Quote:
Originally Posted by Hill Walker
Maybe not. Near where I once lived there was a 'storm drain' which emptied into a local river estuary at below the level of the high tide. The drain was fitted with a one way valve to prevent the river feeding into the drain and also a large underground storage tank for water that could not be discharged when the tide was in. When this system was overloaded it used to blow off a manhole cover and produce a fountain which could be as much as six feet high. All this in the middle of an important road which would then flood and then have to be closed.
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That makes sense, the water in the river is high so it keeps the one way valve closed. Water flowing down hill into the underground storage tank then has nowhere to go so comes out of manholes. The cause is the drains being below the river level