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.
MECHANICS OF LOCAL SCOUR
THE BASIC MECHANISM OF LOCAL SCOUR, I.E., SCOUR DUE TO THE PIER ALONE IS STUDIED. IT HAS BEEN SHOWN THAT THE MECHANISM OF SCOUR AT A BLUNT NOSED PIER IS THE STRONG HORSESHOE VORTEX SYSTEM UPSTREAM OF A BLUNT NOSED PIER. THE MECHANISM WHICH FORMS THE HORSESHOE VORTEX IS THE PRESSURE FIELD INDUCED BY THE PIER. WHEN THE PRESSURE FIELD IS STRONG ENOUGH THE COMING FLOW BOUNDARY LAYER SEPARATES AHEAD OF THE PIER AND ROLLS UP TO FORM THE HORSESHOE VORTEX. PIERS AT WHICH THE HORSESHOE VORTEX SYSTEM DOES NOT FORM ARE CALLED SHARP NOSED. MAXIMUM SCOUR OCCURS AT THE UPSTREAM END OF A BLUNT NOSED PIER. THIS STUDY IS LIMITED TO BLUNT NOSED PIERS . THE MAGNITUDE OF MAXIMUM SCOUR IS INVESTIGATED ANALYTICALLY USING THE MOMENTUM PRINCIPLE, BY ATTEMPTING TO INTEGRATE THE NAVIER-STOKES EQUATION IN THE BOUNDARY LAYER IN THE SCOUR HOLE, AND BY TRYING TO SPECIFY THE RELATION BETWEEN THE STRENGTH OF THE HORSESHOE VORTEX AND LOCAL SCOUR . THIS STUDY HAS DEMONSTRATED THE EXISTENCE OF A BASIC FUNCTION BETWEEN SCOUR DEPTH AND THE PIER REYNOLDS NUMBER. FOR SANDS WHOSE MEDIAN DIAMETER IS LESS THAN 0.52MM, IT APPEARS THAT SCOUR DEPTH IS INDEPENDENT OF SEDIMENT SIZE. WHEN THE SEDIMENT SIZE IS LARGER, SCOUR DEPTH DECREASES AS THE SEDIMENT SIZE INCREASES FOR THE SAME HYDRAULIC CONDITIONS. THE TIME REQUIRED TO REACH 75 PERCENT OF THE MAXIMUM SCOUR DEPTH IS A FUNCTION OF THE APPROACH FLOW VELOCITY. LABORATORY STUDIES INDICATE THAT UP TO VELOCITIES OF 6 FPS, SUCH TIME WILL PROBABLY BE LESS THAN 100 MINUTES.
SCOUR AROUND BRIDGE PIERS IN STEEP STREAMS
This paper describes the results of a laboratory investigation to determine local scour depths around bridges piers. The experiments were designed to simulate steel gravel rivers with slopes between 0.005 and 0.02 and Froude numbers in the range from 0.8 to 1.2. A widely graded sediment, typical of these rivers, was used and equilibrium sediment transport rates were imposed. Effects of shape and skewness of the bridge piers were considered. The observed depths of scour are compared with depths calculated using existing empirical methods, and a new correlation between skewness and shape is proposed.