Effects of Added Rigid Vegetation on the Bar Formation: Results from Full-scale Experimental

Abstract:

River corridors were characterized by island and floodplain development driven by the inter-play of flows, sediments and vegetation. This study used a natural stream as an experimental site with the site was 60m long by 30m wide, which was in downstream of Landao Creek, Huisun forest, Taiwan. The field experiment was designed to investigate the effects of river processes on bar, and the effects vegetation dynamics had on bar formation, which cause by upstream dam broke. To analysis how vegetation impact on bar-form and bar-stabilization, we used wood piles (4.4×4.4×80cm) as natural rigid vegetation, which inserted into the bar upstream.

In the field site, we chose 6 site to set wood piles, each site was designed 4 row and were total 11 piles within, which set at in-row and in- line spacing of 50×20cm. According to the results, two primary phase of forming bar in the fluvial processes were discussed. First, in the flood period, presence of piles could trap large gravel in the front and turn the gravel into deposition. When the deposition gradually accreted, the front of deposition would induce flow to divide, let flow disperse into two branches around the deposition; some coarser particles rather are transported along the bar surface with momentum of flow movement and deposited at the top of bar, than are transported along the diversion channel around the bar (the median grain size of bar top were coarser 50% than before). Second, in the flood recession period, bar quickly accreted portions of passing bed load sheets and grew laterally and in the headward direction. Phenomenon showed that the composition of grain size were decreasing in the headward direction, which flow entrained were getting smaller with flood recession (the median grain size between top and the front of bar differed 30%).