Flow patterns at river bends and its response to surrounding infrastructures

On January 6, 2023, high-intensity precipitation occurred in the Babon River watershed, causing the river overflow and flood in several areas. Severe impact in the surrounding area was observed, such as in the Dinar Indah Housing Complex. Hydrodynamic complexity around a river bend points out the importance of understanding the flow patterns at a river bend. Therefore, this study aimed to describe the flow pattern of the river bend and analyze its response to the surrounding infrastructure using the HEC-RAS 6.3 model. Furthermore, improvement to structural mitigation was also designed. Field observations were also conducted to gather field data on the flood impact. The model results indicate that the highest water level within the river bend is 1.9 m, and the highest velocity is 5.85 m/s, which is on the outer bend. A dyke was then planned to be about 3.75 meters high, accompanied by a retaining wall to protect the river bank using the interlocking permeable revetment (IPR). Dyke and retaining walls provide stability against shear, overturning, and collapse.


Introduction
Flow characteristics in river bends are complex and unique, compared to that of a straight channel.The non-uniformity of velocity pattern and pressure gradient could affect significantly the sediment transport [1].The inertial forces and an unequal transverse pressure gradient cause helical flow, and the high velocity is convicted to the outside and pushed to the riverbed, resulting in high shear stress and erosion [2].Eroded sediments are transported and deposited on the inside as the flow is re-circulated from the outside to the inside [3].Flow characteristics in the river bend could cause more significant infrastructure damage than that of straight rivers [4], [5] due to erosion.
Changes in river morphology are in line with flow characteristics.The river flows in twists and turns to the sea [6] and will adjust the size and shape of both the geometric or roughness of the riverbed.The bed and bank of the river will be formed by the material transported by the river.Horizontal erosion is greater than vertical erosion in meandering rivers.This causes the river flow to move horizontally due to erosion of the river banks by the main flow on the outside of the bends and sedimentation.The river is getting crooked, and the meander will undergo changes and displacement if it occurs continuously.Changes and displacements of meanders occur through translation, expansion, and rotation.Under certain conditions, the bend in the meander is cut off and will become an oxbow lake [7], [8].
Morphological processes in rivers may have significant effects on both the environment and society [9], [10].The evolution of the meanders could endanger the surrounding infrastructure, as happened in 1311 (2024) 012002 IOP Publishing doi:10.1088/1755-1315/1311/1/012002 2 the Babon River bend which is near the Dinar Indah Housing Complex, Meteseh, Tembalang, Semarang City, Central Java (Figure 1).The outer bend of the Babon River is getting closer to the housing complex from year to year due to its morphodynamic changes.On January 6, 2023, an extreme flood occurred due to high intensity rainfall and overflowed the crest of the levee at the outer bend of the Babon River, which had caused the collapsed of several levee segments [11].The flood inundation around the Dinar Indah Housing Complex was quite severe because it was on the outside of a river bend which has the characteristics of high erosion rate and velocity.In addition to that, the overflow of the Babon River was also caused by the narrowing and silting of the river channel near to the housing complex, so that the storage capacity is reduced [12].The overflow had also caused the collapse of the river embankment that protected the houses (Figure 1-d).
This study focuses on the flow characteristics and changes in river morphology at the river bends.Changes in river morphology, especially at bends, will mostly affect the surrounding environment [13].
Understanding the characteristics of flow patterns at river bends becomes important when planning the design of nearby buildings.This study aims to determine the flow patterns at the river bends and their response to surrounding infrastructures using HEC-RAS 1D modelling.In addition, to understand the flow patterns, the 1D modelling using HEC-RAS also helped to estimate the water level [14].

Materials and Methods
Generally, this research was conducted through two main steps: field observation and hydraulic numerical simulation using the HEC-RAS 1D model.

Field observation
The field observation was carried out on 29 February 2023 around the location where the river dyke collapsed.This observation was done to gather evidence of the impact of the flood on 6 January 2023.
During the field observation, interviews were conducted with residents to study the chronology of the event.The field observation coincided with the construction of a temporary dyke.The broken dyke has been replaced with a temporary one made of sandbags, as shown in Figure 2.During the site visit, the river cross-section was also observed.On the outer side of the bend, the riverbed elevation is lower than on the inner bend.This indicates the erosion on the outside of the bend which could endanger the surrounding infrastructure.Erosion causes banks to wear down and river banks to form.Sedimentation causes the inside of the bend to be shallow and sloping.

Hydrology A. Catchment Area
The catchment area boundaries were estimated based on DEM data from the Indonesian Geospatial Agency, which were then processed using the Arc-GIS application.The endpoint of the catchment area was set at the bend of the Babon River near the Dinar Indah Housing Complex.The delineation of the catchment area is shown in Figure 3. From this analysis, it was found that the catchment area is about 47.13 km 2 with a river length of 7.52 km.

B. Curve Number
The Curve Number (CN) was determined based on the land cover variation in the catchment area.The land cover data was obtained from the Topographical Map of Indonesia [15], [16].The CN value is summarized in Table 1.

C. Flood Design
Due to the rainfall data unavailability, we extracted rainfall data from satellite-detected precipitation, PERSIANN (Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks) [17], accessed through http://chrsdata.eng.uci.edu.A series of 10 years of daily rainfall data (2013-2022) were analyzed based on statistical extreme value analysis.Log normal extreme values were then selected to represent the design rainfall.The design flood discharge was estimated using the Nakayasu Synthetic Unit Hydrograph method.The design flood estimation is shown in Figure 4.For the maximum flood discharge used in the simulation, a 50-year return period of 282.03 m 3 /sec was chosen.The hydrograph graph is shown in Figure 4.

Model setup
The river geometry data was approximated based on DEMNAS data [18].The DEM data was processed using the Ras-Mapper Terrain feature in the RAS Mapper to follow the recent condition.The model domain was set following the river course around ±825 m long as shown in Figure 5. Upstream and downstream Boundary Condition (BC) locations were also indicated in Figure 5.To replicate the original cross section at the river bend, station and elevation adjustments were made in the river cross sections to represent the real condition.This adjustment was made due to the unavailability of cross-section data.Figure 7 (a) shows a straight flow cross-section.In these crosssections, the highest velocity of the channel is expected to be in the middle, resulting in a lower elevation for the middle channel.As for the cross-section at the river bend, the deeper the river bend, the deeper the elevation will shift to the outside of the bend, as shown in Figure 7 (b).On the inner side, the more central the sedimentation turns will be more frequent.So that on the inner side, the more in the middle of the turn, the elevation will be higher.In this model setup, the levee was used for flow limiters as in Figure 7 (b).
Correspondingly, the further away from the middle of the turn, the lower the channel elevation will be further away and the sedimentation will decrease as shown in Figure 8 (a).In a straight channel after a turn, the channel will return like a straight channel at the beginning of the turn where the channel with the lowest elevation was in the middle of the channel.The manning value corresponds to the material and river conditions at the reviewed location.For the main channel, right side, and left side a manning value of 0.03 is employed, as the channel features a clean channel with a rock base, as indicated in Table 2.The left side of the river bend (70 m) has a manning value of 0.033 because on that side there are gabions [19], [20], [21], [22], [23], [24] manning values listed in Table 2

Model Verification and Simulation
Preceding the flood event on January 6, 2023, Semarang experienced continuous rainfall for two consecutive days.This excessive precipitation has led to the overflow of several rivers within the city, including the Babon River.The overflow of the Babon River had severe impacts when it breached the dyke of the Dinar Indah housing complex, causing the residents' homes to be inundated.The study conducted two distinct modelling phases: the first phase focused on the existing scenario, while the second phase projected the potential impact after implementing water management structures.
In the existing model, the modelling process considers various parameters such as manning coefficients, cross-sectional dimensions, and levee characteristics.This approach ensures that these parameters closely align with the actual conditions.From the existing model, it was known that at the end of the turn, the velocity was greater than the velocity during the bend, as shown in Figure 9.The darker colour in Figure 9 indicates a higher flow velocity.This seems different from the theoretical considerations where the maximum velocity is expected at the outer bend.In this particular setup, the cross-sectional area at the end of the bend is smaller than that of around the bend making it have a greater velocity.However, if we only examine the cross-section around the bend, the outer bend still shows a larger velocity than that of the inner bend, although it is still smaller than those at the end of the turn due to the smaller cross-sectional area.The maximum velocity at the end of the bend is 5.85 m/s.This observation aligns with the findings of [25], who researched the subject.They concluded that the velocity of the water downstream on the outer side of a bend is higher compared to the velocity of the water in a straight stream.9 illustrates the velocity distribution across each cross-section.In Figure 10, cross-sections of the straight flow before and after the bend.Within these cross-sections, the highest velocity was shown in the central channel.In the section where the embankment collapsed, the flood depth reached 1.9 meters above ground level, as shown in Figure 10(b).The ruptured dyke will be reconstructed, spanning a total length of 36 meters.On the other hand, the flow velocity at the start and end of the river bend is higher compared to within the bend itself.As shown in Figure 9, the beginning and end of the bend were depicted with a darker color.Consequently, this gives rise to a significant risk of erosion along the riverbank on that side.So, a retaining wall is needed along the riverbank to reduce the risk.The proposed riverbank reinforcement will span a length of 120 meters on each side, both at the beginning and the end of the bend, as depicted in Figure 11.

Dyke and retaining walls
The retaining wall was placed along the river bends.Retaining wall construction is now taking place in the field to replace the damaged walls, right around the river bend.However, according to the model simulation, the length of the retaining wall would not be enough.Therefore, in this study, we suggest two additional retaining walls: the pre-and post-turning segments as shown in Figure 11.The post-turn segment was designed to have a height of 2.4 m, a foundation of 0.9 m, a bottom width of 2 m, and an upper width of 1.5 m.For pre-turn anchoring, the height is 2.1 m, the foundation is 0.9 m, the bottom width is 2 m, and the top width is 1.5 m.The dyke was planned to use interlocking permeable revetment blocks with dimensions of 50 cm x 50 cm x 50 cm and a thickness of 7 cm.Each retaining barrier meets shear stability, rolling, and collapse moment requirements with safety factor values exceeding 3. The dyke was planned to have a length of 36 m and a height of 3 m.It will also have a freeboard of 1 m height, a foundation thickness of 0.375 m, a foundation width of 1.875 m, and a top width of 0.25 m.These dimensions ensure stability in terms of shear adaptability, rolling moment, and collapse, with a safety factor value exceeding 3.For retaining walls and dyke, the quality of concrete used is 19.3 Mpa.The dyke will be reinforced with D16-250 reinforcement for the repetition and 15 D8 reinforcement for the embankment body, as well as 9 D8 reinforcement for the foundation.Figure 12 and Figure 13 are drawings of a planned retaining wall and dyke.

Conclusions
Numerical simulations using the 1D HECRAS model of the Babon River case have been conducted in this study.The model uses a 50-year return period discharge of 282.03 m 3 /s, with a Manning's roughness coefficient of 0.3 for regular channels, 0.033 for gabions, and 0.045 for designed channels.From the simulation, the characteristics of the river bend were observed.Around the straight channel, the dominant flow velocity was around the central line, while around the bend of the river, the closer to the center of the turn, the flow velocity would shift toward the outside of the bend.Conversely, if one is farther away from the center of the turn, the maximum flow speed will shift toward the side of the main channel.This results in a higher risk of erosion and sedimentation at river bends.The maximum velocity in the current model occurs at the end of the curve, reaching a velocity of 5.85 m/s.Additionally, the floodwater level on the collapsed dyke is 1.9 m above the ground level.
At the start and end of the 120 m turn, the velocity was greater than at the other cross sections.Therefore, a retaining wall would be necessary.The retaining wall was designed to use interlocking permeable revetment blocks with dimensions of 50 cm x 50 cm x 50 cm and a thickness of 7 cm.The retaining walls were planned to have heights of 2.4 and 2.1 meters, while the dyke can reach a height of 3.75 meters.Each retaining wall was designed to ensure stability against shear, overturning, and collapse, with a safety factor value that exceeds 2. For retaining walls and dyke, the quality of concrete used is 19.3 Mpa.The dyke will be reinforced with D16-250 reinforcement for the main structure and 15 D8 stirrup reinforcements, while the foundation will be reinforced with 9 D8 stirrups.The results of the design modelling show that the flow velocity at the end of the bend has decreased from 5.85 m/s to 5.6 m/s, and the flood depth has increased by 0.2 m to 35.70 m.

Figure 1 .
Figure 1.Map of the study location: (a) river bend near the Dinar Indah Housing Complex; (b) straight flow channel section; (c) the upstream river section; (d) the side of the collapsed river embankment; and (e) the river bend after near the housing complex

Figure 2 .
Figure 2. Temporary embankments found during the field observation: (a) top view and (b) side view

4 Figure 3 .
Figure 3.The catchment area of Babon River at the Dinar Indah housing complex.
Figure 6 (a) shows the cross-section location in the model, while Figure 6 (b) shows the long section of the model.

Figure 6 .
Figure 6.(a) Cross-section locations, (b) View of the river model profile (long section)

Figure 9 .
Figure 9. Distribution of flow velocities in the channel and cross-section

Figure
Figure9illustrates the velocity distribution across each cross-section.In Figure10, cross-sections of the straight flow before and after the bend.Within these cross-sections, the highest velocity was shown in the central channel.Figure10 (b) and (c) show a cross-section of the bend, where the velocity distribution was more inclined towards the outside of the bend.
Figure9illustrates the velocity distribution across each cross-section.In Figure10, cross-sections of the straight flow before and after the bend.Within these cross-sections, the highest velocity was shown in the central channel.Figure10 (b) and (c) show a cross-section of the bend, where the velocity distribution was more inclined towards the outside of the bend.

Figure 10 .
Figure 10.(a) model results in straight channel cross-section 709, (b) model results in cross-section 448, (c) model results in cross-section 423, (d) model results in cross-section 334 straight channel

Figure 12 .
Figure 12.Typical cross-section of the designed retaining wall

Figure 13 .
Figure 13.Details of the designed dyke3.3.Simulation of the proposed designSimulation under the designed scenario was then carried out.This scenario used a manning value of 0.045 in areas with retaining walls.This approach is based on Table3, which involves constructing the retaining wall using IPR (Interlocking Permeable Revetment) and planting vegetation on it.As a result, the flow velocity has decreased.On the other hand, the level of floodwater has increased, due to the increase of manning value.The flow velocity has decreased by 0.25 m/s, and the depth has increased by 20 cm, still falling within the design range.The flow velocity is depicted in Figures14 and 15for the depth of flow.

Figure 14 .Figure 15 .
Figure 14.Comparison of flow velocity on (a) existing and (b) design scenario