I have prepared the River Proximity Map for the Buffer 250m, 500m, 1000m, 2000m and beyond 2000m for the study area. I used the Sentinel 2 satellite image 10m Resolution of Band Green and Near Infrared for the calculation of NDWI. Satellite Image Link.

Tutorial Video Link: Step By Step Part 1

Hands-On QGIS Practical

River Proximity Map for Flood Risk Assessment Using Sentinel-2 Imagery

A complete click-by-click practical exercise: Sentinel-2 → NDWI → River Extraction → Proximity Analysis → Flood Susceptibility Classes → Map Layout → Export

1. Practical Objectives

• Load Sentinel-2 imagery into QGIS.
• Create and interpret a water index (NDWI).
• Extract the main river from Sentinel-2 imagery.
• Create river-based proximity zones using buffer analysis.
• Assign flood-proximity susceptibility classes.
• Clip and symbolize the final analysis.
• Create a professional QGIS map layout.
• Export the final map as PNG and PDF.

2. Prepare the Project Folder

Before opening QGIS, create the following folder structure to keep all data organized:

Flood_Proximity_Project/

  01_Raw_Data/Sentinel2/

  01_Raw_Data/Boundary/

  02_Processed_Data/Raster/

  02_Processed_Data/Vector/

  03_Output/Maps/

  03_Output/Tables/

  04_QGIS_Project/

Open QGIS and immediately save the project using Project → Save As. Suggested name: River_Proximity_Flood_Risk.qgz.

3. Start QGIS and Set the Project CRS

1. Open QGIS and create a new project.
2. Check the CRS shown in the bottom-right corner.
3. Click the CRS indicator and select an appropriate projected CRS for your study area.
4. Click Apply → OK.

Important: Proximity and buffer distances should be calculated in a projected CRS with metre units. Do not rely on geographic coordinates (degrees) for metre-based distance analysis.

4. Add the Study Area Boundary

• Click Layer → Add Layer → Add Vector Layer.
• Browse to the study area boundary file and click Add.
• Right-click the layer → Properties → Information to check its CRS.
• If necessary, reproject it using Right-click → Export → Save Features As and select an appropriate projected CRS.
• Save the reprojected boundary as Study_Area.gpkg.

5. Add Sentinel-2 Bands

For the NDWI calculation, load the following Sentinel-2 Level-2A bands:

Band	Name	Resolution	Purpose
B03	Green	10 m	Water detection / NDWI
B08	Near Infrared (NIR)	10 m	Water detection / NDWI
B04	Red	10 m	Optional visual composite

1. Click Layer → Add Layer → Add Raster Layer.
2. Select B03 and B08.
3. Click Add → Close.
4. Confirm that both layers align correctly.

6. Optional: Create a False Color Composite

A False Color Composite helps visually identify rivers and other land-cover features. A common Sentinel-2 combination is:

Display Channel	Band
Red	B08 (NIR)
Green	B04 (Red)
Blue	B03 (Green)

Water generally appears dark, while healthy vegetation commonly appears reddish. Use this as visual support; the actual water extraction will use NDWI.

7. Calculate NDWI

NDWI (Normalized Difference Water Index) is calculated as:

NDWI = (Green − NIR) / (Green + NIR)

1. Click Raster → Raster Calculator.
2. Insert the exact B03 and B08 layer names from the layer list.
3. Use the expression below.
4. Choose the output location: 02_Processed_Data/Raster/NDWI.tif.
5. Click OK / Run.

(“B03@1” – “B08@1”) / (“B03@1” + “B08@1”)

Note: Replace B03 and B08 with the exact raster names shown in your QGIS project if necessary.

8. Inspect the NDWI Result

1. Right-click NDWI → Properties → Symbology.
2. Select Singleband Pseudocolor.
3. Click Classify and then Apply → OK.
4. Use the Identify Features tool to click river, vegetation, settlement, and other surfaces.
5. Compare the NDWI values and identify a suitable water threshold.

Higher positive NDWI values generally indicate water, but the correct threshold should be checked for the specific image and study area.

9. Create a Water Mask

• Open Raster → Raster Calculator.
• Start by testing a threshold such as NDWI > 0.
• Save the output as Water_Mask.tif.
• Inspect the result and adjust the threshold if necessary.

Example expression: (“NDWI@1” > 0)

Possible alternative thresholds for testing include NDWI > 0.1 or NDWI > 0.2. Choose the threshold that best separates the actual river from surrounding land.

10. Polygonize the Water Mask

• Open Processing → Toolbox.
• Search for Polygonize (Raster to Vector).
• Set Input raster = Water_Mask.tif.
• Set Field name = DN.
• Save the result as Water_Polygons.gpkg.
• Click Run.

The resulting polygons normally include both water and non-water classes.

11. Extract Water Polygons

• Open the attribute table of Water_Polygons.
• Click Select Features by Expression.
• Use the expression: “DN” = 1.
• Click Select Features.
• Right-click the layer → Export → Save Selected Features As.
• Save the output as Water_Only.gpkg.

12. Extract the Main River

• Turn on the Sentinel-2 image or False Color Composite as a reference.
• Zoom to the main river.
• Use the Select Features tool to select the river polygon or polygons.
• Hold Ctrl if multiple disconnected polygons must be selected.
• Right-click Water_Only → Export → Save Selected Features As.
• Save the result as Main_River.gpkg.

Check the extracted river carefully. Ponds, lakes, wetlands, and temporary water bodies may also be detected by NDWI.

13. Define River Proximity Classes

Zone	Distance from River	Flood Proximity Susceptibility	Example Score
1	0–250 m	Very High	5
2	250–500 m	High	4
3	500–1,000 m	Moderate	3
4	1,000–2,000 m	Low	2
5	More than 2,000 m	Very Low	1

These distances are suitable as a teaching example. For scientific research, distance thresholds should be justified using local flood history, river morphology, hydrology, and other evidence.

14. Create the 250 m Buffer

• Open Processing Toolbox and search for Buffer.
• Set Input layer = Main_River.
• Set Distance = 250 metres.
• Set Segments = 5 or use the default value.
• Enable Dissolve result if appropriate.
• Save as Buffer_250m.gpkg.
• Click Run.

15. Create the Remaining Buffers

Repeat the Buffer tool for the following distances:

Buffer	Output File
500 m	Buffer_500m.gpkg
1,000 m	Buffer_1000m.gpkg
2,000 m	Buffer_2000m.gpkg

At this stage the buffers overlap. The next step creates separate, non-overlapping distance rings.

16. Create Non-Overlapping Distance Rings

Open Processing Toolbox and search for Difference. Create the following zones:

Input Layer	Overlay Layer	Output
Buffer_500m	Buffer_250m	Zone_250_500m.gpkg
Buffer_1000m	Buffer_500m	Zone_500_1000m.gpkg
Buffer_2000m	Buffer_1000m	Zone_1000_2000m.gpkg

The 0–250 m zone is simply the Buffer_250m layer.

17. Create the Area Beyond 2 km

• Open Processing Toolbox → Difference.
• Set Input layer = Study_Area.
• Set Overlay layer = Buffer_2000m.
• Run the tool.
• Save the result as Zone_Over_2000m.gpkg.

This represents the Very Low proximity class within the study area.

18. Add Risk Attributes

For each proximity zone, open the Attribute Table and click Toggle Editing → Field Calculator.

• Create a new whole-number field named Risk_Score.
• Assign values from 5 (Very High) to 1 (Very Low).
• Create a text field named Risk_Class.
• Assign the appropriate class name.

Distance	Risk Score	Risk Class
0–250 m	5	Very High
250–500 m	4	High
500–1,000 m	3	Moderate
1,000–2,000 m	2	Low
>2,000 m	1	Very Low

19. Merge the Proximity Zones

• Open Processing Toolbox.
• Search for Merge Vector Layers.
• Select all five proximity zones.
• Choose a GeoPackage output.
• Save as River_Proximity_Risk.gpkg.
• Click Run.

20. Clip the Final Analysis to the Study Area

• Open Processing Toolbox → Clip.
• Input layer = River_Proximity_Risk.
• Overlay layer = Study_Area.
• Save as Final_River_Proximity_Risk.gpkg.
• Click Run.

This ensures that no analysis polygons extend outside the study boundary.

21. Symbolize the River Proximity Map

• Right-click Final_River_Proximity_Risk → Properties → Symbology.
• Select Categorized.
• Select Risk_Class as the classification field.
• Click Classify.
• Arrange the classes in the logical order: Very Low → Low → Moderate → High → Very High.
• Click Apply → OK.

Keep Main_River above the proximity zones in the Layers Panel so that the river remains clearly visible.

22. Create the Final Print Layout

• Click Project → New Print Layout.
• Enter a layout name such as River_Proximity_Map.
• Click OK.
• Select Add Item → Add Map and draw the main map frame.
• Adjust the extent and scale using Item Properties.

23. Add Essential Map Elements

• Map title: River Proximity-Based Flood Susceptibility Map of [Study Area]
• Legend showing the five susceptibility classes and the river
• Scale bar
• North arrow
• Study area boundary
• Data source information
• Projection / CRS information
• Optional inset location map

Example source note: Data Source: Sentinel-2 MSI imagery and study area boundary. Analysis: NDWI-based river extraction and distance proximity analysis. Software: QGIS.

24. Add an Optional Inset Location Map

• Add a second, smaller map using Add Item → Add Map.
• Set its extent to show the broader region, district, or Bangladesh.
• Display the study area prominently.
• Add a simple border and a short location label if needed.

25. Final Quality-Control Checklist

☐ The extracted river matches the actual river visible in Sentinel-2 imagery.

☐ All buffers were created in metres using a projected CRS.

☐ Distance zones do not overlap.

☐ The legend matches the risk classes shown on the map.

☐ The study area boundary is clearly visible.

☐ Labels and map elements are readable.

☐ The map title clearly describes the analysis.

☐ The data source and CRS are included.

☐ The final map has been checked at the intended print size.

26. Export the Final Map

• In the Layout window, click Layout → Export as Image.
• Select PNG for reports and presentations.
• Use approximately 300 DPI for high-quality output.
• Save as River_Proximity_Flood_Risk_Map.png.
• For a printable report, click Layout → Export as PDF.
• Save as River_Proximity_Flood_Risk_Map.pdf.

27. Complete Workflow Summary

Sentinel-2 Level-2A imagery

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Load B03 (Green) and B08 (NIR)

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Calculate NDWI

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Inspect values and select a water threshold

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Create Water Mask

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Polygonize raster

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Extract Main River

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Create 250, 500, 1,000 and 2,000 m buffers

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Create non-overlapping distance rings

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Assign proximity susceptibility scores

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Merge and clip to the study area

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Apply symbology

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Create Print Layout

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Export PNG and PDF

28. Important Scientific Note

River proximity is an important flood-susceptibility factor, but distance from a river alone does not represent complete flood risk. A comprehensive flood-risk or flood-susceptibility assessment may also include elevation, slope, rainfall, drainage density, land use/land cover, soil characteristics, historical flood extent, and exposure/vulnerability. For advanced research, the river-proximity layer can be standardized and combined with other factors using a justified weighting approach such as AHP or another multi-criteria decision method.

Calculate NDVI: Step by Step Tutorial