Flow measurement by vegetated weirs: An investigation on discharge coefficient reduction by crest vegetation

Abstract

Flow measurement using weirs as an engineered hydraulic control has been well studied and understood, but the presence of vegetation on the weir crest or top of weir-like structures in natural waterway has rarely been considered. This technical note presents a quantitative assessment of the effects of crest vegetation roughness on flow estimation. Experiments are carried out to record the discharge coefficient reduction of finite-crest-length weirs when the weir crests are covered by flexible artificial vegetation. The effects of equivalent roughness height of the vegetation canopy on the weir discharge capacity are evaluated, in addition to the variation of flow rate and weir crest length. According to the ratio of upstream head to vegetated crest length, the tested flows are classified as broad-crested and narrow-crested weir flows, and some change in discharge coefficient deterioration rate is observed for the two types of weir flows. Empirical equations are proposed for prediction of discharge coefficient thus flow estimate correction by taking into account the vegetation resistance effects. The results may contribute to improve the accuracy of hydrological modeling and flood forecasting, although vegetation degradation during floods is not considered.