The British Geological Survey have carried out a study to determine the optimum design for 2-cusec irrigation wells in Bangladesh. This wide ranging study has included field studies, laboratory measurements, computer simulations and economic comparisons.
One of the aims of well design is to ensure that water is delivered to the irrigator as cheaply as possible. The cost of water is dependent both on the capital cost of the well (well materials, drilling costs, pump installation, etc.) and on recurrent pumping costs. A large diameter, initially more costly, well will reduce well losses and long term pumping costs so there is a fine balance to be struck between capital and running costs if the optimum well design is to be selected. Well-running costs are closely linked to well discharge and drawdown. To date, no accurate method exists of predicting these factors, so well design has been, to a large extent, based on experience.
A number of irrigation wells of different designs have been constructed in Bangladesh and pump tested: both long-term and step-drawdown tests have been employed and, in addition to drawdowns, flow rates within the wells were monitored. Figure 1 shows one of the wells being tested and Fig. 2 shows the results of one step-drawdown test.
The hydraulic characteristics of over thirty types of well screen, with diameters ranging from 4 to 12 inches, have been measured in a specially designed test rig which was constructed at Hydraulics Research Limited for the British Geological