Vol. 3
No.
6
December 2001
Swiss rationality
Valais, a mountainous state in Switzerland faces extensive dry spells along with high evaporation. This leaves a water deficit of 500 millimeter per year (mm/year) to 700 mm/year. However, the Swiss are rationally maintaining their traditional mountain irrigation system – bisses, to meet this shortfall. Bisses have been constructed by local communities, who share a limited resource relationship. This system is a rare example of an indigenous irrigation technology operating in a developed world context.
BissesBisse is a multi-functional irrigation system, providing the best opportunity to maintain sustainable livelihoods in Valais. They are generally slope off-take systems which divert glacial meltwater rivers, thus ensuring reliable discharge during the irrigation season. Multiple sources are commonly used in non-glacial bisses or long bisses (for example, 5-32 kilometre (km), which can cross several watersheds. They usually consist of a headwork structure, a main conveyance channel and secondary as well as tertiary distribution channels, constructed down slope, taking the advantage of different ecological niches. At some places, they may also have reservoirs.
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| Figure: Diversion angles on a typical head work |
The head works of bisse, diverts or traps water usually from a river at a convenient location. They can be anything from a simple dam to a sluice gate system (an arrangement to regulate the flow) containing sediment trap and an overflow facility. The size of the structure and the materials used in construction depends on the desired life span and the erosional nature of the river. Concrete is the most favoured construction material. The diversion angle, (angle with respect to the direction of flow of source), at which the head work is placed tends to flatten as the velocity and erosional characteristics of the meltwater river increase. (see Figure: Diversion angles on a typical head work)
Principal channel widths range from 0.30 metre (m) to 1.5 m and depth range from 0.2 to 0.8 m. Most channels present a large surface area that helps in keeping the water warm over long distances. Conveyance channels may be upto 32 km long and maintain low average slope percentages between the head work and end of the bisses. All these channels are laid at a slope, with 85 per cent of bisses having average slope less than 10 per cent. Longer bisses required natural (eg waterfall/tributary stream) or artificial drops (eg sluice system/cascade), which are thoroughly regulated to stop erosional damage and bank rupture.
Most of the channels were made on the ground by excavating the earth. Vegetation and excavation debris from construction is used to reinforce the outer banks and provide a walkway. Recently constructed channels are often sprayed with concrete as reinforcement. Artificial channels, made of wood or poly vinyl chloride (PVC) pipes, are needed in unstable and permeable areas. Many modern channels have a dual function, carrying both drinking and irrigation water to the communities.
Water distribution systemDistribution of water from these channels can be by gravity or under pressure (spray irrigation) often by means of an underground irrigation network.
Traditional gravity flow technique is primarily a form of border irrigation, in which distribution is carried out from secondary or tertiary channels and less often from the main canal. Tertiary channels vary in length from about 1-100 m depending on field size.
The responsibility for distribution is entrusted with the farmer or an elected official. However, where the topography is favourable, distribution can be left completely unsupervised for long periods (2-3 hours). The water is either diverted through a sluice or the channel is dammed with a piece of wood, metal, or stone so that water overflows the banks of the distribution channel. This will continue until the required area is flooded. At this point, the stone is removed and placed further along the distribution channel allowing the irrigation of another section of the field.
Spray irrigation, which became available at the start of the 20th century, was introduced as a more economic distribution method than the gravity flow method. This system never requires a pressure of more than two atmospheres, which can be obtained from a 20 m drop. The sloping land of the Valais produces a large natural pressure gradient that often requires reduction and regulation to ensure a uniform distribution of water. Spray irrigation allows more simultaneous use than gravity, which is important for taking commercial crops.
Bisses demonstrate longevity, a lack of long-term irreversible environmental degradation, and a degree of intra – and inter–generational equity, which are essential characteristics of a sustainable system. Modernisation and rationalisation of the bisses have led to increased channel capacities, the covering of once open channels, the reduction of water losses, and shortening of conveyance lengths, usually through tunneling projects. The successful design criteria of bisses were matched to the unique and often harsh environmental conditions found in different communities.
Source: Crook, Darren and Jones 1999, Design Principles from Traditional Mountain Irrigation Systems in the Valais, Switzerland, in Mountain Research and Development, Vol. 19 (2), pp 79-99.