|Design and Operation of Smallholder Irrigation in South Asia (WB, 1995, 134 p.)|
|Chapter 11 - Construction and maintenance problems of drainage works|
The function of the secondary and the primary drainage system is to act as an outfall for tertiary drainage. Nothing inhibits cultivator interest in maintaining tertiary drainage more quickly than having secondary/primary drainage channels back up and flood his fields, submerging the tertiary system. This may be inevitable in some circumstances, but it certainly should be exceptional.
The factors which determine the effectiveness of the primary/secondary drainage system are topography, wet-season water levels in the river into which the primary drains discharge, intensity of precipitation and rate of run-off from the area, and design and condition of maintenance of the primary/secondary system. Only the last item is controllable. Very high levels in the main river system, backing up water into the irrigated area, are not subject to manmade intervention unless by major upstream flood-mitigation storage. Extensive flood embankment construction and possibly major drainage pumping from behind embankments may be considered, but are outside the scope of the present discussion.
Very flat topographic gradients add to drainage design problems, requiring relatively large channel sections. These can be calculated, however, provided that the rate of precipitation and run-off are known. These are the least predictable elements. No economically viable drainage system could accommodate a maximum possible storm. The task is to devise a primary drainage system whose cost is commensurate with the value of the agricultural production, or the physical facilities, which the system protects. This approach acknowledges that the capacity of the system will sometimes be exceeded and damage will result. The usual procedure is to decide upon a certain appropriate magnitude (and frequency) of storm and to design the drainage system to limit the period of inundation of crops during this event to a predetermined figure. It is emphasized that the occurrence of a more intense storm will result in longer period or greater depth of inundation of crops, and proportionate increase in loss of yield. This, however, is a marginal increase in loss. Of greater concern can be the damage to physical structures which may result from a storm greater than the "design storm", or greater rate of run-off than anticipated. A culvert through a railroad embankment may wash out, destroying the line, or breaching a main irrigation canal. Such structures should consequently be designed on much more conservative assumptions as to storm magnitude than in the case of agricultural damage. The incremental cost of providing additional capacity is local only, confined to the structure in question.
Frequently, a primary drainage channel has two separate functions, both influencing its design. The first relates to the outflow of the small but important amount of ground water seepage flowing into the secondary drains through much of the year. The key requirement here is to keep the bottom of the primary channel at as low a level as possible, as this level may control the elevation of the watertable in the area being drained by the secondary drains. The emphasis in channel design from this viewpoint is on depth rather than capacity. The second function is evacuation of surface flows, either irrigation spill or more particularly runoff from heavy rainfall. In this case, t he emphasis is on channel capacity, and bed elevation or channel depth are secondary considerations.
With regard to maintenance, both depth and width present particular problems. The wet ground conditions at the bottom of a drainage channel in which seepage water is flowing year-round promote heavy growth of phreatophyte plants, requiring frequent cleaning. In such situations an ideal, conceptual design would be a composite section with a relatively wide upper portion providing capacity for major storm run-off, and a central narrower, deeper, section providing for seepage outflow. This would minimize the extent of the perennially wet portion of the channel. An inverted near-triangular section can provide the same function, the apex being the narrow deeper portion of the water way.
If mechanical equipment is to be used for maintenance, the problem with channel width is the reach required, particularly if access is available from one side of the channel only. The equipment commonly employed is some type of hydraulically-operated back-hoe, either wheel or crawler-mounted. The reach of such equipment is dependent on its size and weight. Reach has recently been extended by the use of counter-weights, and by the introduction of an ingenious combination of cable and hydraulic actuation amounting to a combination of back-hoe and drag-line. However, there remain problems in procurement of appropriate equipment for drainage maintenance. A contributing factor is that all drainage channels are not of ideal cross section. Many are natural stream-channels, deepened or widened for project purposes. Such channels commonly meander through irregular terrain, are frequently joined by tributary streams which have to be crossed, and are distant from roads. Equipment required in such circumstances should be capable of off-road travel in difficult and often very soft ground conditions. It should also be capable of travel on conventional roads, preferably without the use of transporter. Finally, it should have longer-reach capability than currently provided with medium-capacity back-hoes. Equipment meeting these requirements is not yet commercially available, although it is within the capability of manufacturers.
An issue in some main drainage systems is whether to permit alternative uses of the channel in the non-flood season. This includes pondage of water for fish culture or small scale lift irrigation of specialty crops (usually vegetables) adjacent to the channel. In other circumstances, the channel is made shallow and wide, and is used for cultivation of paddy or of "floating" rice in the wet season, the crop being temporarily inundated during flood-flows. Against such practices it can be argued that it is difficult enough to prevent obstruction of drainage channels without officially encouraging it. However, provided that the level in the channel does not have to be held down in the non-flood season for watertable control there is no technical reason why the alternative use should not be permitted. In any case it is likely to be practiced unofficially. Such use could be more effective if formal bottom-weirs were installed, closed for impondment of water in the dry season and opened for unimpeded flow in the wet.
A related practice which has been developed on a pilot scale, in secondary drains, amounts to organic control of phreatophytes, particularly a variety of bull-rushes (typha) which is particularly troublesome in many area. Typha can be displaced by pare-grass, a virile plant which grows well in a wet environment forming long trailing stems which may be harvested for cattle-feed. Harvesting is accomplished simply by pulling on the stems from the bank, the stems breaking at lower nodes. Such a practice requires organization to ensure that the drainage channel is reasonably clear of pare-grass during the wet season, when its capacity is needed for drainage outflow. Typha may also be displaced by rice, the plant being sensitive to a toxic compound produced in the root system of the rice. However, conditions in the drainage channel must be maintained favorable to rice cultivation.
A very difficult drainage channel maintenance situation is encountered in some highly erosible clay soils. It is almost impossible to maintain a conventional channel section in such soils due to rapid erosion of the channel side-slopes in heavy rainfall. One expedient is to line the channel. This, however, also poses problems as the clays are expansive, a difficulty which can only be overcome by over-excavation and back-filling with non-expansive material prior to lining, a costly procedure for drainage channel construction.
A final example of difficult drainage is the problem of "quick" conditions, in which a channel excavation is rapidly refilled by upward flow of material from the bed. The conditions make either open drainage or closed (pipe) conveyor very problematical. The situation usually occurs in low-lying perennially wet areas, fortunately usually local. A solution consistent with modern environmental views could be to reclassify such a location from a drainage problem area to an ecologically valuable wet-land, and to leave it untouched.