Flood hazard comparison based on geomorphic flood index and hydraulic HEC-RAS (Case study in Ciliwung Watershed, Jakarta)

The Jakarta Special Capital Region is a highly vulnerable area to floods due to its location on wetlands laced by 13 major rivers and borders the Java Sea, with more than 40% of its land below sea level and groundwater extraction leading to the ground sinking. The city has experienced several major flood disasters, and climate change has increased the risk, frequency, and severity of flooding in Jakarta. The present study aims to identify areas with flood potential in the Jakarta Special Capital Region (Provinsi DKI Jakarta) using the Geospatial Flood Index (GFI) method recommended by the National Disaster Management Agency (BNPB). The GFI method is an alternative rapid assessment that utilizes the Digital Elevation Model for National Spatial (DEMNAS) data with an 8-meter spatial resolution and employs ARCGIS and QGIS software to identify areas with high potential for flooding and the extent of inundation. To assess the effectiveness of this method, a comparison is made with the Hydraulic HECRAS model for the section from the Automatic Water Level Recorder (AWLR) MT. Hartono to the Manggarai Flood Gate, considering flood return periods Q50 and Q100. The modeling results indicate that the inundation area estimated by the conventional GFI modeling is 150% larger than that the HECRAS Hydraulic model predicted.


Introduction
Floods are the stream of the river that overflows beyond the river's capacity and, thus, will pass through the riverbank and inundate the surrounding area.The causes of flooding are triggered by several factors, including natural factors such as climate change that increase the frequency, magnitude, and extreme events, high rainfall intensity, and human factors such as urbanization that increases runoff, changes in land use, and enhancement of settlement construction in flood-prone areas [1].
Floods in the Jakarta Metropolitan Area occur every year (see Figure 1) and inundate the entire area of Jakarta City.The floods in 2020 were the most enormous ever.The high precipitation in the Ciliwung Cisadane River system allegedly caused this incident.Besides, it was influenced by the small flow capacity of the Ciliwung Cisadane River system and the slight infiltration in the Jakarta Metropolitan Area, so it was unable to accommodate water runoff from the primary Ciliwung Cisadane River system or its tributaries.Another factor that causes flooding in the Jakarta Metropolitan Area is that the topography of Jakarta is primarily flat, with an altitude between 0-1000 m above sea level and an average slope between 0-8 %.This kind of topography strongly supports the occurrence of soil erosion processes, which carry sediment from the upper part, which in turn sedimentation is deposited in river streams and causes silting of the river so that the continuous capacity of the river for rainwater will cause flooding [2].
Disaster risk mapping is a spatial approach to determine the nature and level of disaster risk by analyzing potential hazards and evaluating existing exposure and vulnerability conditions that may endanger people, property, services, livelihoods, and the environment on which they depend [ [3].Disaster risk assessments include hazard, vulnerability, and capacity assessments [ [4].Mapping areas with a flood hazard level must be done so the government can make the right policies to overcome them [4].

Study Area
The study area is located in the Ciliwung River System watershed.The Ciliwung River watershed covers an area of 76 square kilometers, with a total area of 392.3 square kilometers, encompassing ten cities or regencies, namely West Jakarta, Central Jakarta, South Jakarta, East Jakarta, North Jakarta, Bogor, Bogor City, Cianjur, Depok city, and Sukabumi in the upstream area.
The morphology of the Ciliwung River watershed in the upstream region in the West Java province exhibits mountainous and hilly terrain.Meanwhile, in the DKI Jakarta province, the Ciliwung River watershed has a flat and gentle morphology.The low slope of the terrain in the DKI Jakarta province increases the risk of flooding, particularly in densely populated and office-dense areas affected by floods.
This research specifically focuses on the section of the Ciliwung River from the Automatic Water Level Recorder MT.Haryono to the Manggarai Flood Gate, covering a distance of 7.9 kilometers.This river section passes through three cities: Central Jakarta, South Jakarta, and East Jakarta.The study area of this research is depicted in Figure 1.

Flood
A flood is overflowing a stream or another body of water or accumulating water over areas not usually submerged.Floods include river (fluvial) floods, flash floods, urban floods, pluvial floods, sewer floods, coastal floods, and glacial lake outburst floods."These various classes of floods are generated by different mechanisms [5].
Suppose an event is submerged by water in an area that threatens and disrupts the life and livelihood of the community, resulting in human casualties, environmental damage, property losses, and psychological impacts.In that case, the flood is considered a disaster [4].
Inland flooding in Jakarta is primarily triggered by intense rainfall associated with tropical depressions and continuous low-level wind convergence resulting from cross-equatorial northerly surges.The situation is further aggravated by obstructed waterways, deforestation, and inadequate drainage and flood control provisions, contributing to the worsening of inland flooding [6].

Hazard
A process, phenomenon, or human activity that may cause loss of life, injury or other health impacts, property damage, social and economic disruption, or environmental degradation.Hazard may be caused by natural, anthropogenic, or socio-natural origin.Natural hazards are associated with natural processes and phenomena.Anthropogenic or human-induced hazards are induced entirely or predominantly by human activities and choices.Floods are sociological since they combine natural and anthropogenic factors, including environmental degradation and climate change [7].Determines the area where certain natural events occur with a particular frequency and intensity, depending on the vulnerability and capacity of the area, which can cause a disaster [ [8].

Hydraulic Modeling with HEC-RAS
HEC-RAS is one of the programs used to model river flow.This program was developed by the Hydrologic Engineering Center (HEC).The equations used are as follows: (1) Where: • V (u, v) is the velocity vector (m/s) • H is the water surface elevation (m) • h is the depth (m) • q is the discharge (m³/s) • The vector components ∇ = /, / are then decomposed as follows: Where: • g is gravity (m/s²) •  is the eddy viscosity •  is the Coriolis effect •  is the bed friction

Geomorphic Flood Index
The Flood hazard index employed in this research utilizes the Geomorphic Flood Index method (GFI).GFI represents a cost-effective and highly efficient approach for assessing areas susceptible to flooding, using digital elevation model (DEM) data as the fundamental basis for the analysis [9].The GFI methodology was initially developed as a QGIS plugin [10].The description of this method is pictured in Figure 2.This technique involves the comparison of the water depth variable (hr) at each location with the elevation differential (H) measured in meters.The value of hr is computed based on the contribution of the accumulated flow area (Ar) in square meters from the nearest point within the hydrologically interconnected river or drainage network [11].By considering the estimated water level (hr) in the closest segment of the river or drainage network, the GFI method identifies the nearest river or drainage as a potential source of risk in terms of flood risk evaluation.In this research, the data used comprises DEMNAS data provided by the Geospatial Information Agency, with a resolution of 8 meters.
There are several main processes in the GFI (Geomorphic Flood Index) process, namely Fill Sinks, Flow Direction, and Flow Accumulation.Fill Sinks is a process of filling gaps where there is no outflow of water from these gaps.The Fill Sink method has an advantage in addressing high-value anomalies in valley areas.These gaps can be eliminated using GIS features [12].Flow direction determines the flow direction from each raster cell.The Flow Direction grid indicates the flow direction from each cell to the nearest area with the steepest slope in the DEM [13].Flow Accumulation generates flow accumulation as the sum of all cells that flow into each cell downstream in the output raster.The flow accumulation results can create a flow network by identifying cells with high flow accumulation values [13].4. Materials

Flood Event Data
The data used in this study is flood event data from DSDA DKI Jakarta from the flood event in February 2017 event.The following is the data on the height of floodwaters in the Ciliwung watershed.South Jakarta 0.5 Source: DKI Jakarta Water Resources Agency, 2022 [15] Meanwhile, the data used for GFI (Geomorphic Flood Index) and Hydraulic HEC-RAS analysis are as follows and are presented in Table 2.

Methods
Analysis of flood inundation using the Geomorphic Flood Index (GFI) method involves two approaches.The conventional method is outlined in the Technical Module for Flood Disaster Risk by BNPB (National Disaster Management Agency).In the conventional method, the hr value is calculated as a function of the contributing area (Ar) (Accumulated Flow) at the nearest point of the river network that is hydrologically connected to the point under examination.On the other hand, the GFI method involves conventional rainfall weighting factors, where the value of V represents the multiplication relationship between the amount of rainfall and the accumulation value at each designated grid.The description of the GFI method is attached in the following Figure 4.  Flood inundation analysis is conducted after supporting data such as river geometry, topography, and upstream flood discharge are available.This analysis compares the flood inundation results obtained from the GFI analysis with the data generated from the HEC-RAS 2D software.The following is a diagram of the flood inundation analysis method using the HEC-RAS 2D hydraulic model, visualized in Figure 5.

Hidraulic HEC RAS Modelling
The flood hazard assessment step in Hydraulic HEC RAS Modelling for the Q50 method is pictured in Figure 6.

GFI Modelling
The flood hazard assessment steps in the GFI method are

GFI Modelling
The flood hazard results in GFI modeling are pictured in Figure 8.The analysis results above indicate that the floodwater elevation values from the HEC-RAS modeling are the closest to reality compared to the GFI method.This suggests that additional approaches may be necessary to ensure that the floodwater elevation values closely match the existing conditions in determining floodwater elevation.The results of the analysis indicate that the inundation generated by GFI has a larger area than the inundation produced by HEC-RAS.However, if you observe the flood locations closely, the flood locations between GFI and HEC-RAS differ.This is because, in the HEC-RAS modeling, the flood area is primarily concentrated within the designated river channel area.On the other hand, in the GFI modeling, the flood inundation distribution is focused on the river area and follows the conditions of the analyzed DEM (Digital Elevation Model).This is an advantage of the GFI model, as it can detect potential flood inundation areas that are not covered by river modeling.

Conclusions
The inundation area revealed by the GFI analysis is notably more extensive than the present conditions and the hydraulic HEC-RAS analysis.Conversely, the elevation values obtained from the HEC-RAS analysis closely align with the existing conditions, in contrast to the conventional GFI and modified GFI analysis outcomes.Notably, the GFI analysis results hold the potential for detecting areas at risk of fluvial flood inundation, underscoring the imperative for further comprehensive studies in this regard.

5 Figure 3 .
Figure 3. GFI Representation Of The Parameter h and hr In Cross Section[14]

Figure 8 .Figure 9
Figure 8. Potential Inundation Height by GFI Conventional Method

.
Type of Data

Table 3 .
Flood Height Verification

Table 4 .
Flood Height Error

Table 5 .
Flood Inundation Area Comparison