

Title: Modified Drop-Box Weir for Monitoring Flows from Erosion Plots and Small Watersheds

Author
item Bonta, James

Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 22, 1998
Publication Date: N/A

Interpretive Summary: Erosion and water-quality investigations are often conducted on erosion plots and small watersheds, under natural and artificial precipitation. Sediment-laden flows are difficult to gage because the flow-gaging devices often give incorrect data due to sediment deposition in them. The original drop-box weir was modified to pass the design flow entirely through the box, and to accommodate gutters collecting runoff and sediment from rectangular plots of soil. The effects of these new approach conditions on the performance of the weir were evaluated. The modified weir can be used to give the good measurements of flow where high sediment concentrations are expected. The study gives further applicability to the drop-box weir to steep and sinuous channels under high or low sediment concentrations. Equations that describe the flow were developed that can be used in practical applications. The new weir and approach configuration also allows the Coshocton wheel water sampler to be used for sampling flows. The results are useful to scientists and consultants interested in measuring flows from erosion plots and small watersheds.

Technical Abstract: Sediment-laden flow and chemical-constituent concentration data from erosion plots and small watersheds are often needed for erosion and water quality impact evaluations and modeling. The original drop-box weir was modified to pass the design flow through the V-notch and box sections of the weir. Approach conditions consisted of gutters and aprons that divert runoff water and sediment to the box at right angles. An evaluation of th effects of these approach conditions on the rating for the drop-box weir, and of the effects of not having a false back wall for erosion-plot use was conducted. An extension of the rating for low flows was performed. Results showed that the rating for the drop-box weir with the new approach conditions was different from the published rating. Three rating equations were fit to the data simultaneously for the three flow ranges. Observations of weir performance suggested that the reason for the three equations was related to the stepped side weirs. This and other published studies led to the conclusion that the drop-box weir should not be modified unless a new study of the effects of the changes on the rating are evaluated.

Last Modified: 12/28/2014
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