THE INFLUENCE OF GRASS AND POROUS BARRIER STRIPS ON RUNOFF HYDROLOGY AND SEDIMENT TRANSPORT

摘要

A series of experiments was conducted in a large tilting flume to investigate the effects of buffer strips on flow hydrology and sediment transport/deposition in and around the strips. Changes in flow depth caused by buffer strips of either nails or grass were recorded, photographed, and measured with a high degree of accuracy. Flow retardation took place at some distance ahead of the strips, causing the water level to rise. This distance is dependent upon flume slope and strip density for any given flow rate. With any increase in flume slope, the point at which water depth increased moved closer to the strip, entering it at around 6% slope. An exponential relationship exists between flume slope and backwater length. Backwater length is also dependent on strip density, and the relationship between these two factors is linear. Under our experimental conditions, sediment deposition did not take place within the strips, but before and after it. The lack of deposition inside the strips appears to be contrary to the common expectation from this technique. The bulk of sediment load in the sediment-laden flow approaching the strips was deposited ahead of the strips, commencing at the point where flow depth started to rise. The finer fraction of sediment load that entered the strip with the flow emerged from the other end unchanged. Some deposition took place as fans downstream of the strips, an indication of resistive flow velocity being slower before and after the strips than within them. When the soil or sand were not consolidated, significant erosion took place inside the strips, creating a head fall at the exit end of the strips, which moved upslope within the strips as experiments continued. For the range of slopes and strip widths studied, the efficiency of the grass or nail strips in slowing down the flow and unloading its sediment in the backwater region was independent of the width of strips in the flow direction. Grass strips thus appear to behave more like "grass barriers" or "grass buffers" than "filter strips," as they are referred to in some literature. The process interpretation of these results is discussed in this article