Abstract:
The aim of this research is to investigate the effects of the Ping River morphology changes on flood flow in the vicinity of Chiang Mai city between the RID stream flow gauging Stations P.67 and P.1. The data used in this study consisted of the Ping river configuration and geometry together with observed stage and discharge data collected from the Royal Irrigation Department (RID) and agencies concerned. The study showed the insignificant change of profile characteristics and plan form of the Ping River. However, it has been found the increase in the river section factor (AR2/3) by 40 percent, approximately. By using the HEC-RAS 4.0 to simulate the flood events of the same water level and the same discharge at Station P.1, the results indicated that the conveyance capacity of river estimated using 2007 river configuration data was greater than what estimated from 1999 river data by about 14 percent. In the case of using the same 2006 floods for channel capacity study , it was found that average water level of 2007 channel was lower than 1999 channel about 1.5 and 0.6 m. for out of and in the vicinity areas respectively. Moreover, this study also performed river configuration simulation on flood flows by modifying channel cross section, channel slope and channel cutoff and the combination of all above. The result showed that combined channel modification provided the optimum solution that yielded a decrease in water level approximately 0.6 and 0.8 m. for out of and in the vicinity areas, respectively. The investigation of rainfall/runoff and land use change in the upper Ping river basin indicated an increase tendency in the maximum flood peak and annual runoff at P.1 station whereas, the maximum 1-3 day cumulative rainfall and the annual rainfall showed unchanged tendency. Additionally, an increased in flood peak was lightly and reciprocally related to the forest area reduction.