Aquatic snails are the intermediate hosts of the disease schistosomiasis.
Irrigation schemes have often provided a favourable habitat for these
snails, but rapidly flowing water will carry them away.
The research established 23 stations in irrigation canals and monitored
snail numbers and water velocity at these points. A maximum tolerable
velocity for the snail Bulinus truncatus of 0.23 m/s was established,
corresponding to 0.13 m/sat snail height. Aquatic weed did not invalidate
this velocity, but the use of a single control velocity was found to be
over-simplistic. The long-term mean velocity was found to be the best
measure for snail control when velocity varied with time.
Using specially designed filter-traps, snails being carried along by the
flow were intercepted and counted. Attempts to relate these counts to
velocity revealed daily and seasonal cycles of numbers of snails detaching.
Snails in 130 syphon boxes were monitored and the number of snails compared
with water velocity. The maximum velocity tolerated here was lower than in
the canals. Turbulence theory was examined to explain the difference.
The fieldwork was largely carried out in the Tessaout Amont Irrigation
Scheme in Morocco.
Recommendations are made for the design, operation and maintenance of
irrigation schemes, if velocity is to be considered as a snail control.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.