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United States Department of Agriculture

Agricultural Research Service

Title: Modified Drop-Box Weir for Monitoring Watershed Flows under Extreme Approach-Channel Conditions

Authors
item BONTA, JAMES
item Goyal, V - NAT INST HYDROLOGY INDIA

Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 29, 2001
Publication Date: N/A

Interpretive Summary: It is difficult to obtain quality runoff data when field conditions are not as prescribed for commonly-used flow-measuring devices placed in stream channels. This is because channels can be very steep causing rushing flows, sediment may clog devices or cause them to be inaccurate and channels might not be straight for a sufficient length immediately upstream from a device causing asymmetric flows. A flow-measuring device called the drop-box weir was modified and evaluated for use in such extreme field conditions. Three baffle configurations placed immediately upstream from the weir and 20 combinations of channel slope and channel angle were evaluated. Slopes ranged from 5% to 75% and angles ranged from 0 to 45 degrees. It was found that a simple baffle upstream from the weir worked well to control flows from all slopes and angles tested. One equation that relates stream depth to flow rate was found for all these conditions. The study was in good agreement with two other studies of the drop-box weir. In one of these studies, flows directed to the weir at 90 degrees yielded nearly the same equation parameters as the parameters in the present study. This study extends the utility of the drop-box weir flow-measuring device to extreme channel conditions and to conditions where there is much sediment in the flow. Federal, state and university scientists and consulting firms will benefit from the results of this study.

Technical Abstract: Rapid flows in steep channels, skewed flows in sinuous channels and flows laden with sediment are difficult to accurately measure. The drop- box weir was modified and evaluated for use in steep and skewed stream channels. Modifications to the weir (MDBW) included preventing water from overtopping the back wall of the box (forcing all water to enter the sides of the box), vertical side walls at the V section and use of a baffle. Three upstream energy-dissipation configurations and 20 upstream approach conditions were tested. The approach-channel angles ranged from 0o to 45o and approach-channel slopes from 5% to 75%. An upstream baffle yielded a stable rating curve compared with two other energy-dissipation measures. The rating curve of the MDBW was independent of approach channel angle and slope at all flows, when an upstream baffle was used to dissipate flow energy. The rating curve was composed of five linear segments on a log-log grid. Changes in the slopes of the rating segments were associated with observed flow conditions in the weir. Departures in the low-flow rating may be due to differences in weir fabrication. There was near identical agreement between rating curves developed with a study in which flows were directed at a 90o approach angle by gutters. The rating of this study is applicable to extreme approach angles beyond the 45o angle tested in this study. Guidelines for weir sizing and development of rating-curves for the MDBW are given. This study extends the utility of the drop-box weir to a variety of extreme approach conditions such as eroding gullies, bare erosion plots, pipe outlets and culverts where jets of water occur, mountainous stream channels, individual rills, erosion-control diversions, etc.

Last Modified: 9/29/2014
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