A Laboratory Sensitivity Study of Hydraulic parameters Important in the Deployment of Fixed-In-Place Scour-Monitoring Devices

INDOT is considering the deployment of fixed scour-monitoring instrumentation as part of a systematic response to the problem of scour around bridge piers. Fixed-in-place automated scour-monitoring devices measure the depth of scour at the point where they are installed. Unless they are installed at the location where maximum scour occurs, their readings may give a misleading estimate of scour potential. This raises a number of technical issues concerning their effective deployment and the interpretation of the collected data, which should be resolved in order to maximize the benefits to INDOT from using these monitoring devices. These include the location of maximum scour under different pier configurations and hydraulic conditions, as well as possible effects due to pier skewness and exposed footings. This report describes laboratory experiments that were performed to study these issues for various flow conditions and pier configurations. The experiments were conducted in a straight channel of rectangular cross-section located in the Hydromechanics Laboratory at Purdue University. The pier geometry was the same in all experiments, and was chosen to be similar to that of a prototype pier (in the US 52 bridge over the Wabash River), though exact scaling was not attempted. Both single and double (i.e., one upstream and one downstream) piers were studied. In general, for piers that were not skewed with respect to the flow, the location of maximum scour was as expected found in the immediate vicinity of the upstream nose of the up-stream pier. Nevertheless, even with relatively small skewness, for flow angle of attack less than or equal to 10°, the location of maximum scour may actually move downstream, either to the side of the pier or even away from the more upstream pier towards the downstream pier in the double-pier arrangement. Measurements at the nose of the upstream pier may therefore grossly underestimate scour in downstream regions if even slight skewness occurs during floods. Experiments also indicated that, under clear-water conditions, an exposed footing may actually serve to arrest the deepening of the scour hole. Under the live-bed high-transport conditions more likely to prevail in Indiana streams, the experimental results suggest that this may be more questionable