Formal and informal tertiary drainage systems

The question of whether formal tertiary drains would survive in a particular project situation may be debatable, but the need for such drainage, in principle, is not. There should be a route for out-flow of surface water from every field, whether by formal drainage channel or otherwise. The alternative to a formal channel is flow via natural topographic features, i.e. by natural channels or depressions where they exist or simply down-slope across fields where they do not. Local excavation or land shaping may be required to ensure unimpeded drainage. This apparent disregard for the classic pattern of formal tertiary drainage layout is explained by the realities of the South Asian smallholder situation.

Effective drainage must be provided at the tertiary level, but it must be sustainable, i.e. acceptable to cultivators as a permanent feature. Such acceptability will require provision of culvert crossings wherever drainage-ways intersect village roads or traditional bullock-cart routes. It is noted in this connection that introduction of irrigation into an area may double the annual run-off. Village roads, which in the course of time often become depressed below adjacent field level, may collect irrigation spill and become frequently impassable. The tertiary drainage system should intercept such run-off by construction of road-side ditches and regrading of roads where necessary.

The tertiary surface drainage system must be maintained by the cultivators as maintenance by the government agency would be impractical at this level. for this reason, the system must be designed in consultation with the cultivators. Furthermore, in situations where facilities not available to the cultivator are needed, such as additional culvert crossings or re-grading of a village road, the irrigation department should promptly cooperate.