River Morphological Change and Sedimentation Countermeasures Analysis in Miu River

One major floods problem in Miu River is huge amount of sediment which transported along the river channel. The decreasing of river capacity has been analysed and some channel courses changes also detected. These problems caused river morphological change and it is necessary to find comprehensive solution which not only decreasing the flood damage but also consider appropriate approach to natural environment. The complexity of these problems and possible solutions to overcome flood and sedimentation problem were investigated. Increasing of river capacity by channel excavation, protecting riverbank with revetment and construction of sediment control facilities were proposed as main solution. However, it is necessary to evaluate the effectiveness of these efforts by analysed sediment control facilities location and dimension of the structures. Some alternatives were investigated in order to find most effective solution to solve flood and sedimentation problem in Miu River.


Introduction
On September 28, 2018, a seismic event of significant 7.5 magnitude transpired around 80 kilometers to the north of Palu City in Central Sulawesi, Indonesia.Regional tsunamis were rapidly generated within a matter of minutes and two significant liquefactions occurred in Balaroa and Petobo.Rivers surrounding the area were greatly impacted by the landslides and slope failures resulting from the earthquake.Slope failures leading to significant sediment flow and secondary sediment movement on the riverbed, resulting in a substantial volume of sediment displacement.Fine sediments are often carried in suspension and coarse sediments travel as bedload until their motion eventually ceases and the grains settle at a downstream location [1].The occurrence of sediment flow and sedimentation are recurrent inside the river channel.Related to this issue Miu River as one of prominent branch of Palu River has severe sedimentation problem.Sedimentation problems in Miu River had impacted on four villages, Salua Village, Tuva Village, Omu Village, and Simoro Village.Since March 2020, there has been a substantial surge in sediment discharge, resulting in huge sedimentation and recurrent flood inundation adjacent to the confluence of the Miu River [5].The complexity of flood and sedimentation problems in Miu River are necessary to be solved with an approach from river morphological aspect and sediment countermeasures analysis.Several approaches have been implemented to address this issue.This study suggests conducting further examination, specifically focusing on river cross-sectional area analysis and numerical simulation.These analyses have the potential to generate a comprehensive solution, in conjunction with previous analyses conducted by the Miu River Improvement Project.

Target Study Area
Target study area of Miu River is located upstream from the Simoro Bridge, which determined as the designated reference point for the Miu River (Figure 1), with catchment area around 50,000 hectares.This reach has length about 11.9 kilometers with 16 samples for cross sectional area analysis.Riverbed slope ranging from 1/30 to 1/50, equivalent to a range of 3% to 2%.It is important to note that this section encompasses the transition from the debris flow section to the bed load region.The basin reveals numerous instances of large-scale slope failures, whereby the recent precipitation events have induced the mobilization of unstable sediment.Discharge for 25 years returns period were analysed about 689.722 m 3 /s [5].Geological condition is dominated by sedimentary rock which formed from Holocene period as in Figure 2 [4].

Characteristics of Miu River and Sedimentation Problem
Figure 1 shows the distinctive characteristics of the Miu River.The characteristics could be described as follows: a.The river is prominently braided, characterized by an unstable flow route, which can be indicated to the significant volume of sedimentation present.b.Based on surrounding condition of the Miu River, it is likely that huge amount of sediment will be generated from the branch of the Miu River.c.The potential for flood inundation is mostly located in the sediment section.The potential future inundation may escalate gradually as a result of sediment movement which lead into increasing elevation of the riverbed.d.The risk of road erosion near riverbank becomes indication high velocity and rapid flow path changes.
The morphology of an alluvial river channel is the consequence of sediment transport and deposition by the river.It depends in large measure-but not exclusively-on the transport of bed material, that portion of the transported sediments that constitutes the bed and lower banks of the channel [2].Inside the sedimentation segment, an atypical elevation of the riverbed transpires, leading to the formation of meanders in the river channel.Lateral shift of a meandering channel is achieved by bank erosion at a cut bank and point bar deposition at the opposite inner bank [6].The sinuous course of the river channel facilitates the erosion of its banks, resulting in the transportation of fresh sediment consisting of soil and sand particles.The occurrence of riverbank erosion and an abnormal rise of the riverbed have resulted in the disturbance of the Kalawara -Kulawi Road that runs beside the Miu river, thus leading to a heightened occurrence of slope failures.The absence of a disparity in elevation between the ground level and the riverbed level in the neighbourhoods situated along the river channel results in the occurrence of floods and sedimentation.Hence, it is imperative to mitigate the anomalous buildup of sediment and stabilize the meandering of the river channel.Meandering with Point Bars and Chute Channels refers to channels that characterised by: moderate-to-high width/depth ratios; highly erodible banks; medium-tocoarse grained bed material (sand, gravel and/or cobbles); heavy bed material load; moderate-to-high velocities, and; moderate-to-high stream power [9].According to the longitudinal profile of the river channel, it is hypothesized that the sedimentary layer and the downstream movement of sediment inside the channel are observed in a gradual manner.

River Morphological Aspect
The analysis of cross-sectional data was conducted in order to ascertain the cross-sectional dimension [7].The primary objective of this analysis is to determine the bank full water level.Once the water level at the bank full capacity was established, the subsequent step involved the calculation of the crosssectional area, denoted as Abkf.The variable B (Wbkf) represents the width of the surface water, while H (dbkf) represents the average depth.The parameters were computed at a bank full discharge condition [3].The estimation of mean depth (H) involved dividing the channel area (A) by the surface water width (B).The utilization of this particular methodology is imperative in order to obtain accurate depth measurements for the uneven shape of a riverbed.The data presented in Figures 4a and 4b indicate that the width of the river increases in the direction of flow, accompanied by a corresponding increase in its aspect ratios.The evidence suggests that rivers exhibit a higher degree of braiding and experience more frequent alterations in their flow paths.This mechanism may be attributed to the significant accumulation of sediment and the erosive action of river flow on the riverbank.Outer-banks in meandering bends are vulnerable to erosion processes and to identify the effects of bank erosion on river evolution, it is of great importance to analyse the effect of bends on the hydrodynamics of flow [11].The bed material predominantly consists of coarse sand, as depicted in Figure 5.The median grain size, represented by d50, ranges from 3 mm to 5 mm.The samples were collected from the Miu River branch, specifically the Omu River and Tuva River.These branches reveal comparable conditions to the bed material found in the main Miu River.To get insight into the potential risks associated with erosion in the vicinity of Kalawara -Kulawi Road.A numerical simulation was performed using the iRIC software [8] to investigate the movement of the riverbank during a flood event.The flood discharge rate was set at 690 m 3 /s for a duration of one hour, with a bed material size of 3 mm.The simulation findings indicate that there is a tendency for riverbank movement, resulting in gradual erosion of the Kalawara -Kulawi Road.The present scenario has also resulted in the formation of the deepest elevation at the outer bank, and various critical locations for cross sections have been discerned.

Sedimentation Problems Solution
The necessity to address primary factor which contributing to sediment production in the Miu River had been analysed with priority implementation and determination of high-risk locations.This could be described in the Table 1

Increasing Miu River Capacity (Sediment Excavation).
Dredging and excavation are the prevailing methods for the removal of sediment from a water body.Dredging involves the extraction of sedimentation while it is submerged, whereas excavation entails the removal of sediment after water diversion or drainage has taken place.Both approaches generally require the transportation of sediment to a designated site for the purpose of treatment and/or disposal.In addition, it is common for these systems to incorporate the process of treating water extracted from dewatered sediment before releasing it into a suitable receiving water body.The estimation of anticipated sediment runoff is derived from the findings of surveys conducted on areas of land subsidence within water resource regions, as well as studies conducted on torrential flows.However, where there are recorded measurements of sediment runoff, it is necessary to calculate the values taking into account the recorded data.Average section area of the movable torrent bed deposited sediment (m 2 ) is determined from the average torrent bed width where it is predicted erosion will occur during sediment runoff (m) and the depth of the torrent bed deposited sediment where it is predicted erosion will occur during sediment runoff (m).

Revetment for Severe Eroded
Location.This study focuses on identifying locally damaged regions and determining the optimal structure type for providing revetment reinforcement at the location.The classification of revetment into two categories, namely boulder type and cyclopean concrete type, was established by considering the specific attributes of each installation location, including land acquisition, workability, and economic efficiency.Both types of revetments exhibit a high level of resistance to abrasion and impact.Furthermore, even when subjected to impacts that exceed anticipated levels, these revetments will not sustain critical damage or exacerbate the extent of damage.
The Miu River has a significant sediment load originating from its upper stream, leading to a substantial increase in flow rate during flood events.Consequently, the river's forceful flow is capable of transporting flowstones measuring around 1 meter in length.The heightened local flow velocity and flow rate can generate an unanticipated force that acts upon the abrasion and impact experienced by the moving stones.Despite implementing various reinforcement methods, it is inevitable that damage will occur.Prompt remediation is necessary when there is localized damage to the revetment, necessitating expeditious implementation of partial repairs.
To mitigate the magnitude of destruction, employing cyclopean concrete with a substantial proportion of coarse gravels in substitution of conventional concrete, together with incorporating a significant quantity of flowstone-equivalent material, seems to be efficacious.Hence, it is anticipated that enhancing the inherent robustness of the concrete, such as increasing its compressive strength from K-175 to K-225, will not yield a substantial impact on the stability or fortification of the underlying structure.
The implementation of boulders or similar materials that mimic the natural flow of the construction is an efficacious approach to safeguarding all the stones.In the event that it is necessary to provide protection for the cyclopean concrete revetment, the most suitable option would be to employ a revetment composed of boulders.The Miu River project places significant emphasis on the selection of land, prioritizing factors like as acquisition, workability, and economic efficiency.Therefore, the IOP Publishing doi:10.1088/1755-1315/1343/1/0120398 selection of boulder type revetment within the parameters of the cyclopean concrete revetment offer only restricted advantages.

Effectiveness of Sediment Control Facility
The implementation of sediment countermeasures in river systems necessitates the development of a comprehensive strategy aimed at managing sediment discharge within the basin.This entails doing a thorough analysis of sediment generation, downstream flow, and precipitation patterns across the entirety of the basin.The prioritized investigation will concentrate on regions where roadways have been affected by erosion and sedimentation, as well as locations where villages have experienced flooding and sedimentation.It is suggested that a comprehensive sediment control strategy be developed for the future, with an emphasis on long-term strategies as follows.a.It is crucial to ensure the stability of riverbed slope, mitigating localized sedimentation and scouring.b.It is important to implement regulations pertaining to substantial river channel curvature.c.It is imperative to mitigate the swift secondary displacement of unstable sedimentation inside the expansive river channel.
In the designated area along the course of the Miu River, smaller and medium-sized rivers converge from both the left and right sides.Each of these small and medium rivers has varying characteristics in terms of flood discharge and sediment discharge.The influence on the Miu River is contingent upon the catchment area of individual small and medium-sized rivers, as well as the degree of devastation present within each respective watershed.The noteworthy aspect pertains to the proximity to the confluence of the Salua Rivers.The location in question is a convergence point for four rivers of varying sizes, one of which is the Salua River, situated inside the primary course of the Miu River.The hydrological conditions of the river bed in this particular region exhibit significant fluctuations.Based on an assessment of the Salua Village, namely Kalawara -Kulawi Road, and the extent of farmed land in the vicinity, it is determined that this particular location warrants the utmost attention and priority in implementing countermeasures along the Miu River.

Conclusions and Recommendations
The determination of sediment countermeasures was based on a prioritization framework, which is closely linked to emergency situations.Nevertheless, there are certain time periods during which it becomes imperative to assess the effectiveness of various measures, such as sediment management facilities and revetment, in order to mitigate potential issues.The assessment findings have the potential to inform future flood and sediment countermeasure policies, which may undergo modifications as a result of budgetary constraints and prioritization of emergency response efforts.

Figure 1 . 2 .
Figure 1.Site Condition of Miu River Area Figure 2. Geological Condition of Miu River Area

Figure 4 .
Figure 4. (a) Width and Aspect Ratio of Miu River (b) Maximum and Mean Depth of Miu River

Figure 5 .
Figure 5. Grain Size Distribution of Miu River and Tuva River [5]

Figure 6 .Figure 7 .
Figure 6.The Initial Condition of Miu Riverbank before Flood Event Structure for Sediment Control Facility.The estimation of the Target Amount of Sediment consideration of two criteria.a. Sediment that can be transported by the rainfall The calculation of sediment transport facilitated by rainfall with a planned annual exceedance probability involves determining the total water quantity.This is achieved by multiplying the rainfall with the planned annual exceedance probability (Pp(mm)) by the drainage basin area (A(km 2 )).The resulting product is then divided by the debris flow density during movement (Cd).The runoff correction factor (Kf2) is considered at this time.

Table 1 .
as follows.Risk Types and Corresponded Facilities in Miu River