Magnitude-frequency of Rainfall-triggered Landslide Erosion and its Controls

Landslide erosion is a dominant hillslope process and the main source of stream sediment in tropical, tectonically active mountain belts. The purposes of this study are quantifying rainfall-triggered landslide erosion, as well as investigating their controls and magnitude-frequency patterns. We quantified the amounts of landslide erosion triggered by 24 rainfall events from 2001-2009 in the Kaoping River, Tsengwen Reservoir, and Shihmen Reservoir watersheds in Taiwan by using multi-temporal, event-based landslide inventory and volume-area scaling relationships. The results show that landslide erosion caused by Typhoon Morakot is 534.25±39.37 Mm3 or 186.28±13.73 mm which can be of comparable magnitude to landslide erosion caused by a magnitude MW = 7.7~7.8 earthquake or 24 years of basin-averaged erosion. Therefore, this shows that typhoon-triggered extreme rainfall and great earthquake are both critical triggers on landslide erosion in Taiwan. Also, uniaxial compressive strength and rock mass discontinuity spacing influence the domainant material type of landslide in a watershed. The deeper landslides that mobilize soil and bedrock are dominant in the south characteristized by weak rocks and are triggered by long-duration rainfall. In contrast, shallow landslides are dominant in the north characteristized by hard rocks and are triggered by short-duration rainfall. Deeper landslide mobilized soil and bedrock cause 5~13 times of erosion in the south more than that in the north. Therefore, this study proposed a landslide magnitude scale, amount of landslide erosion in an event over the erosion rate in a watershed, to cooperate the characteristic of watershed in prediction of landslide erosion. Furthermore, the magnitude-frequency analysis shows landslide erosion rate in the Kaoping River watershed is 2.99~5.27 mm yr-1. Extreme-intensive rainfall plays a more important role in hillslope mass wasting than frequent-moderate rainfall, which rainfalls with return period larger than 50 years contribute 54~74% of total landslide erosion. Magnitude-frequency of extreme rainfall can induce at least ±1.2 mm yr-1 uncertainty in estimation of landslide erosion rate. In summary, extreme rainfall is critical triggers in landslide erosion. Magnitude-frequency of landslide should be considered when estimating long-term erosion rate.