A momentum-based prediction model of backwater rise within and downstream of an emergent canopy in nonuniform flow

Abstract

Vegetation is widely used to restore waterways and wetland ecosystems. However, emergent canopies create a nonuniform water surface, and predicting the backwater rise is necessary to evaluate the flood hazard. This paper investigates the water surface elevation from downstream to upstream of an emergent canopy in a steady nonuniform flow and predicts the upstream backwater rise. Laboratory flume experiments are conducted on rigid vegetation with varying coverage exposed to different flow rates. The backwater rise and the nonuniformity of vegetated flow are explored based on a momentum equation, and a general formula for backwater rise within and downstream of an emergent canopy is proposed for nonuniform flow. The results reveal that a unified phase diagram of backwater rise can be derived from the momentum model, which presents the variation of both the upstream and downstream water surface elevations and acquires a threshold to identify strong or weak nonuniformity in vegetated flow. The upstream water depth H1 exhibits a nonmonotonic variation, first decreasing and then increasing with water depth H2 at the trailing edge of the emergent canopy. Two partitions of subcritical flow and supercritical flow for the downstream water depth H3 are quantified by the experimental data. The proposed model can be used to accurately predict the backwater rise based on a dimensionless vegetated parameter Cveg and unit discharge q, as demonstrated by both the present and previous experimental results. This work improves the understanding and predictability of the nonuniformity and backwater rise generated by emergent canopies in natural or ecology-restored rivers.