Geomorphic Responses to a Large Check-Dam Removal on a Mountain River in Taiwan

The ability to understand and predict the impacts of dam removal in river systems is important, especially as dam decommissioning is becoming increasingly popular. In this study, we document the morphological and sediment impact of the removal of Chijiawan Check Dam in May 2011; a 13-m-high dam located on a coarse-grained, steep mountain river channel in Taiwan. An estimated 0.2millionm(3) of sediment had accumulated within the impoundment before its removal. Longitudinal and bankfull cross-sectional surveys and a detailed sediment textural survey were undertaken along a 3.2-km study reach of the Chijiawan Creek between 2010 and 2012. A rotating knickpoint with migration rates of up to 22m/day was observed along the study reach, following dam removal. The rate and character of channel change, associated with the dam removal, appear to be driven as much by channel morphology and distance from the dam as by the hydrology variability. Our results suggested that relatively small amounts of sediment were eroded during the first 3weeks following dam removal because of low discharge conditions. However, after 1 and 15months, 10 and 75% of the sediment that had accumulated within former impounded was eroded, respectively. Sites near the former dam had a sediment texture that reflected the transport of released sediment, and this suggested that basin-wide sediment processes exerted a strong influence. The removal of Chijiawan Dam offers unique insight on how sediment processes can drive river channel responses to sediment pulses may vary with discharge and sediment load, in areas subject to remarkably high flows and sediment loads. Copyright (c) 2015 John Wiley & Sons, Ltd.