Providing the most effective solution for protection of the bed downstream of hydraulic structures constitutes an important field of research. In addition, for safe and cost-effective designs, it is important to be able to predict the maximum scour depth downstream of hydraulic structures. The study reported in this paper examined the effectiveness of two types of countermeasures – a bed sill and riprap – to reduce the maximum scour depth adjacent to horizontal rigid aprons. For tests with the bed sill located downstream of the apron, two distinct scour holes were formed upstream and downstream of the sill, with respective maximum scour depths of ds1 and ds2. The results show that a bed sill can reduce ds1 and ds2 by up to 23% and 47%, respectively, compared with the maximum scour depth for the no-sill condition. However, when the erodible bed between the apron and the bed sill was replaced by riprap, ds1 and ds2 decreased by up to 53% and 90% respectively. Variations of the other characteristic scour hole lengths, including the maximum extension of the scour hole and the maximum height of the mound, were also studied and an attempt was made to examine any geometrical similarities in the scour hole profiles under different conditions.
The influence of rigid vegetation on the longitudinal dispersion coefficient in a compound open channel was examined using an image processing technique. To simulate floodplain vegetation, cylinders of 5 mm diameter were attached to the floodplain surface. Potassium permanganate solution was used as a conservative tracer. Instantaneous velocity components were measured using particle image velocimetry. The results showed that, compared with non-vegetated conditions, floodplain vegetation decreased the depth-averaged longitudinal velocity and maximum tracer concentration by up to 83% and 12·5%, respectively. It was also found that the magnitude of the longitudinal dispersion coefficient, K, increased with the relative flow depth, Dr (the ratio of the floodplain to main channel flow depth). Furthermore, the value of K increased by up to 39·3% for vegetated tests compared with non-vegetated tests. Moreover, the results were compared with several previous empirical equations and the most appropriate equation for prediction of K in compound channels with partly vegetated floodplain was found to be that proposed by Fischer in 1975.
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