Tag: mountain

Rural alpine natural water system

Rural alpine natural water system

By |

I visited a low-tech mountain hut during a walking tour in the Alps in France. At 1500m elevation, it can only be accessed via cable car and then a mountain walk, and it is isolated from both electricity and water networks. This means the only power comes from gas, used for cooking, and wood, burned for heating. Water must be obtained from a nearby stream, yet I found the system to be surprisingly sophisticated.

To check it out, I walked about a hundred metres from where the hut stands, to locate the stream. Basically, the system consists of two plastic tanks, each holding about 100 litres of water, sitting in the stream. Water enters the first tank directly from the stream – it is fresh as it has come straight from the glacier higher up. This tank contains a filter to catch any sediment; at present, this is improvised with an old stocking.

The water that passes through the filter is led via a plastic hose to the second tank, which is a couple of metres down into the valley and is a storage tank: when supply exceeds demand, some water can be held here until it is needed on drier days. An overspill pipe attached to the top allows water to run safely back into the stream if the tank is full.

The main pipe leads from the storage tank, downhill to the hut, where it can be used cold or heated with gas.

Some calculations for quantifying the system

^ situation 1
^ situation 2

We can examine two situations; the first, where the second is not in overspill so nothing flows through location 6; the second where it does. Bernoulli’s equation is useful here. It is an energy balance equation and states that [pressure + kinetic energy + gravitational potential energy] = constant in a flow system, as long as the mass flow rate is constant (there are a couple more technicalities, but I won’t dwell on them here). Bernoull’s equation allows us to consider only start and end point, ignoring what happens in between.

Where p is pressure, rho is density, v is velocity, g is gravitational acceleration and h is height from a fixed point.

Also, with a constant mass flow rate (and incompressible fluid) we can assume:

where A is pipe area.

Situation 1: At both 5 and 1, the density of the water is the same and the area of the pipe is the same. Therefore the velocity of the water entering the house is the same as the velocity of the water from the stream. To calculate the pressure change, we can cancel the second term in Bernoulli’s equation (since velocity and density don’t change), giving p1 + (rho)(g)(h1) = p2 + (rho)(g)(h2). Rearranging gives p2 = p1 – (rho)(g)(h1-h2).

Sticking into this that p1 = air pressure = 1 bar = 100kPa and h1-h2 = 30, p2 is 3.94 bar.

French alpine viaducts – the old and the new

French alpine viaducts – the old and the new

By |

The A40 motorway in southern France is a busy highway through the mountains that provides the most direct route from Geneva to the mountain resort of Chamonix and surrounding towns. Together with a railways along a similar route, they are used all year round by both French and international tourists and local people; the area is famous for skiing, mountain hiking, climbing and local French culture – so the roads and railways along this route are essential.

Two prominent and impressive viaducts caught my eye as I drove down the motorway. For each, I sketched a front-on, fine-lined shape and a more visual sketch of what the bridges actually look like when you’re driving on the road.

First, a modern, slender concrete structure – the Viaduc des Egratz de Passy. This one is part of the westbound A40 motorway.

^ see the column cross sections at the bottom of the drawing

Viaduc des Egratz de Passy, 1981 (road)

  • The (presumably reinforced) concrete posts are generally rectangular, with their short side aligned with the length of the motorway, with one exception. The column on the right of the drawing above is hexagonal instead – the reason is not clear, but it may be required because of the harsh bend at that point on the structure.
  • The deck does not appear to be simply fixed straight on to the columns – at a glance, it looks like it is levitating slightly. This is probably because of the damping system between the deck and the columns. Allowing some small damped rocking, rather than rigidly fixing the two together, helps the structure deal with the vibrations of the road without sudden plastic collapse or fast fracture of the joints.

Second, a more traditional, heavier-weight arch design – clearly from a much older era – the Viaduc de Saint-Marie.

Viaduc de Saint-Marie, opening date unclear (rail)

  • Straight, sturdy columns form the bottom section. Semi-circular arches have been utilised for structural stability in the top section.
  • The main material is masonry – probably stone masonry by observation.
  • Arch bridges are excellent at dealing with the continuous vibrations of railway traffic without the need for external damping systems (which were likley not developed at the time of construction).
  • The project would have been advanced for its time, fitting in with such an uncertain landscape – not to mention massively expensive as it would have been build by human power alone. Impressive!