Slope of alluvial fans in humid regions of Japan, Taiwan and the Philippines

This study constructed a database for 690 alluvial fans in humid regions of Japan, Taiwan and the Philippines, and analyzed their slopes and other morphometric parameters such as the area and relief ratio of the source area. This type of comprehensive geomorphological research on alluvial fans in humid regions has been limited, although numerous studies have dealt with fans in arid and semi-arid regions. Semi-conical depositional landforms larger than 2 km2 and steeper than 0.11° (0.002 m/m) were selected as alluvial fans. About 60% of the fans formed during the Holocene and the rest (40%) formed during the Pleistocene. Mean fan slopes for both all the Quaternary fans and Holocene fans follow a lognormal frequency distribution. The distribution also fits slopes of fans belonging to a certain areal range. The area and relief ratio of the source area, which correlate well with the fan slope, also follow a lognormal distribution, indicating the dimensions of fan/basin systems vary gradually rather than abruptly. These findings contrast with a previous notion that depositional landforms rarely have slopes of 0.5° (0.009) to 1.5° (0.026) and hence only semi-conical landforms steeper than 1.5° should be called alluvial fans.