Effect of emergent vegetation on riverbank erosion with sediment mining

Abstract

The present work investigates the combined effects of the upstream sediment mining pit and vegetation on the riverbank using emergent rigid vegetation beyond the toe on the flow structure and morphological changes due to fluvial erosion. A steep gradient of streamwise velocity and other turbulence parameters such as Reynolds shear stress (RSS), transverse RSS, and turbulent kinetic energy (TKE) at the interface of the vegetated and unvegetated part of the test segment was observed. The cross-sectional analysis showed that vegetation increased the velocity of the unvegetated main channel, and the sandpit increased even the near-bed velocity with a similar trend in its longitudinal variation at the center line of the main channel. The abrupt variation in RSS and transverse RSS at the location of the berm induces instability and erodes the berm present at the toe of the riverbank. The combination of the vegetation and sandpit led to increased TKE of the flow at the near-bed and berm locations. The morphological analysis showed complete riverbank erosion in both cases of the unvegetated riverbank, i.e., without or with an upstream pit. The installed stems of rigid vegetation on the riverbank helped decrease the fluvial erosion of the riverbank, and its profile observed minimal changes over the length of the test segment. However, the main channel erosion was amplified due to the vegetation (in no-pit case) at the beginning of the test segment, which eroded the bed of the main channel by about 67% of the bed thickness. Also, in the vegetated riverbank cases, the upstream pit caused an increase in erosion by 7.66% at the center of the main channel. The study helps establish the hypothesis of negating the effects of sediment mining on bank erosion by using the rigid vegetation on the riverbank beyond its toe location, which performed well by maintaining the riverbank profile.