| Summary: | Typhoons in Taiwan may cause large increase in flow rates in its steep rivers, which in turn causes short-term general scour. This scour, along with other types of scour may undermine bridge safety. There were many studies done focusing on monitoring the change in scour depth during a flood, but not how a water profile is affected by short-term general scour, which can be significant. Furthermore, the water surface elevation is sometimes used as a gauge for the risk of bridge failure. As such, this study was conducted to understand the effect of short-term general scour on water surface profiles and if water elevation is a suitable method of risk assessment of bridges.
In this study, various steady and unsteady state simulations were done using HEC-RAS software. Under steady state simulations, the effect of varying scour length, scour depth and flow rates were tested individually. Under unsteady state simulations, hydrographs modelled after Typhoon Morakot in 2009 and Typhoon Soulik in 2013 were used to understand the combined effects of scour depth and flow rate on water surface profiles. The results of these simulations yielded a general trend that the elevations of scoured water surface profiles during typhoons are always lower than the elevations of non-scoured water surface profiles. The effect on elevations occurs before the start of the scour up to the end of the scour.
The water elevation at the start of the scour was then used to generate a chart of rating curves for varying scour depths. It was found that the decrease in elevation could be as much as 1m for a flow rate of 6000m3/s and a scour depth of 1.5m. However, these results were obtained for a scour hole that was introduced with a very steep slope and the exit from the scour hole was via an adverse slope. When a scour hole was created with a less steep entrance slope and a mild exit slope, the decrease in water elevation at the start of the scour for a scour depth of 2m and flow rate of 6000m3/s was only 0.4m. This suggests that further studies need to be done on how scour varies along the river length, including how it begins and ends to get a clearer model on the effect of short-term general scour on water surface profiles. However, current results show that the effect of short-term general scour can be significant for high flow rates (6000m3/s) and deep scour depths (2m or more).
|
|---|