Author(s): Chong-Yu Sun; Shaohua Marko Hsu
Keywords: Flow pattern; Groundsill; Flume experiment; Permeability; Scouring
Abstract: Due to the geographical features of Taiwan, with steep slope and fast flow, river scouring and deposition are quite severe during flood peaks. In order to stabilize riverbed elevation, groundsill is a common feature as a protective structure on riverbed. While flow passes through the interface between groundsill and riverbed, local scour usually occurs downstream of the groundsill. Hence the need to place energy-dissipation structures downstream of the groundsills as a protective measure is also very common. Once damage occurs to this protective measure, the stability of the groundsill itself will be affected. As a consequence, the subject of downstream scouring at groundsill has been a major research topic in Taiwan. This study is aimed to compare downstream scouring between permeable and impermeable groundsill. Site investigations were first performed at the Dajia River Bridge and then flume experiments, with a scale of 1 to 100, were designed based on field data. This discussion is focused on the differences between permeable and impermeable groundsill. Field investigation showed that severe localized scouring occurs downstream of an impermeable groundsill, while deposition occurs downstream of a permeable groundsill. Laboratory experiments showed that for an impermeable groundsill, down cuts much deeper than that of a permeable groundsill. The main cause is that a portion of flow penetrates through the permeable groundsill and reduces the over-weir flow on top of the groundsill, thereby reducing the impact on the bed surface. Furthermore, increase in the penetrated volume allows for some of the vertical flow to flow laterally downstream due to a decrease in water height. The reduction of over-weir flow is correlated with porosity; a porosity of 0.27 (K≈1.25 cm/s) reduces flow by 7%, a porosity of 0.34 (K≈1.25 cm/s) reduces the flow by 20%. A greater porosity allows for a greater reduction in over-weir flow, thus decreasing the impact on the surface bed.