Flood control at km 130 Padaleunyi Toll

Flood is one of the natural phenomena that often brings loss of property and life. Mostly, it occurs during a high-intensity rainfall event in the catchment area which results in high river flow that cannot be accommodated by river cross sections. In Bandung area, one of the locations that are often hit by the flood is located on km 130 of the Padaleunyi toll road. This flood occurred due to the overflow of the Cilember and/or Cimancong rivers tributary which flows parallel to the toll road, inundating the toll road segment with low elevation at around km 130+500. This paper aims to analyze the effective flood control methods in the above location. With catchment area around 2.3km2, which is relatively small, peak flood discharge calculation was carried out using a rational method. Hydraulics simulation was carried out using HecRas, based on river field measurement data of Cilember and Cimancong river cross-section. Analysis result shows that the combination between flood embankment construction and river normalization provides a significant decrease in flood water level in km 130 Padaleunyi toll road. The reinforced concrete vertical wall was considered as the appropriate flood protections structure due to the limited space available between the river and the toll road segment. This paper also underlined the impact of the increasing loss of water retention areas on an increased risk of flooding.


Introduction
Based on historical floods at KM 130 Padaleunyi Toll in November 2016 and October 2017 there was a flood along the toll with overflowing points originating from the Cimancong River. The main cause of the overflow of water in the Cimancong River is the intensity of rain that occurs in the upstream is very large and the infiltration capacity of the area is small. This condition can be seen from the land use of the surrounding area which was originally filled with rice fields and gardens turned into landfills, as well as a flat-topped watershed topography, the elevation between the Cimancong River and the Padaleunyi Toll has a value not much different so that the rise in the Cimancong River can easily overflow on the road without being infiltrated into the soil properly.
Flood events on KM 130 Padaleunyi toll cause great losses, such as disruption of road traffic which results in total congestion on the left and right sections of the Padalarang and Cileunyi toll. This loss must be controlled and addressed by integrated flood damage mitigation efforts in the form of a reduction in peak flood discharge, reduction/transfer of water discharge in a body of water, flood resistance with infrastructure, flood warning, and floodplain processing. With this flood, control effort is expected to reduce the impact of losses caused by the overflow of the Cimancong river

Location description
Cimancong watershed which extends from coordinates 6°55'22.5"S;107°32'46.7"E to 6°53'47.6"S; 107°33'37" E and has an outlet point located at coordinates 6°55'22.5 "S; 107 ° 32'46.7"E. This watershed has an area of about 2.3 km 2 with land cover consisting of industrial areas, residential area, gardens, parks and rice fields. Cimancong watershed is located in an area with an elevation of about 700 -710 meters above sea level and is included in a fairly densely populated area.
Accessibility of the study location can be reached through the Padalarang-Cimahi residential area, where the location of this study is parallel to the toll road towards Padalarang-Cileunyi. The picture 1 shows the Cimancong: Data used in generating trends are observation data on rainfall from BMKG. The influential rain station was Bandung Climatology Station with the length of rain data used throughout 37 years from 1980 to 2016.

Method
In general, the method used is as follow

Effective rainfall
In the calculation of effective rainfall, it is assumed that infiltration has no effect this is due to the area of the watershed that is small enough so that it can be concluded that the amount of effective rainfall is the amount of rainfall that occurs. The amount of effective rainfall as shown in Table 2 below: The rain that is used for calculation is the effective rainfall value that has been distributed in hours. The effective rainfall distribution that is used is based on Van Dreen investigation where one day of rain is only focused in 4 hours with a large amount of uniform rainfall for each hour. The following Figure 2 explains the effective distribution of rainfall in the T year period.

Flood discharge
The planned flood discharge is calculated using the Synthetic Unit Hydrograph Method which is the Nakayasu Method, SCS and empiric method, which is a practical Rational Method. Based on the consideration of calibration of bankfull discharge, the flood discharge value used is the result of the calculation of the Rational Method. As for the equations used in the calculation of the practical Rational method are. Figure 3 presents the flood discharge plan with a T-year period. In planning flood control buildings in a city with a population more than 2,000,000,000 of people, using flood discharge during the 25th periode of 24.25 m 3 /sec with a peak discharge time of about 1.1 hours.

Hydraulical analysis
Hydraulic analysis when existing conditions is using 1dimensional HEC-RAS software. River geometry input from STA 0 -STA 19 tends to be straight and the simulation discharge is using Q25. The Figure 4-6 shows the river geometry input, cross and long-sectional view of the river after running:   From the figure above, it can be seen that the river cross-section at Sta-9, Sta-10, Sta-11, Sta-12, Sta-13, Sta-14, StA-15, Sta-16, Sta-17, Sta-18, and Sta-19 occurs overtopping / overflow of water. This happens because the cross section of the river is not able to accommodate the flow of water when a planned flood discharge occurs.
In the display figure extends the condition of the Cimancong River above, it is known that a sudden decrease in water level at Sta-1, this condition is caused by the influence of the superkirits flow (Fr> 1) where the channel slope is steeper than the critical slope, causing flow at Sta -1 has acceleration and has an effect on the water level in the cross section  The Table 3-4 explains that normalization planning is able to be a flood solution and safe with the dimention of freeboard used, and based on the calculation of slope stability in STA 4, STA 7, and STA 9, the results are safe for the possibility of landslides throughout the planning.

Alternative 2: Construction of Embankments
Construction of embankments is planned with a number of considerations, including: developer activities in the form of reclamation, land loading factors, safe dimensions of sliding, rolling, and soil carrying capacity as well as border line considerations. Construction of embankments is carried out along STA 5 -STA 13 on right side of the river with the provision of peak elevation at each point of embankment are same. The following on Figure 10 is an illustration of the planning of embankment development at the study location: Then simulated using the help of HEC-RAS software with a cross and longitudinal display as follows on Figure  11- Fig. 11. Cross-section (P14) of Cimancong River The Table 5-6 explains that normalization planning is able to be a flood solution and safe with the dimention of freeboard used, and based on the calculation of slope stability, the results are safe for the possibility of landslides throughout the planning: Table 5. Result Calculation of construction of Embankment Planning Table 6. Result calculation of slope stability

Alternative 3: Construction of Embankments and normalization of Cimancong River
Construction of embankments and river normalization is planned with several considerations, including: developer activities in the form of reclamation, land acquisition factors, safe dimensions of sliding, rolling, and soil carrying capacity as well as border line considerations. Construction of embankments is carried out along STA 5 -STA 13 on the right side of the river with the provision of peak elevation at each point of embankment are same, and normalization of the river is carried out along STA 0 -STA 4. The following on Figure 13 is an illustration of the study location planning in (a) embankments planning and (b) normalization planning:  The table explains that alternative is able to be a flood solution: Table 7. Result Calculation of construction of embankment and normalization planning

Conclusion
Conclusions for planning flood control buildings at KM 130 Padaleunyi Toll are as follows: 1. Problems that occur in the Cimancong River and cause flooding in KM 130 Padaleunyi Toll which is caused by the cross section of the river that is not able to accommodate the water load during a flood. 2. To solve the problem of flooding at KM 130 Padaleunyi Toll will be planned to build embankments with reinforced concrete structures on the right side of the river channel that intersect with toll roads, from Sta-5 to Sta-13 or 413 m. 3. Planning embankment dimensions using flood discharge during the 25th periode of 24.25 m3 / sec and 0.6 m freeboard, and plan dimensions are safe against shear, rolling and soil bearing stability. Accessibility of the study location can be reached through the Padalarang-Cimahi residential area, where the location of this study is parallel to the toll road towards Padalarang-Cileunyi. The picture 1 shows the Cimancong: