Contents preface (VII) introduction 1—37

4.11.2. Backwashing

Reversal of flow through the screen openings agitates the aquifer material, removes the finer fraction and rearranges the remaining aquifer particles. These effects usually cause effective

GROUND WATER AND WELLS

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development of the well. The ‘‘rawhiding’’ method of backwashing consists of alternately lifting a column of water significantly above the pumping level and then letting the water fall back into the well. To minimise the changes of sand-locking the pump, its discharging rate should be gradually increased to the maximum capacity before stopping the pump. During this process, the well is occasionally pumped to waste to remove the sand brought to the well by the surging action of this method of well development. As in the case of the overpumping method, the surging action may be concentrated only in the upper layers of the aquifer. Besides, the surging effect is not vigorous enough to cause maximum benefits. When compared with other methods of well development, the overall effectiveness of backwashing as well as overpumping methods in case of high-capacity wells is rather limited.