Evaluation of pier-scour measurement methods and pier-scour predictions with observed scour measurements at selected bridge sites in New Hampshire, 1995-98

806969PDFTech ReportFHWA-NH-RD-12323EBridge piersScourMonitoringGround penetrating radarStreambedsFloodsMeasuring methodsMathematical predictionEquationsField studiesNew HampshireNew Hampshire. Dept. of TransportationBoehmler, Erick M.Olimpio, Joseph R.Geological Survey (U.S.). Water Resources Division. New Hampshire-Vermont DistrictUnited States. Federal Highway AdministrationNTL-HIGHWAY/ROAD TRANSPORTATION-Bridges and StructuresUS Transportation CollectionIn a previous study, 44 of 48 bridge sites examined in New Hampshire were categorized as scour critical. This report summarizes research conducted to evaluate pier-scour measurement methods and predictions at many of these sites. This evaluation included measurement of pier-scour depths at 20 sites using ground-penetrating radar (GPR). Pier-scour was also measured during floods by teams at 5 of these 20 sites. At 4 of the 20 sites, fixed instruments were installed to monitor scour.At only one bridge site investigated by a team was any pier scour measurable during a flood event. Measurements made using GPR and/or fixed instruments indicated pier scour at 6 sites. The GPR surveys indicated scour along the pier side and further upstream from the nose that was not detected by flood-team measurements at 2 sites.Most pier-scour equations selected for this examination were reviewed and published in previous scour investigations. Graphical comparisons of residual pier-scour depths indicate that the Shen equation yielded scour depth predictions closest to those measured, without underestimating. Measured depths of scour, however, were zero feet for 14 of the 20 sites. For the Blench-Inglis II equation and the Simplified Chinese equation, most differences between measured and predicted scour depths were within 5 feet. These two equations underpredicted scour for one of six sites with measurable scour. The underprediction, however, was within the resolution of the depth measurements.The Simplified Chinese equation is less sensitive than other equations to velocity and depth input variables, and is one of the few empirical equations to integrate the influence of flow competence, or a measure of the maximum streambed particle size that a stream is capable of transporting, in the computation of pier scour. Absence of a flow-competence component could explain some of the overprediction by otherequations, but was not investigated in this study. Measurements of scour during large floods at additional sites are necessary to strengthen and substantiate the application of alternatives to the HEC-18 equation to estimate pier scour at waterway crossing in New Hampshire