Effect of plant basal cover on velocity of shallow overland flow

Abstract

Accurate estimation of mean flow velocity is imperative for the prediction of hydrographs and sediment yield. The Chezy and Manning equations are the most widely used for calculating flow velocity in runoff and erosion models. The suitability of both equations for overland flow on bare land has been evaluated, and conflicting results were obtained in previous studies. The suitability of these equations for overland flow on a planted slope requires further investigation. The purposes of this study were to evaluate the suitability of the Chezy and Manning equations under planted conditions and evaluate quantitative effect of vegetation on Manning’s n and then on flow velocity. Flume experiments were conducted at the plant basal cover ranging from 0 to 30%, slope gradient varying from8.8% to 25.9%, and flow discharge ranging from 0.5 to 2.0 × 10−3 m3 s−1. The flow depth and flow velocity were measured. The coefficient of determination (R2) and Nash–Sutcliffe model efficiency (NSE) were used to compare the performance accuracy of both equations. The results showed that both equations provided a bad accuracy when plant basal cover was greater than 15% on a planted slope. The NSE values were negative for Manning equation and close to 0 for Chezy equation. It indicates that Manning and Chezy equations were not suitable for the planted slope. Manning’s n decreased with the flow depth for 0% plant basal cover and increased with flow depth for plant basal cover greater than 1.25%. Thus a new equation for calculating Manning’s n was developed based on plant basal cover and flow depth and was used in the Manning equation. The new flow velocity equation including plant basal cover provided satisfactory accuracy with a NSE of 0.995. The results indicate that the Manning’s n was significantly affected by flow depth and plant basal cover.