Knickpoint evolution across anticline structure: A case of uplifted reach in the Taan River, Taiwan

摘要

The behaviour of knickpoint migration is a key to understand how the river morphology responses to surface uplift orrnbase-level drop. Annual rate of knickpoint migration in bedrock channel may govern the incision rate that a stream canrnreturn to equilibrium. Generally, the migration rate ranges from sub-meter to sub-centimeter. Several researchersrnhave attempted to correlate knickpoint migration rate to a varieties of factors, including upstream drainage area, rockrnproperties and geological structure, among others. Usually, the migration rate is determined from the elapsed time andrnthe knickpoint retreat distance. Very often, the knickpoint migration process is hard to be realized due to incompleterndata of long term longitudinal profiles.rnA case study on the knickpoint migration of an uplifted reach in the Taan River, Taiwan was carried out through therncollection and analyses of multistage annual longitudinal profiles, discharges in major flood events, and geologicalrndata. The longitudinal distance of the reach is about 1 km. The uplifted reach contains a pop-up structure with 10rnmeters of the maximum vertical uplift formed by the tectonic deformation due to the thrust faulting in the 1999 Chi-Chirnearthquake. Since then, incision developed rapidly into the soft bedrock. The knickpoint migrated upstream in anrnastonishing rate of tens meters annually. It took less than 10 years for the channel to cut through the pop-up structure.rnThe changes in multistage annual longitudinal profiles enable the understanding of the processes of knickpointrnevolution. The studied reach covers an anticline structure so that contains three distinct types of knickpoint evolution.rnThe major factors affecting the knickpoint evolution in the studied reach was identified and characterized.