Altered regional sediment transport regime after a large typhoon, southern Taiwan

Analyses of river suspended sediment response to record-breaking regional rainfall in southern Taiwan during typhoon Morakot, 7–9 August 2009, reveal systematic changes in the regional sediment transport regime as characterized by rating curve parameters, with decreasing b (typically considered to be a constant for a given gauging station) and increasing intercept K values in concentration-discharge ( C - Q ) relations ( C = KQ b ). These changes result in much greater sediment concentration, and thus sediment transport, in subsequent low-flow events after the typhoon, an effect that amplifies and extends the influence of such extreme events through increased low-flow sediment transport. Our finding that the exponent b is not constant contrasts with the conventional assumption that large events influence sediment yields through increased intercept ( K ) values, thereby supporting the interpretation that basin sediment delivery influences both rating parameters, and increases post-event low-flow sediment transport. Surveys of landslide density and riverbed grain sizes before and after typhoon Morakot support the interpretation that the observed decrease in b values and the synchronous shift from an inverse correlation between K and mean catchment slope before the typhoon to a positive correlation afterward reflect an altered sediment transport regime and a change from channel migration and bank erosion to reworking of landslide debris and enhanced bed mobility as the dominant processes supplying fluvial sediment.