Cellular systems implementing both rain fall flooding and signal propagation in a complex geography.
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This is a trace of flooding simulation, city of Brest, showing in blue spots a valley (Kerinou) that finish in the river Penfeld, bottom left.
In case of heavy rains, this place can be flooded up to 1 m of water.
See also https://www.google.fr/maps/@48.4021315,-4.4735296,14z/data=!5m1!1e4