Jeff,
Thank you very much for your reply! There are a few things I don't understand, I'm a basic user of ArcGIS. I have the DTM of the area and I could also digitize the river as a vector file. At the end of the river there is a culvert for which I have a point of 58m height at the bottom and a point above it on the bridge at 62m height.
What I'm trying to see is where the water will go if the level of the river will rise and goes over the top of the bridge, so over 62m. They told me I can reclassify my DTM in order to achieve that but I don't understand how to do it. I should end up with a shapefile, polygons etc showing where the water will go?
In the photo you can see the cuvert and the river during heavy rain; if the level of the river will go above that bridge, where it will go? That's what I'm trying to figure out, how to show that...
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Regards,
Stratos