Landslide-amplified flash floods—The June 2008 Panay Island flooding, Philippines

The role of landslides in generating or enhancing a flash flood event in Aklan, Panay Island, Philippines, resulting from Typhoon Fengshen, was investigated. The flash flood occurred between 1100 h and 1400 h (GMT + 8) of 21 June 2008 and inundated Sitios Agbatan and Abaya in the municipality of Libacao. ischarges at Kipot, a channel constriction in Aklan River about 10 km upstream, were independently derived using a hydrologic model (302.6 m3/s) and Manningʹs equation (7000 m3/s). The modeled flood peak reached Kipot at 0300 h on 21 June 2008 and the communities between 0600 h and 1000 h on June 21, at the earliest. The actual flood peak arrived at Sitios Agbatan and Abaya between 1100 h and 1400 h on June 21, at least 1 h earlier than the modeled arrival. the flash flood may have been caused by the preceding rainfall alone (234.4 mm from 0600 h June 20 to 0600 h June 21), the discrepancies in peak discharges and the delayed arrival time suggest the possibility of damming and eventual breaching of the dam. Evidence of the damming was found at the channel constriction in Kipot. A landslide at this site was mapped, characterized, and inferred to have completely blocked the channel and caused the damming. Implications to flash flood risk assessment are discussed.