Long-Term Impact of Extra Sediment on Notches and Incised Meanders in the Hoshe River, Taiwan

The extra sediment load induced by typhoons and rainstorms in the Heshe River, Taiwan, are the principal reason for severe sediment-related disasters. The total sediment load during Typhoon Morakot in 2009 was 31 × 10⁶ m³, accounting for 95% of the annual sediment discharge. Large amounts of sediment load entered the Hoshe River, causing the braiding index (BI) to increase. Subsequently, the BI became positively correlated with the channel width in the Hoshe River. The specific typhoon and rainstorm events decreased after Typhoon Morakot, the sediment input decreased, inducing the fluvial morphology of the braided river to develop into a meandering river. The extra sediment load induced the deposition depth to increase and produce a headward deposition in the main channel and its tributaries. In addition, the river bend and the topographical notch restrained the sediment from moving downstream and being stored locally, indirectly increasing the erosion density of the river banks from 2.5 to 10.5 times.