This study compared downstream scouring between permeable and impermeable groundsill. Site investigations were first performed at the Dajia River Bridge and a flume experiment with a scale of 1 to 100 was designed based on field data. The discussion focuses on the differences between permeable and impermeable groundsill. Field investigation showed that severe localized scouring occurs downstream of a impermeable groundsill, whereas deposition occurs downstream of a permeable groundsill. Laboratory experiments showed that for an impermeable groundsill,
scouring is much deeper than for a permeable groundsill. The main cause is that a portion of flow penetrates the permeable groundsill and reduces the over-weir flow over the groundsill, thereby reducing the impact on the surface bed.Furthermore, increased penetrated volume allows for some of the vertical flow to flow laterally downstream because of a decrease in water height. The reduction in over-weir flow is correlated with porosity. A porosity of 0.27 (K≈1.25cm/s) reduces the flow by 7%, and a porosity of 0.34 (K≈1.25 cm/s) reduces the flow by 20%. Greater porosity cause greater reduction in over-weir flow, the impact on the surface bed.
Key Words: flow pattern, groundsill, flume experiment, permeability, scouring at base point, penetrating flow, overweir
flow, porosity