Sedimentation Management Good Practice (SMGP) to Manage Sedimentation at Kawanua Flood Disaster, Seram Island

Flood natural disasters are almost certain to occur every rainy season. Rain with very high intensity occurred on July 9th, 2023 causing an increase in water discharge which made the river overflow and the Kawanua Bridge collapse. In the upstream area of the river, there is Kawa check dam which has experienced a huge amount of sedimentation due to the top of the crest check dam being covered by fallen trees. This study aims to determine the magnitude of the rate of erosion and sedimentation that occurs in the catchment area in the upper reaches of the river and find out where the source of sedimentation comes from, then the most appropriate solution can be decided, and a similar disaster does not occur. This research used a USLE method to calculate the erosion and sedimentation rate. The results showed that 37.88% of the catchment area was classified as a low level of erosion. Meanwhile, the moderate level of erosion represented 29.42%. Sedimentation Management Good Practice (SMGP) is a concept used to analyze erosion and sedimentation problems. The recommendations given by the SMGP concept have sustainable characteristics that minimize damage to the water environment. Additional recommendations are given by collaborating between government institutions related to catchment area and forest area management authority.


Introduction
Flood natural disasters are almost certain to occur every rainy season [1].Likewise, what happened to Ambon Seram River Basin, high-intensity rain caused the river to overflow and caused the Kawanua Bridge on Seram Island collapse.One of the reasons for the overflow of river water was that the top of the Kawa check dam crest was covered by fallen trees, triggering high sedimentation.In order to determine the sedimentation rate at the Kawa check dam, it is necessary to understand the characteristics of the catchment area at Kawa check dam.
The catchment area is an area bounded by mountain ridges where rainwater that falls on the area will be accommodated by the mountain ridge and flowed through small rivers to the main river [2].Damage to catchment area is generally caused by uncontrolled land changes in the upstrea, resulting in changes to the hydrological cycle [3].The condition of the catchment area, especially the upstream part, is relatively critical enough to be able to withstand high rainfall, causing flooding in the downstream areas.Another cause of flooding is the large pile of sediment upstream and changes in land use [4].River sedimentation that occurred in the Ambon -Seram river basin was due to river bank erosion caused by topographical conditions at the site.
Based on data from the nearest rain gauge, high rain intensity occurred on July 9, 2023 reaching 218.7 mm in a single day.It is causing an increase in water discharge and accumulation of sedimentation at the Kawa check dam.The flooding that occurred resulted in the breaking of the Kawanua bridge and the flooding of one of the main roads with an inundation height of up to 0.60 m.This incident resulted IOP Publishing doi:10.1088/1755-1315/1343/1/012034 2 in the paralysis of the activities and transportation of the residents of the villages of Saunulu, Yaputih, Hatu, and villages around Laimu District, Central Maluku Regency, and even Siwalalat District, East Seram Regency [5].Optimization of land catchment area management is very important in the conservation principles that are applied to obtain economic and sustainable benefits in order to reduce the magnitude of the value of erosion, sedimentation, and flooding [4].
To determine the rate of erosion and sedimentation that occurs in the Kawa check dam, an analysis will be carried out using ArcGIS software.Furthermore, solving the sedimentation problem that occurs at the Kawa check dam will be carried out using the Sedimentation Management Good Practice (SMGP) concept.This method was chosen because the solution provided has a sustainable concept that minimizes damage to the water environment so that the right solution is obtained while still considering the sustainability aspect.

Site Description
The location of study area is in the Kawanua catchment area which is administratively located in Maluku Province, West Seram Regency that can be seen in Figure 1.The Kawanua catchment area is a combination of 2 rivers in West Seram Regency.Those two rivers are Kawa River and Nua River.In the upstream of the Kawanua Bridge has a Catchment Area of 422 km 2 .
Upstream of the Kawanua Bridge there is the Kawa Check Dam which is full of sediment.There were even fallen trees covering the top of the check dam.The condition of the check dam, which is covered in sediment, causes river flow to overflow on the right and left side of the check dam.

Datasets Rainfall Data
Rainfall data used in this study was obtained from the Kobisonta Rain Station with data for 2015-2022.This data is used for the analysis of the erosivity of rain.

Soil Map Data
Soil data was obtained from FAO. FAO as an international organization that focuses on agriculture, food, and sustainable development, has made great efforts to map and classify soil types in various countries, including Indonesia.The Indonesian Soil Type Shapefile data provided by FAO is a geospatial dataset that describes soil types based on predetermined classifications and criteria.
The soil erodibility factor (K) indicates the resistance of the soil to the release and transport of soil particles from the kinetic energy of rainfall.In general, soils with larger aggregates tend to be more resistant to rainfall and runoff.This is because it requires more energy to transport them.In addition, fine-textured and highly cohesive soils generally have high resistance.Meanwhile, soils with fine dust and sand particles are generally less resistant, so the erodibility factor is greater.
This soil map is used to obtain soil type information which is then used to obtain the soil erodibility factor (K).At Kawanua catchment area, there are two types of soi, namely Acrisold and Cambrisols.The area of Acrisols is 66.88%, while the other 33.11% is Cambrisols.

Land Use Data
This land use data was obtained from the Directorate of Inventory and Monitoring of Forest Resources, Directorate General of Forestry Planning and Environmental Management -KLHK with an accuracy scale of 1:250,000 and classified/categorized into several types of land cover.
In general, the tillage factor (C) shows the overall effect of vegetation, litter, soil surface, and land preparation activities on erosion.C is the ratio between the amount of erosion of a vegetated area and the management of certain plants to the amount of erosion of identical soil and without plants.While the land conservation factor (P) is the ratio between the magnitude of erosion on conservation land and land without conservation.Conservation efforts include terracing, planting parallel to the contour, crop rotation, and fertilizing.It is easier to determine C and P factors in the field when combined, because these two factors are closely related.
The results are as shown in Figure 3, secondary forest is the most dominant type of land use with a percentage of 95.64%.Then the percentage of bush, savanna, and dry land farming was 2.06%, 1.9%, 0.24%, respectively.

Land Slope Data
The National DEM was built from several data sources including IFSAR data (5m resolution), TERRASAR-X (5m resampling resolution from 5-10 m original resolution), and ALOS PALSAR (11.25 m resolution), by adding mass point data used in making topographical maps.Indonesian (RBI).The spatial resolution of DEMNAS is 0.27-arcsecond, using the EGM2008 vertical datum.
At Kawanua catchment area, the value of the slope length factor (LS).Based on the results of the analysis, most of the Kawanua catchment area has steep slopes.The percentage of slope class IV (steep) is 38% and rather steep (class III slope) is 37%.Then the proportions of class I, class II, and class V slopes are 7%, 13%, and 5%.

Research Methodology
The method used in this study is a combination of quantitative and qualitative.Qualitatively, the rate of erosion and sedimentation will be calculated upstream of the Kawa check dam.Meanwhile, qualitative or descriptive analysis will provide a solution to the sedimentation problem using the Sedimentation Management Good Practice (SMGP) method.
Quantitative analysis of Digital Elevation Models (DEM) according to different models using Geographic Information Systems (GIS) supports users to extract various types of information about the landscape where hydrological and topographic information is the most important.Modeling using GIS will produce the erosion rate using the USLE formula and will be followed by calculating the sedimentation rate.Field investigations were carried out to see the current conditions at the Kawa check dam so that it would help in identifying problems to find out the main causes of sedimentation and combined with modeling with GIS.
After knowing the main problems that occur in the catchment area, further solutions will be provided using SMGP.The aims of SMGP are to set out the environmental aspects that should be considered when undertaking engineering works, and to help applicants choose sustainable engineering solutions that reduce impacts on the water environment [6].
IOP Publishing doi:10.1088/1755-1315/1343/1/0120344 2.3.1.Erosion.Erosion is the event of moving or transporting soil or soil particles from one place to another naturally by water or wind [7].Naturally, the occurrence of erosion is determined by climatic factors (rain intensity), topography, soil characteristics, ground cover vegetation, and land use.In a wet climate, the climatic factors that influence it are rain, the amount and speed of runoff, and erosion damage (Directorate of Expansion of Agricultural Areas, Jakarta, 1986).

Rain Erosivity.
Rain erosion (R) is a driving factor for erosion both due to the influence of falling rainwater onto the soil surface and partly due to surface runoff.Bols (1978) in Luliro makes an empirical equation for regions in Indonesia that relates the erosivity of rain with P (annual rain area in mm) [8].In this study, the erosivity factor of the rain was calculated using the following equation developed by Bols: where EI is monthly rain erosion index; MAXP is maximum rainfall in one month (cm); RAIN is monthly average rainfall (cm); and DAYS is average number of rainy days per month.

Slope length factor and slope factor (LS).
The length (L) and slope (S) factors describe the effect of the length and slope of the slope on the amount of erosion that occurs.L is the distance down the slope from the point of upward runoff to the point of entry of runoff into the river/canal or b the point of erosion resulting in deposition [7].Determination of the slope length factor with a mathematical formula involving an exponential number that represents the slope of a particular slope.Wischmeier and Smith (1978) in estimating erosion using the USLE equation the L and S components are integrated into the LS factor for slopes S< 20%, using the following formula: ( ) Where LS is factor of length and slope (no dimension); S is slope (%); L is slope length (m) The above formula is obtained from experiments using erosion plots on slopes of less than 20%, so it is not sufficient for topography with steep slopes.Harper (1988) in Asdak (1995) shows that on soils with a slope greater than 20%, the use of the formula above gives results that are too high [9].It is recommended for steep slopes to use this equation: where L is slope length; g is acceleration due to gravity (9.81 m/s 2 ); S is slope (%)  6) drainage network such as slope, length, shape, and size; (7) runoff [10].To estimate the source of sediment entering the river, an empirical model based on the USLE (Universal Soil Loss Equation) formula is used.This universal equation was first developed by Wischmeier and Smith in 1965.According to Vis.R in Soewarno (2013) the USLE formula is widely applied in various tropical countries, but Indonesia still needs to adjust various factors [11].The USLE equation developed by Wischmeier and Smith (1978): where A is amount of soil eroded (tons/ha/year); R is rainfall factor/ rainfall erosivity; K is soil erodibility factor; LS is length and slope factor; C is factors of ground cover vegetation and plant management; P is factors of special soil conservation measure.2.3.5.Sedimentation.SDR is an estimate of the ratio of soil carried away due to soil erosion during run off.SDR value is affected by contours and environmental factors.In theory, the gentler the average slope and the larger the size of the catchment area, the smaller the amount of sediment produced per unit area (Wischmeier and Smith, 1978).

SY = SDR x Ea
(5) In this case, SY is sediment production (ton/ha); SDR is Sediment Delivery Ratio; Ea is the erosion rate (ton/ha).Sediment production is influenced by the ratio of the amount of soil carried by river flows and into the reservoir or SDR (Sediment Delivery Ratio).The SDR equation was proposed by the USDA (United States Department of Agriculture) in 1979 and is described in Equation 6. [16] SDR = 0.41 A -0.3 (6) where SDR is sediment delivery ratio; A is watershed area (km 2 )

Flowchart of this research
This research uses Sedimentation Management Good Practices (SMGP) method which is shown in Figure 2.

Basic Principles of Sedimentation Management Good Practices (SMGP)
Erosion and sedimentation countermeasures using SMGP consist of three stages, namely analysis of sediment addition, determination of the causes of erosion and sedimentation, and implementation of Sediment Management Good Practice by dredging sediments on a regular basis both periodically and as a long-term prevention effort [6].These are three basic principles of good practice [6] 1. Demonstrate need Sometimes engineering activities may be carried out to address a perceived problem rather than a real problem, such as removing sediment from an area where sediment deposition is not increasing.
Quantifying the problem will help to determine if engineering is required and will ensure that any solution is proportionate to the scale of the problem.

Consider a range of options
Most engineering requirements can be addressed in a number of ways so a range of options to address any problem or need should be considered.The cause of any problem should be identified and options that address the cause, not the symptoms, should be considered.Without considering a range of options it is not possible to determine if the chosen approach represents the most suitable option -ie the option that minimises ecological harm at a cost that is not disproportionately expensive.

Include mitigation
All reasonable mitigation should be identified and implemented eg ensure measures are taken to reduce the risk of pollution when works are being carried out.

Erosion and Sedimentation Rate Analysis
Analysis of the addition of sediment is carried out to check whether there is additional sediment in the river which has an impact on infrastructure functions (intakes, check dams, bridges) or other sustainable activities (navigation, flood risk management, water supply).If there is no significant risk with the addition of sediment then the requirement for sediment dredging should be reconsidered.Sediment dredging can result in changes in river morphology (making rivers deeper, wider and smoother) mean that more water can be carried downstream faster during flood flows, which can increase water levels downstream and increase the risk of flooding [6].Erosion evaluation is an assessment of the possibility of the amount of erosion that will occur in an area.The assessment of the probability of the amount of erosion that will occur is called the potential for erosion or the assessment of the threat of erosion.The evaluation of erosion aims to determine which parts of the land or areas have the potential to experience erosion and the possible threat of erosion that will occur.Based on the results of the analysis, the level of erosion risk in the Kawanua catchment area is very light, as indicated by class I of 56.61%, then class II (mild) of 43.02% of the area.The rest is divided into class III and IV, amounting to 0.34% and 0.016%.The results of this approach are presented in Figure 3. Based on the analytical results showed that 37.88% of Kawanua catchment area was classified as a low level of erosion.Meanwhile, the moderate level of erosion represented 29.42%.The results of this approach are presented in Figure 4.
Sediments are the result of erosion processes, both due to surface erosion, gully erosion, and cliff slides.Sediment rate is the amount of soil and part of the soil transported by water from an eroded place in a catchment area and into a river or body of water [7].The sediment that is carried into the river is only part of the soil that has eroded from its place, some of the other eroded soil will settle somewhere at the bottom where erosion occurs.
The sedimentation rate in the Kawanua catchment area is classified as 'good' based on KLHK sedimentation rate classification as shown in table 3. The total amount of sedimentation rate is 1,04 mm/year which is less than 2 mm/year (classified as 'good') [12].

Identifying Issues
Determining the causes of erosion and sedimentation is carried out in order to be able to choose the right solution and be sustainable in the long term to reduce the need for periodic maintenance and reduce maintenance costs [6].The results of the analysis showed a low level of erosion and sedimentation in the catchment area upstream of the Kawa check dam.The build up of sedimentation in the check dam occurred due to the large number of fallen trees that closed the area above the crest check dam so that the suspended load type of sediment was retained.
Erosion and sedimentation occurring in the upstream catchment area of the Kawa check dam is at a low level.This can also be seen in the land cover upstream of the check dam, which is 95% secondary forest (DAS is not disturbed by human activity).The upper part of the check dam crest which was covered by fallen trees resulted in large amounts of sedimentation occurring and blocking the river channel as shown in Figure 5.When it rained with high intensity on July 9, there was an increase in water discharge from the upstream direction.As a result of the check dam being covered with sediment, the water overflows and makes a new channel in the river body (the right and left of the check dam) which can be seen in Figure 6.
In the USLE analysis, debris flow is not included because debris flow analysis requires actual discharge data from Automatic Water Level Recorder (AWLR), which is limited at the site.From the results of the analysis using the USLE method, the rates of erosion and sedimentation in the water catchment area are classified as low.The erosion and sedimentation that occurred was most likely caused by high-intensity rain during the last 3 days prior to the flood disaster which resulted in the breaking of the Kawanua bridge.The high intensity of rain in the upstream area with steep topography causes natural flash floods (natural damage).Flash floods not only cause high land erosion, but also cause landslides and uproot many trees.These fallen trees also caused the top of the check dam Kawa to be closed.
Flash flood events in river areas can occur naturally and as a result of human activities such as mining and illegal logging.The disaster that occurred upstream of the Kawa check dam is classified as natural disaster because the condition of the catchment area is still not disturbed with human activities and there are no visible signs of mining or illegal logging.From the results of monitoring in the field, it was shown that the condition of the fallen trees that covered the top of the Kawa tree check dam was still intact up to the roots.The occurrence of natural disaster begins with lateral erosion.
Lateral erosion occurs laterally or horizontally to the side.This erosion resulted in deposition in certain areas which became the end point of lateral erosion [13].So areas that experience erosion will lose part of the surface.While the thrust of lateral erosion can be caused by wind or water flow that occurs due to the changing position of the current rope (thalweg) which is not always in the middle of the river channel, the river bank walls that are impacted by the current rope erode/erode in the lateral direction.
The occurrence of the process of meander formation is one of the impacts of lateral erosion on the outer band of the valley wall, so on the other hand, namely in the inner arch, currents that are weaker than the strong currents in the outer arch occur, resulting in sediment deposition in the inner arch.inner band) intensively occurs to form a point bar.And so on, lateral erosion/erosion takes place over a long period of time, and the river's meander (sonousity) becomes larger, and eventually forms very large IOP Publishing doi:10.1088/1755-1315/1343/1/0120349 bends (meanders) and changes the shape of the river [14] [15].After lateral erosion occurs, it is usually followed by fallen trees.In vulnerable areas it is necessary to plant trees that have a strong root system so they do not fall easily.

Recommendations
SMGP is implemented to address erosion and sedimentation problems using long-term (preventive) and short-term (mechanical) approaches.The long-term erosion control approach is done vegetatively with the addition of vegetation or trees in the catchment area (buffer zone) and especially in the upstream area which can strengthen the carrying capacity of the soil on the river bank.If it is not possible to restore with long-term erosion control approach, then ongoing management or short-term erosion control approach such as sediment removal or larger scale dredging may be required.It is carried out mechanically by applying sediment management good practice, namely the removal of sediment at impoundments.Sediment cleaning in the check dam area is more of a maintenance nature, namely by cleaning the boulders that are stuck and dredging the sediment upstream of the check dam.Sediment cleaning in check dams needs to be done regularly to ensure the check dam can function optimally and sustainably.
3.4.1.Sediment dredging.Sediment dredging (making rivers deeper, wider, and smoother) mean that more water can be carried downstream faster during flood flows, which can increase water levels downstream and increase the risk of flooding [6].Then the function of river normalization is the river increases the capacity of the river by dredging the riverbed so that the channel capacity increases [17].In the Kawanua catchment area, sediment dredging needs to be carried out in sediment storage at Kawa check dam.Sediment dredging needs to be done so that the Kawa check dam can carry out its function In the SMGP concept, an analysis is required before dredging the sediment.Sediment dredging needs to pay attention to the existence of infrastructure in the area, if there is no dredging infrastructure it can make the river deeper and increase the potential for flooding in the downstream area of the river.

Ring net barriers.
The concept of the ring net barrier system is to hold back debris material but not to hold back the flow of water so that there is no hydrostatic pressure on the net so that water can still flow through the gaps in the net.The ring net barrier system can be an alternative to the Sabo Dam retaining structure with a more efficient and economical cost.
The ring net barrier system consists of an arrangement of steel wires made of high tensile wire mesh material, generally placed in the catchment area, between two river bank walls.The span length of the net barrier can reach 15 -25 m, and the net height can be set at 2 -6 m.The net is made of steel and is supported by special ropes / slings which are anchored to the bank of the river bank with the depth of anchorage depending on the design load and the capacity of the local soil or rock material.The example of Ring Net Barrier can be seen in Figure 7.In the ring net barrier system, the bearing capacity of the subgrade and uplift are not the main issues because the net is not resting on the subgrade.
The purpose of installing a ring net barrier is to block the flow of debris and if a flood occurs, the trees carried by the current will stop in this place.With the ring net barrier, the occurrence of trees covering the top of the Kawa check dam will not be repeated, so that river water does not overflow to the side of the check dam.The location plan of Ring Net Barrier in the Kawanua watershed is shown in the Figure 8.Based on PERMEN LHK No.16 of 2022 that forest management authority lies with the Directorate General of Sustainable Forest Management -Ministry of LHK.To prevent trees from falling when there is flooding in the upper reaches of the river, it is necessary to enhance cross-sectoral coordination between the Directorate General of Natural Resources -the Ministry of PUPR and Sustainable Forest Management -the Ministry of Environment and Forestry.The aim of this coordination team is to organize forest areas that have a high potential for landslides and need to be adjusted to the existing vegetation in the area so that erosion can be controlled.

Limitation
In general, the applications and limitations of direct and indirect methodologies for erosion evaluation are numerous; in particular, no approach or method can be considered completely reliable when used individually.To obtain sedimentation rate modeling that is close to conditions at the site, calibration is required using sediment discharge calculations (suspended load and bed load), but this analysis cannot be carried out due to limited data due to the absence of AWLR in the Kawa and Nua rivers.The Kawanua Check Dam area there is also no scientific research that could help in preparing this research.No major research has been carried out on the Kawanua Check Dam or near the location to determine the sedimentation problem.Apart from that, limited road access to the upstream river makes it difficult to determine the condition of the upstream catchment area.

Conclusion
The following is the conclusion of the research that has been done: 1) The total amount of sedimentation rate in Kawanua catchment area is 1,04 mm/year which is less than 2 mm/year.The sedimentation rate in the Kawanua catchment area is classified as 'good' based on KLHK sedimentation rate classification.
2) The large rate of sedimentation that occurred was due to high-intensity rain which resulted in landslides in the upstream of the river and carried a lot of debris material including fallen trees; 3) There are several solutions based on this analysis, the first solution is to build a ring net barrier that works holding debris material but not holding back the flow of water so that there is no hydrostatic

Table 1 .
The Value of factor slopes The rate of catchment area erosion is determined by a formula developed based on the physiographic and climatological conditions of the catchment area concerned.

Table 2 .
Erosion Hazard Classification

Table 3 .
Sedimentation Rate Classification

Table 4 .
Kawanua catchment area erosion hazard classification IOP Publishing doi:10.1088/1755-1315/1343/1/01203411 pressure on the net so that water can still flow through the gaps in the net.Then, the second solution is sediment dredging in sediment storage at Kawa checkdam.4) The additional solusion is Inter-corporate cooperation, which is necessary because of the authority of each institution in accordance with existing regulations.Based on PERMEN LHK No.16 of 2022 that forest management authority lies with the Directorate General of Sustainable Forest Management -Ministry of LHK.Inter-corporate cooperation is classified as an additional solution because this solution is not included in the SMGP concept.