The outlet - drainage
How the outlet is constructed depends on whether or not there is a collection basin for the fish, and if this is inside or outside the pond. The design of the level control will also influence the construction of the outlet system. If open channels are used, there are separate ones for the outlet water and level control.
Normally a standpipe inside the pond functions as a level control. The water has to pass through this standpipe, which may be variable or fixed, to flow out of the pond. A swivel can be used to control the level of the standpipe; this will again control the water level in the pond. If using a double standpipe, it is possible to take the outlet water from some depth in the pond. The standpipe and level control may also be placed outside the pond.
Special material must be used at the end of the outlet pipe to prevent erosion when tapping down the pond (shock tapping); concrete is usually used here. Concrete can also be used to construct the collecting basin for fish so that fry can be harvested or collected. When the outlet pipe is laid through the pond levee it is important to use mooring blocks, for instance of concrete, which are clamped on the pipe to prevent it being dragged out by the frictional forces of the water on the pipe. When both the flow rate and velocity are high, this is especially critical.
If channels are used for drainage, a specially design outlet is commonly placed in a small channel where the water leaves the pond; this is known as a weir gate or monk (Fig. 14.7). It is normally constructed with two plates vertically installed in the channel, like hatches. The water has to pass below the first and above the second. The level of the second controls the level in the pond. From the pond the outlet water continues through a common drainage channel out of the farm. For large water flows open channels represent a simple cost effective solution.
The methods chosen for handling the fish may influence the construction of the pond and hence the outlet system. To collect the fish from the pond it is normal to either reduce the water level with the help of the outlet or to use a seine net, or eventually a combination of both methods. When using water reduction as a collection system, a collection basin, either inside or outside the pond, can be used to collect the fish. If the basin is inside the pond it is normally small and at the bottom of the pond; it is commonly made of concrete or wooden planks. The outlet pipe from the pond is taken out from this basin. When the fish have been collected they can be removed from the basin using a net, pump or screw. The other possibility is to have the collection basin outside the pond. In this case the fish are tapped directly out of the pond together with the water and collected in the external collection basinBoth these solutions improve capacity and reduce the cost of fish handling.
A seine net may also be used for collection. Typically it is dragged through the pond and the fish are collected. The net can be hauled from the pond levees. If the pond is too wide, this will, however, be quite difficult, so widths of more than 20 m are not recommended. The seine net may also be dragged mechanically, for instance by a tractor.