Design, measurement, and sampling with drop-box weirs

Rangelands, surface mines, construction sites, unprotected and long slopes, gullies, eroding stream channels, and erosion plots 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, resulting in 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 sizes 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, was compiled. 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 tables and weir-sizing guidelines were summarized. Low-flow rating curves must be developed from field data 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. Water 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.