A Laboratory Study of the Shallow Flow Field in a Vegetated Compound Channel

Abstract

The significant defensive role of vegetation in general and mangroves in particular for coastal and estuarine regions has been increasingly recognized. However, understanding the shallow flow field in and around the region of vegetation is still limited. In order to gain more insight, a laboratory experiment of a vegetated compound channel was conducted. The emerged circular cylinders are a representative model for the emergent mangrove forest. Scenarios of different widths and densities of vegetation were considered. Data acquired from Acoustic Doppler Velocimetry (ADV) and force sensors have been analysed. The influences of vegetation on the shallow flow field were clarified by comparing different scenarios with and without vegetation. Furthermore, the results confirm the presence of the large horizontal coherent structures (LHCSs). The large coherent structures formed in the mixing layer at the vegetation interface emerge, promoting the transverse momentum exchange toward the forests. The LHCSs has not only boosted flow penetration into the vegetated floodplain area but also strongly disturbed the flow inside the forest. As the LHCSs move, they cause the fluctuation of the force on the cylinders. The fluctuations are largest at the vegetation edge. Negative values of stream wise forces were also recorded.