2.1.4 Hydrological characteristics
The most important data needed for site selection can be gathered from such sources
as Irrigation Departments or other Water Authorities. The following are needed:
data for discharge, yield, floods and water elevations of existing water
sources (rivers, irrigation channels, reservoirs, springs, etc.).
2.1.5Soil characteristics
Field investigations to determine surface and sub-surface soil conditions at the site
should be made as early as possible. Often money can be saved if proper soil
explorations are made before the site is procured. They may reveal soil conditions
undesirable for pond construction, in which case another site may have to be found.
Investigations should be carried out in order to ascertain the suitability of soil both for
construction and operation of ponds.
For engineering purposes, the techniques used for soil investigations vary from
relatively simple visual inspection to detailed sub-surface exploration and laboratory
tests. Visual inspection of the site is an essential preliminary step. In order to provide
data on sub-surface soils, a test pit measuring 0.80×1.50 m with a depth of 1.50 to 2.0
m, depending on the land form and the elevation of the water table, should be dug in
each hectare of the site. Digging of a test pit permits visual examination of soil and also
makes it possible to obtain disturbed and undisturbed samples of soils encountered in
the different layers below ground level.
Soils have characteristics that can easily be determined by sight and feel. Visual
examinations are employed in place of precise laboratory tests to define the basic soil
properties. A sandy clay to clayey loam is the best type of soil both for pond
construction and growing natural foods at the pond bottom. Areas with a layer of
organic soil over 0.60 m in thickness are unsuitable for any kind of fish pond, because it
would be difficult to maintain water levels in the ponds due to high seepage; also, it
would be necessary to transport suitable soils for dike construction to the site, and this
will be costly. Big surface stones or rock outcrops may make an area unsuitable for
anything except lined ponds or concrete raceways.
In general, a site will be suitable for construction of fish ponds if soils below the
proposed pond bottoms have a grain-size curve which can be plotted to the left side of
the grain-size curve A in Figure 1, together with a coefficient of permeability less than k
= 5 × 10-6 m/sec. Dikes without any impervious clay core may be generally built from
soils having grain-size curves plotted between the grain-size curves A and B in Figure 1
or having a coefficient of permeability between k = 5 × 10-6 and 1 × 10-4 m/sec (MI.,
1972).