Experimental study on fluid energy reduction through a flood protection forest

Abstract

A flood protection forest is a conventional river-management measure that can fit well into the natural environment. This study focused on single-line tree models placed laterally across open-channel flows. We conducted horizontal particle image velocimetry (PIV) and drag force measurements in a laboratory flume to examine the reduction efficiency of mean kinetic energy due to the tree members. The results showed that the flow structure is related closely to a wake phenomenon for sparsely arranged trees, whereas it is similar to a jet for a dense arrangement. It was found that the drag force is related to the separation zone downstream of the trees. In particular, two neighbouring high-speed streaks can merge and become a uniform flow. The characteristic length of this process and incident velocity define the drag force. Furthermore, a semi-theoretical formula was developed that was found reasonable in predicting energy reduction efficiency.