Technical Studies


Study on Channel Migration and Stable Water Supply Alternatives of Kao-ping Weir Reach After Typhoon Morakot

During 2005~2006, several significant typhoon floods resulted in serious depositions near the intake of Kao-ping weir. To ensure the stable water supply, South Region Water Resources Office, WRA had carried out several stable water supply countermeasures in 2006~2008, including the rising of Kao-ping weir, building of left-bank spur dikes, etc. However, typhoon Morakot occurred in 2009 had an extreme discharge with the peak near 35,000cms at Kao-ping weir that exceeded the design-discharge at Kao-ping weir (26,800cms). The water stage over-topped the embankment, and resulted in huge fatalities and property damages. After typhoon Morakot, the meandering pattern of Kao-ping River and sediment concentration increase obviously. Hence, the study of channel migration and stable water supply countermeasures of Kao-ping weir reach is required.

In this study, a two dimensional mobile-bed model called CCHE2D was adopted to simulate the channel migration of Kao-ping River. The study reach is from the upstream Li-ling bridge to the downstream Kao-ping bridge. Inflow sediment concentration, bed-material grain size and cross-sections of Kao-ping River were surveyed and analyzed. These basic data were used to calibrate and validate the numerical model. Meanwhile, five stable water supply plans at Kao-ping weir were proposed to evaluate the effects and to suggest further possible countermeasures. The over-topping of embankment under the desigh discharge and flood of typhoon Morakot based on the present morphology were also evaluated.

According to the simulation results, the CCHE2D model had the capability of simulating the velocity magnitude, sediment transport and bed changes in the study reach. The model was able to correctly simulate the variations of water levels of unsteady flow at the Li-ling bridge and Kao-ping weir. The performance of CCHE2D is satisfactory, judged from the validation results.

In the simulation of stable water supply plans, all plans had deposition trends along the Kao-ping River under several large flood events. Local scouring was occurred near the Lao-nong Creek where connected with the Kao-ping River. The downstream reach of railway bridge also had significant local scouring. Based on the present bed morphology, the deposition depth were nearly 0.5~1.0 meters along the stream after Q10 flood. Further study on the mid-term and long-term channel migration is needed to propose a suitable stable water supply countermeasure at Kao-ping weir.