Drop-Box Weir for Sediment-Laden Flow Measurement

Laboratory development and field experience indicates that a drop-box weir will perform extremely well under wide ranges of discharges and sediment loads. This weir has a central V-notch to improve the sensitivity at low flow rates. The drop box into which the water falls is placed immediately upstream of the measuring section of the weir to keep entrained sediment in suspension. The calibration of this weir can be established for a given size weir from a dimensionless relationship of gage head, H, and discharge, Q. This relationship holds for any weir size as long as the gage head does not exceed 0.8 times the V-notch depth. At greater gage heads, upstream approach conditions, i.e., streambed slope and the approach flow angle, influence the calibration and corrections must be made. Field performance has, to a great extent, verified laboratory predictions.