Water Mills in Nepal

Himalayan society is primarily agrarian. Hydropower is widely available in this mountainous terrain, and water-powered mills are commonly used in agro-processing. According to Nepal’s Center for Rural Technology (CRT), over 25,000 of these water mills (ghattas) are operating throughout remote Nepal, with power output ranging between 200 - 500 Watts. Traditionally this power is only harnessed for the ghatta's sole purpose, grain grinding.

Ghattas are not limited to the Nepali Himalayas, but can be found throughout India, Pakistan, China, and even parts of Turkey. There are an estimated 200,000 traditional water mills in India alone (referred to there as a gharat, a chuksor, a panchaki or a chuthak.) Although the names are different, the technology itself is the same.

ghatta

Traditional Ghatta

  

Ghatta Operation & Construction

The fundamentals of a ghatta are fairly basic. Water is diverted from a stream or river and flows down a chute towards the ghatta’s turbine. The vertical shaft of the turbine runs up through the floor of the ghatta house and turns the top stone of a pair of grinding stones. Grain is stored above the stone in a suspended woven basket. It slowly falls out through a feeder mechanism at the bottom of the basket. The top stone has a hole carved in its center all the way through it. Grain from the feeder mechanism falls down through this hole, and is then ground between the pair of stones.

The grinding stones are always inset slightly below the floor of the ghatta house in a square depression. Typically, the top stone is a little above the surface, but the bottom is well below. This square depression, well defined by four wooden boards, keeps the flour from spilling out onto the floor. The ghatta owner (ghattera) scoops the flour out of this sunken "box" when the grinding is completed.

Water is typically diverted through a long (20-200 m) channel. Nearby the chute there is an alternative path for the water to flow when the ghatta is not operating. On the top of the chute is a gate that regulates water flow, prevents surface debris from entering, and serves as overflow protection. The chute is generally made of a hallow tree trunk, is placed at an angle of 40 to 50 degrees, and has a 3 to 7 meter head (vertical drop).

ghatta sketch

Layout of a Ghatta

  

The water flows down this chute and hits the angled 18x25 cm wooden planks that are the blades of the turbine. The main "shaft" of the turbine is a large piece of wood. The wooden plank-blades taper down to 6x6 cm pegs. These pegs are pounded into slots carved into the shaft. The slots are slightly angled relative to the vertical axis of the shaft, and thus the plank-blades are angled as well.

A long metal rod (something like a very large nail) is hammered into the top of the wood shaft. This extends the shaft from below the ghatta house, up through the floor, through a hole in the bottom grinding stone, and connects finally to the top grinding stone. Where it passes through the bottom grinding stone, small slivers of bamboo or other light wood surround it. These slivers act as a collar for the extended turbine shaft to rotate in. They are replaced by the ghattera as they wear down, and are sometimes oiled as well to keep the ghatta running smoothly.

Keyed Slot

Keyed slot in bottom of the grinding stone

  

The top of the turbine shaft comes to a square post. A metal "key" – a flat rectangular piece of metal – is manufactured to have a square hole in its center that exactly fits about the post. The top grinding stone has a rectangular inset carved into it. The key fits into this inset, and this is how the shaft is ultimately connected to the top grinding stone. The key is easily replaced as necessary, as it just lifts off the turbine shaft; it relies on the considerable weight of the grinding stone to keep it firmly in place.

A second metal rod is also hammered into the bottom of the wood turbine shaft. This rod comes down to a point, and it is upon this point that the turbine rotates. A point connection is a good way to avoid friction, and the turbine turns rather easily. The metal point turns either on a metal plate or small piece of stone.

This metal or stone plate is inserted into a long horizontal beam running underneath the ghatta house. This beam is attached to a vertical beam, which pops up through the floor a few inches outside the sunken box used to collect flour. Raising the vertical beam pulls the horizontal beam up slightly as well, which in turn raises the shaft and pushes the top grinding stone up as well. The height of the top grinding stone can thus be varied as the ghattera sees fit. It can even be raised up high enough to spin very quickly without touching the bottom grinding stone.

Ghatta Variability

Ghattas are made from local materials, so naturally each is to some extent unique. Although they have the features outlined above, the sizes or materials can significantly vary from location to location. The grinding stones can come in many different sizes, for example. Their diameters generally range from 24 to 34 inches, and the thickness can range from 3 to 10 inches (although we’ve heard of stones being up to 18 inches thick).

Features themselves are also never as regular as described, for they are most all hand carved or shaped. Thus the plank-blades of the turbine aren’t really rectangular, the hole through the center of the top grinding stone does not stay on axis, and the top grinding stone is not centered on the top of the turbine shaft (and thus wobbles back and forth as it rotates). Variation does not, however, impede the ghatta from grinding grain.

Ghatta houses vary with the building materials as well. They generally are constructed exactly the way neighboring houses are built, but have the added requirement that they must be partially suspended over the water pathway underneath. Usually this means digging out a trench underneath the house and then covering it over with large stones or beams of wood. We have seen instances, however, where the ghatta was built entirely on a raised wooden platform over the top of a stream.

ghattera

Ghattera with "home improved" turbine

  

The power output of the ghatta also varies not only from place to place, but season to season as well. Typically, however, the flow rate of water varies from about 40 to 100 liters per second. Ghattas are estimated by CRT to be less than 25% efficient. This efficiency can be improved upon, and the power output boosted, by replacing the wooden turbine with a metal runner. CRT has a program in Nepal to do just this, and their "improved ghattas" have operational efficiencies of 30-50%. We’ve also seen "home improved" ghattas where the individual ghattera has taken the initiative to replace parts of his system with metal and boost the efficiency himself. This is common only near the developed parts of Nepal and, for the most part, ghattas remain as they have for centuries.

Curiously, there is one feature that seems not to vary: the direction in which the grinding stone turns. Technically, there is nothing about the ghatta design that dictates which way it turns. The plank-blades could easily be angled in the opposite direction, and the water directed by the chute to strike the other side of the turbine. Our understanding, however, is that tradition dictates that the stone always turn counterclockwise. The small hand-grinders found throughout Nepal and other surrounding agricultural countries – called jnato in Nepali – are always operated with a counterclockwise motion. The ghattas, coming from this tradition, are constructed to operate in this manner as well.

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