Water velocity as a control of aquatic snails in concrete canal systems for irrigation
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.