Weirs

“Weirs are preferred to flumes for measuring flow in small open channels at embankments because a weir traps sediment and debris possibly produced by internal erosion, increasing the likelihood the sediment is noticed. However, the open channel conveying the seepage must have enough vertical drop for a weir. If the channel grade does not allow the weir to function as an unsubmerged weir, a flume may be necessary instead”.

“Sharp-crested weirs are simple open channel structures well suited to measuring the range of seepage and drainage discharge from most embankments. The discharge is a function of head, determined by measuring the water level in a stilling basin on the upstream side of the weir notch. The head is equal to the difference between the water surface elevation at that stilling basin and the lowest point along the weir crest. The notch is a standardized shape cut into a metal plate or other material capable of sustaining a sharp edge… Selection of a notch shape for a weir is a trade-off between the capacity and accuracy afforded by V-, rectangular-, and trapezoidal-notch weirs… V-notch weirs are typically used for low flow, are very accurate for low flows less than 1 cubic foot per second (cfs) (0.0283 cubic meters per second) and are reasonably accurate for flows as great as 10 cfs (0.283 cubic meters per second). Rectangular-notch weirs can accurately measure greater flows than a V-notch weir, and a minimum crest width of on foot (30.5 cm) is recommended. Trapezoidal-notch weirs are used for greater flows but have a lower accuracy than V or rectangular notches”.

“Due to the difficulty of predicting seepage rate, weirs sometimes prove too small or too large for the flows encountered. However, weirs should first be sized for the expected range of flow and then modified if ill-suited to the encountered flows. Poorly sized weirs are inaccurate due to undesirable nappe formation, fault head measurement, or submergence”.

Revision ID: 4929
Revision Date: 12/05/2022