Prediction of Maximum General Scour Depth during a Flood for Intermittent Rivers

摘要

Most rivers in Taiwan are intermittent rivers. A short-term general scour usually occurs in the river during the risingrnlimb of a flood due to the transitory sediment imbalance and the steep slope gradient (Lu et al., 2008). However, duringrnthe recession limb the entrained sediment particles tend to deposit on the riverbed to nearly the initial mean bed levelrnbefore the flood if the river is in quasi-equilibrium condition over the long term. This type of short-term general scourrnhas caused many casualties in Taiwan such as bridge and embankment failures, e.g. the Houfeng Bridge failure inrnDajia River (Typhoon Sinlaku, peak flow Qp= 4,225 m3/s, 2008) and the Shuangyuan Bridge failure in Gaoping Riverrn(Typhoon Morakot, Qp= 27,360 m3/s, 2009). In this study, the maximum general scour depths of the floods werernmeasured using the “numbered brick column” technique for both gravel-bed and sand-bed rivers. A maximum generalrnscour depth of 4.5 m was measured near the river reach of Highway Bridge hydrological station (D50 = 96 mm, S0 =rn1/90) in Dajia River, which was caused by a flood induced by Typhoon Sinlaku with a corresponding peak scoured flowrndepth of 7.73 m. The experimental results indicate that the short-term general scour for the gravel-bed intermittentrnrivers is usually very significant and cannot be neglected as it is one of the most important factors causing the bridgernfailure (Lu et al., 2007). Based on the data collected in the Dajia River and Choshui River, a general scour formula wasrntentatively developed with consideration of both the flow and sediment characteristics to predict the peak scoured flowrndepth. It was found that the proposed equation gave better predictions as compared with the Blench’s (1969) formula.rnIn conjunction with the real-time ultrasonic water level gauge, the formula can be used as an efficient tool for thernbridge closure or emergency evacuation purposes.