Overview of watershed development and river sedimentation condition in Sorong, Papua, Indonesia

Sorong Regency is developed to be a special economic zone, for enhancing economic in the eastern part of Indonesia. Few studies or investigations, which are related to river sedimentation due to the development, are rarely available. In September 2023, we conducted field investigation to four watersheds in Sorong (Remu, Mariat, Klasaman, and Warsamson Watersheds), in which each of them has different services. The normal flow at the upstream of Remu River was not visually turbid, and the watershed was well conserved for drink water purposes. The Mariat River was visually turbid as the watershed was developed for transmigration and agriculture area, which is likely related to siltation at the downstream segment of the river and sedimentation in the irrigation facilities. The sand mining and washing activities in the Klasaman Watershed are likely related to the difference of visual turbidity of the upstream and downstream river flows relative to the mining site. We think that the erosion protection of manmade slopes are needed to reduce the amount of sediment entering the Warsamson River. Erosion and sedimentation monitoring efforts are crucial and urgent toward sustainable water resources development in Sorong, Papua, Indonesia.


Introduction
The government of the Republic of Indonesia [1] has developed 20 special economic zones until 2023, one of them is at Sorong, Papua.The Sorong special economic zone (Sorong SEZ) was planned to improve the economy in the east part of Indonesia.Sorong SEZ is projected to have factories, offices, agricultural areas, residential areas, and transportation hubs [2].
To support the development of Sorong SEZ, the government is building water supply, water sourced energy, settlements, and transportation infrastructures [3][4][5][6][7].The aim is to attract investments, such as for manufacturing factories, industrial facilities, agricultures, culinary and hospitality resorts [8].The government hopes that the investments may create jobs and economic leaps [1], especially in the east part of Indonesia [9].
There is lack of studies and investigations that reveal the relation between infrastructure developments and river sedimentation in Sorong SEZ.It is not clearly known whether infrastructure developments in Sorong SEZ are related to the river capacity reduction and the risk of river floods.In 2016 to 2023, after the establishment of Sorong SEZ, there were several flood occurrences in Sorong Region [10][11][12][13], regardless to some temporal local inundation during the rainy situation.We also notice the difficulty to find reports and publications that explain whether the river sedimentation may disturb the performance of the constructed water infrastructures in Sorong SEZ.We have few scientific reports [10,[14][15][16] that explain the needs of dredging in ponds, reservoirs, weirs, or irrigation channels due to river siltation and sedimentation, especially in Papua, Indonesia.
This paper aims to initiate an effort to survey the river sedimentation profile in Sorong SEZ.We would like to see the overview on the possible relation between watershed development and river sedimentation condition in the study area.We want to discuss the chance of implementation of the best practice in sustainable water infrastructure development in the study area.

Methodology
In order to fulfil the research objectives, we conducted field survey and investigation at four representative watersheds in Sorong, Papua, Indonesia.The survey was conducted between 3 September 2023 and 9 September 2023.It was during the dry season but scattered rainy days were occurred within the monitoring period.
The four representative watersheds were selected based on the watershed services (Figure 1).The Remu Watershed was selected because its function as drinking water supply for the Sorong City.The Mariat Watershed was surveyed because it supplies irrigation to the Aimas Transmigration Area, Sorong Regency.The Klasaman Watershed was sampled because it has sand mining activities that supply sand to construction projects in Sorong City.The Warsamson Watershed was studied because it has raw water intake to supply the Sorong SEZ in the future.The most downstream survey locations at Remu River was 0°52'20.59"S131°16'58.90"E,at Mariat River was 0°59'50.43"S131°19 '11.09"E, at Klasaman River was 0°54'36.65"S131°21'16.98"E,and at Warsamson River was 0°49'30.52"S131°23'27.16"E.We conducted river tracking to the upstream of the surveyed locations and captured the situation of the surveyed river segments using a camera and a DJI Mini 2 Drone [17] with flying height up to 25 m from ground and flying area 1500 m 2 .
We visually observed water flow turbidity and river morphology at several river segments.Camera and drone were used at all locations, except drone was not flown at Warsamson River.At Remu River, we take a look at riffle and pool segments upstream of the drinking water intake facility.At Mariat River, we compare the condition at the upstream and downstream of the Mariat Irrigation Weir.At the Klasaman River, we compare the condition at the upstream and downstream of the sand mining location.At the Warsamson River, we surveyed two locations that were at the upstream and downstream of the constructed new road at Kampung Klasigi, Giwu, Kota Sorong, Papua Barat, Indonesia.The coordinates of survey points will be stated in relation to its detail context in the result section.
We also scientifically discuss the surveyed condition with river erosion and sedimentation profiles at other locations.We critically and objectively assessed the surveyed condition based on scientific literatures to provide recommendation for the update of the sustainable water infrastructure development master plan of the Sorong SEZ.

Results
We found that the erosion and sedimentation general overview is different among the four studied watersheds.Based on our visual observation using satellite images, drone, camera, and our field investigation as the validation, the visual flow turbidity of each watersheds are distinctly different.The visual turbidity of the flow are likely to be related to the river services and watershed utilizations.
In Remu River, the visual turbidity is low and the water is relatively clear in the dry season and no rain condition.Nevertheless, the Remu water intake guard reported that the water is very turbid during the floods.The Remu river authority also highlight the sedimentation problems in the downstream of the Remu Watershed (downstream of the intake).The torrential part of the Remu Watershed is conserved by the local community as it supplies clean water to the people of Sorong.
The Mariat River was visually more turbid compared to the other three rivers, in the dry season.The visual turbidity at the upstream and the downstream of the Mariat irrigation weir were closely similar to each other.The Mariat watershed was highly developed and cultivated.Large part of the watershed was converted to agricultural and residential areas.It was not directly intersect with the palm oil cultivation area in Klamono (1°04'29.8"S131°33'42.4"E),Sorong.The soil in Mariat River are mostly clay and there are several clay brick factory in the surrounding area.
There are two distinct developed areas of the Klasaman Watersheds, which where the sand mining quarry and the residential area.The sand mining quarry was existed at the upstream of the Sorong Tourism Forest.We focussed at the mining site while exploring the visual turbidity of the river.At the downstream of the Forest, the area was fully developed for residential and low to middle size industry, such as home industry and farms.River siltation and sedimentation were noticed at the downstream river segment relative to the forest.
We noticed that there were many uncovered and non-vegetated slopes between the newly constructed roads on the watershed.We also noticed that there were several open quarries and agricultural in the area.When the water level at the main Warsamson River was high (some riparian trees were flooded), the discharge from river branches cannot flow freely to the main river.The main stream of Warsamson at the most downstream survey location was around 80 to 100 m wide, which was the largest watershed among the study watersheds.

Remu River
The visual turbidity of Remu River during the dry period (no flood) are low (clear water flow), see Figure 2. We can see that the river bed in the study location consist of stone and gravels, whereas fine sediment was scarce.The torrential area of the Remu River are well preserved but in the upstream area, there were road and settlement constructions.There was no large sand quarry (more than 10 ha) are sensed and reported in the watershed.Sedimentation problem are existed at the downstream of Remu River (at the segment from the water intake structure to the estuary).

Mariat River
The flows in Mariat River were visually turbid, either at the upstream or downstream of the Mariat Weir (Figure 3).The soil of the river bank area were generally silt and clay, with few coarse aggregate can be observed.There is a different water turbidity (colour) between the river water and the water at the nearby oxbow lakes.We noticed some logging activities at the downstream area of the weir, as the loggers floated the timbers on the river and transported it to the downstream.We noticed many agricultural areas managed by local farmers at the upstream segment of the Mariat Watershed.

Klasaman River
We captured a distinct different between the water visual turbidity at the upstream and the downstream of the sand mining quarry (Figure 4).The method that was commonly used by some sand miners in Sorong Area was to use water to separate between the collected sand and fine sediment.We hardly found rocks at the downstream river segment relative to the mining location.We reckoned many clay bars at the downstream river segment relative to the Sorong Tourism Forest.

Warsamson River
The land cover of the upstream part of the Warsamson River were relatively pristine, where endemic flora and fauna settled.The downstream part of the Warsamson River was more developed than the upstream as many roads, settlements, resorts, and infrastructures were on going to be built (Figure 5).There are some quarries nearby the construction areas from which the construction materials were efficiently collected.Some cut and fill location were open bared and the surface material were prone to be eroded by surface runoff and rainfall.The sedimentation in the Warsamson was seen at the newly built raw water intake facilities, which was hindering the operation of the intake.

Discussions
We have presented some erosion and sedimentation overview in Sorong Area, based on the sample of four different watersheds.The presented sediment information can be used by local people and government authority to evaluate the current master plan of the Sorong Special Economic Zone (SEZ) development.The stakeholder can see the current profile of the river sedimentation, so that some adjustment in the ongoing construction projects can be implemented, in order to reduce the sedimentation load to the watersheds.Several best practices from other area in developing economic centre without neglecting the sustainability of the river can be considered [18,19].
We are confident that the water turbidity in Remu River was the best among the other study rivers because the society abstract the water of Remu River and preserve the watershed.The local communities does not hesitate to warn individuals or companies if they conducted some harmful activities to the Remu River [20,21].The government should put extra care to the road and settlement projects at the upstream of the Remu Watershed so that there will be no uncontrolled sediment entering Remu river due to the execution of the project.We think that the sediment brought by the flood water was likely to be related from the upstream side of the Remu River.
The stakeholders need to conduct further study related to the environmental capacity of the Mariat River, as the erosion and sedimentation problem in Mariat Watershed was the most severe compared to the other study watersheds.We also have to consider that the soil around the river bank was mostly composed of fine sediments, which are easy to be eroded [22,23].The irrigation weir needs to be flushed at appropriate time and frequency as the erosion from the upstream of the weir was visually substantial.Agricultural activities needs to be intensified so that less area should be converted from forest covered area into agricultural area.
We think that the excessive river sediment that was transported in Klasaman River must be trapped before it is entering the river segment in the Sorong Urban area.Generally, it will be more difficult to enlarge river capacity in the urban area rather than dredging sediment in the sand/ sediment pocket [24][25][26].The designed river sand or sediment pockets should not damage the environment and the hydrological cycle of Klasaman River.
We do recommend that the construction bodies in Warsamson Watershed can conduct sediment monitoring in their project so that they can control the volume of sediment entering the Warsamson River.As an example, some slope cover may reduce slope erosion in a slope cut and fill project [27].The construction of an infrastructure should not dampened the functionality of other infrastructures.Practically, excessive sediment that is entering the river tends to cause problems to water intake structures [28,29].Saltation and sedimentation in the river might cause river shallowing and floodplain enlargements, which may cause damages to the riparian ecosystem and riverside infrastructures [30,31].This research have not provided a monitoring data that covers erosion profiles that caused by seasonal and spatial differences of rainfall in the area.This research do not deliver any quantitative turbidity parameters that can be used as references in comparing the sedimentation levels between rivers.The visual survey data in a short period of time do not able to represent the annual sedimentation situation in the studied site.
Nevertheless, the visual site investigations conducted in this research have reveal that the existence of sand mining and road cut and fill activities in the study sites have contributed to the portion of sediment entering the rivers.We do believe that more detail and sufficient quantitative sediment data are needed in the future study if we want to reveal the seasonal and spatial river sediment profiles of the area.

Conclusions
The erosion and sedimentation profiles of Remu, Mariyat, Klasaman, and Warsamson Rivers in Sorong, Papua, Indonesia, were vary across the river's services and the anthropogenic activities in the watersheds during the surveyed time.The site investigations referred that erosion problem is minimum in Remu River as the upstream area was conserved.Infrastructure constructions should not supply large amount of sediments to Remu River in order to maintain clean water supply capacity to Sorong City.The land IOP Publishing doi:10.1088/1755-1315/1343/1/0120299 use master plan of the Mariat watershed needs to be updated to accommodate current erosion and sedimentation pattern in the watersheds, so that the irrigation intake can be optimally operated and maintained.The mining activity in Klasaman River must not produce fine sediment supply to the river and the excess sediment supply during the flood time must be prevented from entering the downstream segment as much as possible, to avoid river shallowing in Sorong City.The land infrastructure development in the Warsamson River must not supply excess sediment to the river, as it might cause malfunction to the water infrastructures in the area (see the erosion prone areas presented in Figure 5).The river authority of the Sorong Special Economic Zone needs to support the development with representative river sediment data as a mitigation action towards river erosion and sedimentation risks.

Figure 1 .
Figure 1.The map of Sorong Region and the most downstream survey locations (outlets shown in red round shapes) at Remu River (orange line, top left), Mariat River (white line, bottom left), Klasaman River (yellow line, middle left), and Warsamson River (light blue, middle right).The background is from Google Earth and the inset map is the map of Papua, Indonesia (source: Wikimedia).

Figure 2 .
Figure 2. The profile of Remu watershed taken from Google Earth (12 July 2022).The Remu river was presented as the blue line (arrows show flow direction) in (a), in which the red line is the watershed boundary, the orange line is the constructed new road that connect Giwu and Saoka, and the yellow triangle is the Remu water intake structure (0°52'21.1"S131°16'59.3"E).The drone view and the cross section view of the riffle study location (0°52'16.6"S131°16'53.8"E)were presented in (b) and (c), whereas the pool study location (0°52'03.3"S131°16'47.4"E)were in (d) and (e).

Figure 3 .
Figure 3.The flow condition in Mariat River from Google Earth (24 November 2022), where an irrigation weir has been built (1°01'25.7"S131°20'34.5"E)(a).The yellow triangle shows the intake and the blue line shows flow direction.There were two survey locations, in which the drone view (b) and the cross section view (c) were captured at the upstream of the weir (1°01'11.7"S131°24'33.0"E),whereas (d) and (e) consecutively were from the downstream of the weir (1°00'24.5"S131°20'19.9"E).The green dashed line was the border of the nearby 4 ha oxbow lake.

Figure 4 .
Figure 4.The profile of Klasaman River.The Google Earth view (26 May 2022) of the Klasaman River shows the location of the sand mining (yellow hexagon) (a), in which the blue line is the river (the arrows show flow directions), K1 and K2 were the surveyed location 1 and 2. The drone view of K1 is shown on (b) whereas K2 on (d).The cross section view at K1 (facing upstream) is on (c) and at K2 (facing downstream) is on (e).This figure shows the visual condition of the river flows at location K1 (0°54'40.5"S131°21'52.9"E)(b) and at location K2 (0°54'35.4"S131°21'17.4"E)(e).

Figure 5 .
Figure 5.The profile of Warsamson Watershed.The Google Earth perspective (26 May 2022) of the Warsamson study site shows infrastructures that has been built in the area (a), in which the orange line is the new constructed road, the blue line is the Warsamson River (arrows show flow directions), the red lines are the watershed border, the yellow square is the sanitary landfill, the yellow pentagon is the sand quarry, the yellow triangle is the raw water intake, W1 and W2 were the studied location 1 and location 2. The cut and fill project to create new road was ongoing (c).There is the raw water intake at the side of the Warsamson River (d).This figure shows the visual condition of the river flows at location W1 (0°53'01.4"S131°23'03.8"E)(b) and at location W2 (0°51'50.9"S131°23'32.4"E)(e).