Upstream morphological effects of a sequential check dam adjustment process

Sediment flushing and the morphological responses to the procedure of check dam removal are still unclear. Following laboratory experiments that revealed three stages (deepening, widening, and volume release) of check dam adjustment, a check dam built in 2007 at Landao Creek in central Taiwan was adjusted in 2015 by removing central bars and cutting 2.5 m from the middle two piers (stage 1 + 2), with the purpose of regulating sediment transfer and keeping the thalweg at the center of the channel, while also preventing hill slope toe erosion. In 2019, four central piers were removed (stage 3) to increase the volume of sediment released. Annual surveys were conducted after the initial adjustment in 2015 through to 2020 using unmanned aerial vehicles (UAVs). The check dam adjustments revealed that the channel had narrowed and stabilized as indicated by regenerating riparian vegetation. Additionally, distinct terraces had formed on the hill slope toes of the creek channel in proximity to the check dam. The meander upstream weakened following the dam adjustments. This study combining laboratory experiments with actual field observation contributed immensely to check dam decommissioning. Additionally, this study illustrated how an adjustable check dam may aid regulation of sediment transport and thereby sediment balance. It can be adjusted accordingly based on the prevailing channel condition. The check dam removal guide consists of three stages. Stage 1: cutting the central portion deeper. Stage 2: widens the opening. Stage 3: complete removal. While the adjustable check dam can be adjusted freely in each stage according to the demand of the sediment budget.