I've been asked to produce a risk map at work which will show the possible flood route from sewers that surcharge (flood) during heavy rainfall. This is basically where I know the x-y coordinate (OS National Grid Ref. as I'm in the UK) of a number of manholes we know sometimes flood, and I have a 2 metre x 2 metre Digital Elevation Model (Raster) from the UK Environment Agency that covers the area in question.

Basically I need a method that looks at the elevation at the point each manhole occurs at, and then looks at the elevation of each raster square around that point. If a square is at the same or lower elevation then assume that square is at risk of flooding, and for each of THOSE squares, look around again and identify further squares at the same or lower elevation. Keep going until there are no more directly connected squares at the same or lower elevation, and this is the total area that could be at risk.

It doesn't need to account for volumes or the speed at which this might happen, just literally all connected raster squares at the same or lower elevation as the origin point.

I know how to use QGIS fairly well and it's all I have access to at work.

  • @Kazuhito Maybe this is a really stuppid question, but what are manholes exactly? I only have a DEM of the area. I want to look at the floodings from the Brahmaputra river in the Barpeta district in India. So I have the DEM of the Barpeta district, but what is my next step?
    – Hiske
    Dec 16, 2020 at 8:36

1 Answer 1


It is called Lake Flood.

I recommend SAGA GUI, but if it has to be done in QGIS, you can find it in the Processing Toolbox | SAGA | Terrain Analysis - Hydrology | Lake flood.

Your manhole points has to be rasterized to be a Seeds raster beforehand. Please make sure the dimension (cell size, extent) of the Seeds raster is the same as your DEM.

Lake Flood itself is simple and robust. Making the Seeds raster to align with DEM is rather hard. QGIS 2.18 would be more easier than QGIS 3 to handle this, using existing layer as the base raster.


(1) Data loading

Start SAGA and load your DEM and manhole shapefile. To do that, just drag and drop your files onto SAGA Window.

(2) Rasterizing

  1. Select Shapes to Gridtool. In the Settings 2. Select your manholes in the Shapes and 3. the height field. Then 4. set Target Grid system (select grid or grid system). Click on 5. Apply to confirm the settings and 6. Execute to run the process.

enter image description here

(3) Lake Flood

  1. Select Lake Flood tool. 2. Set Grid system, and 3. Select your DEM and Seeds raster (the one you have just created). 4. Check on Absolute Water levels so that SAGA thinks your manhole height is above sea level. Click on 5. Apply to confirm the settings and 6. Execute to run the process.

enter image description here

(4) Final check

Click on Data tab and see if you have your original DEM and final output Surface data. Double click to display them on the map window.

enter image description here

(5) Export as GeoTiff

Import/Export > GDAL/OGR > Export GeoTIFF

  • Hi...thanks for the reply. I have found the Lake Flood tool in QGIS but haven't yet tried to run it, as I'm not sure how I'd rasterise the seed point. I was looking at SAGA GIS though (your post is the first time I've heard of it). Would it be easier to do in that?
    – user88596
    Oct 13, 2018 at 16:28
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    Hi @TheRobster Yes, to your first comment. Let me add an example working in SAGA. To the second, not necessarily. (You can give mean of these points, or maybe you would prefer to select more elevated manhole. Your choice.)
    – Kazuhito
    Oct 13, 2018 at 16:54
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    @TheRobster Sorry, I am confused now. Your original question is quite accurate description of a lake flood operation. Your latest comment suggests you need channel network or drainage. They are different algorithms. You may have to edit your original post to clarify your intention better.
    – Kazuhito
    Oct 13, 2018 at 17:23
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    @TheRobster Or, perhaps you can post a new question. A drawing will be helpful.
    – Kazuhito
    Oct 13, 2018 at 17:32
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    Got that, thanks @TheRobster To build channel network from point sources, please look into Terrain Analysis | Channels | Channel Network. You will be able to use the rasterized manhole data as Initiation Grid.
    – Kazuhito
    Oct 13, 2018 at 17:44

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