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.