International Journal of Advanced Science and Technology Vol. 29, No. 4, (2020), pp. 6195 - 6208 Analysis of Changes in Bathymetry of the Palu River Estuary and Its Effect on Flow Characteristics Mohammad Antariksa, M. Galib Ishak, I Gede Tunas* Department of Civil Engineering, University of Tadulako, Indonesia *Email: [email protected] Abstract The tsunami triggered by the 7.4 magnitude Palu earthquake has caused a change in the shape of the Palu Bay coast. The results of field investigations have proven the existence of changes in the bottom waters around of the Palu River estuary. This has the potential to cause changes in the characteristics of the flow pattern of ocean currents that lead to and leave of the Palu River estuary. This study aims to determine and identify the impact of these coastal changes on the pattern of current flow to and leaving of the Palu River estuary by conducting hydrodynamic simulations using the Surface water Modeling System (SMS) software. obtain a bathymetry map, the results of which illustrate that the 2018 earthquake and tsunami caused a decrease in the elevation base at the Palu River estuary. From the results of the hydrodynamic SMS simulation before the earthquake and tsunami of 2018, the maximum value obtained for Point 1 is in step 11th with a flow velocity value of 0.11 m/s, in Point 2 there is in step 24th with a flow velocity value of 0.03 m/s . The minimum value for Point 1 is in step 17th with a flow velocity value of 0.08 m/s, in Point 2 there is a step 17th with a flow velocity value of 0.02 m/s. Simulation results after the earthquake and tsunami in 2018, the maximum value obtained for the simulation model is for Point 1 found in step 1st with a flow velocity value of 0.14 m/s. For Point 2, found in step 1st with a flow velocity value of 0.05 m/s. The minimum value for Point 1 is in step 19th with a flow velocity value of 0.08 m/s. Whereas for Point 2 found in step 19th with a value of flow velocity = 0.03 m/s. As for the direction of current flow, before the earthquake and tsunami of 2018, the flow of current to the estuary is mostly from the west and east direction, then when it reaches the estuary, turn left the estuary perpendicular to the river toward the north. Whereas after the earthquake and tsunami of 2018, from step 1st to step 11th, the flow of the current from the sea towards the estuary is perpendicular to the river through the north, when it reaches the estuary, the current flow is divided by turning west and east leaving the estuary, then in step 12th to step 24th there was no change in direction between before and after the earthquake and tsunami in 2018. Keywords: Tsunami, bathymetry, hydrodynamic simulation, coastal current. 1. Introduction The beach has a dynamic balance that tends to adjust the shape of its profile so that it is able to destroy the incoming wave energy. Normal waves that come will be easily destroyed by the beach mechanism while large waves that have large energy even if they occur in a short time will cause erosion (Sumbogo, 2007).Sea level rise is one of the most damaging effects of global climate change which will have far-reaching consequences for the majority of the world's population and natural systems(Din etal., 2017). Sea level change is of great interest for two fundamental reasons. First, changes in the rate of sea level rise are intimately related to changes in the Earth’s climate. Second, sea level change has important socioeconomic consequences for populations living near the current mean sea level. (Cazenave etal., 2004). The coastal erosion activity normally occurs when the sand are ISSN: 2005-4238 IJAST 6195 Copyright ⓒ 2020 SERSC International Journal of Advanced Science and Technology Vol. 29, No. 4, (2020), pp. 6195 - 6208 washed off from the coastline and gets smaller, and the opposite process, in which accretion, starts to take place when sand or other materials are accumulated to the coastline and becomes bigger (Azid etal., 2015). The dynamic equilibrium of the beach is not only interrupted by the forces of nature, but also by man-made activities. The building of constructed seawalls, breakwaters, the advancement of the shoreline artificially by land reclamation, and the removal of beach material from the coastline caused various effects to the coastal area (Yaacob etal.,2018). Currents caused by wind are generally seasonal in nature, where in one season the currents flow in one direction steadily and in the next season they will change direction in accordance with changes in wind direction. If the flow is faster then the sediment concentration decreases, whereas if the current is weak the sediment concentration increases(Rachmat, 2018). Tides are a phenomenon of the movement of the ups and downs of sea levels on a regular basis caused by a combination of gravitational forces and attractive forces of astronomical objects, especially by the sun, earth and moon. The influence of other celestial bodies can be ignored because of the greater distance or smaller size (Willy, 2017). Water level elevation (tide elevation) and low tide water levels are very important for planning coastal structures. For example the elevation of the top of a breakwater structure, jetty, etc. is determined by the elevation of the tide level, while the depth of the shipping lane/port is determined by the low tide(DISHIDROSAL, 2018). Bathymetry is a measure of height and height of the seabed, so the bathymetry map provides information about the seabed, where the information can provide benefits in several fields related to the seabed(Febrianto etal., 2015). River is a channel system formed by nature to flow water and transport sediments contained in it. River as one of water resources has important benefits and roles in human life and various urban activities such as industry, housing, trade, facilities and infrastructure. (Gunadi, 2017). River flow discharge is the volume of river water flowing in a certain time unit, or river water flow is the height of the river water level measured by the river water level gauge. The measurement is done every day with Automatic Water Level Record (AWLR), or in another sense the flow or flow of a river is the flow rate of water or the volume of water that passes through a cross section of a river unity of time. In the SI unit system the amount of discharge is expressed in units of cubic meters per second (Ishak, 2017). The river estuary can be divided into three groups, which depend on the dominant factors that influence it. The three dominant factors are waves, river discharge and tides (Yuwono, 1994). Sediments are fragments of material generally consisting of physical and chemical breakdowns of rocks. Particles like this have sizes from large/boulder to very fine/colloid, and vary in shape from round, oval to square. In general, particles that move by rolling, gliding and jumping are called bed-load transports, while floating particles are called suspended load transport (Anasiru, 2006). Proses sedimentasi ini meliputi proses erosi, transportasi atau angkutan, pengendapan atau deposition, dan pemadatan atau compaction. Konsentrasi sedimen digunakan sebagai indikator proses pengiriman sedimen di saluran alami karena memiliki efek besar pada seluruh sistem sungai (Mahendra etal.,2018). Central Sulawesi has an area of 61,841.29 km2 and there are several bay’s in which one of them is Palu Bay.In Central Sulawesi, especially in the areas of Palu City, Donggala and Sigi, an earthquake with a magnitude of magnitude 7.4 has caused a tsunami and liquefaction in the Palu City area. The tsunami wave moves from the north, into the bay, then hits Palu City. The existence of the bay makes sea water travel in a narrower location, so that the resulting pressure is even greater. Based on the position and depth of the earthquake center, the earthquake event was caused by active fault activity in the Palu-Koro fault zone. Coastal inundation surveys showed that the tsunami inundation height was 3–4 m on average around the Palu bay and, was as high as 6.8 m in localized areas above the mean sea level and 6.2 m above the astronomical tide level when the earthquake occurred (the splash was excluded) ISSN: 2005-4238 IJAST 6196 Copyright ⓒ 2020 SERSC International Journal of Advanced Science and Technology Vol. 29, No. 4, (2020), pp. 6195 - 6208 (Sassaetal.,2018).A simplified numerical model suggests that the landslide tsunami propagated as an edge wave and split into two separate waves due to the presence of an underwater shallow area just north of Palu City. Both waves arrived to the coast of this city within several minutes: one from North-West and the other from the North(Takagi, 2019). The Palu City area is dominated by alluvial deposits which are soil characteristics that are not strong against water, if there are liquefaction and earthquakes occur, the soil structure will easily move to reach its stable point(Han etal., 2018). In the case of Palu City, the shape of a narrow, shallow, narrow bay near the coastline is most likely to contribute to increasing the size of the tsunami (Sochuetet.al., 2018). In this study a simulation model of the Palu River estuary flow will be made before and after the 2018 earthquake and tsunami. As a result of the earthquake and tsunami, based on the results of field monitoring there has been a change in the shape of the coast around the the Palu River estuary.