Computing urban flooding of meandering river using 2D numerical model (case study : Kebon Jati-Kalibata segment, Ciliwung river basin)

This research is an extension from a previous study titled modelling the diversion channel at Kalibata-Kebonjati meandering segment in Ciliwung watershed. Based on http://pusatkrisis.kemkes.go.id data, there are five sub districts that were flooded because of the rainfall in 5 February 2018 i.e : Pasar Minggu, Pancoran, Cilandak, Jagakarsa and Tebet. Their flood levels were achieved 300 cm, and the worst disaster happened in Pancoran, at Kebonjati to Kalibata segment in particular. Thus, this segment needs to manage accurately with appropriate structures. Formerly, the last study using 1D HEC-RAS, but the model has several lackness related to the rates of meandering velocities and discharges at the river edges, whereas, these numbers have significant effects for river scouring. Since several studies which applied 2D and 1D/2D coupling hydraulics modelling have been providing satisfactory results on complex river, this research was redeveloped and compared the meandering segment by using 2D HEC-RAS and Coupling 1D/2D HEC-RAS. Using rainfall data from three stations (Bendung Gadog, FT UI and Gunung Mas), the models indicated better results comparing with 1D model beside relations between velocity and discharge, respectively. Further, a diversion channel and stilling basin will be held in order to manage the flooding and scouring issues.


Introduction
As a capital city of Indonesia, Jakarta encounters severe problems related to water management specially annual flooding. Based on https://bpbd.jakarta.go.id data, there are more than 7 (seven) times flood disasters occured and inundated 15 sub districts every month in a year. Another data held by http://pusatkrisis.kemkes.go.id data per February 14th 2018, there are five counties were flooded by 5 February rainfall i.e Pasar Minggu, Pancoran, Cilandak, Jagakarsa and Tebet with 10 -300 cm of flood level. The worst disaster happened in Pancoran, at Kebonjati to Kalibata segment in particular.
Formerly, the hydraulics modelling used 1D HEC-RAS. But there are several lacks related to flood characteristics, specially velocities and discharges, when being compared with observation data [1].
This study aims to achieve an optimal modelling in order to provide basis data and to decide the most appropriate structure for manage flooding. Several studies had conducted in order to reduce flooding that claimed could be ruinning until billions dollars per years [2,3].
Others studies implied, when comparing 1D and 2D Numerical model in complex condition, for some cases, 1D approach is commonly may not be sufficient [1,4]. On the other hand, a 2D model was successfully used to simulate a high-magnitude outburs flood in Kverkfjoll, Iceland [5] or using Shallow Water Equation [6]. However a full 2D model may be consuming time and facing stability fragile issue, thus, the kopling method that combines 1D approach in the river and 2D approach for the overland flow effectively succeed for large and complex river system with good results [4,[7][8][9][10].
Ciliwung watershed are large and complex, therefore a coupled 1D-2D model and 2D model will be used for simulating the flood in order to obtain an effective and efficient solution for flood management in this area.

Study area
Ciliwung is one of commodious watershed which is mostly located in the capital city of Indonesia. Therefore, the administration boundaries made the watershed becomes equally important as capital city of Jakarta itself. Based on spatial analysis, Ciliwung watershed has 438.34 km 2 area and 116.32 km main river length, respectively.

Methodology
Main objective focus of these modelling i.e for comparing two schemes of hydraulics modelling (2D and coupling) in Ciliwung river specially at the meandering segment, the methodology of this certain problem is shown on Figure  2.

Fig. 2. Methodology of modelling
From groundstation climate data and watershed characteristics, the hydrology model will be developed in order to obtain upper boundary condition for hydraulics model. Due to fact that the particular segment located far away from the sea, model will use a calculated rating curve for the lower boundary. Furthermore, hydraulics river characteristics will be applied for others model variables.
Next, the coupling and 2D hydraulics models have been developed, then a comparison between model characteristics specially discharge and velocity will be started to identify.
The rainfall data have been obtained from three ground stations i.e Gunung Mas, FT UI and Bendung Gadog and calculated by Polygon Thiessen method in order to obtain area rainfall as follows :  The method shows that FT UI rainfall station has larger effect compared with two others stations, eventhough not very significant (37.44%).
From the stations weighting, the maximum annual rainfall data was calculated as follows : Since the hydraulics computings will be held along 11.824 km length from Pancoran/Kramatjati until Tebet sub district, the upper boundary is the hydrograph with 319.30 km 2 area and 84.73 km river length. On the other hand, the lower boundary calculated by rating curve and the long river alignment is divided by 190 cross section per 62.56 meter in average.

Fig. 4. Hydraulics model delineation
The calculation of profile in 1D modelling conducted by energy equation for solving water surface difference between cross section with standar step method procedure. As follows : Where Z1 and Z2 are the elevation of the main channel inverts (m), Y1 and Y2 are the depth of water at cross sections, V1 and V2 are the average velocities (m/sec), α1 and α2 are the velocity weighting coefficients, g is the gravitational acceleration (m/sec 2 ) and he is the energy head loss.
Where L is the discharge weighted reach length, f S is representative friction slope between two sections and C is the expansion or contraction loss coefficient. L derived from average value of cross section reach lengths for flow in the left, main channel and right overbank respectively.
Since the energy equation is not applicable for modelling water surface in critical depth condition, HEC-RAS is applying momentum equation for solving this problems with functional equation as follows : Whereas the flow in 1D equation assumed as a gradually varied because the basis premise hydrostatic pressure distribution exists at each cross section and total energy head is the same for all points in a cross section, these equation is limited for modelling flow in the curb river section. It is the Navier-Stokes equation which been used for describe the motion of fluid. Combining the momentum equation in two dimensional form of the Diffusion Wave Approximation with mass conservation yields, known as the Diffusive Wave Approximation of the Shallow Water (DSW) equations.

( ) ( )
Where t is time, u and v are the velocity components in the x dan y direction respectively and q is the source/sink flux term.
Where u and v are the velocities in the Cartesian directions, g is the gravitational acceleration, vt is the horizontal eddy viscosity coefficient, c fis the bottom friction coefficient, R is the hydraulic radius and f is the Coriolis parameter.

Hydrological Modeling
Based on an analysis of spatial map that distributed by National Mapping Agency of Indonesia (BIG), the landuse coefficient is 0.75.

Hydraulics model
The model is used to simulate existing condition using 2 flooding scenarios i.e 2 and 50 years of return period.
The model input are the topography data, river network and cross section for 1D simulation and combined with Digital Elevation Model (DEM) for 30 x 30 m grid size conducted from NASA SRTM.

Coupling 1D and 2D numerical model
The inundation maps of the existing condition for the flood discharge with return periods of 2 and 50 years are shown in Figure 6

2D numerical model
The inundation maps of existing condition from the flood discharge with return periods of 2 and 50 years are shown in Figure 8 and Figure 9.

1D AND 2D numerical modelling
Eventhough 1D model previously succeed for modelling diversion canal and its impact for decreasing flood in Kebonjati-Kalibata segment, it is obviously important for comparing 1D and 2D hydraulics existing model related to flood characteristics specially in meandering part.
The flood elevation for 2 and 50 years of returned periods calculated as follow :

Conclusion and future work
In summary, the 1D and 2D hydraulics modelling obviously resulted differ values. In meandering section, the gap between them for 2 and 50 years of returned periods achieved 61.64 -62.62%.
Beside that, there is an indication about the possibility of relationship between coupling model and 2D model based on flood characteristics i.e : velocity and discharge. The ratio between models spared between 0.21 -0.27 at inner curve and 0.27 -0.57 at outer curve for discharges and 0.22 -0.43 at inner curve 0.18 -0.35 at outer curve for velocities.
For future work, in order to manage flood disaster that annually occured on its segment, it is important to develop diversion canal. Furthermore, since from previous work that 1D HEC-RAS model had faced problems related to velocity and hydraulics jump at the end of canal [1], it is also considerable for applying stilling basin at the outlet of canal.