Hydraulic equipment of ponds
Water intake arrangements
The intakes are used only in the case of basins or ponds fed from the deviation of a water flow, where it is therefore possible to regulate the inflow. These installations have to be installed in counter-current to avoid the accumulation of various clutter (dead leaves, branches) in the mechanism, and if possible, upstream of a sill ensuring a minimum level. The grills prevent the fish from escaping upstream or entering from upstream.
The concrete structure includes a grill with vertical bars, then a mechanism to allow the adjustment of the entering flow.
The complete emptying of quantities of water (basins or ponds) must be possible.
The diameter of the pipe depends on the volume of water to be evacuate and the size of the fish which must pass:
- 20 to 30 cm in the case where the fish is recaptured inside the pond.
- 30 to 50 cm when the fishery is behind the dam.
The pipe has its openning 30 to 40 cm below the bottom of the pond, 10 cm lower than the “pan“ (see Fig.1, page 137) to allow complete draining.
Three systems exist:
* the sluice-gate
* "le moine"
* the sluice-gate "moine" (or drowned "moine") which is a combination of the two above.
* the "piston moine".
They can be supplemented downstream by a gravel bed filter which totally avoids the escape of the fish towards downstream and retains the sediment.
The methods of recapturing the fish
Advantages of the fishery:
* it allows the retrieval of the fish without pulling in of the netting or penetration of the pond; landing nets, a sorting table and containers are sufficient.
* it facilitates the fishing operations, especially when steep banks render the access difficult.
* There is immediate recuperation as they exit, the fish do not stay in the emptied water which is full of mud.
Constraint: the slope of the ground behind the dam must be sufficiently great (1 or 2 cm/m) in order for this device to be efficient. The fishery is situated behind the dam, at the exit from the evacuation pipe.
To facilitate the passage of the fish, the diameter of the emptying pipe must be quite significant: 20-30cm, even more if it is planned to retrieve large fish.
It is made in masonry. Its size depends on the quantity and the size of fish that it will retrieve. Its depth is from 0.6 - 0.8 m to facilitate the catching of the fish into the landing net.
As an indication, for a pond of 6 - 8 hectares in semi-extensive production, the fishery measures 2 m width by 6 m length. A centre longitudinal partition extends along 2/3 of the length: the pipe emerges from one side and the fish can shelter from the current in the other half. Grooves at the exit allow sliding grills, then some small boards to regulate the water level. Clean water coming from the supply channel stops the fish from finding themselves in water which is too muddy and allows the fishery to be cleaned. For want of something better, a water supply coming from a cistern reservoir positioned nearby can be used. (Fig.1)
For small ponds, (1 hectare or less; with a production of 300- 400 kg/ hectare) a fishery of 1m x 2 m (possibly without a centre partition) is the minimum.
Experience has shown that a length of 2m is the minimum to avoid the current on the exit grill trapping the fish.
It is necessary that in the immediate proximity of the fishery, there is a surface of clean and stabilised ground to allow the installation of a sorting table, stocking vats filled with clean water, various containers, scales...
The installation of an internal fishery is the only way of making use, from the fishing point of view, of a reserve which was devised initially for irrigation, apart from opening the dam and replacing the initial emptying pipe (diameter: 100 - 150 mm) with a more significant pipe. The water evacuation device should be a submerged "monk", preceded by a fishery. The sluice-gate can be situated on the "monk" or at the exit of the pipe, downstream. The dimensions of the interior structure are: 0.8 - 1 m in height, 1m width and 3 - 5 m long. The double grooves at the entrance of the fishery and the "monk" allow the use of small boards and grills to regulate the entrance of the water and fish. To limit the ascent and the descent of the dam carrying the landing net (staircase essential!), the sorting operations should take place next to the fishery, on a concrete surface of around 5 x 5 m.
Certain reserves are not fitted with an emptying pipe (there is direct pumping in the water). In this case, a pumpage drain tank is installed in place of the emptying "monk" which takes a submersible pump or the strainer of a pump installed on the dam.
Management of ponds
At the location planned for the pond, the sub-soil must be as homogenous and as impermeable as possible so as to avoid the inconvenience of incorporating costly waterproofing.
The quality of the sub-soil is checked with a series of drill holes, spades or diggers which descend beneath the bottom level. Stony terrain is not suitable (too permeable), nor peaty zones: permanently humid, drainage is not possible, the water is often acidic and access is difficult for excavation machines.
The bottom of the pond should not be lower than the top (the surface) of the water table, which will make it impossible to drain.
The bottom of the pond should be on an even slope (0.5 - 2%) towards the dam, the lowest point of the whole set-upshould be just in front of the location of the emptying system.
To ensure good drainage, a system of drainage ditches should follow on from the lowest point (width: 1 to 2 m). Currently, a "fish bone" arrangement is used: a main ditch receives the water drained by the lateral ditches. It is essential to maintain this network in a good state by cleaning it out after each emptying.
The work starts with the clearing of the top soil for the entire surface planned for the base of the dam. The dam must be watertight: from the pond, the water must not encounter any irregularities or weakness. The body of the dam and the soil form one body due to a ditch or anchoring key over-deepened vertically along the axis of the dam. When the height of the latter is only moderate, a mechanical harrowing which causes scarifications of 15 - 20 cm on the soil can be enough.
If there is not enough impermeable material on the site for the dam to be created, or if the material is too permeable, it becomes necessary to create a watertight nucleus in the dam body with imported clay material. The dams must be made up of homogenous earth (no blocks, branches or stumps) to ensure both solidity and impermeablity.
The compaction is done in layers of about twenty centimetres with special vehicles (with special rollers, not caterpillar track vehicles, which have a very weak pressure on the soil).
Total height of the dam: 20 - 50 cm more than the anticipated maximum water level; 60 - 80cm around large areas, to take into account waves provoked by the wind.
The width of the top must, as much as possible, allow access to the emptying and fishing points for vehicles (2 - 2.5 m).
- slope of exterior side :1/1
- interior side, the slope is less, 1/2 in compact, firm soil and up to 1/3 or 1/4 in crumbly soil or if the area is exposed to much wind. To reduce the erosion of the dams due to waves, leaving reed zones as a shield some distance from the banks has proved to be effective.
The planting out of short marsh plants (sedge...) on the dams is equally possible. A turf cover, sown from the end of the embankment, reduces erosion by the rain.
Corniche du Bois Sacré
83500 La Seyne sur Mer
Tél. : 04 94 10 26 26
Fax : 04 94 10 26 30