Study of water discharge in the Ulee Lheue Port Channel, Indonesia

Ulee Lheue serve as ferry ports in Banda Aceh, covering an area of ±8 ha. The information on water flow entering the port channel is needed to determine the quantity of water and facilitating transportation at the port. This study includes the measurement of water depth, channel width, current velocity, water discharge, and tides which was conducted every 3 hours at three different water depth. On Jumada I (Jumadil awal) 1-29, 1435 Hijra, the highest and lowest water depth of 6.06 and 3.82 meters occurred in the 23rd and 10th day at 09:00 and 16:00 local time, respectively. The highest and lowest velocity of 0.357 and 0.01 m/s occurred in the 15th and 18th day at 09:00 and 12:00 local time, respectively. The strongest and weakest discharges of 177.93 m3/s and 9.47 m3/s occurred in the 12th and 28th day at 09:00 local time, respectively. The results found a strong correlation between water discharge and current velocity. Thus water discharge and currents play an important role in the channel stability and in controlling sea transportation in terms of scheduling the departures and arrival of ships.

shows that is the current velocity of the 5th day at 03:00 pm was the highest in the first week, that was 0.26 m/s." "Based on Figure 7, the current velocity on 12th Jumada I at 09:00 am was 0.35 m/s because the depth of the water also headed towards the tide, resulting in the large current into the port channel."

Introduction
Aceh province is located at the western end of Sumatra island, and the capital is Banda Aceh. The land area is 56,758.85 km 2 or 5,675,850 hectares, while the sea area is 12 miles or 7,479,802 hectares [1]. Aceh province has strategic waters because it is directly facing the Indian Ocean. Therefore, adequate facilities and infrastructure, such as sea transportation, that can support the activities of the society are required.
Ulee Lheue Port is the only port in Banda Aceh with an area of ± 8ha. The condition of the port was very poor after the 2004 tsunami, with no buildings left. The port used to serve several routes, namely the ferry to Sabang, Lhokseumawe, Kuala Langsa and even Belawan, Medan. In relation to port, the seawater discharge needs to be maintained as it will ultimately influence the seawater depth in the port environment due to tides. The study concerning water discharge is usually undertaken in a watershed (DAS) or reservoir because such research serves as a flood control to prevent flooding [2].  [6]. By scientific law, the higher or the further the distance, the smaller the gravitational forces. This occurs because the distance from the earth to the sun is further compared to the moon. Thus, the gravitational force of the moon has more influence on tides.
Some researcher argued that the combined influences of the moon and solar could increase or decrease the height of tides occurred [3][4][5 [6]. During the new moon and the full moon, where the earth, moon and sun are in an orbital line, the lunar tides are strengthened by the tides of the sun. During this period, the tides have a maximum height, called 'spring tides'. In the first and third places, where the moon and the earth are perpendicular to the sun, the lunar tides are weakened by the solar tides.
Water discharge is the velocity of water flow per unit time, used to monitor water capacity. Positive and negative signs of water discharge and velocity point out direction of flow towards North (out of the port) and South (entering the port), respectively. This study aimed to calculate the water discharge in the port channel based on measurements of tidal currents, water depth, and channel width. Therefore, the water discharge serves as the control and provides a recommendation concerning the channel equilibrium to the government via the ministry of transportation.

Time and study location
The research was conducted at the Ulee Lheue Ferry Port in Meuraxa Regency, Aceh Province. Data collection lasted one month (every day at 08:00 am -06:00 pm) from middle of February to March 2014. The data collection was carried out four times daily (at 9 am, 12 pm, 3 pm, and 6 pm). The data included the channel width, current velocity, and channel depth.

Research procedure
The procedure employed in this study was a survey method conducted at three depths. This research included various oceanographic measurements, namely the measurement of sea level fluctuation, water depth, current velocity, port channel width and water discharge. The sea level fluctuation was measured using a Roll Meter weighted with plumb weight. The stone serves as a ballast so that the meter transect remains uptight. This sea level fluctuation was measured for one hour to obtain more accurate data. The water depth data collection was based on tides, where the plumb weight placed at the end. The plumb weight also functions as ballast in measuring the depth of the three points of the port channel. The Lagrangian or Eulerian approach was employed to measure the current measurement. The Lagrangian approach was undertaken by observing the movement of surface water masses over time, usually done using a float or known as floating dredge. This velocity measurement was held four times a day at the three depths, as presented in Figure 1. The velocity measurement was conducted by using a standard floating dredger. The velocity measurement consists of the magnitude and direction of flow current. The floating dredger was then released, and the length of the floating dredger that moving was calculated to obtain the floating dredger traveling time. The result was divided by the distance and the time of the floating dredger to get the current velocity. Whereas the direction of the current was obtained from the direction of motion of the floating dredger. This measurement also was undertaken at three depths, as displayed in Figure 1. Global Positioning System (GPS) was employed to measure the channel width. This tool works by recording the coordinates of one pier point to another pier; the coordinates determine the distance between one pier to another. The water discharge was calculated by the area of the cross section and the current velocity, as presented by the following equation [7]:    The depth determines the location of the bottom of the waters; therefore, it must be considered when measuring water discharge. The deeper the waters, the further the observation results from the factor of meandering or turbulent current. Water depth is the vertical distance between the seafloor and the bottom of the water, generally expressed in meters. The changes in water depth influenced by the tidal forces in the first week, that on the 7th day the highest water depth occurred at 9.00 am. This occurred because the water depth on the 6th day at 06:00 pm was 5.48 m; thus it can be predicted that the tide would occur at night and then recede in the morning. On the 8th day, the highest depth was 5.53 m at 08:00 am, and the lowest was 4.12 m at 03:00 pm. The water depth pattern on the 8th and 9th days did not show a significant difference; the highest depth was 5.46 m at 8 am, and the lowest depth was 3.88 m at 03:00 pm. The lowest depth of the month occurred on the 10th day at 04:00 pm that was 3.82 m. This was caused by tidal forces, which may be the lowest tide at that time, affecting the water depth. The highest depth was on the 13th day at night because it was a full moon. The highest depth on this day reached 5.36 m at 1 pm, and the lowest was 4.27 m at 08.00 am. The highest depth in Jumada I occurred on 23 Jumada I 1435 H, at 09:00 am, caused by tidal forces in the Ulee Lheue port channel.
Four ships for transportation purpose are anchored in Ulee Lheue port. Each ship has a different size and function, depending on its needs. The fast ship or Mitra Bahri ship is for passengers only. Papuyu and Agency for the Rehabilitation and Reconstruction (BRR) ships are for both passengers and cargo (goods and vehicles). Whereas, the National Search and Rescue Agency Ship (BASARNAS) is as a rescue ship for the unexpected event in the ocean. Each ship is equipped with ship draught to balance the ship stability when running [8]. BRR ship hold 2.7 m draft, BASARNAS Ship hold 2.9 m draft, Papuyu ship hold 2.5 m draft and Mitra Bahari ship hold 2.1 m draft.    Figure 9 The current velocity of the fourth week. Figure 6 shows that is the current velocity of the 5th day at 03:00 pm was the highest in the first week, that was 0.26 m/s. At that time, the water depth moved towards the tide so that the current velocity was large and entered the port channel. Whereas, the lowest current velocity was on the 4th day at 09.00 am. Based on Figure 7, the current velocity on 12th Jumada I at 09:00 am was 0.35 m/s because the depth of the water also headed towards the tide, resulting in the large current into the port channel. Current is always related to depth. The change in the direction of complex currents occurs as the water depth increases, causing the highest and lowest current velocity. Besides, the water current velocity is also influenced by the wind [2]. The current velocity can be decreased rapidly along with the increasing water depths, and finally, the wind does not affect it [4]. When the wind blows in the ocean, energy is transferred from the wind to the sea level. Some of this energy is used to form the surface gravitational waves, moving the water from the small wave towards the wave propagation to create currents at sea. The faster wind velocity leads to the higher the frictional force at the sea level and the greater surface currents. The friction between the wind and the sea level can create water movement, including laminar and turbulent water movement [9]. That's why, the water current velocity is difference. When the wind blows harder, it will cause friction between the wind and the sea level, creating greater water movement. The maximum current velocity at the Ulee Lheue port channel was on the 15th day (0.35 m/s) and the minimum current velocity on the 18th day at 12:00am (0.01 m/s).

Water disharge
The water discharge from 1 Jumada I 1435 H (2 March 2014) to 29 Jumada I 1435 H (30 March 2014) was measured every 3 hours at Ulee Lheue port and the average was 90.94 m 3 /s. To better understand the graph, it was divided into four weeks and presented in Figure 10 to 13. Positive and negative signs of water discharge point out direction of flow towards North (out of the port) and South (entering the port), respectively.  Figure 10. Water discharge of the first week. Figure 11. Water discharge of the second week.

Figure 12.
Water discharge of the third week. Figure 13. Water discharge of the fourth week.
Based on first week Figure 10, the largest discharge of the first week was on the 5th day, at 03:00 pm (151.94 m 3 /s) and the smallest discharge was on the 4th day, at 09:00 am (13.18 m 3 /s). Figure 11 shows that the biggest discharged was on the 12th day at 09:00 am (177.93 m 3 /s), the maximum discharge in a whole month. This happened because the current velocity entering the port channel was also strong. Figure 11 also indicates that the lowest discharge of the second week was on the 8th day at 03:00 pm (11.99 m 3 /s). As for the third week (Figure 12), the largest water discharge was on the 19th day at 06:00 pm (173.88 m 3 /s) and the lowest was on the 18th day at 12:00 am (10.23 m 3 /s). The water discharge on the fourth week was average because the change in current velocity in this week was slow. The highest discharge of the fourth week was on the 28th day at 12:00 am (156.02 m 3 /s), and the lowest was on the same day but at 09:00 am (9.47 m 3 /s). The average discharge at the Ulee Lheue port channel was 81.28 m 3 /s. The large water discharge at the Ulee Lheue port channel will affect the ship activities of leaving or entering the port. If the water volume entering the port pond is large, the incoming ship will choose the berth based on the water depth. Before technicians build a port, they have considered the appropriate sites for the vessel to berth based on the depth of the waters [10].

The correlation between water discharge and current velocity
Correlation is a value indicating a relationship. This study discusses the relationship between water discharge and current velocity. The greater the correlation value, the greater the relationship between the two variables. The correlation between water discharge and current velocity at the port was not significantly different. The values at 09:00 am, 12:00 am, 03:00 pm and 06:00 were 0.968, 0.982, 0.985 and 0.988 respectively. The correlation between water discharge and current velocity is displayed in Figure 14. The correlation between the current velocity and water discharge at the port channel is presented in Figure 14. The correlation values between the two variables (current velocity and water discharge) at 09:00 am, 12:00 am, 03:00 pm and 06:00 pm were 0.968, 0.982, 0.985, and 0.988 respectively. The correlation presented in Figure 14 shows that the average correlation between water discharge and current velocity was 0.981. The greater the correlation value or, the closer to 1, the greater the correlation between water discharge and current velocity. Figure 6 and 10 explain that the higher the current velocity, the more the influence on the water discharge in certain areas. Based on the patterns in Figure 6 and 10, it can be seen that at 09:00 am, the highest was light blue and the lowest was purple. The light blue was the 7th of Jumada I and the purple was the 4th of the Jumada I. The trend in Figure 6 and 10 was similar for each hour because the data in Figure 12 and 10 were collected at the same time. On the 23rd day, the average water level was 6.05 m, measured at 09:00 am, and the current speed was 0.09 m/s. The current velocity is very slow at the highest tides [11] [12], so the water discharge entering the port was 59.67 m 3 /s. On the 10th day, at 04:00 pm, the lowest water depth was 3.82 with an average current velocity of 0.11 m/s so that the water discharge flowing to the port was 95.23 m 3 /s. Based on Figure 7, it is observed that the water discharge at the highest tide was smaller than the average discharge, that was 81.28 m 3 /s. Whereas, at the lowest tide (on 10th Jumada I 1436 H), the water discharge was higher than the average, because 03:00 am was the lowest ebb tide, so that the water depth shifted from ebb tide to the high tide at 04:00 pm. Thus, the current velocity was also high resulted in a large water discharge, as shown in Figure 11. The 13th of Jumada I was a full moon and the highest tides occurred at night due to the full moon. The magnitude of the current velocity and water discharge are closely related. The higher the current velocity, the greater the water discharge because the mass of the water volume that moves at a specific time is higher per unit time.

Conclusion
This study showed that the highest water discharge was on the 12 th of Jumada I 1435 H at 09:00 am (177.93 m 3 /s) and the average water discharge was 81.28 m 3 /s. The results also indicated that the highest water depth occurred at 09:00 am on the 23rd of Jumada I (6.05 m) and the average water depth was 5.02 m. The largest current velocity at the Ulee Lheue port was on 15 Jumada I 1435 H (March 16, 2014) at 09:00 am (0.35 m/s), while the slowest current velocity was on 18 Jumada I 1435 H at 12:00 pm (0.01 m/s). The average of the current velocity at the Ulee Lheue port is 0.14 m/s. Water discharge correlates with current velocity, the higher the current velocity occurred, the greater the discharge entering or exiting the port channel. The average correlation value between water discharge and current velocity in this study was 0.981. The correlation value that is close to 1 shows that this study is valid.