Modelling of flow characteristics in the access channel of Bali tourism port using DualSPHysics

The access channel is a structure formed as a basin on the seabed. Based on previous studies, excessive sediment accumulation in shipping lanes or access channels carried by ocean floor currents is one of the factors that cause damage to the hull under the ship. Therefore, this study aims to investigate the characteristics of seabed currents that cross the access channel. This research was carried out by modelling the numerical test of the access channel model using the Smoothed Particle Hydrodynamics (SPH) method with DualSPHysics software. The results of this study show the movement and characteristics of seabed currents that occur in the access channel at the Bali Sanur Harbor. This study obtained the average speed of seabed currents, with variations in the speed of ocean currents and forecasts regarding sediment dredging maintenance on cruise lines.


Introduction
In general, port areas have a specific structure or pathway for ships, designed to provide access for ships to enter or exit the port without causing friction on the ship's bottom surface, which could lead to damage or unsafe conditions for the ship and its cargo.This specialised pathway for ships is called an access channel.
The access channel is a waterway that allows ships to navigate in and out of the port and access the port facilities.It serves as a connection between the open sea or the main water body and the port area.Access channels are designed and maintained to accommodate ships of various sizes and weightsto ensure safe and efficient navigation [1].The channel is dredged to a minimum depth of 2 m, and its width is determined based on the size, draft, and maneuvering requirements of the accessing ships [2].
The access channel plays a crucial role in the safe navigation of sailing ships, as it is hazardous if the channel is too shallow or the basin too small [3].The accumulation of sediment and erosion in the access channel could raise the sea floor and cause potential collisions that must be avoided in ports.
Underwater currents, also known as bottom currents, refer to the movement of water near or along the sea floor.They differ from surface currents that occur in the upper layers of the sea.Bottom currents are mainly influenced by factors such as tides, sea winds, and other forces [4].These currents transport small particles, such as sand, in large quantities, shaping underwater landscapes, such as beaches and submarine canyons through erosion and sediment deposition.
As underwater currents continuously transport sediment to the access channel, there is a risk of sediment accumulation in the channel.Therefore, it is essential to study the characteristics of bottom currents.Various studies have been conducted to understand the characteristics of bottom currents using numerical methods.One software used for this purpose is DualSPHysics, a computational fluid dynamics (CFD) software specifically designed to simulate free-surface flows [5].DualSPHysics uses the Smoothed Particle Hydrodynamics (SPH) method, a Lagrangian technique, for solving fluid flow problems [6].It can accurately model free-surface flows such as waves, tsunamis and underwater currents by analysing the movement of individual particles, resulting in more accurate numerical outputs.
This study aimed to model the characteristics of bottom currents in the access channel of Bali's tourist port varying bottom current velocities.The numerical output provides data on the average bottom current velocity in the vicinity of the access channel.These data can be used to understand sediment movements and determine potential sediment accumulation in the access channel, for further research purposes.

Seabed Currents Analysis
The horizontal movement of water near or along the sea floor is different from the surface currents that occur in the upper layers of the sea.Bottom currents are primarily influenced by factors, such as tides, sea winds, and other forces.These currents play a crucial role in transporting sediments, shaping underwater landscapes, and influencing the distribution of marine organisms and nutrients.Understanding bottom currents is essential for various marine activities such as navigation, sediment transport and coastal engineering projects.
The access channel or navigational pathway is a water body that is considered safe for ship navigation in terms of depth, width, and other navigational clearances, whether in the sea, rivers, or lakes.It is marked on navigation charts and is typically found in sailing guidebooks and announced by authorised authorities [7].An access channel is used to guide ships along river or lake routes, and regular maintenance is necessary to ensure safe sailing and environmental preservation.Proper planning of the access channel can enhance port loading and unloading productivity, facilitate smooth ship movement, and ensure ship safety during navigation.Hydrographic conditions in the access channel that require attention include channel depth, tidal currents, channel width, changes in current geometry/alignment, and a clear space above the water surface [8].

Research Methodology
Smoothed Particle Hydrodynamics (SPH) is a Lagrangian meshless method that discretises a continuum using a set of material points or particles.It simulates fluid dynamics by integrating the discretised Navier-Stokes equations locally at the position of each particle.The neighbouring particles are determined based on a distance-based function with a characteristic length denoted as "h" At each time step, new physical quantities are calculated for each particle, and they move accordingly.This method uses interpolation functions, known as kernel functions (W), to estimate values at specific points.These functions represent the physical properties of particles within a defined integral approximation [9,10].
3 Kernel smoothing must satisfy several properties, such as positivity within the defined interaction zone, compact support, normalisation, and monotonically decreasing with distance and difference.The function, , can be discretely approximated based on the set of particles.In this case, the function is interpolated at a particle of interest where the integration is performed over all particles that fall within the support region, as defined by the smoothing length h.[9]. is calculated by Equation 1below: () = ∫ ( ′ )( −  ′ , ℎ)′ (1) where  and ′ are the position of the particle of interest and the neighbouring paricles respectively.
The cases generated by the Gencase program underwent numerical testing.The numerical modelling simulations were conducted using the DualSPHysics program.In these numerical tests, DualSPHysics were utilised a computer's GPU to accelerate data processing (data computation), and once the data processing is complete, the results are returned to the program.
The mathematical theory for these tests is based on the principles of the Navier-Stokes theory of fluids.This theory is further explained through continuity and momentum equations, as described in the fundamental section.The continuity equation and momentum equation were solved using the numerical method called smoothed particle hydrodynamics (SPH).

Results and discussions
From the analysis from SPH, the diagram in Figure 3 can be extracted to a numeric table, the average seabed flow is found, and the results are presented in Table 2. Unstable sediment refers to sediment that moving at that velocity.For instance, at a bottom sea current velocity of 0.03 m/s, sediments with sizes below 0.28 mm be carried by the current, while those larger will remain undisturbed.The measurement speed was obtained by averaging the data from 50 to 60 s, resulting in 10 s of movement, and calculated every 0.01 second.Thus, 1000 data points were averaged.

Conclusions
The analysis of bottom sea current characteristics in Sanur Harbor's access channel using the DualSPHysics method yielded the following conclusions: a.At a current velocity of 0.03 m/s, the bottom sea current speed was 2.8 cm/s, carrying sediment particles below 0.3 mm diameter.b.At 0.05 m/s current velocity, the bottom sea current speed is 3.7 cm/s, carrying sediment particles below 0.35 mm diameter.c.At 0.08 m/s current velocity, the bottom sea current speed is 5.4 cm/s, carrying sediment particles below 0.48 mm diameter.
d.At 0.1 m/s current velocity, the bottom sea current speed is 7.4 cm/s, carrying sediment particles below 0.59 mm diameter.e.At 0.3 m/s current velocity, the bottom sea current speed is 26.9 cm/s, carrying sediment particles below 2.1 mm diameter.f.At 0.6 m/s current velocity, the bottom sea current speed is 55.3 cm/s, carrying sediment particles less than 6 mm in diameter.g.At 0.9 m/s to 1.2 m/s current velocities, the bottom sea current speed ranges from 80.1 cm/s to 106 cm/s, carrying sediment particles below 10 mm diameter.Overall, the bottom sea currents exhibit considerable strength owing to the convergence of currents from the north and east in the central location of Sanur Harbor.

Figure 1 .
Figure 1.Visualisation of basic sea currents for a duration of 60 seconds in the DualSPHysics software.

Figure 2 .
Figure 2. Time history of average speede of bottom currents with the inlet flow velocity of 0.3 m/sFrom the above diagram (Figure2), it can be observed that new stability occurs after 50 s (100 time step per second).The data from the diagram were exported to Microsodt Excel, and the average values for each second were obtained.To determine the stability of the average speed of the bottom sea current, the average of these data points was calculated.The oldest green line represents the average line, the green lines indicate q1/q3, and the light green line represents the minimum/maximum speed of the bottom current.

Figure 3 .
Figure 3. Fall velocity of sediment

Table 1 .
Average current speed at Sanur Harbour

Table 2 .
Average seabed current calculated using SPH