In addition, the water in your water pipes and toilet tanks (not the bowls) is drinkable if you closed the valve on the main water line before the flood.

此外，如果你在洪水到来前将主水阀关闭的话，那么水管中的水和厕所水箱(不是便池)里的水是可以饮用的。

youdao

In order to stop flood waters overwhelming central London and to save the massive expense of building fresh flood walls and defences, some land could be set aside to act as a safety valve, catching flood waters and storing them until the danger has passed.

BBC: London's drowning