Hydrometeorological and landuse attributes of debris flows and debris floods during typhoon Toraji, July 29-30, 2001 in central Taiwan

The 921 Chi-Chi earthquake, a 7.5 M earthquake with center located near Chi-Chi of Nan Tou County occurred on September 21, 1999 and caused more than 2400 fatalities and totaling billions of dollars losses due to destruction and damages to private and public buildings, roads, bridges and hydraulic structures in central Taiwan. This 921 Chi-Chi earthquake also triggered close to 26,000 landslides with a total area of 15,977 ha and severely disturbed mountain slopes, thus setting the conditions for occurrence of more landslides and other related disasters. Post-earthquake watershed restoration works progressed without disruptions by major typhoon rainstorms until July 29-30, 2001 when typhoon Toraji hit eastern and central Taiwan with heavy rainfall having peak hourly intensity exceeding 70 mm/h. This rainstorm caused several major debris flows and debris floods that destroyed or severely damaged dams, roads, bridges, dikes and houses, in addition to causing 103 casualties, 111 missing (presumed dead), 189 injured. This study (1) explores the characteristics and impacts of debris flows/floods and (2) investigates hydrometeorological and land use attributes of debris flows and debris floods associated with this typhoon Toraji in central Taiwan. Debris floods, different from more familiar debris flows, are extreme peak streamflows containing large quantities of muds, rocks and debris normally occurring in third and fourth order watersheds. Data analysis and field investigations revealed that the disastrous debris flows and debris floods during typhoon Toraji were essentially caused by very heavy rainfalls on inherently unstable mountain slopes that were loosened further earlier by the 921 Chi-Chi earthquake. The extremely intense rainfall of typhoon Toraji provided not only additional materials by creating new and reactivating or enlarging old landslides but also high peak stream flows required for occurrence of debris flows and debris floods. The destruction and damaging power of debris flows and debris floods witnessed during typhoon Toraji suggests that the effectiveness of engineering structures for disaster prevention is limited. For disaster mitigation, it is required to have more comprehensive strategies such as (1) land use zoning that regulates and restricts human encroachment on hazardous areas and (2) implementation of evacuation drills and relocation plan for people currently living in the hazardous zones.