Protective Measures for Scouring Effects in Serayu Micro Hydro Integrated Barrage

Adding a new micro hydropower component to an established barrage, such as the Serayu Barrage in Central Java Province, poses significant structure risks and potentially compromises the stability of the structures. To mitigate these negative impacts, physical modeling was conducted at our hydraulic laboratory to accurately replicate 600 meters of the Serayu River, including the 110-meter-wide Serayu Barrage structure at a scale of 1:40. Multiple scenarios were examined to simulate hydrodynamic conditions and sediment transport along the barrage structure. Notably, downstream scouring conditions were identified as a crucial parameter affecting the structure’s stability. Results obtained using a design discharge of 2,470 m3/s revealed concerning findings, with an observed 4 to 6 meters deep scouring downstream near the end sill of the weir. Immediate mitigation measures using riprap are imperative to prevent further damage to the weir structure. This paper investigates several configurations for riprap protection to address the scouring problems. After implementing a full riprap arrangement downstream of the end sill, the scouring depth was reduced significantly and pushed far downstream. Looking at it from an economic standpoint, placing a full riprap configuration for downstream protection is a more financially efficient option over an extended period.


Introduction
The utilization of barrages in rivers has grown from traditional irrigation and water supply purposes to various other functions such as hydroelectric power generation and flood control [1,2].However, this phenomenon has resulted in significant modifications in terms of channel morphology both upstream and downstream of the barrages.The alteration of river flow due to weir or sluice gates can cause highvelocity flows and turbulence downstream, leading to the erosion and scouring of riverbed [3,4].Several cases show that ignorance of this issue can result in failure and structural damage of riverine infrastructure, posing significant technical, economic, and societal challenges.This raises a concern about the long-term stability and sustainability of barrages and their associated structures.
A viable solution to mitigate the erosion of riverbeds downstream of weirs or barrages is the implementation of riprap.Riprap, or stone revetment or rock armor, involves strategically placing large rocks or stones along the banks and riverbeds to provide stability and counteract erosive forces caused by water flow [5,6].Extensive research has demonstrated that riprap can effectively reduce downstream scouring effects by dissipating energy from flowing water and preventing excessive erosion of surrounding areas.Numerous studies have been devoted to analyzing patterns in local scours below weirs and barrages using physical modeling that results in promising scour predictions, being beneficial for mitigation measures [7,8].Trends to upgrade existing barrages for hydropower generation have also led to the alteration of riverbed profiles and increased the risk of scouring downstream of weirs.Therefore, it is necessary to implement effective protective measures to mitigate these scouring effects and protect the integrity of river system.Among Indonesian's existing barrages, many small-scale structures are prone to scouring problems due to their limited design features and maintenance practices.Especially the old structures that were built without proper considerations for scour mitigation measures can experience significant damage and sedimentation issues downstream of the weirs.Related to those issues, the Serayu Barrage in Serayu River Indonesia (see Figure 1) will be upgraded for micro hydro by utilizing two gates as shown in Figure 2. The barrage structure was built in 1993, and its primary function is to provide a water supply for the surrounding irrigation area.However, the objectives have yet been upgraded, and it is expected to generate 5 MW of electricity [9].Therefore, proper mitigation measures are crucial to protect the integrity of rivers and prevent the scouring effects downstream of the barrages.On the other hand, placing micro hydro components in the existing weir alters the flow patterns and ultimately triggers downstream scouring.Therefore, this study aims to analyze the downstream scouring effects on the Serayu Barrage using physical modeling and to provide proper mitigation measures.

Technical Data
Serayu Barrage has a total width of 110 m and is equipped with eight radial gates to flow 2,470 m 3 /s (100-year flood discharge).The technical data were provided by [10] and are summarized in Table 1.The gate width and height are 10.7 m and 7.2 m, respectively.The structure has an intake channel to provide water for about 24,531 Ha of irrigation area.As previously mentioned, the barrage is planned to be upgraded by adding a micro hydro component.Six small and submerged turbines were planted to replace the two radial gates and are expected to generate 5 MW of electricity, see Figure 3.For practical purposes, the location of the turbines has been determined in gates no.7 and 8.  and sediment transport process in the river.Physical modeling allows researchers to study the flow patterns and sediment transport in a controlled environment, providing insight into the behavior of the river downstream of the barrage.However, some limitations of physical modeling include high cost and time-consuming nature of constructing and conducting experiments, and the difficulty in accurately scaling down the model to represent real-world conditions [11].Due to the availability of pump capacity and laboratory spaces, the physical model was built with a scale of 1:40.The scaling factors can be seen in Table 2. Our physical model consists of 300 meters upstream and 300 meters downstream of Serayu River, including the barrage and its appurtenance structures.The upstream model was constructed using cement mortar and reproduced based on the bathymetry drawings with a fixed riverbed upstream and a movable riverbed downstream using sand material, see Figure 4.The sand with an average diameter (d50) of 0.15 mm (Figure 5a) and riprap stone with a diameter () of 0.75 cm (Figure 5b) at model scale were prepared for the downstream scouring simulation.Note that it was not possible to downscale the riprap bulk density; nevertheless, the density used at model scale was comparatively heavier than its prototype with respect to the resistance to the flow.It is therefore more conservative to be applied in reality.Model calibration and validation are essential steps in ensuring the accuracy and reliability of physical modeling.Researchers can verify whether a model accurately represents real-world conditions by comparing the data obtained from the physical model with field measurements and observations.In our case, the error in the dimension of the laboratory structure model compared with its prototype is less than 5%.Field measurements were conducted upstream of the barrage during high and low discharge events to validate the model.The field measured data are then compared with the laboratory model measurements, showing an error of less than 2%, see Table 3.Our experiment analyzes the rating curve and scouring effect using 100-year discharge.This is in accordance with the standard (KP-04) that recommends using 100-year discharge as the maximum value to be mitigated [12].For the rating curve analysis, two scenarios are considered consisting of the existing design of the Serayu Barrage before and after turbine placements, see Figure 6.Meanwhile, for the scouring simulation, three scenarios are conducted as follows: 1. Scenario 1, Serayu Barrage with turbines in gates no.7 and 8 without downstream protections, 2. Scenario 2, Serayu Barrage with turbines in gates no.7 and 8 with downstream riprap protections at the side corner of the end sill, 3. Scenario 3, Serayu Barrage with turbines in gates no.7 and 8 with downstream protections along the downstream end sill.The illustration of these three scenarios are given in Figure 7.Note that all the results in our study will be shown at prototype scale.

Discharge Capacity
Reducing barrage gates opening during flood discharge will increase the water level upstream of the barrage and potentially overflow the dike, which can result in flooding for the surrounding areas.The backwater effect, caused by the reduced gate opening, can also lead to increasing sediment deposition upstream of the barrage, further exacerbating the structure's scouring downstream.To understand the flood level changes before and after turbine placement in gates no.7 and 8, the upstream water levels during the low and high flood are measured.Based on the simulation, the upstream water level for both conditions is still below the top dike elevation of +15.5 m during 100-year discharge, see Figure 8.The upstream water level during 100-year discharge reaches +12.2 m for the existing conditions without turbines and provides a freeboard of 3.3 m.On the other hand, the upstream water level increases to +13.6 m and still provides 1.9 m upstream freeboard.The comparison of both rating curves can be seen in Figure 9.

Downstream Scouring
In this experiment, the downstream scouring pattern is analyzed using 100-year discharge for three scenarios as shown in Figure 9. Understanding the flow pattern is critical to assess water flow dynamics, sediment transport near the barrage, and its impact on riverbed scouring.During 100-year discharge, the model shows some turbulences, vortex formations, high-velocity zones, and changes in flow direction that eventually contribute to scouring effects (Figure 10a)-utilizing gates no.7 and 8 for turbine placement will block the upstream flow directions.During the flood events, the turbine in gates no.7 and 8 will be closed so that water will dominantly flow through gates no. 1 -6.Supercritical flow occurs at the spillway and stilling basin from gates no. 1 -6.Due to the uneven flow from the barrage outflow, the downstream flow distribution changes and a vortex occurs, see Figure 10b.The flow pattern shows a significant turbulence pattern on the right side of the downstream area.The mean downstream velocity ranges from 12 to 15 m/s and contributes to downstream riverbed scouring.Therefore, protection materials must be applied to prevent further riverbed degradation and disturb the stability of structures.Protective measures were applied using riprap downstream of the end sill.Three scenarios were implemented with different configurations, as shown in Figure 9.To optimize protection effectiveness, careful considerations must be given to design aspects such as the length, arrangement, and hydraulic conditions in Scenario 2 and Scenario 3.

Scenario 1
Scenario 1 shows the unprotected downstream condition (Figure 9a).Based on the simulation results, it was found that it leads to a massive downstream scouring near the end sill without protection.The maximum scouring depth reaches 5 to 6 meters and is spread along the end sill width of approximately 80 meters.

Scenario 2
Scenario 2 with riprap was designed only at the right and left sides, and left a gap in the middle to optimize the use of materials (Figure 9b).According to the Indonesian Design Standard, riprap stone's diameter ≥ Ø 0.3m (prototype scale) is recommended to be used for low-head hydraulic structures.Scenario 2 with riprap protection shows better results than Scenario 1.The scouring pattern has changed where the area is reduced with the maximum scouring depth of approximately 5 -6 meters.The riprap gap in the middle makes the river bed vulnerable and ease the sediment transport.

Scenario 3
Scenario 3 suggests riprap with a full from side to side in front of the end sill (Figure 9c).This configuration aims to compare the effectiveness of different riprap arrangements in mitigating downstream scouring.Scenario 3 (Figure 11) shows a significant scouring protection by reducing the scouring area in front of the end sill, lowering the maximum scouring depth to 4 meters, and the scouring location is far from the structures.The flow pattern near the end sill plays a crucial role in the scouring process downstream of weirs and barrages.The turbulent flow created by the impingement of water on the end sill leads to high velocities and shear stresses, causing erosion and scouring of the riverbed downstream.The results of the scouring simulation from different scenarios can be seen in Figure 12, Figure 13, and Table 4. Due to 100-year discharge, the flow velocities and shear stresses are exceptionally high, resulting in significant scouring depths.The experiment results show that Scenario 2 and Scenario 3 effectively reduce the extent of scouring downstream of the weir.However, Scenario 3 with the full riprap arrangement provides better scouring mitigation than Scenario 2 by pushing the scouring location more downstream from the nearest structures as shwon in Figure 12.Looking more closely at the longitudinal section of the downstream river bed (section A-A) in Figure 13, the action to put riprap protection significantly stabilizes the river bed.The scouring location has been pushed far downstream of the structures.This is important to ensure the safety of the structures.Riprap as a countermeasure to mitigate downstream scouring has shown promising results in reducing the extent of scouring near the end sill of barrages.

Conclusion
Based on the findings of our experiment, it can be concluded that the arrangement of riprap plays a significant role in mitigating scouring downstream of the barrage.The results showed that with riprap arrangements, Scenario 2 and Scenario 3 effectively reduced the extent of scouring downstream of the weir.Scenario 3 that involved a full riprap arrangement in front of the end sill was proven more effective than Scenario 2 in mitigating the downstream scouring.Hence, our study suggests that a full riprap arrangement using 20-meter long and 130-meter width of riprap provides better protection against scouring near the end sill of barrages.From an economic perspective, using a full riprap arrangement may be a more cost-effective solution in the long term, as it provides better protection and reduces the need for frequent maintenances and repairing works.Integrating the existing barrage with turbines for micro hydro generation can be considered an additional measure to enhance the structure's functionality and sustainability.However, it comes with consequences such as potential effects on the flow patterns changes and scouring downstream.Additionally, it is important to consider the flow pattern and turbulence intensity near the end sill when designing countermeasures for downstream scouring.The physical modeling might simplify several hydraulic conditions due to its limitation to model the prototype.Further research is needed to validate different downstream mitigation plans such as baffle blocks and bed sills in combination with other riverbed protections to enhance the effectiveness of scouring mitigation downstream of the barrage.

Figure 3 .
Figure 3. Plan view bathymetry of Serayu Barrage (prototype scale)2.2Physical ModelTo understand the downstream scouring effects of micro hydro placement in the existing gates, a physical model test was conducted in the Hydraulic Laboratory of Technical Implementation Unit for Hydraulic and Geotechnics, owned by the Indonesian Ministry of Public Works and Housing.Using physical modeling has many benefits, including its ability to represent the actual hydraulic conditions

Figure 6 .Figure 7 .
Figure 6.Cross section of the Serayu Barrage, a) design without turbines, b) design with turbines installed

Figure 10 .
Figure 10.Flow conditions with 100-year discharge, a) laboratory view, b) flow pattern

Table 1 .
Technical data of the Serayu Barrage

Table 2 .
Scaling factor for the hydraulic parameters

Table 3 .
Validation with field measurements (all values are shown at prototype scale)

Table 4 .
Scouring results from 3 scenarios