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close this bookDesign and Operation of Smallholder Irrigation in South Asia (WB, 1995, 134 p.)
close this folderChapter 13 - Village schemes and small tank projects
View the documentBackground
View the documentFarmer-constructed diversion systems
View the documentVillage schemes with storage

Village schemes with storage

In view of the highly seasonal nature of rainfall in monsoonal regions, any irrigation scheme would benefit from reservoir storage, including small village schemes. However, construction of even a small storage (usually referred to as a "tank") is usually beyond the capacity of a farmer group and involves the participation of government agency. The farmers served by the system usually operate and maintain it, with minimum further government intervention. There are large numbers of such schemes in existence, some centuries old. The area served ranges from as little as fifty up to several thousand hectares. New construction of tanks continues and tank rehabilitation programs are also in progress.

The tank scheme has all the ingredients of a highly desirable field for participation of international financing agencies. It has a considerable element of self-help and farmer management, it is ecologically sound, and it undoubtedly improves quality of life in the village. Factors influencing the scope for further development of such schemes include the availability of sites suitable for storage construction and a number of financial and economic issues.

With regard to sites, key considerations are topography, which determines the cost of storage capacity, and hydrology. A typical location is in an open valley, combining the possibility of obtaining storage capacity at relatively modest cost of dam construction and a service area immediately downstream from the dam, partly in valley-bottom and partly on the valley slopes. Alternatively the reservoir may be located in the more steeply sloping head reaches of the valley, a supply canal extending down to a service area in the more gently sloping lower reaches.

The hydrology of a site is of interest in two respects, yield and flood flow. As there are rarely river-flow or rainfall records of long standing at small project sites, the estimation of seasonal yield and of flood-flow are likely to be quite approximate, as evidenced by the number of existing tanks which have never filled and others which have over-topped due to inadequate spillway capacity. There is need for further study of the hydrology of small catchments in semiarid areas and further installation of hydrological and meteorological recording stations in areas of prospective further tank development.

The spillway is often the major item of cost in a small tank scheme, particularly where several such schemes occur in series, down the length of a valley. The yield available to each of the schemes is then only a proportion of the total yield from the catchment, but the flood flow which has to be accommodated at each is the whole flood from the catchment, subject only to the small amount of flood regulation in upstream tanks. This can lead to disproportionately high spillway costs per unit of area irrigated, particularly for very small schemes. The problem is aggravated by the fact that conventional design requires founding the spillway and its outlet channel and terminal energy dissipating structures on sound rock, which is often at considerable depth in the topography common to small tank schemes. There is room for a less conventional approach to the design of small tank spillways, including greater use of flexible stone-filled wiremesh crib-work, which reduces the need for founding on bed-rock and the various means of protecting small embankment dams against wash-out in the event of overtopping, currently receiving considerable attention elsewhere.

The long-term hazard with small tanks is siltation. Many older tanks are operating at much reduced capacity or are virtually out of service, due to depletion of storage capacity by silt accumulation. Removal of silt from existing tanks by mechanical excavation is of doubtful economic viability. In some cases the storage capacity lost by siltation can be compensated by raising the crest of the dam, but only where the higher reservoir level would not present a problem of encroachment. The use of some form of automatic spillway gate, limiting the surcharge on the spillway crest during passage of floods, can convert flood surcharge pondage to active pondage, thereby increasing net storage capacity. The gates can be hinged wooden shutters arranged to fall, or to tilt, when the reservoir reaches a certain level. The shutters are reset after passage of the flood. More sophisticated automatic shutters are available, which conserve part of the flood volume, but sophistication is not a desirable feature in a small tank with minimum maintenance and maximum exposure to interference with the spillway structure. Increasing the effective length of the spillway crest by employing, in principle, the duck-bill weir arrangement previously described (the crest in this case has a zig-zig configuration, in plan) can also reduce the height of flood-rise, thereby increasing live storage capacity.

Reducing the rate of siltation is of course highly desirable, although not generally easy to accomplish. The catchment area may be cultivated lands, with high sediment run-off when heavy monsoon rains fall on newly ploughed fields or it may be in over-grazed deforested lands also subject to heavy run-off. Anti-erosion measures are available, but generally involve changes in land use or cultivation practices in the catchment area, raising questions of jurisdiction and recovery of cost. Much of the catchment erosion is commonly focused on local areas, such as deeply incised stream channels which are back-eroding in soft material. Remedial measures may be initially directed at such areas, which do not raise the problems of changing land use or cultivation practices. A more comprehensive method of erosion control involving contour planting of vetiver grass is being promoted by the World Bank. It has been particularly successful in some areas.

The concept of conserving excess monsoonal runoff at source, in a multiplicity of small pondages, is very attractive ecologically. The storage may be as small as farm ponds, the somewhat larger "seepage tanks" which serve solely to recharge groundwater for supply to immediately downstream dug-wells, or the regular irrigation tanks with associated small canal systems. Construction of farm ponds can be undertaken by the cultivator himself The spillway problem is minimized due to the very small size of the catchment involved, the whole storm runoff generally being accommodated in the pondage. Alternatively, a simple spillway channel, protected by appropriate vegetative cover, may be provided. The hydraulic head involved is small, and erosion during flood discharge is not a major consideration, nor is the stability of the very small embankment dam.

However, in moving up the scale to the conventional small tank the situation changes. Construction of an embankment dam by manual methods (head-basket or bullock-cart) is in some respects an excellent village enterprise, particularly during periods of "scarcity" when there is little alternative employment. However, many such dams have failed due to slides on the upstream or downstream slopes because of design deficiencies, unfortunate choice of fill material, or lack of compaction. Such incidents can be avoided by providing appropriate technical assistance to the villagers concerned, also with respect to spillway provision.

From the viewpoint of the international agency, a key issue in the construction of tank irrigation schemes is economic viability. This problem has already been discussed in relation to small direct diversion schemes. In the case of a tank scheme the availability of water storage permits higher crop benefits, but adds considerably to construction costs, particularly due to spillway works. Conventional economic analysis based on conventional cropping patterns, as would be applied to a major irrigation project, frequently indicates that a prospective small tank scheme is economically non-viable. However, such simple analysis does not do justice to the full range of benefits of a tank scheme. Particularly in a semi-arid area, a tank scheme is a catalyst for many village activities, often effecting a remarkable transformation from the pre-project condition. While the "quality of life" aspect of a tank scheme in such an area should not be ignored, such benefits are not readily evaluated in economic terms. There are much more tangible benefits which can and should be evaluated, in deciding the fate of a prospective tank scheme. The deficiency may not be in the scheme, but in the scope of the economic analysis.