A TREATMENT OF THE CAPILLARY FRINGE IN THE THEORY OF DRAINAGE

Abstract

SummaryWhen a drainage system copes with both artesian and precipitated water the streamline dividing the two flow zones passes through a stagnation point in the medial vertical plane parallel to the drain lines. The approximation is made that the water‐table and capillary fringe boundary may be regarded as those appropriate to precipitated water only, in the presence of an impermeable bed passing through the stagnation point. While the absolute heights may be in error it is shown that the partition of the capillary fringe about the water‐table location in the absence of a fringe is given with only small error. This partition is calculated from hodograph theory with the aid of Edsac 11 for combinations of three values of precipitation rate, seven values of fringe thickness, and nine values of depth of impermeable bed. By interpolation, curves are presented to show the partition for combinations of three selected precipitation rates, five depths of bed, and continuous variation of fringe thickness. It is shown that for high precipitation rates the greatest fringe thicknesses are accommodated almost wholly above that surface which would have been the water table in the absence of a fringe, while for the lowest rates of precipitation and zero bed depth only 30 per cent. of the fringe is so accommodated; the remainder is below.