A Study of Flood Causal Priority in Arui Watershed , Manokwari Regency , Indonesia

Papua is the final fortress of forests in Indonesia. However, floods recently occurred in Jayapura, Paniai, and Manokwari. If causal factors are not identified and early anticipated, the flood will become an annual disaster with more severe impacts. This study aims to identify main factors causing floods using scoring analysis. Data collected include: rainfall, river gradient, water discharge, drainage density, watershed shape, slope and land use. The -1 research results show that the main factor of floods are average daily rainfall in wet month by 86.53 mm day (rather high) with a score of 28, land use dominated by forest and plantation (rather low and medium) with a score of 21, watershed slope dominated by 1<8% (high) with a score of 5, drainage density of 6.4 (medium) with a score of 3, watershed shape (Rc of 0.30 < Re 0.71) with somewhat/elongated shape with a score of 2, and river gradient of 0.0062 (low) with a score of 2 respectively. High rainfall causes greater flow of water and runoff. Land use dominated by plantations (21.46%) is thought to be one of the main causes of floods in Masni District. Flood mitigation does not mean that rain should be reduced, but regulates/manages such as stopping the conversion of forest into palm oil plantations, reducing surface runoff by applying soil conservation, making bypass from meandering, dredging sediments, and constructing retaining walls.


Introduction
In Indonesia, there were 22 priority watersheds (to be restored) in 1984, increased to 39 watersheds in 1994, 62 watersheds in 1999, 108 watersheds including Arui and ( Watershed 2009.According to the in Manokwari Regency) Decree of Minister of Forestry .328/Menhut-Number II/2009, Arui Watershed is stipulated as a watershed that need immediate priority handling (watershed to be restored).Since 2012, referring to Government Regulation Number 37 of 2012 concerning watershed management, watershed is classified into 2, namely watershed to restored and watershed to be retained.Watershed damage is accelerated by the use of natural resources as a result of population growth, economic development, conflict of interests and lack of integration between sectors, between upstream and downstream areas especially in the era of regional autonomy (Adamson & Cussen 2003;Girolamo & Porto 2012 Paimin 2012) ; et al. .The watershed of which carrying capacity to be retained is watershed whose land condition, quality, quantity and continuity of water, socioeconomic aspects, investment of water buildings, and utilization of space work properly (Government Regulation N .37 of 2012).Meanwhile, umber the watershed of which carrying capacity to be restored is watershed whose quality, quantity and continuity of water, socioeconomic aspects, investment of water buildings, and utilization of space do not work properly.Several impacts of watersheds that must be restored include: floods, landslides, sedimentation and erosion that may disrupt economy and order of life of the community.According to MoLEF (2017), Arui Watershed is a watershed to be restored characterized by high erosion rates, increasing sediments, low score in the 2017 watershed monitoring and evaluation and floods in 2016.
Flood is a natural phenomenon that occurs when the intensity of falling rain is very high, while the soil is not able to absorb it well and then runoff occurs ( After the flood occurred in Arui watershed area on 29 February 2016, regional and central government visited the affected areas and they promised to normalize the watershed.However, until now such promise has not been realized.The normalization of a river needs data of siltation, rainfall, narrowing, sedimentation, erosion, conversion land functions, etc., tudies rainfall, watershed shape, river S on bend, river gradient, drainage density, watershed slope, land use and water debit will detect and prevent floods in the area as early as possible.Therefore, hydrology, land and morphometrics of watershed (watershed shape, river bend, river gradient, drainage density) in the Arui River Basin should be investigated to avoid floods in the future.
Flood has become a never-ending problem in Indonesia.Several causes are known, but the actual causes are unknown for certain, whether the main causes of floods are sedimentation, land function conversion, river bend, river gradient, drainage density, moisture, topography or only high rainfall.Identifying their rates can determine what should be prioritized to be managed by c conserving soil and water and handling floods, so that the floods will not recur.On the contrary, if the main cause is not known for certain, floods will be handled carelessly so that the floods will be an annual disaster and sometimes occur more severely.No matter how small the impact of floods, it is still a disaster that must be addressed and the root causes of flooding problems should be sought.This is to prevent such disaster turning into a greater disaster or floods becoming a doom that will destroy all living creatures on the surface of the earth.
This study aims to identify the factors including: rainfall, , watershed shape, river bend, river gradient, drainage density, slope of watershed and land use and river border that cause , floods in Arui watershed especially in Masni District that occurred on 9 September 2009.It is expected that the results of this research can a model or design to be developed into handle and prevent floods, provide information to the government (PU, BPDAS, BPWS, Agricultural Agency), Private organization, NGOs, communities and other , stakeholders concerning watershed management so that it can be used as a study material in planning, implementing and deciding further policies for managing Arui watershed.

M s ethod
The research has been conducted at PDAS Laboratory, BWS West Papua Province, BPDAS & HL Remu Ransiki and Arui Manokwari.Materials and equipment Watershed used in this research are: administration map (scale of 1: 110.000), rainfall data 2013−2017, rollmeter, stopwatch, AWLR logger, ARR (automatic rain recorder), current meter, map of arui watershed, calculator, citra landsatd, computer, and printer.Data collected in the form of primary data, i.e. form of watershed, river curves, river gradient, river density/drainage and water discharge.While secondary data consist of: rainfall, slope and land use 2014−2017.

Climate
Rainfall data obtained from BWS of West Papua Province include: daily rainfall, monthly, yearly, wet month, dry month and rain intensity which were then used to classify the climate according to Schmidt & Fergusson (1954) as citated by (Soewarno 1991)

4
rainage Density D River density, long river counts including tributaries and wide watersheds are obtained through Geographic Information Systems.River density is an index that shows the number of tributaries in the watershed.The index is obtained by the following equation (Soewarno 1991) 1.The parameters and the weight of the flood formulation are as following: Average daily rainfall in wet months (mm day ) (35%) watershed form (5%), river -1 gradient (10%), drainage density (5%), average slopes (5%), and land use (40%).The identification of the flood factor begins with determining he scores for each parameter i.e.Score 1 = Low, 2 = Somewhat Low, 3 = Medium, 4 = Somewhat High, 5 = High.The main factor causing the flood is obtained from the multiplication of weights by score.The largest total sorted score is the most influential main factor for the occurrence of floods.

Results and iscussion
D Flood has become a disaster not only in Indonesia but also throughout the world.Floods that occurred in Masni District (Meiforga, Macuan, Aska, Ririnfos, Bowi Subur, and Membowi vilage) one of the districts in Arui Watershed area on 29 February 2016 have caused damage to agricultural land, settlements, roads, and bridges.Using the flood formulation paramater from Paimin, .( 2010), the results et al of flooding analysis were presented in Table .2 2 Table shows that the flooding factors with highest score is rainfall (score 28), and then followed by land use change (score 16), slopes (score 5), drainage density (score 3) form of watershed (score 2), and river gradient (score 2) respectively. -1 The rainfall of 718 mm day became the main factor causing the flood, preceded by the previous 5 days precipitation by 153 mm day and the previous 6 days by 173 -1 mm day .The floods that occurred in Masni District were -1 classified as surface runoff, because they only lasted 46 hours.The results of interviews with the community, showed that before the flood on February 29, 2016, in the area occurred a very heavy rain.
Changes in land use that dominated by oil palm plantations (20 2821.4% Arui were the second , ) watershed main factor causing flooding.Prior to the opening of transmigration and oil palm plantation area the Arui watershed area has never experienced a flood.The topography of about 60.01%Arui is is flat watershed which is high potential forly.Flat topography of rainwater from upstream areas with steep terrain to very steep uninfiltrated water into the soil will become surface runoff and accumulate in a place that could become a puddle.According to Paimin et al. (2010) the watershed area with slopes less than 8%(flat) has high potential for flooding.
Arui watershed drainage density is moderate, however, if high rainfall 4 rivers converge water flow will be greater to reach the peak discharge.Similarly, Arui Watershed shape is somewhat oval based on flood formulation, so it is categorized rather low for flooding (score 2).However, at the location of the flood there is meandering /curves with a sinuosity value of 2. .The existence of river curves reduces 54 the flow of the flowing river that impacts runoff flooded and causing flooding.If the factors causing floods occur simultaneously such as rainfall is rather high, although the form of overburden and drainage density is rare, but if the river body had many sediments and river curves the runoff will easily overflow, so that they will cause flooding.Moreover, Arui watershed area in the last years has an 5 average rainfall of mm.If rainfall in Arui Watershed 3998,2 area is high, flat topography, four rivers if together and there is a lot of silting, then easily peak discharge occurs.

Rainfall
The rainfall is the input of water on the earth's surface.Distibuti and intensity of rain affectedsoil on absorption of rain water At the beginning of the rain the .water will be temporarily suspended in the crown stand, then flow through the stem stream and the crown canopy and absorbed by the soil.However, if the rain with a troughfall high intensity, then the stem flow will be faster, and the ground cannot absorb all water, so that it will be pooled or flowed on the ground surface.The subsequent flow of water leads to a lower area such as the (river).
average daily rainfall The during the wet month of the last 5 years in Arui watershed area is presented in .Table 3 -1 ( ) Average daily rainfall in wet months is 86.53 mm day , based on flood formulation it was categorized as rather high or has a score of 4 and weight 7, so that thetotal score was 28.High rainfall number that reached 718 mm day -1 on 29 -1 February 2016, 173 mm day on 6 days before and 153 mm day on 5 days after was the main factor that causeding -1 flooding in .In the area where high Masni District (Figure 1) intensity rain then the soil quickly fall occurred, while saturated, so that the rain water is could not absorbed in longer time by the ground, it will then flow in the surface of the soil become a surface flow High rainfall besides .beneficial also threatens water quality, landslide, sedimentation and even flood (White & Howe 2010  2011; Zhang .2017).When water accumulates in the et al canal, water is not easily absorbed into the soil, given the shallow groundwater depth.Consequently water flows at the soil surface or only floods if there is no lower channel.In addition to the presence of forests, the water that reaches the tree-covered land next rainwater will be immediately reevaporated back into the atmosphere and interception (retained by the canopy which will further evaporate into the atmosphere), to the ground either through trough or droplet and flowing to the ground through the stem stream (Asdak 2010).
Annual rainfall data is further classified based on the criteria of wet, dry and dry months to obtain climatic type in Arui watershed area as shown as Equation [ 6 sufficient old with uneven distribution and excessive river capacity so the flood can t be inevitable.But according to ' Maryono (2004) rainfall is not merely the cause of floods.The flood is more caused if there is land conversion, sedimentation, river narrowing, and damage DAS.The flood will not occur despite high rainfall, provided that much water is absorbed in the soil, the river flows smoothly, the river is able to hold water and people do not build in the river border.By contrast, according to Neuvel & Knaap (2010) floods will occur if the rain can't be absorbed into the soil, the river can't hold water, the water flow is not smooth due to the narrowing of rivers and slum communities in the river border., the form of a bird feather basin (elongated) produces relatively small flood peak discharge values with relatively long peak flood times.The watershed forms more round, there are many turns and if the river is more shallow then when the rainfall is high the runoff will be easily restrained and cause puddles.However, if the watershed forms are more rounded even though the rainfall is high but there are no turns and deep rivers then the water easily flows/does not stagnate to get downstream and easily water up to the sea.The shape of the watershed is rounded if there is a change of land use will decrease the water quality, the peak discharge value will be bigger and relatively fast to flood (Wirosoedarmo et al.

2010).
Spherical and watersheds are not entirely the cause oval of floods if they are not influenced by others such as: land use change, high rainfall, slope, river slope, and drainage density.The area will be prone to flooding if rainfall is high, land use is dominated by wake area, many turns, round, and river basin shape becomes more flat.The flood is caused by rain water easily up to the surface of the soil, flowing up into the body of water, the water with a large amount concentrated in the body of water, the body of water ca 't overflow the excess n water then the flood is inevitable.

River meandering
The meandering River occur naturally because the nature of water that always flows to lower ground sometimes undermines the base, left-right rivers and precipitates sediment on the river bed as the water flow s .The more turns the debit will surely increase before the turn and decrease after turn.Some impacts of stagnant runoff water will erode the river border/landslides of river cliffs.If the river is more shallow then the peak discharge will often occur in the river where there are meander., According to Nur (2009) the cause of the river meandering is litology of the river composer in the form of easy loose rock and soil type that is easily eroded.Constructing easily eroded soil such as clay and easily loose rocks like alluvial soil will facilitate the flow of the river to grind and turn.Alluvial soil types derived from sediments are easily carried by currents and at low river currents will easily settle.River curves naturally will always vary depending on river currents, types of soil that exist in the river body and the slope of the river.Any water debit in any volume can lead to a winding path, will erode the sediment from the outside of the corners and settle them on the bottom of the river.Large river currents easily carry basic river materials, river slides and sediments to flow into lower river bodies.If the flow of small rivers even no flow begins to occur sedimentation/ sedimentation that sometimes formed a river bend.

River Gradient
The magnitude of river gradient /slope effect on flood, through large/small river currents.This slope is influenced by the slope of Arui watershed which has 3 slope categories: 08% (flat), 1525% ( ) and > 45% steep (very steep).The upstream river flow usually has a larger flow velocity than the downstream, but in the Arui watershed, which is dominated by a 60% flat river flow, is slowing down.So if large rainfall of water that does not enter into River Gradient runoff.
in flood location in   response to rainfall in the area.River flow in the upper and middle watershed is relatively more rapid, considering the gradient of the river is sloping and very steep.According to Barokah & Purwantoro (2014) the upstream part of the river is a source of erosion , because in general the river flow through mountainous areas, hills, or mountain slopes that sometimes have a considerable height from sea level.The middle of the river is a transitional area from upstream and downstream.The slope of the river bottom is more gentle so the flow velocity is relatively small than the upstream.This section is an area of balance between the process of erosion and sedimentation that varies greatly from season to season.While the downstream of the river is usually through a plateau that has a slope of the ramps of the river bottom so that the flow speed is slow (Barokah & Purwantoro 2014).With a low river gradient in the Arui basin the actual flood potential is low.However, if the rainfall is large, there are river bends, the number of sediments at the bottom of the river and the change of land use is dominated by oil palm plantations then the river flow into surface runoff will be able to cause flooding.
Drainage Density There are five rivers flowing in Arui watershed.The rivers are rivers in the South such as, the Mariam and the Macuan, Mantedi and Nembowi rivers that converge into the Arui River and into the Sarera Kecil Bay.The drainage density is classified as a score of 1 with a weight of 3 then a total score of 3. The flow pattern (drainage pattern) of the Arui river channels generally watershed resembles the shape of tree branches (dendritic patern).Normal water flow in five rivers in Arui watershed in Table 6.
Normal water discharge between 0.487.05m s and river 3 -1 water level with bridge 36 m.But on the Macuan river bridge with 3.5 m high and 10 m wide river when heavy rains can cause clogged water.As floods in 2016 in , Masni District river water overflowed due to shallow river and high river basin with bridge only 4.2 m.When shallow rivers and bridges are low the flow of water is inhibited, collected and inundated.Based on Figure shows the drainage of Arui 6 watershed drainage that is there are 4 four rivers that merge into one river (main river).Again, the only bush-shrinking river border can't function to reduce the flow of surface, sediment and mud to the river.So that water easily into the river, the capacity of the river quickly filled and eventually the water overflows.In fact the river border (riparian buffer) serves to preserve the function of the river by planting or catching eroded soil (sludge) and nutrients and chemicals  carried from the land on the left and right of the river so as not to enter the river (Dempsey .2017).Riparian buffers also et al stabilize the river.Trees planted along rivers also further cool the rivers that create a good environment for the growth of various types of aquatic animals.
Watershed Slope Slopes/topography of land surface appearance caused by high differences in two places.Slope is one element of the occurrence of floods, erosion, and landslides.The steeper slope and the large, the amount of so surface flow for erosion or landslides the greater.In large sloped lands, the surface flow has a large velocity so that infiltration tends to decrease (Helman .2017).Flat area et al and tend to concave will easily occur flooding, considering that in this area the water is easily concentrated to a lower area.The slopes in the Arui Watershed area are listed in 7 Table .7 Based on Table shows  topography if the area is open when rain water flows easily in the soil surface, towards the river and when the river is full then the water will overflow either to the settlement or downstream .Conversely, the topographic area is watershed rather steep and very steep well maintained (designated as protected forest) it will serve to keep the soil fertility, protect the soil, water from landslide threats and provide more water in the dry season.
Land use as forest (13.62%) is still found even in the downstream watershed.In a downstream basin that still encounters forest will enlarge the water seep into the soil, reducing the flow of the surface so as to avoid flood disaster.In the middle watershed (22.58%) and upper watershed 15.37% are designated forests with steep slopes (15 rather <25%) and very steep (> 45%).Thus, in Arui watershed area 51.58% is still designated as forest and only 48.42% as non forest.The forest area of 51.58% in the Arui can watershed cause floods of course if the forest area is less likely to flood will be relatively larger.Whereas with the still-forested areas will be able to receive, accommodate, store and drain rainwater from the tributaries to tributaries and to the main river.So steep slopes (15 <25%) and very steep (> 45%) still found forests will be able to and control the flow of the surface.In this slope the threat of surface flow, erosion and landslides is very large, so it is necessary in this area remain as a forest area.Sloping slopes, slope length and , particle-size slope shape will affect the extent of erosion and surface flow (Arsyad 2010;Helman . 2017).

et al
According to Dessalegn et al. (2014), particle-size distributions were also influenced by topography such that the upland soils were dark red and coarse textured, whereas the lowland soils were dark grayish and clayey.Regarded topography as the dominant factor influencing soil property variation due to its influence on runoff, drainage, microclimate.The topographic control of the soil quality and site characteristics are attributed to varying pedogenesis processes along the toposequence as evidenced by differences in transport and deposition of soil materials.
Floods that occur in the Arui watershed are not only caused by flat topography and silting of the river.But more due to heavy rain/high intensity.Although Arui watershed 51.58% is still forested but if rain with high intensity, the soil is quickly saturated.When saturated infiltration/water seeps into the soil will stop, consequently the water or inundating flowing in the soil surface as .Floods that are almost runoff happening around the world as a result of excessive rain while the soil can't absorb water, river water overflows and creates puddles.The area with flat topography until somewhat concave when the rain water easily concentrated, flooded and if the amount of water is greater then the inundation flood is not inevitable (Marfai 2011).
Land Use increasing and development of the population there has been an increase in land use for settlement and agricultural purposes, as well as increased water demand.Arui was originally predominantly forest, but the watershed beginning of 1980 opened transmigration programs imported from Java, Nusa Tenggara and Bali.Forest areas have been converted into agricultural land, plantations, agriculture and settlements.Reduced areas of forest and forest destruction in Papua should be stopped so that floods do not recur.The area of protected forest/nature conservation in Arui watershed is still 50.6%( ) flood properties than forest plants.The strength of plantation crops in retaining rainwater is not as large as the strength of forest crops that are typically tens of years old with deep roots deep into the soil.Therefore, the risk of landslides and mud floods is still a threat to this area.Land changes have affected the natural hydrological cycle, rainwater that could have been stored longer in the soil has been converted into surface streams that are directly discharged into the sea. 5 Based on Figure out of 11 land use types the percentage of land use change tends to vary.Land use types of water bodies, primary swamp forests, plantations, and transmigrations tend to remain.Primary dryland forest, mixed dryland farming, dryland and rice fields tend to decrease between 0.01 and 2.2%.However, secondary dryland forest and open soil tend to increase between 0.6 to 1.4%.Alleged sedimentation in all rivers is the impact of land use change on Arui watershed and oil palm-dominated plantations.Since the last 4 years land use change has not changed much, but in 2016 Arui watershed area floods.Of course for subsequent years the change and transfer of forest functions must be stopped so that the floods will not recur.Likewise, the traditional agricultural system of the people of Papua with shifting cultivation systems should turn to sedentary agriculture and impose soil and water Several oil palm plantation sites have entered the second cycle, allegedly clearing land for planting oil palm seedlings to trigger high sediment on the river bed in Arui watershed.Sediments (ground particles) or parts of a material transported by water from a site that is eroded and enters the water.According to Mueller et al. (2009), found that land use change have larger impacts on sediment yield than climate change.Damage to soil erosion occurs in the form of deterioration of chemical and physical properties of soil such as nutrient loss, increased density and resistance of soil penetration, decreased infiltration capacity and soil capability in water retention (Arsyad 2010).

Conclusions
The results of the research show that the main factor of floods are the average daily rainfall in wet month by 86.53 -1 mm day (rather high) with a score of 28.Meanwhile land use dominated by forest and plantation (rather low and medium) with a score of 21, slope of the watershed dominated by 1<8% (high) with a score of 5, the drainage density of 6.4 (medium) with a score of 3, shape of watershed (Rc of 0.30 and Re 0.71) with somewhat oval/elongated shape with a score of 2 and river gradient of 0.0062 (low) with a score of 2 respectively.High rainfall causes greater flow of water and runoff.Land use dominated by plantations (21.46%) is thought to be one of the main causes of floods in Masni Distrik.Flat topography (60%), dense river drainage, ovalshaped/meandering watershed so that when it rains, water will easily become surface runoff that will overflow on the ground and to the settlement.
climate; Bk = Number of dry months; Bb = Number of wet months 2 Watershed shape The shape of the watershed has significance in relation to river flow/flow velocity.To determine the shape of the basin, it is needed first to determine the value of circulatory ratio or Rc and elongated ration or Re (Strahler 1964) (Schum 1956) with the formulaas following (Equation [2] and Equation [3Basin data circularity, elongation ratio, watershed area, total river length, and circumference were obtained through Geographic Information System.In addition to the watershed data form river bends at the location of the floods includeThe slope of the river is obtained by the quation(Soewarno 1991)    note : G = river gradient; Vertical Distance = high difference between upstream and downstream of flood location; horizontal distance = stream lenght of flood location.River gradient data was obtained through direct survey of the study sites.
.7% (tropical wet / climate type A) From Table 3 it shows that almost 10.8 months (11 months) Arui watershed experienced rain wet months (> 100 mm).Based on the Schmidt and Fergusson systems, Arui watershed has a tropical climate type of wet /climate type A with a value of Q = 0 -<15.3%.As the impact of tropical climate wet Indonesia gets sunshine all year round.The , area fertile soil, year-round sunshine and the not so great temperature difference between day and night impact the abundant biodiversity of Indonesia.Influenced by the movement of the circulation of the sun causing circulation of wind patterns and make the tropical climate region has two seasons, the rainy season and drought, in the absence of winter.In areas with wet tropical climates of vegetation that grow in many forests are green and dense.May affect global climate in case of significant change.et al. , Hajani et al. as impacts with et al. et al. high rainfall will cause enormous surface flows great rainfall impacts if the area is damaged water easily reaches the soil surface causes water flow and runoff the bigger the lower the infiltration, it is very possible that flood caused by surface runoff will often occur if the capacity of the river is full will surely overflow to the surface of the land and to the settlement.According to (201 ) if the rainfall is Vannier et al.

2
Watershedshape watershed Arui has area: 232,010,000 m , circumference of watershed: 98,950 m , length : 2 watershed 2,407,0 71 So Re>Rc 59,410 m then RC is 0.29 and Re is 0. mean (elongated/rather oval) 3 .As Figure forms a slightly oval watershed based on flood formulation has a score of 1 and weight 2 then the total score of 2. According to Paimin et al , .(2012) the watershed shape affects the time of water flowing into the outlet.The more rounded the watershed means the shorter the time it takes to reach the outlet, the higher the flooding fluctuations occur.Conversely increasingly oval form of watershed, water the longer the outlet.Similarly according to Wirosoedarmo (2010) et al.

Figure 3
Figure 3 The shape and density drainage of Arui watershed.

Figure 4
Figure 4 Meandering and location flood in Meiforga village time (det); D = river depth (m); J= distance; L= width of the river (m); A = cross-sectional area (m ); V = current velocity (m s );
Average daily rainfall (mm day ) of wet months of the last 5 years at Arui watershed area 62% and very steep 15.37%.Thus the government and the public must maintain a somewhat steep and very steep terrain.Given the area with a rather steep and very steep Table 7 Water discharge, water level and riparian river

Table 8
Jurnal ManajemenHutan Tropika Vol. 24, (2): 81-94, August 2018  EISSN: 2089-2063  DOI: 10.7226/jtfm.24.2.81conservation.According toYan et al.(2013,)the impacts of land use changes on changes in streamflow and sediment yield were evaluated and quantified.Therefore quantifying the effects of land use change on spatio-temporal change patterns of sediment yield is crucial to development of soil-water resources and land use management at a catchment scale(Cai et al. 2012).