Abstract -- This project is a study on the warning model of debris flow occurrence by using the rainfall and the electrical conductivity of groundwater. The purposes are mainly to investigate the hydrological and physiographic factors which trigger debris flows, and to evaluate the critical rainfall lines of debris flow occurrence and their revisions for the warning of debris flow after the Chi-Chi earthquake. Besides, a preliminary study was conducted on the observation and theoretical analyses of the electrical conductivity (EC) of groundwater.
In this study, the Nan-Tou county was selected as the test site and 28 watersheds in this area were selected as the samples. After examination by using the Mann-Whitney-Wilcoxon test, it was found that among the 14 factors, four factors were relatively highly related to debris flow occurrence. The four factors were : landside area, landuse factor, percentage of the soil particle greater than sieve No.4 and effective watershed area. Equations for predicting debris flow occurrence in the Nan-Tou area were proposed by using Fisher’s discriminant function of multivariate statistical analysis. After the Chi-Chi earthquake, the potential of debris flow of the streams increased. Accordingly, the peak ground acceleration (PGA) of the earthquake and a function decreasing with time were used to revise the critical rainfall line for each watershed after the great earthquake. In regard to the relationship between EC and debris flow occurrence, to evaluate the feasibility by using the EC of groundwater to predict the occurrence of debris flow, a number of tests by using seepage box and water quality tests were conducted. It was found that four compositions, namely, Cl-, SO4-, Ca2+ and NO3-, induced the EC value to increase.
In the site, considering the factors of environment and electrical power supply, a real-time analyzing system was installed at the Hsin-Hisn bridge which was about 2 km away from the Long-Hua elementary school. The real-time rainfall and EC data observed from the station at the Hsin-Hsin bridge was transmitted to the system at the Long-Hua elementary school by using the GSM (wireless transmission) system. After analyzing, the information was transmitted to the monitoring center in Taipei through the internet system. Aside from these, the information could be used for on-line demonstration and could be shared with other.