Impacts of rainfall variability on river discharges and suspended discharges : A Case Study in Chenyulan Watershed, Taiwan

In this study, the variations of rainfall, river discharges and suspended sediment discharges were analyzed in the Chenyulan watershed in Nantou County, central Taiwan. The hydrological data, such as rainfall, daily discharges and daily suspended sediment discharges, was collected based on Neimaopu hydrology station during the period from 1972 to 2020. The yearly costs of structure conservation to prevent sediment disasters and slope hazard events were implemented in the watershed between 1999 and 2020 as well. The Rating Curve Method with the formula Q_s=aQ^b is adopted to estimate sediment discharges with the corresponding discharge events. The impact factors that caused the variation of discharges and suspended sediment discharges were also analyzed to provide the references for the influence of geological and hydrological changes on sediment yielded on slope and following suspended sediment discharges in the rivers in the watershed. The analyzed results show that the suspended sediment discharges in 1972-1989 are less than the average value in 1990-2009 at the same discharges. The suspended sediment discharges in 2010-2020 are gradually reverted to that in 1972-1989. The causes of decreasing the suspended sediment discharges in last decade are analyzed, including: 1. The variations of rainfall were gradually calmed in last decade; 2. the loose soil on slopes in the watershed caused by Chi-Chi earthquake became concreted with time; 3. the landslide and debris flow disasters obviously decreased in last decade and the soil yield from slopes has slowed down; 4. the local government involved a lot of money to build the conservation structures in upstream creeks to trap the loose soil and control the volume of sediments from flowing into rivers.