DESIGN, MEASUREMENT, AND SAMPLING WITH DROP-BOX WEIRS

Authors: Bonta, James - Jim; Pierson Jr, Frederick

Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/6/2003
Publication Date: 12/1/2003
Citation: BONTA, J.V., PIERSON JR, F.B. DESIGN, MEASUREMENT, AND SAMPLING WITH DROP-BOX WEIRS. APPLIED ENGINEERING IN AGRICULTURE. 2003. v. 19(6). p. 689-700.

Interpretive Summary: Unprotected land surfaces such as rangelands, surface mines, construction sites, gullies, etc. will cause large sediment loads under high intensity rainfalls. Conventional flow-measuring devices can easily become clogged with sediment and debris during a major runoff event, with the loss of runoff and sediment records. The drop-box weir (DBW) was developed to overcome many of the problems encountered in sediment-laden flow measurement. The weir creates turbulence in a box that entrains and passes sediment through the weir. It is not a well known device and it has not been widely used, yet it is only one of two devices suitable for measuring flows with large sediment concentrations. Information on what is known about the design and operation of the DBW, and of sediment sampling approaches using the DBW, were accumulated, and weir sizing, rating-curve development, and sampling strategies were presented to facilitate its use, and to identify its limitations. There are four known configurations of the DBW; a modification of the DBW for erosion plots; a modification of the DBW for small watersheds in steep channels; and a Korean version of the weir. For each of the four configurations, rating table and weir-sizing guidelines were summarized. Low-flow measurements that define a relationship between flow rate and depth of water (rating curve) must be developed for individual weirs, but laboratory-developed curves can be used for larger flow rates. Mathematical procedures are outlined specifically for determining rating-curve equations where field data are obtained. Samplers designed specifically for use with DBWs are described. Other design considerations are discussed for practical use of DBWs including measurement of stage, maintenance, and sediment traps. Research needs for hydraulic modeling and sediment sampling are presented. The results have practical utility for researchers to accurately measure flows having large sediment concentrations where conventional flow-measuring devices would fail.

Technical Abstract: Rangelands, surface mines, construction sites, unprotected and long slopes, gullies, eroding stream channels, and erosion plot will yield large sediment loads under high intensity rainfalls. Conventional flow-measuring devices can easily become clogged with sediment and debris during a major runoff event, with the loss of runoff and sediment records. Flow measurements can also be inaccurate using conventional flow-measuring devices in steep channels. The drop-box weir (DBW) was developed to overcome many of the problems encountered in sediment-laden flow measurement. The weir creates turbulence in a box that entrains and passes sediment through the weir. It is not a well known device and it has not been widely used, yet it is only one of two devices suitable for obtaining flow records with large sediment concentrations. It has utility for a range of watershed size from small erosion plots to large watersheds. Information on what is known about the design and operation of the DBW, and of sediment sampling approaches using the DBW, were accumulated and weir sizing, rating-curve development, and sampling strategies were presented to facilitate its use, and to identify its limitations. There are four known configurations of the DBW, the original weir with upper weir lips; a modification of the DBW for erosion plots (removal of upper weir lips); a modification of the DBW for small watersheds in steep and skewed channels (removal of upper weir lips and use of baffle); and a Korean version of the weir (larger chute opening to minimize blockage of trash - suitable for large and small watersheds). For each of the four configurations, rating table and weir-sizing guidelines were summarized. Low-flow rating curves must be developed for individual weirs, but laboratory curves can be used for larger flow rates. Curve-fitting procedures are outlined specifically for determining rating-curve equations where field data are obtained. Samplers designed specifically for use with DBWs are described. Other design considerations are discussed for practical use of DBWs including measurement of stage, maintenance, and sediment traps. Research needs for hydraulic modeling and sediment sampling are presented.