GIS
How to Create Floods Hazard Map using ArcGIS
Study case West Java Province, Indonesia
Floods in Indonesia have been an annual disaster.
Although floods posing a relatively lower risk impact than other disasters, they have a higher frequency of occurrence.
I want to show how I made those floods hazard maps of West Java, Indonesia, using ArcGIS software.
How I built the floods maps?
Dataset
The initial data I used was DEM-SRTM (Digital Elevation Model: Shuttle Radar Terrain Model) 1 Arc Second (30 meters), which I got from USGS (Earth Explorer United States Geological Survey) website.
To validate the map, I used historical data of floods in West Java that happened in 2012–2018, which I got from the Indonesian National Board for Disaster Management (BNPB).
Workflow
The floods vulnerability factors are arranged based on the slope, main river network, and MTWI (Modified Topography Wetness Index) value.
The Slope map was calculated using Slope tools in ArcGIS.
The main river network map was calculated using some functions. First, the Fill function served to correct each pixel in the SRTM DEM data. The following function used was Flow Direction to determine the flow direction by calculating the height difference or slope. The last function I used was Flow Accumulation to determine the shape of the river network, which is calculated based on the calculation of the results of the Flow Direction function.
The results of the river network formed from the Flow Direction function will be entered into the Euclidean Distance function. Which is to create a buffer (a layer that covers an object) on data that has a raster format with a buffer distance of 1 kilometer. The result was a map of the main river network with a 1-kilometer buffer
Lastly, calculate the MTWI value using this formula :
Where ad is the flow area per unit length of the contour or flow accumulation value based on the DEM data analysis (depending on the DEM resolution), tan (β) is the slope angle (based on the DEM data analysis), and n is the exponential value calculated by the formula n = 0.016 x 0.46, where x is the DEM resolution.
This MTWI value describes the tendency of water to accumulate at a point based on the force of gravity where water always flows to a lower place. That’s why the MTWI value will be more significant on very flat slopes and more minor on steep slopes.
The flood hazard index is estimated based on the slope and distance from the river in the flood-prone area using the Fuzzy Membership method.
After created three main maps: Slope Map, Main River Network Map, and MTWI Map, the next step is to apply the Fuzzy Membership function to each of these maps. This Fuzzy Membership function assigns a weighted value ranging from 0 to 1 to each data in the map.
You can read more about how Fuzzy Membership works through the link below.
Result
In terms of vulnerability to flooding, West Java Province is divided into 5 levels of vulnerability, from areas not prone to very vulnerable.
Based on Historical Data Map of West Java Floods during 2012–2018 from the National Disaster Management Agency (BNPB), I found several links between flood event points and flood-prone maps:
- There are flood event points in the high vulnerability zone; this means that the area can be validated to become an area with a high level of vulnerability.
- There are no flood event points in the high vulnerability zone; this can happen because the method used to analyze the flood vulnerability zone is based on the calculation of the slope and the main river network. In the northern part of West Java, almost all parts are zones with a high level of vulnerability, but not all parts have flood points.
- There are flood event points in moderate and low vulnerability zones; this can be interpreted as an anomaly caused by factors causing flooding other than the level of wetness (potential for standing water), slopes, and the main river network. One of the other factors that allow flooding is very high rainfall in the area.
- There are no flood event points in the medium and low vulnerability zones; this means that the area can be validated as having medium and low vulnerability levels. The area has the following factors for making the flood hazard map: low wetness (potential for standing water), steep to steep slopes, or an extensive river network. So the area is a low to moderate vulnerability zone.
My Mitigation Recommendation for Floods Disaster
From the map above, I recommended some mitigation solutions for flood disasters, such as:
- Repair or construct the drainage systems (waterways and rivers)
- Addition of water catchment areas
- Creation and installation of an early warning system (EWS) in flood-prone areas
- Enforcement of regulations and laws regarding actions that may result in flooding by the authorities
- Increased socialization to the community in flood prevention.
Final Words
Congratulations for keeping up to this point! Hopefully, you have learned something new from this article. Hopefully, this article can give you good references about Floods Hazard Map.
If you love the content, please follow my Medium account to get a notification about my future posts!
About Me
A Geological Engineering graduate from Padjadjaran University who is consistently developing and challenging himself in various aspects, eager to learn, and open to new experiences. Currently contributing as Data Analyst Intern at Grouu Baby Food.
Check out Ariq’ GitHub to know more about him! Lastly, if you have any queries or any topics to be discussed, please reach out to Ariq via LinkedIn.
References
BNPB. 2016. Risiko Bencana Indonesia (RBI). BNPB.
Irawan, M.F., Hidayat, Y. & Tjahjono, B. 2018. Penilaian Bahaya dan Arahan Mitigasi Banjir di Cekungan Bandung. Bogor: Jurnal Ilmu Tanah dan Lingkungan, IPB.
https://www.researchgate.net/project/Delineation-of-Flood-Prone-Areas-using-Geomorphic-Methods
