The open canal is a common structure for conveying water. Piped systems may be more expensive but are better from the point of view of vector control because they produce virtually no vector breeding or vegetation growth. The greatest risk for mosquito production is expected in the minor distribution and drainage channels as they are more suitable for vector production than the larger canals. In addition, their maintenance is given less attention, particularly when it is no longer the responsibility of the irrigation authority, and is under the care of the users. These drainage canals are mostly poorly maintained and they have little direct relationship to increasing the average crop yield, but become excellent mosquito and snail habitats.
4.1 Favourable vector habitats
Slide C34 Water velocity: erosion vs snail breeding
Slide C35 United Republic of Tanzania, Zanzibar: seepage and conveyance water losses
Slide C36 Sudan: canal side breakage due to animal burrowing
Slide C37 Sudan: profuse weed growth, and too closely sited settlements
Slide C38 Haiti: backwater pools due to lack of proper drainage of an irrigation scheme
Disadvantages of earth canals favouring vectors:
• water velocities higher than 0.7 m/s are not tolerable because of erosion. The low operating velocities require large cross-sectional areas, thus a wide strip of land is flooded producing many vector habitats.
• high seepage and conveyance water losses result in waterlogging of adjacent land.
• danger of canal bank breakage caused by overtopping, erosion and animal burrowing.
• profuse growth of aquatic weeds retards the flow and causes heavy maintenance costs.
• Waterflow in drainage ditches is less uniform and constant than in irrigation canals. Choking vegetation and pools of stagnant water causes the waterflow to be even more erratic and more conducive to vector breeding. In these circumstances, problems of silting become more serious than those of erosion.
These disadvantages may be reduced or overcome by lining earth canals.
• Due to high costs of a lined canal, often water flowing from the impoundment to the irrigation plot travels several kilometers through a natural stream. The storage regulation causes continous slow to rapid flow, forming backwater pools, marginal pockets and isolated seepage ponds.
4.2 Environmental management measures
• Use of pipes instead of open canals especially for drainage (e.g. burried coconut shells as drainage).
Slide C39 Malaysia: burried coconut shells as drainage
• Canal lining: from the viewpoint of vector control, the main advantages of canal lining, especially with a hard surface, are:
i) increase of water velocities, thus preventing stagnant or sluggish water. However, this may introduce vectors of onchocerciasis in parts of Africa and S. America,
ii) reduction of weeds when properly maintained,
iii) reduction of the needs of drains, because seepage is less.
Slide C40 Egypt: canal lining
Slide C41 India: seepage and water velocity problems at the transition from lined to non-lined canals
• Canal flushing and periodical drying of canal: snort periods of increased water velocity may dislodge and expose vectors, stir up bottom sediments burying mosquito larvae, and lowers the invasion of the marginal vegetation that reduces water velocity. This is feasible where there is plenty of water, otherwise might not be advisable. The use of automatic syphons in Malaysia has been shown to be most useful for this purpose.
Slide C42 Malaysia: canal flushing using an automatic syphon
Slide C43 Malaysia: canal flushing using an automatic syphon
• Effective canal maintenance to ensure that the canals are in good shape and generally free from vegetation and silting at all times.
• Resettlement and settlement of population: As not all natural streams and long parent canals may be lined, the same requirements are suggested for the location of settlements and the sanitary facilities as in the “impoundments” section.
Slide C44 Thailand: effective canal maintenance with community participation
Slide C45 Morocco: washing in the canalettis is more confortable than in the river