Abstract
The Kaoping River (KPR) is a small mountainous river with a length around 170 km, the area of 3250 km2 and a very high sediment yield (11,000 t km-1 yr-1). Its main trunk and major tributaries are characterized by braided channel pattern wherever the valley is wider. As many river valley around the world, its floodplains have experienced significant human modification over the 20th century. In 2009, Typhoon Morakot, with extremely intensive rainfall (> 150-year return period), caused severe flood hazards and has aroused great concern of the floodplain management especially when it is suspect that the frequency of extreme weather events is rising in Taiwna. This study aims to investigate the landcover change of the valley floodplains of KPRs two major tributaries, the Nantszhsien Stream and the Laonon Stream, over a century and its implication on the flood hazard. Landcover and geomorphic interpretation are completed by using historical maps, aerial photographs and orthomaps from early 20th to 2009 (after the typhoon) on a GIS platform. It is found that along with the process of channelization, the former floodplain has been reclaimed, mainly for agricultural purpose. The flooded area induced by Typhoon Morakot was within the former active-channels zone identified from 1948 aerial photos and historical maps published in 1920s for the study area. These pictorial materials together provide a good baseline for delineating the so-called fluvial territory that is wide and continuous enough to guarantee the floodable areas to minimize the risk to the inhabitants. It also contrasts the space of the river reservation zone, in which the development is prohibited by law