Effect of Drain Tube Openings on Water-Table Drawdown

Conventional methods assume that the drain tube is completely open and offers no resistance to the entry of water. An approximate method of determining the effect of drain tube openings on water-table drawdown is presented, based on the use of Hooghoudt's equivalent depth to account for convergence near the drain during water-table drawdown. Data from the literature are used to define an effective drain tube radius, r_e, for drains with a finite number of openings. The r_e value is then used to define the Hooghoudt equivalent depth which is in turn used in solutions to the Boussinesq equation for water-table drawdown. The use of an envelope wrap material will permit a somewhat wider drain spacing for all cases with a larger increase in spacing for the deeper profile. The allowable increase in spacing for both cases was relatively small and the total system cost with the envelope wrap was about 6% higher than without the wrap for the shallow profile and about 7% lower for the deep profile.