SCOUR AROUND BRIDGE PIERS APPLYING STREAM POWER APPROACH
The current research aims introducing the problem of scour around bridge piers from a new point of view. The classical approach for tackling this problem usually depends on applying empirical equations developed from the analysis of experimental results with the use of dimensional analysis. Thus, in the current research an intensive study for evaluating the existing scour predictor was performed. The comparison processes through comparing the results of some laboratory experiments performed for scour around bridge piers and the predicted scour depth from some selected available predictor equations. About 156 experiments were carried out to perform the current study in accordance with 15 predictor equations. The results show that the scour predictors have a low reliability for following a specific trend or giving accurate results. Thus, it has been considered that it is of great importance to study the scour problem around bridge piers through a new approach. Stream power approach has been selected for achieving this objective. A one dimensional numerical model for open channel hydraulic was selected to stimulate the problem of flow and scour around bridge piers. Stream power variation is studied versus several parameters such as scour depth, sediment type and flow conditions. Several laboratory experimental studies were tested and stream power values in these different cases were analyzed. Results show that there is a significant relationship between scour depth escalations and stream power value. The relationship between stream power and scour depth was found to have a specific trend through which a new predictor could be developed. The new predictor was applied on some experiments and the estimated scour depth was compared to the experimental results. It was found that there was a good agreement between the experimental results and the scour calculated using the stream power approach. Finally, it can be concluded that the new approach is considered more reliable than the classical methods for estimating the scour depth at bridge piers. However more studies are needed to perform an inclusive idea about the application of stream power approach for pier and abutment scour problem.
Clear-water scour development at bridge abutments
Development of local scour depths at vertical-wall bridge abutments of varying lengths was investigated in several series of experiments of ranges of uniform sediments and clear-water flow intensities. For each flow-abutment-sediment combination tested, the change with time in maximum local scour depth from plane-bed to equilibrium conditions was recorded and analysed. The results of similar earlier abutment-scour experimental studies were also incorporated in analyses of scour development. Variations in scour rates and depths with flow and sediment parameters are found to be different for short (flow depth/abutment length = y/L > 1) and long (y/L ≤ 1) abutments. In order to provide a consistent framework for assessment of the development of scour depth with time, a revised definition of the time to achieve equilibrium conditions is developed. Based on this definition, the dimensionless time to equilibrium for scour development from plane-bed conditions can be expressed as a function of relative flow intensity and relative abutment length. This expression can be adopted into existing methodologies to predict scour magnitudes occurring at bridge abutments. Utilising extensive data sets, expressions enabling the determination of scour depths at abutments, and also piers, are reassessed for the present definition of time to achieve equilibrium conditions. Application of the developed expressions is highlighted in an example.
Effects of Time and Channel Geometry on Scour at Bridge Abutments
Experiments are described to investigate local scour at bridge abutments. The experiments were performed in a two-stage channel using abutments that extended different distances onto the floodplain including right up to the edge of the main channel (Melville, Type III). To ensure the largest scour depths the conditions on the floodplain upstream of the abutment were close to critical conditions for the bed material. The time evolution of the scour and the ultimate scour depth were measured. The time development of the local scour corresponded well with the theories of Ettema and Franzetti and the theory of Whitehouse for scour at horizontal cylinders in the marine environment. Melville has suggested that scour at abutments on floodplains can be approximated by scour in rectangular channels if an imaginary boundary is assumed, separating the flow in the main channel from that on the floodplain. The experimental results confirm the validity of Melville's suggestion for the configurations tested in the experiments.