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the flood water had surrounded the village Erftstadt, eventually it found the way to the brown coal mine pit, building up hydrolic pressure

the moving underground soil water accelerated by the sheer mass of it, further weakening the soil texture. so the soils started gliding and calving in

video of the canyon site


My idea: this is swamp area, which produced the brown coal over millenia. The route of the stream Erft had been deviated so the caol could be mined.
But the civil engineers used an unsound rule: the probability of a 100 year flood.
They should have asked advice of geologists; or scientists with experience in long period events

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After this a dam failed in the area; this happened recently so a video first, info will follow later

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what happened the last 48 hoursin Germany:
It's in German, but the images make irt worthwhile

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this is a compilation of the floods in EU, mostly shot in Germany

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To put things into perspective:
I believe the top 3 in Germany was 278, 220 and 218 mm/ m² per day.
In reality most of it fell in about a 2-3 hour period.

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instructional vid from Switzerland:
a local 1 meter wide stream, on a moderate steap terrain, widens to 12 meters in seconds
models tend to underestimate the water levels, cos of sediments, debris and rocks are mixed up in the mud
thats why in the lower part of a decent stream dredging the sediments is very important