Effect of density of riparian vegetation on effective tractive force

Abstract

This study aims to clarify the effect of density of riparian vegetation on effective tractive force for various bedload transport conditions. Hydraulic model experiments were performed using a straight channel, and the methods used to calculate the effective tractive force necessary for estimating bedload transport rate in channels with riparian vegetation were studied. An apparatus was developed to directly measure the effective tractive force in a channel with riparian vegetation. Applying the apparatus in a hydraulic experiment allowed accurate direct measurement of effective tractive force, formerly estimated indirectly. Based on the experimental results, the effect of the density of vegetation on the effective tractive force was examined. Then the methods for estimating effective tractive force and bedload transport rate were studied. As a result of the experiment, it was found that the coefficient of velocity U′/U′_* and the ratio of coefficient of velocity (U′/U′_*)/(U/U_*) are negatively correlated with the density of vegetation. A very high negative correlation was also obtained between recession coefficient of effective tractive force (β = effective tractive force of a flow with riparian vegetation/effective tractive force of a flow with no riparian vegetation) and the density of vegetation. The friction factor of the channel bed f′ was positively correlated with the density of vegetation. Methods were presented for accurately estimating effective tractive force using equations including the relationships of the density of vegetation to the coefficient of velocity, the ratio of the coefficient of velocity, and the recession coefficient of effective tractive force. Using the estimated effective tractive force and an existing bedload equation enabled very accurate calculation of bedload transport rate in flows through riparian vegetation. The methods presented in this study have potential to be developed as a tool that can provide guidance in the planning and designing of riparian vegetation that are intended for the prevention and mitigation of floods and resultant sediment transportation.