Fractal characteristic of watershed is an important parameter which influences the formation of synthetic unit hydrograph. Based on a previous study, hydrology response of watershed expressed in hydrograph form could be well presented by hydrology network characteristic as a form of fractal characteristic of watershed [1]. Fractal characteristic of watershed was stated as fractal dimension which was presented in three parametersi.e.river branch ratio (RB), river length ratio (RL) and watershed river area ratio (RA). The purpose of this research was to analyze fractal characteristic and to verify its fractal dimension stability as preliminary research to prepare variables which would be used to develop synthetic unit hydrograph model in the future. Analysis was undertaken using two methodsi.e.Horton’s Coefficient Ratio and Box Counting Dimension. Analysis result revealed that fractal dimension of river network from 8 watersheds calculated using those two methods could give almost the same result, ranged from 1 to 2. Fractal dimension from calculation also showed similarity with study conducted by Tarbotonet al. [2], Balkhanovet al. [3], Khanbabaeiet al.[4], obtaining that fractal dimension of river network is generally in the range of 1 to 2.

No abstract

Flood disaster in Palu River has repeatedly occurred with varying discharge magnitudes, especially in the downstream segment near and around the estuary. The most recent flood occurred in July 2018 has inundated some areas of Palu City and resulted in a considerable impact on the socioeconomic life of the community in the city of Palu. Actually, flood prevention efforts have been undertaken by the Palu City Government and River Basin Board of Sulawesi III, one of which is by constructing levee combined with revetment along more than 5 km measured from the estuary to the upstream reach. The levee is made of soil material, while the revetment is a structure to protect the levee made of concrete. These structures were built on both sides of the river banks. However, the flood disaster always happens almost every year in this area. This paper intends to evaluate the performance of the flood control structure using Geographic Information System and HEC-RAS hydrodynamic model. The use of these tools provides the ease and efficiency of flood simulation along the river being modeled. The analysis results show that the bank capacity of Palu River is currently only effective for flood discharge below 550 m 3 /s, where the river bank capacity at the beginning of the levee and revetment design is approximately 550 m 3 /s, equivalent to the 25 years return period of discharge. The river bank capacity decreases due to sediment deposition on the river bed which were originated from the upstream watershed. This decline in cross-section capacity is estimated to be the cause of the flood disaster in parts of Palu City.

One of the important factors in water resources management is the determination of design flood associated with determining the size, capacity and age of the water resources structures to be built. Determination of design flood can be done in various ways, one of which is very popular to date is discharge prediction using synthetic unit hydrograph (SUH) approach. The use of unit hydrograph models has been widely applied in various parts of the world, especially in Indonesia, some of which are Snyder, Nakayasu, GAMA I and ITB-1. These methods are considered to have a good performance because it has to accommodate the characteristics of watersheds in a model parameter that greatly contributed to the process of rainfall-runoff transformation. However, in some cases it also provides a sizeable deviation, especially in Indonesia, considering that watersheds in Indonesia have different characteristics with watersheds in the United States where Snyder Unit Hydrograph developed. To overcome these problems, the unit hydrograph performance must be improved so that it can be used in various watersheds to obtain the smallest deviations. This research was conducted in 8 watersheds located in Central Sulawesi Province, Indonesia. This study aimed to improve the performance of Snyder Unit Hydrograph Model, covering Snyder, Nakayasu, SCS, GAMA I, ABG and ITB-1. The improvement of model performance was conducted by adjusting model parameters, in this case using Solver Tool on Microsoft Excel. Evaluation was done by the error indicator such as coefficient of Nash-Sutcliffe model efficiency (E). The study result showed that model parameter adjustment could decrease a deviation of SUH model parameter for peak discharge and average peak time up to 30% and could increase Nash-Sutcliffe Model Efficiency Coefficient (E) up to over 80%. The decrease of a deviation of SUH model parameter and the increase of E coefficient revealed that optimization using solver facility was effectively undertaken. However, not all deviations decreased but even increased significantly after optimization. It happened because the process of parameter optimization occurred simultaneously, and it was only based on a purpose function by maximizing Nash-Sutcliffe Model Efficiency Coefficient (E). The adjustment in this coefficient caused the increase or decrease of a parameter deviation of SUH Model depending on E value achieved on the optimization process. Overall, it could be declared that the decrease of a parameter deviation of SUH model was accompanied by the increase of Nash-Sutcliffe Model Efficiency Coefficient (E). Keywords : performance improvement, synthetic unit hydrograph, model parameters, flood prediction I. INTRODUCTION One of the important factors in water resources management and planning was flood discharge estimation functioned to determine optimum discharge size associated with dimension and a life of structures. The aim of optimum flood discharge estimation was to plan structures which did not have over a dimension (over estimated) implying ...

High magnitude flash flood has occurred several times in some areas in Central Sulawesi Province after the 2018 Palu Earthquake, one of them is in the Bangga River, Sigi Regency, Indonesia. It has caused massive impacts such as damaging agricultural and plantation areas and submerging public facilities and infrastructure and even causing fatalities. The flood carries a variety of materials, especially high concentration sediments which are thought to originate from eroded soils due to landslides induced by a 7.5 magnitude earthquake. These materials are eroded and transported by the flow at the upstream watershed due to heavy rainfall. This study intends to investigate the potential of landslides, factors that trigger floods and increased flooding after the earthquake. This research was conducted by investigating the landslides potency based on field surveys and interpretation of the latest satellite imagery, analyzing the characteristics of rainfall as a trigger for flooding, and predicting the flood potency as the primary impact of these two factors. Rainfall-flood transformation was simulated with the HEC-HMS Model, one of the freeware semi-distributed models commonly used in hydrological analysis. The model input is the configuration of river networks generated from the National DEM (DEMNAS), hourly rainfall during floods and other watershed parameters such as land cover, soil types and river slope. The similar simulation was also carried out on the condition of the watershed before the earthquake. Based on the results of the analysis, It can be inferred that flash floods in the Bangga River are mainly caused by heavy rainfall with long duration and landslide areas in the upper watershed triggered by the 2018 Palu Earthquake with an area of approximately 10.8 km 2 . The greatest depth of rainfall as a trigger for flooding is 30.4 mm with a duration of 8 hours. The results of the study also showed that landslides in the upper watershed could increase the peak flood by 33.33% from 118.56 m 3 /s to 158.08 m 3 /s for conditions before and after the earthquake.