DRAINAGE OF UNDULATING SANDY SOILS WITH HIGH GROUNDWATER TABLES .2. VARIABLE HYDRAULIC HEAD RATIO

In a preceding paper a new drainage formula has been given, the derivation of which was mainly based on an analysis of the question of how much groundwater flow depends on the permeability of the underlying ground and on the often rather complex geometric properties of the channel network in the drainage basin considered. In this paper due consideration has also been given to the open-water flow, requiring the investigation of other hydrologic properties, such as the average relations between drainage area, channel gradient, roughness coefficient and channel size. The hydraulic head ratio between groundwater flow and open-water flow strongly depends on the bottom depth of the drainage channel and on the depth of the phreatic surface. By substitution of an adequate expression for this hydraulic head ratio into the basic drainage formula the final expressions could be improved, especially regarding the distribution of the groundwater discharge to the large drainage channels. For wet conditions this is not of major significance on the relation between total discharge and groundwater depth. © 1978.