THE
INLET AND OUTLET
The diameter of
the inlet pipe should be less than that of the outlet pipe in order to prevent
overflow from the pond in the event that someone accidentally leaves the inlet
open. The inlet pipe should be at least 20cm above the water surface to prevent
fish from escaping. If the inlet is set at or near the level of the water, fish
will swim against the current of the inflowing water and escape from the pond.
When the inlet is above the water level and properly screened, fish are unable
to jump into the pipe ad escape through the inlet pipe. The screen as inlets to
commercial ponds. Such channels are difficult to screen effectively catfish are more likely to escape from the
pond through then inlet as opposed to climbing out of it on the pond sides.
The also escape
when the pond overflows totally due to a blocked standpipe, during heavy rains.
Old literature recommends that the pond inlet and outlet be located at opposite
ends of the pond to facilitate flushing (good water in and poor water out) when
poor water quality becomes an issue. However, this is not all that important if
you flush the recommended way (see chapter 5). Pond outlets should have
anti-seep collar and an anchor-collar. The anti-seep collar prevents water
seepage from the “joint” where the outlet pipe and clay soil meet. These two
substances do not bond together and water tends to follow the outside of the
pipe unless interrupted by an anti-seep collar. Anti-seep collars are standard
construction principles but have been ignored in most pond construction and
most ponds have leaks as a result. When not in use, the drain pipe is full of
air, which makes the pipe tend to float. If the PVC pipe is not secured at the
pond bottom, the pipe can be dislodged at the bend or leaks can develop due to
the pipe floating up slightly. Having an anchor- collar just after the pipe
bend prevents this by keeping the drain pipe and its bend down.
4.2.5 THE
FREE-BOARD HEIGHT
The recommended
freeboard height is 20 to 30cm for the following reason:
a.
It allows for more free movement of air
currents above the pond water surface which improves mixing and oxygenation of
the water.
b.
It becomes easy to undertake routine
tasks such as feeding, seining, checking water quality and removing dead fish.
c.
It is cheaper to construct and maintain
d.
There is less surface area for erosion
into the pond. Therefore, this contributes less eroded silt to the pond water
and soil. However in ponds of over one hectare a freeboard of up to 50cm can be
accommodated (table 4.2)
Table 4.2 Recommended
free-board height for catfish grow-out ponds
|
Pond size rang
|
Freeboard Height
|
|
· Smaller
ponds (100m2 up to 1 ha)
· Large
ponds (> 1ha)
|
· 20
to 30 cm freeboard is sufficient
· 50
cm is recommended because there can large be waves on the pond
|
It is undesirable to have a
free-board higher than what is recommended because:
a.
It is an unnecessary added construction
cost – expensive to make.
b.
It attracts predators and burrowing
animal such as nutria and muskrats.
c.
High freeboards above the water favor
the nesting flying ants and termites, which later leads to leaks.
d.
It makes working on the pond difficult
and dangerous.
e.
A high freeboard prevents air currents
from reaching water surface, thereby preventing mixing and reducing oxygen
exchange.
f.
Looks ridiculous.
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