Renewable Energy Resources for Irrigation - Practices of Irrigation and On-farm Water Management - page 505

measure of flood control; large mill complexes may have dozens of sluices con-
trolling complicated interconnected races that feed multiple buildings and industrial
processes.
13.6.3.2 Types
Watermills can be divided into two kinds:
(a) one with a horizontal waterwheel on a vertical axle
(b) the other with a vertical wheel on a horizontal axle.
The oldest of these were horizontal mills in which the force of the water, striking
a simple paddle wheel set horizontally in line with the flow, turned a runner stone
balanced on the rynd which is atop a shaft leading directly up from the wheel. The
bedstone does not turn. The problem with this type of mill arose from the lack of
gearing; the speed of the water directly set the maximum speed of the runner stone
which, in turn, set the rate of milling.
Most watermills in Britain and the United States had a vertical waterwheel, one
of three kinds: undershot, overshot, and breast-shot. The horizontal rotation is con-
verted into the vertical rotation by means of gearing, which also enabled the runner
stones to turn faster than the waterwheel.
13.6.4 Tide Mill
A different type of water mill is the tide mill. This mill might be of any kind, under-
shot, overshot, or horizontal but it does not employ a river for its power source.
Instead a mole or causeway is built across the mouth of a small bay. At low tide,
gates in the mole are opened allowing the bay to fill with the incoming tide. At high
tide the gates are closed, trapping the water inside. At a certain point, a sluice gate in
the mole can be opened allowing the draining water to drive a mill wheel or wheels.
This is particularly effective in places where the tidal differential is very great, such
as the Bay of Fundy in Canada where the tides can rise fifty feet, or the now derelict
village of Tide Mills in the UK.
Other water mills can be set beneath large bridges where the flow of water
between the stanchions is faster. At one point, London Bridge had so many water
wheels beneath it that bargemen complained that passage through the bridge was
impaired.
13.6.5 Exploring the Potentials of Water Power
Hydropower provides a developing economy with opportunities to develop appro-
priate technologies. One of the first things a country can do is to assess its
opportunities for developing alternative energy sources. Hydropower in developing
countries has the following distinguishing characteristics: sustainability, depen-
dence on local resources, cost effectiveness, durability, flexibility, simplicity, ability