Analysis of the behaviour of the Tukad Buluk Poh flood water at the Gilimanuk - Denpasar National Road Crossing

BIO Web of Conferences 159, 04004 (2025) SAFE 2024 https://doi.org/10.1051/bioconf/202515904004 Analysis of the behaviour of the Tukad Buluk Poh flood water at the Gilimanuk - Denpasar National Road Crossing 1 1 1 Acep Hidayat , Suprapti , Muhammad Isradi , Riska Dwi Praptoyo2 Civil Engineering Study Program, Faculty of Engineering, Mercu Buana University, 01 Meruya Selatan Street, Jakarta, Indonesia. 2 Undergraduate of Civil Engineering Study Program, Faculty of Engineering, Mercu Buana University, 01 Meruya Selatan Street, Jakarta, Indonesia. *Corresponding author: 1 Abstract. In most of Indonesia, floods are a common natural disaster that spreads into several watersheds during the rainy season. When river water outflow increases to the point where it overflows and floods the surrounding area, this is known as a flood. Over the past few years, there has been an increasing number of flood incidents throughout the rainy season, which has resulted in significant losses for those impacted by this calamity. One of the rivers in Bali's Jembrana district is Tukad Biluk Poh. It has a watershed area of around 84 km2, is about 28 km long, flows into the Indian Ocean, and is one of the rivers that floods the most. Access to Gilimanuk to Denpasar via the national road was not immune to the flood overflow at the time in Tukad Biluk Poh. To prevent flooding on this portion of the road, this study aimed to ascertain the flood water level on the Gilimanuk-Denpasar National Road bridge that crosses Tukad Biluk Poh. When the flood discharge was analyzed using the Nakayasu method's synthetic unit hydrograph, the design flood discharge had return periods of 299,720 m/sec for two years, 436,764 m3/sec for five years, 529,525 m3/sec for ten years, 648,435 m3/sec for twenty-five years, and 1134,737 for a thousand years. The HEC-RAS tool was then used for a hydraulic analysis to ascertain the flood water level, and the findings were excellent. For two years, MAB yields 1.78m, for five years, 2.31m; for ten years, 2.64m; and twenty-five years, 2.64m. 3.03 years, with MAB returning 3.34 years, 1 MAB returning 3.51 years, MAB returning 5.76 years, and MAB returning 6.21 years. The bridge is referred to as the current one by obtaining the flood water level at the 200th year return period of the bridge's condition, which is 4.65 meters on the pavement and 3.10 meters on the bottom of the girder at the design flood water level. Based on the analysis, the bridge's height was raised to 7.71 meters at the base of the top structure, considering a 1000-year design flood and a 1.50-meter guard height. © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (https://creativecommons.org/licenses/by/4.0/). BIO Web of Conferences 159, 04004 (2025) SAFE 2024 https://doi.org/10.1051/bioconf/202515904004 1. Introduction Floods are natural disasters that often occur during the rainy season, spreading across various watersheds (DAS) in most parts of Indonesia. Flood is a condition with increased river water discharge that overflows and inundates the surrounding area. The number of incidents of flooding during the rainy season over the last few years has continued to increase, causing various losses to the people affected by this disaster [1]. Tukad Biluk Poh is a river in Bali's Jembrana district drains into the Indian Ocean. It is about 28 km long and has a watershed area of around 84 km2, making it a perennial river prone to flooding. This river has seen multiple floods, the most severe of which was in early 2018 and resulted in significant losses for the local population of Tukad Biluk Poh due to flash floods. Numerous homes in this residential area were flooded due to the national road between Gilimanuk and Denpasar, which at the time was also not immune to the water overflow from Tukad Biluk Poh. This area has previously experienced many floods. Due to heavy rains and suspended water discharge from the bridge's lack of high guard, floods frequently occur at the national road's inbound bridge. According to the author's observations, the state of the bridge, which disrupts the flow because of the lack of guard height and width, and decreased infiltration in the Tukad Biluk Poh watershed as a result of illegal logging are the leading causes of flooding in the area where the National Road intersects with Tukad Biluk Poh. According to information gathered from the Jembrana Forestry Department, over three-quarters of the 383 hectares of production and 299 hectares of protection forest have been degraded and damaged. This situation results in a higher water discharge on the surface than if there is infiltration by tree cover because the water flows straight into the main river rather than seeping into the earth. Since the current bridge cannot handle the water discharge, it is required to demolish it and construct a new, more effective bridge on the national road that crosses the Tukad body [2, 3]. Flood management measures are required to lessen the likelihood of flooding-related damage due to the issues that arise. Knowing the anticipated flood discharge will help with flood control plans in a watershed [4]. The capacity of the Tukad Biluk Poh section on the National Road area, where the river hydraulics analysis is intended to analyze the profile of the flood water level in the river with various return periods of the flood discharge, must therefore be examined, as well as the planned flood discharge in the watershed area, using a hydrological analysis. strategy. 2. Method The method used for flood discharge analysis is the rational method and the Nakayasu synthetic unit hydrograph method, while the flood water level simulation uses the HEC-RAS program. 2.1. Study location The National Road bridge that spans Tukad Biluk Poh is the site of the study. Tegal Cangkring Village, Mendoyo District, Jembrana Regency is home to the bridge that spans the distance between Jalan Gilimanuk and Denpasar. situated 12 meters above sea level at the coordinates 50L 247077.00 m E, 9072639.44 m S. [5–8] 2 BIO Web of Conferences 159, 04004 (2025) SAFE 2024 https://doi.org/10.1051/bioconf/202515904004 Figure 1. Study location 2.2. Data collection Primary and secondary data were the two categories of data collected for this investigation. primary data from firsthand field observations and secondary data from sources whose information can be verified. Among the information gathered are: (1) Original Information. In September 2020, when the river was not inundated, a firsthand observation of its location was conducted. From the site of the investigation to three kilometers upstream, the author carried out a survey right at the river's position. A roll meter has been used as a measuring tool to take measurements on the National Road bridge. Two locals, Bli Dede and Bli Moncong, who live near Tukad Biluk Poh, were interviewe (...truncated)