Irrigation by means of canals with water flowing under the force of gravity is an agricultural practice normally associated with arid environments such as the American Southwest (Howard and Woodson 2018), coastal Perú (Huckleberry, et al. 2012), and the Middle East (Rost 2017). Switzerland is renowned for certain agricultural (e.g., dairying) and environmental (e.g., mountains) characteristics, but not for canal irrigation and aridity (Netting 1981). Nevertheless, the northeast-southwest oriented section of Rhône Valley upstream of Lake Geneva, and known as the Valais, has a sunny and dry climate, the result of being in the rain shadow of the high Pennine Alps that form the border with Italy to the south. This is Switzerland's largest wine producing area (Kahn 2011). Vineyards in the broad and flat bottom of this U-shaped glacial valley are irrigated using modern drip technology and melt-water from the Rhône River itself. The south-facing slopes, north and east of the city of Sion, are the driest part of Switzerland. Vineyards and meadows on these slopes have required irrigation by means of a series of canals dating back centuries (Reynard 1995). These canal systems, known as Bisse (pronounced "bis") are the topic presented here. They are not simple canals dug into the earth, but involve highly unusual techniques to transport water over, under, through, and around some of the most rugged topography on the planet. The Grand Bisse d'Ayent is offered as a case study. It was constructed in 1442, is 18 kilometers long, drops from 1,775 meters to 1,318 meters above sea level, and is simply stunning.
Google Earth Terrain View of the Rhône River valley--the Valais--and the dry south-facing slope of mountains northeast of the city of Sion, Switzerland (46°14'06"N 7°21'38"E). Roads with many switchbacks due to the steep terrain indicate the area characterized by bisses.
The Grand Bisse d'Ayent [red line] carries glacial melt-water directly from the Tseuzier Dam on the Liène River, which heads on the Wildhorn. Water flows through the bisse from June to October and irrigates vineyards and meadows in the municipalities of Ayent and Grimisuat. Source: Carte La Suisse Mobile.
Agriculture on the north side of the valley--on the south-facing slopes of the Berense Alps--requires steep terraces that are irrigated with water that is transported 5-25 kilometers by at least 20 canal systems, or bisse that are truly remarkable features on the cultural landscape. The earliest bisses were constructed in the 1400s, 600 years ago! They continue to function, albeit after numerous repairs and renovations.
Throughout much of their lengths these canals are like others elsewhere. They were excavated into the earth with a slight downhill gradient. They are for the most part unlined and have to be cleared periodically of vegetation growth, and undergo annual repairs due to bank erosion and sedimentation. The trail alongside the bisse was used historically only by a hired guard whose job it was to monitor and regulate the flow of water, and to prevent users from stealing more water than they were allocated (Baur 2019).
In some cases, canals are lined with metal, wood, or concrete that reduces water loss due to infiltration and makes cleaning and maintenance relatively easier than is the case with unlined canals. In this particular locale, tabular rocks line the canal bank on the outside of the curve to prevent bank erosion.
As evident in the previous two images, a 1- to 2-meter wide trail parallels each bisse. Today, these trails are used by hikers and other tourists as well as the guards. Cool, clear refreshment is made available to passersby where water is diverted out of the canals and into wooden troughs every few hundred meters.
Hiking trails alongside the bisses are well-signed, especially where they are crossed by paths and roads.
In addition to hikers and others who stroll along the bisses are local residents who need to cross the canals from time to time. Small footbridges were constructed to accommodate cross-canal pedestrians. These are not infrequent features on the cultural landscape.
The addition of paved roads and automobile traffic in recent times required that bridges 4- to 5-meters wide with heavily reinforced abutments be constructed over bisses.
In order to maintain a constant gradient of .025 percent, or a drop of 2.5 centimeters every 100 meters, and a maximum flow of 500 liters per second, canals traverse and cross sloping terrain that is quite challenging. Slopes are sufficiently steep in most places that one side of the canal is excavated into the slope (left) with the opposite side built-up with a wall of earth or concrete (right).
Some hard, rocky slopes along which canals were excavated result in nearly vertical rock faces. These rocks are sufficiently hard that falling debris is minimal to non-existent. Damage to the canal, therefore, is not a problem. Construction, of course, was labor intensive.
Earthen slopes in some places are easily erodible such that small landslides can block canals and prevent them from functioning--delivering irrigation waters to otherwise dry vineyards. To ensure this does not happen today, pipes are used in lieu of open canals in hazardous locales. In days gone by a great deal of shoveling was required.
In a few particularly problematic places canals cross vertical-sided gorges cut by tributary streams. Here, an aqueduct bridge on the Grand Bisse d'Ayent crosses one such gorge. Built of steel and wood, this obviously recent bridge has a steel canal substructure carrying water under a superstructure walkway. An earlier bridge at this locale was doubtless made entirely of wood.
The walkway on this aqueduct bridge has wooden side rails and steel grate flooring, allowing one to see the water flowing beneath.
In addition to crossing ravines and gorges, there are a few places where bisses and walkways pass through tunnels carved out of solid rock. The walkways are elevated above and next to the water, and have concrete surfaces.
The distance from the walkway to the top of the tunnel is approximately 1.6 meters, thereby requiring most people to walk bent over. A low handrail helps from falling into the canal while ensuring one stays bent over so as to not hit their head while passing through. Unfortunately, the lighting system does not always function as planned. This tunnel on the Grand Bisse d'Ayent was constructed in 1831.
Prior to 1831 this bisse did not pass through a tunnel but instead ran along the face of a cliff known as Torrent-Croix, more than 200 meters above the bottom of a gorge. Constructed of wood, this feature can only be described as a hanging canal. Clearly this is a place where no water has gone before 1442.
Construction of this hanging canal clearly required mountaineering skills. Not without consequence, the earliest account of such an activity was the ascent of nearby Mount Ventoux in 1336 (Petrarch 2023). Although no evidence exists as to how construction was carried out, it probably involved hammering anchors [A] into the rock face so that a rope (now a steel cable) [B] could be suspended above the proposed route of the canal. Suspended from this rope, workers then bored holes into the rock face followed by driving tapered wooden stakes, known as putlogs [C] into these holes. A canal made of wooden planks [D] was supported by these stakes. Once the canal was completed, the rope or cable served as a hand rail during maintenance and cleaning.
A close-up view of the downstream end of the hanging canal shows the wooden-plank canal [A] resting on the putlogs driven into the rock face [B]. The vertical sides of the canal are supported by tapered pegs called tenons [C] on both sides of the canal. These tenons are based in rectangular holes called mortises in the putlogs. Boards with mortises through which the upper ends of the tenons pass were lain across the top of the canal [D]. These ensure that the tenons and sides of the canal remain vertical. Joints between boards forming the canal were caulked before water was allowed through.
Today water in this bisse flows through a tunnel (right) whereas prior to 1831 it flowed through the hanging canal (left). Why did the builders not bore a tunnel in the first place? The answer lies in labor inputs and available technology. Hardened steel tools developed in the 19th century made tunneling easier than in the 15th century. If tunneling was considered during initial construction, it was decided against in favor of the less labor-intensive, but much more dangerous hanging canal.
A water wheel helps the bisse guard manage the flow of water. Flowing water [A] pushes paddles [B] thereby turning the axle [C]. As the axle rotates, a cam [D] pushes down the handle of a hammer [E], thereby raising and then dropping the head of the hammer [F] on piece of metal protruding from the side of the canal. The time between clinks, and hence the velocity of flow, can be measured with a watch.
If water in the canal [A] is flowing too fast, a sluice gate [B] can be opened to allow some water to flow out of the canal and down the natural slope. A sluice gate is merely an opening in the downslope side of the canal in which a board can be slid up and down in grooves built into the sides of the concrete sluice. In earlier days, prior to the use of concrete, farmers would use a shovel to break the earthen bank thereby allowing excess water to flow out of the canal. To stop this flow, they would simply shovel earth back into the opening in the canal bank.
The bisses of Valais, exemplified by the Grand Bisse d'Ayent, exemplify tradition--the transmission of customs from generation to generation. These fascinating features on the cultural landscape of Switzerland have been operating for more than 30 generations. The people who conceived and built these irrigation systems so many years ago faced some monumental problems during construction. The physiography was complex. Builders faced the challenges of having water flow along the contours of a steep rocky slopes, crossing steep canyons cut by tributary streams, and even hanging canals along vertical cliff faces several hundred meters above land below. Later users faced a variety of maintenance issues including bank erosion and sedimentation, landslides that required using pipes instead of open canals, and tunneling through bedrock so as to reduce the danger of maintaining the hanging canals. Most recently, bridges were built over canals to facilitate automobile travel, and the trails alongside the canals are now used more by touring hikers than water guards. To accommodate the latter, troughs provide fresh drinking water along the way. All of these complex construction and maintenance issues were addressed for one primary reason—to irrigate meadows and especially vineyards. The growing of grapes clearly required water going where it had not gone before.