Long term change observation of river terrain due to dam removal in Central Taiwan

This research focuses on the long-term geomorphologic change in the upstream of the silt dams in the Lan-daw rivers watershed in central Taiwan, adopts the long-term rainfall records in the Lan-daw rivers watershed to calculate the 1-day, 2-days, 3-days accumulated rainfall with different return period, and analyzes the relationship between the geomorphologic change and the accumulated rainfall. This research builds the Digital Surface Models based on the photos shot by UAV at 9 different times. The river in upstream of the Lan-daw rivers watershed is sinuous. The research classifies 3 time periods from 2010 to August, 2019, including the first time period from 2010 to June, 2017, the second time period from June, 2017 to Nov. 2018, and the third time period from Nov. 2018 to Aug. 2019. The target in the first time period is to observe the geomorphologic change after the first dam removal, that in the second time period to observe the geomorphologic change in the 2 years after dam removal, and that in the third time period to observe the geomorphologic change after the second dam removal.

The longitudinal slopes in the first, second, and third time periods are -30.3%, 14.8%, and 5.98%, and the knickpoint in the longitudinal profile in the first and second time periods occur in the upstream 20 m of the silt dam and that in the third time periods occurs in the upstream 45 m of the silt dam. The research classifies the cross-sections profiles into 3 groups, including the first group from C1 to C7 cross sections, the second group from C8 to C14 cross sections, and the third group from C15 to C22 cross sections. The geomorphologic change in the first group near the silt dam is the most obvious in the three groups. The geomorphologic change in the three groups in the first time period are -6.43 m to -8.13 m (scouring), those in the second time period are 0.23 m to 0.34 m (deposition), and those in the third time period are 0.46 m (deposition) to -1.78 m (scouring). Based on the analysis of the long-term rainfall record in the Lan-daw river watershed, the return period of the heaviest rainfall from 2015 to Aug. 2019 is less than 20-year return period. This means that the geomorphologic change in upstream of silt dam in the Lan-Daw river watershed is easy induced in the short time after dam removal.