Effects of sluice gate operation on sediment flushing in Bekasi weir using a 1D numerical model

. Sedimentation upstream of a weir is a common phenomenon, especially for a river with high sedimentation. In the long term, this can lead to problems since the discharge capacity will decrease due to the rise of the riverbed, as experienced in Bekasi Weir, West Java, Indonesia. The upstream accumulated sediments increased up to 1.5 m from its original riverbed in a decade. One of the easiest ways to overcome this issue is to return the riverbed elevation to its actual elevation by manual excavation or sediment flushing using a sluice gate. This paper discusses the effects of operating three sluice gates of the Bekasi Weir for flushing the deposited sediments. The analysis was carried out through 1D modeling using MIKE-11 software to analyze the morphological effects after flushing the sediments with variations of sluice gate opening scenarios. Finally, it was found that without any manual excavation, the flushing method with one fully-opened gate was the best solution by giving a significant upstream bed degradation of up to 1 m, reducing the upstream flood level, and maintaining the upstream-downstream freeboard. The solution could be more effective when combined with river normalization.


Introduction
Urban floods have become common in many countries, including Indonesia.Recently, floods have occurred in Bekasi City, where the water from the Bekasi River overflowed the river banks.The floods were caused by the accumulated sediments, which have increased up to 1.5 m from its original riverbed in a decade [1][2].If the surface flow rate entering the river exceeds its flow capacity, there will be an excess of river flow and a risk of flooding in the area [3][4].Bekasi River is one of the rivers flowing through Bekasi City, originating from the Cileungsi and Cikeas Rivers.Bekasi Weir, in the middle of the Bekasi River, separates the upstream and downstream rivers, as shown in Fig. 1.
Bekasi Weir ensures the Bekasi River's water level to supply the water to Jakarta for irrigation.Meanwhile, during the flood season, three gates are designed to drain the flood discharge.Therefore, assessing the weir performance as well as the river condition is of importance.Many methods have been developed to analyze river issues, for instance, using numerical models.In the past decade, numerical modeling was used to analyze river morphology [5][6][7][8].These studies proved the effectiveness of numerical modeling for hydrodynamic and sediment transport phenomena.However, there is only few literatures on the gate system's relations to the river's morphological response.This paper aims to analyze the effects of different gate operations on riverbed changes and water level conditions.

Numerical Model
The current developments of numerical models have been helping engineers deal with complex phenomena in hydraulic modeling, one of which is one-dimensional (1D) numerical modeling that can be advantageous to study sediment flushing cases because it allows for a detailed analysis of the sediment transport process in a simplified manner.The following are some benefits of 1D numerical modeling for flushing sediment: 1.The movement of sediment particles in a body of water can be properly simulated by 1D numerical models by considering several variables including the sediment size, water flow velocity, and sediment deposition and erosion rates.This makes it possible to comprehend the flushing procedure in greater detail and to know how well flushing operations will work.2. 1D sediment transport modeling requires less computational effort than 2D or 3D modeling but can still ensure numerical accuracy to a certain level.3.For less complex domain configurations and longterm simulations, 1D numerical modeling is a practical method to investigate the sediment flushing process.Hence, it can be used to evaluate the viability of the existing flushing operations and improve their design with several scenarios.Finally, the design of the sediment flushing operations can be optimized.
One of the common software for 1D sediment transport modeling is MIKE 11 hydrodynamic model that uses 1-D implicit, dynamic wave routing based on the Saint-Venant equations for unsteady flow [9].The model is intended to do in-depth river modeling, with several considerations for floodplains, road overtopping, culverts, gate openings, and weirs.The software has the option to calculate the morphological changes that are updating the bed levels or to calculate the sediment transport without bed level updates.The governing equations used in MIKE 11 are given in Equations 1-5.The model setup is conducted for 33 km long of the Bekasi River by 448 cross-section data, including the Bekasi Weir structure, see Fig. 5.Some parameters used in the modeling are the sediment density, water density, porosity of bed material, Manning coefficient, and diameter of bed material (d50), which were set to 2,650 kg/m 3 , 1000 kg/m 3 , 0.4, 0.031 s/m 1/3 , 0.025 mm, respectively.The discharge hydrograph of Q2, as shown in Fig. 6, was set for the upstream boundary conditions.The overall domain for the numerical simulation is shown in Fig. 7.  To ensure the accuracy, the numerical model must be calibrated.In this regard, the calibration was performed using the velocity and surface water level values.The field measurement upstream (Sta.10+700) was used as the reference point for calibrating the model.The water level and velocity measured at this location are 18.55 m and 1.45 m/s, respectively.At the same time, the calibration results showed an acceptable agreement with an error between the field measurement and the modeling results of below 5% after using the Manning coefficient of 0.031 s/m 1/3 .Therefore, the model setup and parameters are considered reliable for the subsequent simulations.Next, we consider four simulation scenarios to analyze the river bed changes during different gate conditions, described as follows:

Results
Two simulation results are provided: the hydrodynamics module that shows the water level profiles and the sediment transport module that shows the river bed changes.

Hydrodynamics
The hydrodynamics analysis presented here focuses on the performance of a weir under different gate-opening scenarios.The numerical model was used to simulate the flow over the weir and the resulting water levels upstream and downstream of the weir.This analysis can provide an essential insight for the behavior of the weir, thus helping engineers and planners design some effective strategies for managing water flow during the flood season.Scenario 1 describes a situation in which all gates of the weir are closed, and the resulting water flow creates the backwater effect that significantly raises the upstream water level, see Fig. 9.The hydrodynamics model predicts that the water level overflows the weir crest and inundates the left and right banks.The model also shows no freeboard available in this scenario, indicating that the weir cannot deal with the flow rate during the flood season when all gates are closed.This result suggests that opening some gates could help alleviate these effects.
For Scenario 2 to Scenario 4, the water flows through below the gate (gate opening) and causes the water level upstream of the weir to decrease, as shown in Fig. 10 Our results can be used for the design and management of the weir system.For example, the results suggest that opening multiple gates during the flood season can help prevent the backwater effects and reduce the risk of flooding upstream of the weir.However, it is also important to consider other factors, such as the capacity of the weir and the potential for downstream flooding.This highlights the importance of using numerical modeling techniques to inform the design and management of complex water management systems.

Sediment transport
Sediment transport analysis is a crucial aspect of river management and engineering, as it determines the effect of different scenarios on the sediment load and bed level of the river.In this case, we evaluated the impact of varying gate-opening scenarios on sediment transport upstream of the weir.Scenario 1, where all gates are closed, leads to sediment aggradation upstream of the weir, with an increase in the bed level ranging from 0.25 to 1 m.This is as we expected because the sediment cannot pass through the weir, leading to the deposition upstream, see Fig. 11.However, downstream of the weir, especially for Sta.11+000 to Sta. 14+000, the bed level decreases by around 1 to 1.5 m due to the high flow from the weir and lack of sediment supply from the upstream part.This sediment degradation downstream of the weir is a concern as it can weaken the weir structure due to the increased erosion near it.The changes in the sediment transport phenomena from Scenario 2 to Scenario 4 can be observed in Fig. 12-15 and summarized in Table 1.These figures show that the sediment transport rate increases with the number of gates open, and the sediment moves downstream.The increase in sediment transport leads to increased bed level degradation upstream of the weir.The results suggest that the weir management plan needs to consider the sediment transport aspect carefully.During the flood season, it is recommended to close only some gates as it leads to sediment aggradation upstream of the weir and sediment degradation downstream.Instead, opening some gates allows for the sediment transport phenomena and leads to the riverbed degradation upstream of the weir.However, opening too many gates can lead to excessive sediment transport mechanisms and high river bed degradation, which can also be detrimental to the weir structure.Overall, the sediment transport analysis of the different gate opening scenarios indicates that weir management plans need to consider sediment transport as a crucial aspect.

Conclusion
The results of the modeling for Bekasi Weir have been presented.It was shown that the operation of the gates significantly impacted the riverbed changes.Our study indicated that the presence of gate opening was a critical parameter that significantly affected the results.As all gates were closed, the water could only flow through the crest of the weir, causing the water level to rise significantly upstream of the weir.This resulted in the overflow of water above the river banks, which may cause flooding in the surrounding areas.Furthermore, closing all the gates could disturb the equilibrium of sediment transport, leading to bed aggradation upstream and bed degradation downstream of the weir.On the other hand, opening some gates during the flood season could lead to a decrease in the upstream water level with a freeboard available to minimize the risk of floods.This also provided better sediment transport conditions, thus preventing sediment aggradation upstream of the weir.However, it is essential to note that opening too many gates may result in excessive sediment transport mechanisms and high river bed degradation, detrimental to the weir structure.Thus, a balance between sediment transport and weir structure safety is necessary to develop an effective weir management plan.
Flushing the sediment to the downstream area is recommended to extend the river discharge capacity and lower the river bed evenly upstream of the weir.Based on the modeling results during the Q2 flood, Scenario 2 with some gates open, was the most effective approach.This scenario led to the flushing of the upstream sediment by around 1 m, with a freeboard above 0.75 m for both upstream and downstream parts.This flushing activity can be done without manual excavation, thus saving money.Furthermore, the downstream area still had enough sediment supply to maintain the river morphology.It is important to note that the solution could be more effective when the flushing is combined with river normalization.This approach can improve the river capacity to deal with floods, reduce erosion, and enhance the habitat for aquatic species.Therefore, integrating river normalization with the flushing of sediment downstream can provide a more sustainable solution for managing the Bekasi Weir area.
The research highlights the importance of managing the operation of weir gates in the Bekasi Weir area and its impact on river bed changes.It emphasizes the need to find a balance between sediment transport and the safety of the weir structure to develop an effective weir management plan.
In conclusion, the operation of the weir gates significantly impacts the river bed changes around the Bekasi Weir area.Opening some gates during the flood season can provide better sediment transport conditions and minimize the risk of floods.Closing all the gates can lead to bed aggradation upstream and bed degradation downstream of the weir.Flushing the sediment downstream is recommended to extend the river discharge capacity and lower the river bed evenly upstream of the weir.However, it is crucial to balance sediment transport and weir structure safety when developing an effective weir management plan.Integrating river normalization with the flushing of sediment downstream can provide a more sustainable solution for managing the Bekasi Weir area.

Fig. 3 .
Fig. 3. Bekasi Weir cross-section.The gate operation system in Bekasi Weir is developed based on controlling the water level and discharge from the upstream to the downstream river, especially during high floods.There are two conditions considered such as closed-gate and open-gate.During the closed-gate condition, the water will overflow the top elevation of the gate to flow to the downstream river.Meanwhile, during the open-gate conditions, the gate will be opened 3 m so that the water will flow through the opening, as shown in Fig. 4.
. The freeboard values available for Scenarios 2, 3, and 4 are 2 m, 3.2 m, and 3.5 m, respectively.The more gates are open, the lower the water level upstream of the weir.The highest water level reduction is shown from Scenario 1 to Scenario 2, around 4.5 m, while the other scenarios show only around 1 to 1.5 m of the water level decrease, as shown in Fig. 8-10.

In Scenario 2
to Scenario 4, the sediment transport pattern changes and the bed level degradation occurs upstream of the weir from Sta. 0+000 to Sta. 10+800.In Scenario 2 with one gate open, the river bed degradation is around 0.25 to 1 m.In Scenario 3 with two gates open, the river bed degradation increases to around 1 to 1.25 m.Finally, in Scenario 4, with three gates open, the highest river bed degradation is about 1.2 to 1.3 m.These results indicate that as the number of gates open increases, the sediment transport increases, leading to more river bed degradation upstream of the weir.

Fig. 15 .
Fig. 15.River bed changes in comparison with different scenarios.