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E. STEP 5. LOCAL SCOUR AT ABUTMENTS 1, General, Equations for predicting scour depths are based almost entirely on laboratory data, For example, Liu, et aI's (1961) (25), Laursen's (1980) (26) and Froehlich's (1989) (27) equations are based entirely on laboratory data. The problem is little field data on abutment scour exist, Liu, et aI's equations were developed by dimensional analysis of the variables and a best-fit line was drawn through the laboratory data. Laursen's equations are based on inductive reasoning of the change in transport relations due to the acceleration of the flow caused by the abutment. Froehlich's equations are derived from a regression analysis of the available laboratory. data. EQUATIONS FOR ABUTMENT SCOUR ARE FOR THE WORST-CASE CONDITIONS. THEY WILL PREDICT THE MAXIMUM SCOUR THAT COULD OCCUR FOR AN ABUTMENT PROJECTING INTO A STREAM WITH VELOCITIES AND DEPTHS UPSTREAM OF THE ABUTMENT SIMILAR TO THOSE IN THE MAIN CHANNEL. The reason for this is the way the experiments were conducted which do not represent many of the conditions in the field. For example, Liu's experiments were made in a rectangular laboratory flume with a sand bed. The abutments projected out various lengths from one wall or occasionally both walls of the flume . When they projected out from one flume wall then the other wall was taken as the centerline of the bridge. Other researc~ was conducted similarly. Thus, the velocity, depth and sediment transport upstream of the abutment were about the same as in the main channel. Field conditions may have tree lined or vegetated banks, low velocities and shallow depths upstream of the abutment. If there is overland flow it often is at a shallower depth and lower velocity, with little bed material transport. THEREFORE, ENGINEERING JUDGEMENT IS REQUIRED IN DESIGNING FOUNDATIONS FOR ABUTMENTS. IN MANY CASES FOUNDATIONS CAN BE DESIGNED WITH SHALLOWER DEPTHS THAN PREDICTED BY THE EQUATIONS . , AND THE FOUNDATIONS PROTECTED WITH RIPRAP PLACED BELOW THE STREAM BED OR A GUIDE BANK (SPUR DIKE) PLACED UPSTREAM OF THE ABUTMENT. COST WILL BE THE DECIDING FACTOR. A METHOD TO DETERMINE THE LENGTH OF A GUIDE BANK IS GIVEN IN APPENDIX C. 2. Abutment Site Conditions. Abutments can be set back from the natural stream bank or can project out into the flow, They can have various shapes (vertical walls, spill through slopes) and they can be set at an angle to the flow Scour at abutments can be live-bed or clear- water scour, Finally, there can be varying amounts of overbank flow intercepted by the approaches to the bridge and returned to the stream at the abutment. 45