As the bottom of the ponds will be above the highest ground water fluctuations, there is no danger of reverse hydrostatic pressure deforming the liner. There may be some slight deformation caused by continued subsidence of the subgrade. PVC has a high degree of elongation however and, as the largest part of the soil subsidence should be over by the time the liner is installed, excessive elongation is not felt to be a problem.
PVC membrane does not have good resistance to continuous sunlight and wind exposure. Therefore, a protective soil layer of 30 cm will be placed over the lining. This will also prevent damage to the liner by aquaculture operations and personnel. A problem may present itself in the erosion of this protective soil cover by wave action. Although the soil used is near its natural angle of repose, it will be noncohesive sandy material and easily eroded.
Several methods are suggested as means of reducing potential wave erosion if found to be excessive. These include:
The use of an inexpensive polythylene film skirt, or overlay, secured on the top of the dike and extending on top of the protective soil down over the wave zone to about 30 cm below the normal pond water level. This skirt will be weighted at the bottom by sand or other material secured in a continuous pocket formed by turning up the bottom edge of the film and sealing it with adhesive over the weight. Although this film does not weather well, it is inexpensive and readily available through the Ministry of Commerce and Agriculture, as it is procured in several weights in quantity to provide farmers with an inexpensive irrigation ditch liner.
The use of flat rock rip-rap to cover the wave zone and for a short distance below normal wave action level. This rip-rap will be laid over the protective soil cover to a thickness of about 15 cm. The stones will be dry-laid with no mortar.
The use of a woven polypropylene fabric cover, or skirt, over the protective fill in the wave zone. This material is used extensively to slow river bank erosion and may have a fish pond application.
The various protective methods will be installed on sections of the dike to enable a comparative study of their effectiveness versus the ability of the nature soil cover to withstand wave action. Because of the relatively small size of the ponds and their orientation, with the long dimension perpendicular to the most important prevailing wind, the fetch is not great and large waves will not develop. Wave erosion could become more of a problem on larger ponds, however.
As an alternative to a PVC liner protected by soil, thought has been given to the use of a Hypalon (reinforced chlorosulfanated polythylene) liner without a soil protective cover (See Appendix 3). Hypalon is resistant to wind and sun and can be made non-toxic to fish. The cost per square metre is greater than 15 mm PVC, but is less when it is considered that the Hypalon liner is 30 mm thick. By elimination of the protective soil cover the final costs are comparable 1:
|PVC installed with soil cover||BD 2.51/m2 US$ 6.78/m2|
|Hypalon installed without cover||BD 2.57/m2 US$ 6.94/m2|
|(See Cost Estimates, Appendix 4)|
The US Fish and Wildlife Service reportedly has used this material and they will be contacted after the consultant returns to the USA. Also, further investigation into the erosion of liner earth coverings at the water line will subsequently be carried out on pond operators' experience. The results and further recommendations will be forwarded to the Directorate of Fisheries and to FAO.
1 US$ 1.00 = BD 0.380 (September 1983)