FLUVIAL BEDROCK INCISION FOLLOWING TYPHOON IMPACT ON TAIWAN

Of all the mechansims of continental denudation, fluvial erosion of uplifting bedrock is thought to be crucial because it sculpts the local relief upon which the mass-wasting of hillslopes depends, and creates the conduits for transfer of the erosion products. Despite its importance, the process is poorly understood. Key questions are: what are the principal mechanisms of fluvial bedrock incision, what is the rate of the process, and what controls that rate? These questions are addressed in a field experiment located in the eastern Central Mountains of Taiwan. There, networks of recessed benchmarks were installed, spanning bedrock channels from low-flow line to above extreme flood level. Between benchmarks, the channel bed topography is surveyed at regular intervals over representative lithologies in order to obtain an accurate measure of fluvial erosion.

Surveys were performed in February and December 2000. Channel bed erosion rates are captured by the differential of the two surveys. We report on a detailed statistical analysis of these surveys. The wet season of summer-autumn 2000 was exceptional in that Taiwan was hit by several typhoons. One of these was a rare supertyphoon, "Bilis", which crossed directly over the island on 22-23 August, generating very high discharges and concomitant hillslope mass-wasting in the study catchment. Accordingly, the measured, real-time average rates of erosion (up to 80mm at some places) match or exceed long-term uplift rates of 5-7mm per year at most points on the channel, indicating the importance of low-frequency, high-magnitude events in fluvial incision. As well as measuring short-term rates of erosion over a wet season, we have been able to analyse the spatial distribution and patterns of bedrock erosion within the active channel. Topography, lithological characteristics and position within the active channel all affect the rates and processes of bedrock incision.