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The success of any system of artificial drainage depends upon there being a suitable outlet for the water, and this is often difficult to find. Sometimes, underlying impermeable rock or clay, there is a permeable drift of gravel or sand. Then the main drain may be discharged into a pit or well. If the water in the pit rises above the level of the drain it must be pumped out. In suburban allotments an outlet is often available into a street drain or a
channel.
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