Improving bridge afflux prediction for overbank flows

Tests were carried out to investigate bridge afflux phenomena for bridge constrictions in a compound channel flume. The flume had different roughness configurations in the main channel and floodplains. All tests were undertaken for the condition of no eccentricity and no entrance rounding. The normal crossing (ø = 0°) and two different skew angles (ø = 30° and 45°) were tested for different arch and flat soffit type bridge models. For each set of experiments, the Froude number for normal depth, afflux ratio and blockage ratio (area of bridge below water level to total flow area) were obtained based on an analogical method developed previously. These quantities were analysed to obtain a simple practical afflux equation. This afflux equation was also applied to field data collected by the US Geological Survey and the results were compared with those obtained using the more complex methods such as HEC-2, WSPRO and USBPR.