Abstract
Wesseling (1964) stated that standing water above drains as a result of submerged outlets creates a radial flow in the vicinity of the drains which promotes flow conditions so that a smaller rise of the water table height midway between drains results. Wesseling (1979) concluded the same for standing water above drains as the result of too high entrance resistance. Standing water above drains may also be due to overpressure in the drains as a result of too small pipe diameter or to irregular drain slopes. With the exception of submerged outlets the resulting water table rise midway between drains is however in the same order of the water table rise above the drain as can be derived from theoretical analysis. This conclusion was confirmed by measurements at an experimental field where the standing water above drains, as a result of overpressure, and the water table midway between drains were monitored in a field located at the northwest of the Nile Delta. In spite of the low discharge rates, overpressure was observed in the drains. It was mainly attributed to irregular drain slopes. The analysis of field data showed that the water table midway between drains rises at least the same as the water table height above the drains. Since overpressure in drains causes a decrease of the dewatering zone, a careful and accurate installation is of utmost importance for the proper functioning of a drainage system.
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