fluids

Article Insight of Numerical Simulation for Current Circulation on the Steep Slopes of Bathymetry and Topography in Bay,

Mohammad Lutfi

Department of Petroleum Engineering, STT MIGAS, Balikpapan 76127, Indonesia; lutfi[email protected]

Abstract: The steep slope of the bathymetry and topography that surrounds is a unique morphology of the area that affects the currents. A simulation was carried out in three regions with seven scenarios to understand the effect of wind, tide, and discharge on currents. The results showed that the average current pattern in Palu Bay is more dominantly influenced by tides at the open boundary and in the middle of the bay, steered by wind directions. The velocity decreases when it reaches the end of the bay and eventually reverses back to the mouth of the bay through both sides of the bay. The current in the estuary with a discharge of 36 m3/s moves out of the river mouth. On the other hand, results with a discharge of 2 m3/s revealed that the tidal current in the middle layer to the lower layer moves in the opposite direction to the current generated by the discharge in the layer above. It means that the tidal current velocity is lower than that generated by the river discharge. The computation revealed a good agreement with observed current velocity at the selected observation points.   Keywords: numerical modeling; current; steep slopes; bathymetry; topography; Palu

Citation: Lutfi, M. Insight of Numerical Simulation for Current Circulation on the Steep Slopes of Bathymetry and Topography in Palu 1. Introduction Bay, Indonesia. Fluids 2021, 6, 234. Palu Bay is located on the west coast of Island, which is in the province of https://doi.org/10.3390/fluids6070234 , Indonesia (see Figure1). The bay is a shipping lane, where there are several ports that play a role in reviving and developing the regional economic sector. The Academic Editors: Amparo López capital of Central Sulawesi Province is Palu City [1]. The southern part of the bay is an estu- Jiménez and Modesto Pérez-Sánchez ary [2], and the surrounding area is a popular tourist attraction and commercial (e.g., malls and restaurants) [3] spot. Received: 13 May 2021 Palu Bay is surrounded by a series of high mountains [4] in the shape of a bowl with Accepted: 25 June 2021 Published: 29 June 2021 an altitude of more than 500 m above the sea level (masl). The elevation of the hill in Mantantimali reaches up to 1500 masl. The location, altitude, temperature, and wind conditions are very conducive for paragliding. This hill is one of the best paragliding Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in locations in the Southeast Asia and even in the world [5]. In addition, Palu Bay border with published maps and institutional affil- the . These factors strengthen the wind gusts in the bay. iations. Recently, hydrodynamic numerical models have become the center of attention by researchers for the investigation of the Palu, Sigi, and Donggala (Pasigala) disaster on 28 September 2018 at 18:02:44 local time (10:02:44 UTC). It was triggered by an earthquake with the epicenter at 0.18◦ S and 119.85◦ E, Central Sulawesi, Indonesia [6]. This disaster was the deadliest natural disasters throughout 2018. This resulted in thousands of deaths [7] Copyright: © 2021 by the author. and infrastructure damage such as houses, public facilities and transportation routes [8]. Licensee MDPI, Basel, Switzerland. This article is an open access article Field observations at the four ports in Palu Bay, namely the ports of Wani, Pantoloan, Taipa, distributed under the terms and and Donggala, revealed that several ships were stranded on land, whereas one of them conditions of the Creative Commons weighing 500 tons was dragged about 80 m ashore [9]. Attribution (CC BY) license (https:// Many studies related to tsunami numerical modeling to investigate the causes and creativecommons.org/licenses/by/ impacts of the disaster have been published [10–12], etc. One of the uniqueness of this 4.0/). disaster is that the cause is not directly related to the strike slip earthquake, since this

Fluids 2021, 6, 234. https://doi.org/10.3390/fluids6070234 https://www.mdpi.com/journal/fluids Fluids 2021, 6, x FOR PEER REVIEW 2 of 11

Many studies related to tsunami numerical modeling to investigate the causes and impacts of the disaster have been published [10–12], etc. One of the uniqueness of this disaster is that the cause is not directly related to the strike slip earthquake, since this mechanism rarely generates a tsunami [13]. Seabed sediment landslides due to earth- quakes are the direct cause of tsunamis [14,15]. One of the causes of the instability of the Fluids 2021, 6, 234 seabed sediments during the earthquake is the steep slopes of bathymetry. It is proven2 of 11 that the water depth that rapidly changes near the coastline and it gets steeper into the middle of the bay based on the Global Bathymetric Data of the General Bathymetric Chart of the Ocean (GEBCO) [16]. mechanism rarely generates a tsunami [13]. Seabed sediment landslides due to earthquakes The steepness of the topography and bathymetry in this área are two important fac- are the direct cause of tsunamis [14,15]. One of the causes of the instability of the seabed tors that influence the current movement pattern and the velocity. Numerical models are sediments during the earthquake is the steep slopes of bathymetry. It is proven that the needed to represent events in the past (e.g., disaster investigation), the present (e.g., early water depth that rapidly changes near the coastline and it gets steeper into the middle of thewarning bay based systems), on the and Global the future Bathymetric (e.g., future Data development of the General planning). Bathymetric Therefore, Chart ofthe the re- Oceansults of (GEBCO) this study [16 are]. expected as an insight for stakeholders to develop the Palu Bay and vicinity with an environmental perspective that is responsive to disasters.

FigureFigure 1. 1.Location Location of of Palu Palu bay, bay, Sulawesi Sulawesi island, island, Indonesia Indonesia on on the the world world map map (source: (source: GEBCO). GEBCO).

2. MethodsThe steepness of the topography and bathymetry in this área are two important factors that influence the current movement pattern and the velocity. Numerical models are This model was built to analyze the water current movement in Palu Bay which is needed to represent events in the past (e.g., disaster investigation), the present (e.g., early warninginfluenced systems), by wind, and tide, the and future river (e.g., discharge. future developmentThe three components planning). are Therefore, used as input the resultsdata for of the this model study separately are expected to understand as an insight the for effect stakeholders of each component to develop on the the Paludynamics Bay andof current vicinity movement. with an environmental The simulation perspective is also carried that out is responsive by combining to disasters. several components to understand which component is more dominant. 2. Methods This model was built to analyze the water current movement in Palu Bay which is influenced by wind, tide, and river discharge. The three components are used as input data for the model separately to understand the effect of each component on the dynamics of current movement. The simulation is also carried out by combining several components to understand which component is more dominant. Estuary Coastal Ocean Model and Sediment Transport (ECOMSED) developed by Hydroqual [17] is used for hydrodynamic modeling. This model was developed based on the Princeton Ocean Model (POM) [18]. it uses finite difference methods by applying Fluids 2021, 6, 234 3 of 11

a sigma coordinate system to vertical cross sections. The system depends on the bottom of the water based on transformation [19] to analyze the physical processes that occur in the vertical layer. Wind data was used at the surface boundary. This data is obtained from the Meteorological, Climatological, and Geophysical Agency (BMKG), where the wind vector on land is transformed into a wind data vector with a height of 10 m above sea level (U10)[20]. Coastline and riverbank coordinates obtained from google earth imagery was imposed at closed boundary (areas bordering land). Tidal data was imposed at the open boundary at the mouth of the bay. Meanwhile river depth obtained from the Public Works office of Palu City and bathymetry obtained from the Hydrographic and Oceanographic Center, Indonesian Navy (PUSHIDROSAL) were used at the bottom boundary by interpolating the contours on the sea map and the sketch of a cross section of the river. The boundaries of the research area in geographic coordinates are as follows: 119◦4304200 western most longitude, 119◦5205800 eastern most longitude, 0◦350300 northern most lati- tude, and 0◦5301200 southern-most latitude. The simulation area consists of three models covering the entire Palu bay (model I), a small part of the Palu river estuary (model II), and the mouth of the Palu river (model III). Each model consists of several simulations with certain scenarios. Model I consists of three simulations, where the first and second simulations were carried out separately using wind and tides as generating forces. The third simulation was conducted by combining the effects of wind and tides for one month simultaneously. The output is the monthly average current direction and its velocity. The stability test for model I was carried out by assuming that the bay conditions are considered uniform. In this condition, the depth, tidal conditions, and wind that blow over the water surface are considered constant for each grid. Furthermore, the stable simulation results in the previous stage are used as the initial conditions in the next stage simulation. At this stage, the current generating forces are no longer considered uniform. Wind and tidal data with 1 h intervals were used during simulation. This computation was run with horizontal finite difference meshes are 126 × 63 grids. Model II uses tides and discharge of 2 m3/s and 36 m3/s as generating forces to analyze the influence of small and large discharge on current movements in the estuary. This simulation was run on a horizontal section with horizontal finite difference meshes are 125 × 125 grids and vertically discretized with 11 ¦Ò-levels. Meanwhile, model III is a nested model to model II. The initial conditions of this model use the output of model II as input data. It was built to increase the resolution in the delta and vicinity at the mouth of the river with the same scenario as model II. Initial conditions for both models were obtained from observation data of temperature, salinity, river discharge, and tide. The initial and boundary conditions for salinity and temperature are assumed to be constants 0 ppt and 28 ◦C at the river upstream and 30 ppt and 29 ◦C at the sea respectively. A Floater Current Meter connected to a GPS was used to measure the velocity and direction of the current for model verification. Data were taken at two locations, namely, Pantoloan Port and at the mouth of the Palu River. This measurement was conducted at low slack, low slack, high slack, and highest water conditions in both locations.

3. Results and Discussion 3.1. Large Model The effect of wind, tide, and the combination of these two components on the current circulation is discussed in this section. The river discharge has not been included in this model because the width of the Palu river which empties into the Palu Bay is very small compared to the size of the Palu Bay. A high spatial horizontal resolution is required so that the Courant–Friedrich–Levy (CFL) criteria are met. The effect of river discharge will be discussed in the small model through a separate simulation. Fluids 2021, 6Fluids, x FOR 2021 PEER, 6, x REVIEW FOR PEER REVIEW 4 of 11 4 of 11

Fluids 2021, 6, 234 that the Courant–Friedrich–Levythat the Courant–Friedrich–Levy (CFL) criteria (CFL) are criteria met. The are effectmet. The of river effect discharge of river discharge will will4 of 11 be discussedbe discussed in the small in themodel small through model a through separate a simulation. separate simulation.

3.1.1. Wind3.1.1. Patterns Wind Palu Patterns Bay Palu Bay 3.1.1. Wind Patterns Palu Bay The windThe that wind enters that Palu enters Bay originatesPalu Bay originates from the northfrom theof the north Philippines of the Philippines and then and then enters theenters Makassar theThe Makassarstrait. wind It that divides strait. enters It into divides Palu two Bay andinto originates onetwo branchand from one enters thebranch north Palu enters ofBay the Paluwhich Philippines Bay is which and is then enters the Makassar strait. It divides into two and one branch enters Palu Bay which is narrower narrowerthan the Makassar than the Makassarstrait. The strait. maximum The maximum value of wind value velocity of wind in velocity the Makassar in the Makassar narrower than the Makassar strait. The maximum value of wind velocity in the Makassar strait in thestrait east in monsoon the east monsoonis more than is more 4.5 m/sthan [21]. 4.5 Them/s [21].topography The topography around Palu around Bay Palu Bay strait in the east monsoon is more than 4.5 m/s [21]. The topography around Palu Bay causes mountaincauses mountain winds to blowwinds from to blow the westfrom andthe westeast sidesand east of the sides bay. of The the windbay. The that wind that causes mountain winds to blow from the west and east sides of the bay. The wind that blows on blowsthe surface on the of surface Palu Bay of watersPalu Bay is focusedwaters is and focused blows and faster blows than faster the surrounding than the surrounding blows on the surface of Palu Bay waters is focused and blows faster than the surrounding area (areasarea affected (areas by affected global byclimate). global Figureclimate). 2 shows Figure the 2 shows wind classthe wind frequency class frequency distribu- distribu- area (areas affected by global climate). Figure2 shows the wind class frequency distribution tion for windtion velocityfor wind and velocity its direction and its indirection September in September measured measured at BMKG atstation BMKG in stationPalu in Palu for wind velocity and its direction in September measured at BMKG station in Palu City. City. City.

Figure 2. WindFigure class 2. Wind freque Figureclassncy distribution.freque 2. Windncy classdistribution. frequency distribution. It can be seen in Figure2 that the most dominant wind velocity other than calm It can be seenIt can in be Figure seen 2in that Figure the 2most that dominantthe most dominant wind velocity wind other velocity than other calm than con- calm con- condition occurs at a velocity above 11.10 m/s with a percentage of 12.9%. This means dition occursdition at aoccurs velocity at aabove velocity 11.10 above m/s 11.10with am/s percentage with a percentage of 12.9%. Thisof 12.9%. means This that means the that the that the average wind velocity blowing in Palu Bay is stronger than the wind that blows average windaverage velocity wind blowing velocity inblowing Palu Bay in Paluis stronger Bay is thanstronger the windthan thethat wind blows that in theblows in the in the Makassar strait with the highest velocity intervals of 5.14 m/s to 7.71 m/s in Makassar Makassarstrait with strait the highestwith the velocity highest intervals velocity ofintervals 5.14 m/s of to5.14 7.71 m/s m/s to in7.71 September m/s in September September [22]. The prevailing wind occurs in the north with a percentage of 7.3% based [22]. The prevailing[22]. The prevailing wind occurs wind in theoccurs north in thewith north a percentage with a percentage of 7.3% based of 7.3% on thebased wind on the wind on the wind rose (Figure3). rose (Figurerose 3). (Figure 3).

Figure 3. Figure 3. WindFigure rose 3. Windin PaluWind rose City. rosein Palu in Palu City. City. 3.1.2. Model of the Average Sea Surface Current Generated by Wind It has been understood based on previous research [23] that there are complex non- linear interactions and energy transfers between ocean wind-waves and the mean flow. Fluids 2021, 6, x FOR PEER REVIEW 5 of 11

Fluids 2021, 6, 234 3.1.2. Model of the Average Sea Surface Current Generated by Wind 5 of 11 It has been understood based on previous research [23] that there are complex non- linear interactions and energy transfers between ocean wind-waves and the mean flow. Surface winds winds can can affect affect surface surface currents currents [24]. [24 Th]. Thee Coriolis Coriolis force force causes causes the current the current to turn to perpendicularturn perpendicular to the to direction the direction of the of wind the wind [25]. [25However,]. However, the deflection the deflection caused caused by the by Coriolisthe Coriolis effect effect is so issmall so small in Palu in Bay Palu because Bay because the location the location of the bay of theis close bay to is closethe Equator to the andEquator narrow. and It narrow. reveals It that reveals surface that currents surface currentsare more aredominantly more dominantly influenced influenced by the local by windthe local system wind that system blows that over blows the water over the surface. water This surface. is consistent This is consistent with previous with previousresearch [26],research where [26 ],the where Coriolis the Coriolis effect of effect the Earth of the disappears Earth disappears so that so the that current the current moves moves in the in directionthe direction of the of thewind wind [27]. [ 27]. Prevailing windwind revealsreveals that that the the dominant domina windnt wind direction direction blows blows from from the north the basednorth basedon wind on rosewind (Figure rose (Figure3). This 3). is This in line is in with line the with simulation the simulation result (Figureresult (Figure4a) which 4a) showwhich showthat the that average the average current current pattern pattern in one month in one moves month from moves the openfrom boundarythe open boundary (northern part)(northern into part) the middle into the of middle the bay of withthe bay a maximum with a maximum velocity velocity of 0.73 of m/s. 0.73 Them/s. velocityThe ve- locitydecreases decreases when itwhen reaches it reaches the end the of end the bayof the and bay eventually and eventually reverses reverses towards towards the mouth the mouthof the bay of the through bay through both sides. both Thesides. reversal The revers is causedal is caused by wind by deflectionwind deflection due to due the to steep the steepslopes slopes of topography of topography around around the bay. the bay. Meanwhile, Meanwhile, the decreasethe decrease in velocity in velocity is caused is caused by bythe the narrowing narrowing of of the the bay bay due due to to the the steep steep slopes slopes of of bathymetry. bathymetry. TheThe simulationsimulation resultresult (Figure4 4a)a) revealsreveals thatthat thethe velocityvelocity inin thethe middlemiddle ofof thethe baybay isis fasterfaster thanthan thethe shallowshallow waters along the coast. This result is supported by previous studies that reported that Palu Bay bathymetry has a parabolic cross-sectional shape [[28,29].28,29].

Figure 4. TheThe average average of of ssa ssa surface surface current current pattern pattern for for model model I ( Ia ()a generated) generated by by wind; wind; (b) ( bgenerated) generated by bytide; tide; and and (c) (generatedc) generated by bywind wind and and tide. tide. Fluids 2021, 6, 234 6 of 11

3.1.3. Model of the Average Surface Currents Generated by the Tides Current movements in the bay,in general, are more dominantly influenced by tides [30–32]. The simulation result (Figure4b) show that tidal currents at a relatively high velocity occur only in the open area with a maximum velocity of 1.37 m/s. In this area some of the currents originating from the north enter the bay while others turn westward. From the western part in the open area the currents enter the bay at a velocity of less than 0.5 m/s, where there are no reversed currents in the area bordering the land (closed boundary area). This model does not include the influence of wind as input data, so there is no current deflection along the coast.

3.1.4. The Average Sea Surface Current Generated by Tides and Winds The simulation results (Figure4c) show that the current pattern in Palu Bay is more dominantly influenced by tides at the open boundary with a maximum velocity of 1.5 m/s. It can be seen that the current with high velocity occurs at the mouth of the bay in an area with open boundary with the dominant current coming from the north. This is caused by pressure from the Indonesian throughflow (ITF) system [33–35]. The flow is a transportation route that connects the northern equator of the Pacific Ocean directly with the Indian Ocean. The currents from Mindanao flows into the Sulawesi sea towards the Kalimantan coast, and then join other currents coming from the south China sea [36,37]. Meanwhile, in the middle of the bay the current pattern follows the current pattern generated by the wind. It reveals that as a whole the current movement in the bay is dominated by the influence of the wind.

3.2. Small Model There are many small rivers that empty into Palu Bay. However, the Palu River is the largest one among them. Therefore, this model only focuses on the Palu River estuary. Due to the lack of river depth data, the domain model II only covers a small part of the estuary, namely a small part of the bay and a small part of the river. A high horizontal grid resolution is required to make the model domain cover the entire bay and estuary. But this is difficult to do, since the comparison between the horizontal grid size and the maximum depth in the model domain will be large, and the CFL criterion cannot be satisfied which will cause a run time error [38]. The high resolution covering Palu Bay and Palu River is also not sufficient to describe the real situation. The position of the delta in the middle of the river mouth causes the river flow to be divided in two directions, namely to the east and west. Therefore, the increase in resolution in the delta area is carried out in model III.

3.2.1. Surface Current with Discharge as Generating Force The direction of current movement at high slack wáter and low slack wáter conditions are opposite each other [39,40]. The maximum tidal flow velocity is reached under these conditions. From the simulation results (Figure5), the direction of movement of surface currents when the maximum tidal current velocity moves out of the river mouth. This reveals that the influence of river discharge of 36 m3/s is more dominant than the effect of tidal currents. The maximum velocity of the current at high slack water is 0.47 m/s smaller than at low slack wáter with a maximum speed of 0.53 m/s. At high slack wáter, the tidal currents move into the river so that the direction of the river flow from the upstream is opposite to the direction of the tidal currents. This condition weakens the flow from upstream. Meanwhile, at low slack wáter condition, the direction of tidal currents moves in the same direction as the river flow from the upstream so that the resulting currents amplify each other. Fluids 2021, 6, x FORFluids PEER2021 REVIEW, 6, 234 7 of 11 7 of 11 Fluids 2021, 6, x FOR PEER REVIEW 7 of 11

Figure 5. Current pattern at high slack wáter condition (a) model II and (b) model III. FigureFigure 5. Current 5. Current pattern pattern at high at highslack slackwáter w conditionáter condition (a) model (a) model II and II ( andb) model (b) model III. III. The location of the delta in the middle of the river mouth causes the current to spread TheThe location location of of the the delta delta in in the the middle middle of of th thee river river mouth mouth causes causes the the current current to to spread spread west and east after the current passes through the delta. The simulation results (Figure6) westwest and and east east after after the the current current passes passes through through the the delta. delta. The The simulation simulation results results (Figure (Figure 6) 6) show that the currents generated by the tides and the discharge move at a higher velocity showshow that that the the currents currents generated generated by by the the tides tides and and the the discharge discharge move move at at a a higher higher velocity velocity to the west, then weaken towards the north due to changes in depth. The current velocity toto the the west, west, then then weaken weaken towards towards the the north north du duee to to changes changes in in depth. depth. The The current current velocity velocity is greater in the deeper area compared to the surrounding area. Depth degradation causes isis greater greater in in the the deeper deeper area area compared compared to to the the surrounding surrounding area. area. Depth Depth degradation degradation causes causes the flow to formthe eddies flow to by form slowing eddies current by slowing velocity current due to velocity friction due at the to frictionbottom atof thethe bottom of the the flow to formwater eddies and increasingby slowing current current velocity velocity by due silting to friction waters [at41 ,the42]. bottom of the waterwater and and increasing increasing current current velocity velocity by by silting silting waters waters [41,42]. [41,42].

Figure Figure6. Current 6. Current pattern pattern at low atslack low wáter slack condition wáter condition (a) model(a) modelII and ( IIb) and model (b) modelIII. III. Figure 6. Current pattern at low slack wáter condition (a) model II and (b) model III.

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3.2.2.3.2.2. Current Current Circulation 3.2.2.Circulation Current on on CirculationVertical Vertical Cross Cross on Section Vertical Section Cross Section 3 TheThe simulation simulationThe results results simulation for for a a discharge discharge results for of of a36 discharge36 m m3/s/s (Figure (Figure of 36 7) m7) show3 show/s (Figure that that the 7the) showcurrent current that the current movesmoves out out of of the themoves river river outat at all ofall layers. thelayers. river The The at current allcurrent layers. velocity velocity The current decreases decreases velocity from from decreasessurface surface to to frombottom. bottom. surface to bottom. ThisThis is is caused caused byThis by the the is current causedcurrent velocity by velocity the current being being velocitydecelerated decelerated being by by deceleratedfriction friction with with by the the friction bottom bottom with of of the thethe bottom of the water.water. Thi This sresult resultwater. is is in Thisin line line result with with is the inthe lineresults results with of of the previous previous results ofstudies studies previous which which studies state state which that that the statethe de- de- that the decrease creasecrease in in velocity velocityin velocityis is proportional proportional is proportional to to the the increase increase to the increase in in bottom bottom in bottomfriction friction friction[43]. [43]. [43].

3 Figure 7. CurrentFigureFigure pattern7. 7. Current Current on thep patternattern vertical on on crossthe the v v sectionerticalertical c forrosscross discharge s ectionsection for offor 36discharge discharge m3/s (a )of of low 36 36 slackm m3/s/s (water a(a) )low low condition slack slack and (b) high water condition and (b) high slack water condition. slack waterwater condition. condition and (b) high slack water condition.

3 TheThe simulation simulationThe results results simulation with with a a resultsdischarge discharge with of of a2 2 dischargem m3/s/s (Figure (Figure of2 8) m8) show3 show/s (Figure that that the 8the) showcurrent current that the current velocityvelocity in in layer layervelocity 6 6 ( 휎(휎6)6) into to layer layer 611 11 ( σ( 휎6)(휎11)11) to moves layermoves 11 in in ( σthe 11)the opposite movesopposite in direction direction the opposite to to the the direction current current in toin the current in thethe layer layer above. above.the This This layer show show above.s sthat that This the the shows current current that velocity velocity the current at at 휎휎6 velocity6 to to 휎휎1111 atwhich whichσ6 to is σis 11indicated indicated which is by by indicated by the thethe sign sign ( -()- )is is influenced signinfluenced (-) is influenced by by the the tidal tidal by currents thecurrents tidal entering currentsentering enteringthe the river. river. the The The river. simulation simulation The simulation results results results reveal revealreveal that that river riverthat discharge discharge river discharge dominates dominates dominates the the direction direction the direction of of current current of current movement movement movement from from the fromthe up- up- the upstream to streamstream to to the the do dothewnstream.wnstream. downstream.

3 3 Figure 8. CurrentFigureFigure pattern8. 8. Current Current on thep patternattern vertical on on crossthe the v vertical sectionertical c ross forcross discharge s ectionsection for for of discharge 2discharge m /s (a )of lowof 2 2 m slackm3/s/s ( a water(a) )low low conditionslack slack water water and (b) high slack watercondition condition.condition and and ( b(b) )high high slack slack water water condition. condition.

CurrentCurrent is is a a manifestation manifestationCurrent is a of of manifestation the the movement movement of of of the wáter wáter movement masses. masses. ofThe The w áoutput outputter masses. of of the the cur- The cur- output of the rentrent simulation simulationcurrent c canan be be simulation used used as as input input can beinto into used the the asmodel model input on intoon the the the water water model quality quality on the module module water quality in in the the module in the ECOMSEDECOMSED program. program.ECOMSED One One program.of of the the polluted polluted One ofrivers rivers the pollutedthat that empties empties rivers into into that Palu Palu empties Bay Bay is intois the the Palu Poboya Poboya Bay is the Poboya River.River. Massive MassiveRiver. use use of of MassiveMercury Mercury use in in the ofthe Mercuryillegal illegal gold gold in themine mine illegal in in the the gold Poboya Poboya mine area inarea the has has Poboya polluted polluted area the the has polluted the waterwater of of t hethe Poboya Poboyawater river. ofriver. the Further PoboyaFurther research river.research Further is is expected expected research to to be isbe expectedable able to to model model to be the ablethe dispersión dispersión to model the dispersión ofof pollutants pollutants that thatof pollutantsaccumulate accumulate that in in Palu accumulate Palu Bay Bay based based in Palu on on th Baythisis model. basedmodel. on this model.

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3.2.3. Verification 3.2.3. Verification Figure9 9a,ba,b showshow aa velocity-time graph for for the the simulation simulation results results and and observations observations at 3 atthe the port port ofof Pantoloan Pantoloan and and at at the the mouth mouth of of the the Palu River withwith aa dischargedischarge ofof 2 2 m m3/s./s. ItIt showsshows aa similarsimilar patternpattern withwith thethe goodgood rootroot meanmean squaresquare errorerror (RMSE) (RMSE) values values of of 0.01 0.01 m/s m/s and 0.560.56 m/s,m/s, respectively. The The RMSE RMSE value at the Pantoloan port was betterbetter thanthan thethe RMSE value at at the the mouth mouth of of the the Palu Palu river. river. Th Thisis is because is because the theinput input of river of river depth depth data dataat the at verification the verification location location obtained obtained from fromthe Pu theblic Public Works Works Office Office of Palu of City Palu is City the isresult the resultof interpolation of interpolation from the from sketch the sketch of a cross of a se crossction section of the ofriver. the river.The results The results of depth of depth inter- interpolationpolation should should not be not used be used as input as input data data in the in themodel model for forverification verification purposes purposes in the in theriver river mouth mouth that that experience experience high high sedimentat sedimentationion dynamics. dynamics. In order In order to obtain to obtain better better sim- simulationulation results results at the at verification the verification location, location, the water the water depth depth observation observation data at data the atverifi- the verificationcation points points must mustbe used be usedas input as input data datafor the for initial the initial conditions conditions of the of simulation. the simulation.

Figure 9. VerificationVerification ofof currentcurrent velocityvelocity ((aa)) PantoloanPantoloan portport andand ((bb)) riverriver mouth.mouth.

4. Conclusions The simulationsimulation results results revealed revealed that that the the average average current current movement movement pattern pattern in Palu in Palu bay isbay more is more dominantly dominantly influenced influe bynced tides by attides the at open the boundaryopen boundary with a maximumwith a maximum velocity ve- of 1.7locity m/s, of whereas1.7 m/s, whereas in the middle in the of middle the bay of the the current bay the pattern current follows pattern the follows pattern the generated pattern bygenerated the wind. by Thethe velocitywind. The decreases velocity whendecreases it reaches when the it reaches end of thethe bayend andof the it turnsbay and back it towardsturns back the towards mouth ofthe the mouth bay throughof the bay the throug two sidesh the oftwo the sides bay of due the to bay the due steep to slopesthe steep of theslopes bathymetry of the bathymetry and topography and topography around the around bay. the bay. The movement of surface currents in the estuaryestuary of the Palu River with a river dis- charge ofof 36 m3/s moves moves out out of of the the river river mouth. mouth. Mean Meanwhile,while, the the simulation simulation results results with with a 3 adischarge discharge of of2 m 23 m/s revealed/s revealed that that the thetidal tidal currents currents in the in middle the middle layer layer to the to lower the lower layer layermove move in the in opposite the opposite direction direction to the to the current current generated generated by by the the discharge discharge in in the layerlayer above, tidal current velocity velocity is is smaller smaller than than the the current current generated generated by by discharge. discharge. It can It can be beconcluded concluded that that river river discharge discharge is more is more dominant dominant in the in estuary the estuary of the of Palu the Palu river. river. The sim- The simulationulation result result seems seems to tobe becorresponding corresponding to to observed observed current current velocity velocity with with RMSE valuevalue of 0.01 m/sm/s at at the the Pantoloan Pantoloan port port and and 0.56 0.56 m/s m/s at at the the river river mouth. mouth. TheThe bathymetry bathymetry used used in in this this study study still still uses uses data data prior prior to the to tsunami the tsunami disaster disaster caused causedby underwater by underwater landslides. landslides. It has been It hasknow beenn from known many from previous many previousstudies that studies bathyme- that bathymetrytry in Palu Bay in Palu has undergone Bay has undergone many changes many changessince the since incident. the incident. Therefore, Therefore, updated up-ba- datedthymetry bathymetry data is needed data is to needed rebuild to the rebuild current the model. current model.

Funding: This research was supported by a grantgrant fromfrom thethe STTSTT MIGAS,MIGAS, withwith contractcontract numbernumber 013/SP/Penelitian-LPPM/Desember/2020. InstitutionalInstitutional ReviewReview BoardBoard Statement:Statement: Not applicable. InformedInformed ConsentConsent Statement:Statement: Not applicable. Fluids 2021, 6, 234 10 of 11

Data Availability Statement: Not applicable. Acknowledgments: I would like to express my gratitude to International Development and Coop- eration, Hiroshima University for the GIS software facility in the laboratory for the completion of several simulations. My special thanks go to Mardeli Jandja for the insight during his life. Conflicts of Interest: The author declare no conflict of interest in this research.

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