Prediction of land conversion impact on flood peaks using the ITS-2 synthetic unit hydrograph model in Sausu River, Sulawesi, Indonesia

Abstract

Land conversion in the Sausu Watershed, Sulawesi, Indonesia has been started massively since 1990. During this period there have been changes in land cover from forest areas to more than 5,000 hectares of plantation land. The land conversion in the middle and upper watersheds is predicted to have triggered major floods which have resulted in large losses, especially in the downstream area as residential and agricultural land with an exposure area of more than 2,000 hectares. The flood in 1996 also has caused the collapse of the Upper Sausu Weir as a supplier of irrigation water for more than 7,000 hectares on the right and left sides of the river channel. This study was conducted to analyze the effect of land conversion in the watershed on increasing the flood peak using the ITS-2 Synthetic Unit Hydrograph Model, one of the flood peak prediction models developed at the Institut Teknologi Sepuluh Nopember in 2017. The research was preceded by data collection, especially rainfall data (daily and hourly), flow data and area conversion of forest land. Daily rainfall data were obtained from Sausu Rainfall Station with a range of 2000-2018, and hourly rainfall and discharge data were obtained from Nippon Koei Co Ltd. with a short range in 2002. The main research was carried out in two stages, namely, calibration to determine the optimal parameters of the ITS-2 Model and the application of the model for flood peak analysis. Rainfall as the input of the model can be defined as design rainfall with 50-year return periods, which are previously transformed to net rainfall. The losses due to evapotranspiration and infiltration were analyzed using Curve Number Method based on cover type and hydrologic condition of the watershed. The results of the analysis have indicated that land conversion in the watershed has increased the potential of peak flow to 16.5% when evaluated on these return period of flows. The peak of this flood does not seem to increase significantly compared to the great floods of the past. However, it must be understood that flooding can occur not only due to changes in land cover but can also occur due to various other factors such as changes in the characteristics of rainfall which leads to extreme rainfall with high intensity and long duration.