Extreme rainfall is the main factor triggering flooding in various regions of the world including Indonesia. The increase in intensity and duration of current extreme rainfall is predicted as a result of global climate change. This paper aims to analyze the impact of extreme rainfall to the peak discharge of flood hydrographs at a watershed outlet in Palu, Sulawesi, Indonesia. Maximum daily rainfall data for the period 1990-1999 recorded at the Palu Meteorological Station, Central Sulawesi were selected using the Annual Maximum Series Method, and grouped into two types. Type I is the maximum daily rainfall data with extreme events and Type II is the maximum daily rainfall data without extreme events. Frequency analysis was applied to the two data groups using the best distribution method of: Normal, Normal Log, Pearson III Log, and Gumbel to obtain the design rainfall of each data group. In the next stage, the design rainfall transformation into a flood hydrograph is performed using the Nakayasu Synthetic Unit Hydrograph based on a number of return periods in one of the rivers flowing into Palu Bay, namely the Poboya River. The analysis results show that the design rainfall graphs with both extreme rainfall and without extreme rainfall are identical at the low return period and divergent at the high return period with a difference of up to 21.6% at the 1000-year return period. Correspondingly, extreme rainfall has a greater impact at the peak of the flood hydrograph with increasing return periods ranging from -1.28% to 26.81% over the entire return period.

The development of computer technology, especially the hydrodynamic modeling package, provides convenience in many things including flood modeling in the river. One of these modeling packages is HEC-RAS Hydrodynamic Model which can be used to simulate both steady flow and unsteady flow. On the other side, the development of Geographic Information System (GIS), is now rapidly evolving for a variety of purposes with a wider range of fields and scope, including the preparation of river geometry data based on Digital Elevation Model (DEM) in Triangulated Irregular Network (TIN) format as the input of the model. The aim of this study is to perform flood routing for determining the river capacity and for estimating the factors that cause floods by integrating TIN data into HEC-RAS Hydrodynamic Model, using Lantikadigo River in Central Sulawesi, Indonesia as a model. In this river, almost every year flooding occurs with fluctuating intensity of inundation. Integrating data is the process of synthesizing geometry data that is processed in the GIS environment as input for the HEC-RAS Model. Data integration provides the effectiveness of the use of simulation time due to input geometry data is done using import data facility when compared manually input geometry data. The results of the study show that the maximum water level of the 1-year return period has exceeded the river bank elevation both on the left and on the right side of the entire segment. The peak discharge of hydrograph for 1-year return period is 55.3 m3/s at the outlet of Lantikadigo Watershed. This means that the average channel capacity is far below the peak discharge. Based on simulation results it can be predicted that the cause of flooding in Lantikadigo River is due to morphological change of river geometry.