FLOOD AND MORPHOLOGY STUDY IN JAMBU RIVER (DONAN TRIBUTARY) CILACAP REGENCY CENTRAL JAVA

• Main Author: Aulia, Humam
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/72063
• Institution: Institut Teknologi Bandung

This research was conducted in Jambu River, which is a 3rd order river in the Donan Basin in Cilacap Regency, Central Java Province, precisely around the Cikuya Bridge. There are several problems that occur around the research location, such as flooding on July 21, 2021, bank failure, and local erosion that resulted in a decrease of soil retaining wall under the Cikuya Bridge abutment by around 6 to 8 cm. The purpose of this research is to explain information and facts related to water resources problems in the Jambu River, especially around the Cikuya Bridge, with the aim of determining the relationship and interpreting the information and facts obtained through data analysis so as to provide answers related to the causes of flooding around the Cikuya Bridge, changes in morphology due to sediment transport and bank failure caused by instability and toe erosion, and the cause of local erosion under the Cikuya Bridge abutment. This study will also evaluate flood and bank failure management carried out by Balai Besar Wilayah Sungai Serayu Opak in 2022 and provide recommendations for further actions to handle the problems in the Jambu River, especially around the Cikuya Bridge. In this research, hydrological analysis was conducted using HEC-HMS 4.1.1 software, while hydraulic analysis in the form of 1D/2D flood simulation, sediment transport simulation with Bank Stability and Toe Erosion Model (BSTEM) 1D, and 1D local erosion under the Cikuya Bridge abutment was carried out using HECRAS 6.3.1 software. Flood simulation uses Unsteady, Combined 1D/2D Model, a flood discharge with a return period of 20 years was simluated to evaluate the capacity of the Jambu River section before and after the construction of parapet based on the design criteria for the capital area of the district/city in Minister of Public Works and Public Housing Regulation No. 25 of 2015. The flood event discharge on July 21, 2021, was also simulated to interpret the flood event during extreme rainfall. Predicted tidal water level elevation with a time interval of 1 hour during the flood event is used as a downstream boundary condition. Sediment transport simulation to predict vertical and lateral river morphology changes for the next 10 years was conducted using a Quasi-Unsteady, One-Dimensional (1D), Mobile Bed Sediment Model, which was integrated with modeling of foot erosion and bank erosion using ARS-USDA Bank-Stability and Toe Erosion Model (BSTEM). Abutment scour simulation was conducted from the results of hydraulic simulation using the Flow Hydrograph consisting of a 100-year flood recurrence period as the upstream boundary condition and three Stage Hydrograph scenarios in the form of tidal water level elevations of Highest High Water Level, Mean Sea Level, and Lowest Low Water Level as the downstream boundary condition. Results from the analysis indicate that the cause of the flood on July 21, 2021, was the high rainfall which resulted in a flood discharge of 416.10 m3/s, or equivalent to the maximum planned discharge with a return period of 47.37 years. The elevation of the riverbank condition before the construction of the embankment around Cikuya Bridge was quite low, so the river channel was unable to accommodate the flood discharge, and the phenomenon of tide caused a lag in the decrease of the flood water level. Flood control in the Jambu River in 2022 by building an embankment downstream of the Cikuya Bridge can hold the planned flood discharge with a return period of 20 years but cannot withstand extreme flood events like the one that occurred on July 21, 2021. Results from the sediment transport simulation for the next 10 years predict that the thalweg elevation at 17 river cross-sections around the Cikuya Bridge will experience sedimentation with the highest thalweg thickness increase of 0.59 m occurring at the cross-section below the Cikuya Bridge, 15 river cross-sections are predicted to experience erosion, and 7 river cross-sections are predicted to experience bank failure. Local scouring below the abutment of the Cikuya Bridge occurs because the flow is constricted on the right side of the bridge and is predicted to cause scouring up to a depth of 1.6 m if there is a flood discharge with a return period of 100 years. Keywords: Jambu River, flooding, river bank collapse, local scour, flood simulation, sediment transport, scour abutment bridge.