I. Tóth
Fish Culture Research Institute
Szarvas, Hungary
1. FISH POND MAINTENANCE
2. RECONSTRUCTION WORKS
3. MECHANIZATION OF MAINTENANCE AND RECONSTRUCTION WORKS
1.1 Maintenance of Earthworks
1.2 Maintenance of Biological Slope Protection
1.3 Maintenance of Water Control Structures
1.4 Steel Structures
1.5 Stone and Concrete Linings
During the first years of operation fish ponds need the most maintenance, as the unforeseen, hidden faults occur at this time. For example, the unexpected subsidence and settlement of dikes, and the more serious erosion on the surface of dikes and excavations occur . Maintenance must be carried out continuously from the first year of operation. One can expect the following troubles with dikes: slope failure, soil slip, notch, slight erosion of the slope, etc.
- Slight erosion in most of the cases is caused by precipitation and its trickling down the slope. Thorough soaking and drying of the surface and negligence in dike maintenance can also cause spalling of the upper surface of the slope. If there is a more serious erosion of slope, shoulders must be constructed on the slope and new horizontal layers must be compacted onto the top. After reconstruction the dikes have to be protected against erosion.- Soil slip is caused most frequently by bad quality of compacting, drenching of the dike or quick water discharge. In such a case a bigger quantity of soil slips down from the slope in some places. The slipped soil has to be replaced with well compacted horizontal layers of earth and possibly with a retaining wall.
- Slope break-through is caused by the same things as soil slip. In this case a part of the slope slips down in a round shaped form. For reconstruction of such a failure first of all a foot wall should be constructed then good quality earth material should be filled and compacted in thin layers.
- Slumping is generally caused by a lack of proper compacting of the dike. It can also occur if the layers containing humus or organic matter were not removed from beneath the dike. Reconstruction in this case is carried out by putting a new layer of earth onto the top of the dike.
Not only the base of the dike but the dike itself may also settle. Settlement of smaller dikes is generally not higher than 2-4% of the height of the dike. If the settlement of the dike is higher the dike may have lost its load capacity after construction or its load capacity is not suitable. Protection generally can be made by constructing protecting shoulders/foot walls on one or both sides of the dike.
Bigger scale erosion generally occurs in canals of higher water current capacity or after a longer operation time.
Maintenance must ensure the original water supply capacity of the canals. Erosion occurring on the bottom level and on the surface of the slope, soil slip and notch must be corrected at least once a year. At the same time accumulated silt and mud must be removed and vegetation obstructing free water flow must be moved or removed.
On the bottom level of the fish ponds the mud must be removed from the internal harvesting canals after the drainage of the pond. Maintaining of good condition can be assisted by setting the feeding places in these canals.
Biological slope protection needs constant thorough checking and maintenance, as wave action can cause erosion of the slopes. Usually a strip of planted reed is applied for biological slope protection. Where the reed is not strong enough temporary brush work protects the earth slope.
The deficiencies in the reed strip must be replaced. The protecting reed strip must be treated continuously.
Recently new biological slope protection methods have been elaborated, when some plastic materials (geotextile, plastic net) are applied on the earth slope desired to be protected and the plastic together with the plants and roots forms a protecting layer.
Checking of structures must be carried out by separate checking of the elements (concrete parts, steel structures, flash boards). Sediments and the mud must be removed from the structures before operation begins.
Leakages occurring at the joints of structures can cause cavities which must be filled with properly compacted earth, and where it is necessary, with concrete. Leakage can be eliminated by repairing the joint or by application of bitumen filler.
If there are settlements of the structures or as a consequence of them there are cracks in the structures, the method of reconstruction must be determined separately in each ease depending on the given problem.
Elements of steel structures must be protected against corrosion. Each year, built-in structures must be cleaned with a wire-brush during breaks of operation then one layer of rust-preventing paint and two layers of oil paint must be applied. Moving elements of the structures must be regularly oiled or lubricated.
If the stones become loose in the linings or the filling material is damaged the stones must be replaced. Settlement of the dike can cause damage of the lining. The same damage can occur if waves wash away the bed of lining or the supporting earth. In such a case the lining must be removed from the damaged areas, the slope must be filled up with bedding material, then new lining must be made.
The most frequent fault of concrete lining is under-washing. Reconstruction is the same as in the case of stone lining.
Erosion at the ends of the lining must be filled with gravel as soon as possible, as otherwise the lining will be destroyed very easily.
2.1 Reconstruction of Earthworks
2.2 Reconstruction of Water Control Structures
The aim of reconstruction is the rebuilding of earth and concrete structures to their original size and shape, and the replacing of some seriously damaged structures. Reconstruction means a constant modernization as well.
2.1.1 Dikes
2.1.2 Internal drainage canals and harvesting pits
2.1.3 Canal
When a large size earth dam is reconstructed, specially classified construction material should be applied on the basis of individual testing, while the dikes with lower load can be constructed of material dug out from the pond bottom in dry condition or by hydromechanical work.
In case of reconstruction of an old dike or construction of a new dike the existing biological slope protection should be saved as much as possible.
When dikes are reconstructed or new dikes are built space should be left along the dikes for the free movement of canal cleaning machines and for the deposition of the mud removed from the canal.
When a new dike is designed its crown width should be wide enough to allow movement of farm vehicles on the top of the dike if necessary.
Internal drainage canals must be reconstructed with a slope angle at least 1:4 and 5.0 m bottom width. The worked out earth must be removed from the pond or must be spread on the surface of the pond bottom. To make the harvesting easier construction of an external harvesting basin or a central external harvesting basin can be useful.
When the canals are reconstructed their shape and size have to meet the requirement of those machines that are used for cleaning.
Purpose of the reconstruction is to resettle the structures to their original condition. At the joints of elements of structures the water resistance must be renewed (by bitumen fixation, etc.). Cavities occurring along the joint linings must be reconstructed as described in the chapter on lining.
During reconstruction, some new structures can be added if necessary for the modernization of the farm.
3.1 Mechanization of Fish Pond Maintenance
3.2 Mechanized Maintenance of Canals
Mechanization of maintenance and reconstruction in pond fish farms is needed both because of the decreasing level of labour supply and the increasing level of technical development. Moreover it has been also recognized that economical production can be carried out only in well maintained fish ponds. Taking into account the possibilities of mechanization in pond fish farms the maintenance works can be divided into three groups:
- maintenance and reconstruction of the pond bottom and the dike,
- maintenance and construction of water supply canals and drainage canals,
- maintenance of water control structures and their surroundings.
The works of the first two groups can be mechanized easily, but maintenance and reconstruction of inlet and outlet structures must be made manually.
After a few years operation, fishponds are silted up and their dikes are damaged. The length of this time depends on the dike material, the method of operation and the level of dike protection. Ageing is highly influenced by the capacity of duck production in the ponds. That is, it is advantageous from the point of view of economics, but requires more maintenance.
The necessity for maintenance and/or reconstruction is shown not only by external visible factors (the damages to dikes, pond bottom and the slope protecting lining) but by the decrease of production level of fish ponds. As an effect of erosion, the operational water level has to be lowered, thus the useful water volume in the fish pond decreases as well.
During reconstruction of fish farm facilities the future needs of production have to be taken into account as well.
Where the construction of fish ponds was carried out by machines, the conditions of mechanized reconstruction work are suited to this type of work. But in case the ponds and their connected facilities were constructed by manual labour, as is done many times in practice, a complete renewal is needed. Conditions of mechanized maintenance of such pond systems can be ensured only by total rational reconstruction of the ponds, or more exactly the pond fish farm itself.
When mechanized reconstruction is carried out, the following procedures are needed:
- reconstruction of crown of dikes,
- replacement of eroded slopes,
- elimination of unevenness of pond bottom,
- partial removal of mud in order to ensure harvestability and drainage,
- removal of water weeds,
- reconstruction of dike protection.
The larger extent of reconstruction work can be done only when the fish pond is dried out.
But in some cases it is impossible to have dry conditions during the time of reconstruction. In such cases maintenance and reconstruction can not be carried out without special machines. For this purpose single-purpose machines must be applied. Mechanical construction appeared in the early fifties of this century, though there were some simple machines utilized in pond construction before this time. For maintenance and reconstruction the same machines are used as for construction.
For excavation works different types of crawler and wheel excavators dredges can be used with mechanical or hydraulic operation (Figures 1, 2, 3, 4, 13, 14).
For transportation of earth, dumpers of 3-6 m3 volume or dump trucks are generally applied.
Application of these machines is economical only over a 600 m transportation distance.
For compaction of soils smooth steel drum, sheepsfoot and rubber-tyred rollers as well as vibratory compactors (Figure 5) can be used. They may be self-propelled or towed. For slight compaction the earth transporting trucks can also be used.
For short distance earth transportation at the site, crawler (Figures 6, 7) or rubber-tyred bulldozers or motor grader (Figure 8) can be applied economically. These machines are of such a design that they can work on the slopes too.
The spreading of earth material and levelling of the surface can be carried out with good efficiency by the above-mentioned bulldozers and the self-propelled or towed grader (Figure 9). The most efficient machines for maintenance and reconstruction are those which can be applied on slopes too. Self-propelled or tractor-drawn scrapers (Figure 10) can be applied for such purposes.
The scraper can be used both for the spreading and for the levelling of earth transported to the field. At the same time it compacts the dikes as it has to move on the dike being constructed. The scraper can be utilized for both the processes needed in construction, maintenance and reconstruction of fish ponds, but for economical application of it no longer than 600 m transportation distance is allowed.
Tractor-drawn scrapers with a capacity of 3-6 m3 are the best for maintenance works of fish ponds (Figures 11-12). They need 50-100 kW tractive power. Their capacity is 30-40 m3 at 200 m transportation distance if crawler tractors are applied. Those scrapers that are pulled by rubber-tyred tractors have a higher capacity, but their ability to move on muddy surfaces is much less, therefore use is not advised for fish pond construction. The tractor-drawn scrapers generally have one container, that allows easy mobility. Application of scrapers with two containers is advisable only for construction of new large size fish ponds.
If needed, the above-mentioned processes can be carried out by rubber-tyred tractor shovels or rubber-tyred front end loaders too. Using them - although they increase the costs - the quality of the earthwork can be improved.
Figure 1. Crawler excavator
Figure 2. Operation diagram of crawler excavator
Figure 3. Buckets and booms for excavators
Figure 5. Vibratory compactor
Figure 6. Crawler bulldozer
Figure 7. Crawler bulldozer
Figure 8. Motor grader
Figure 9. Towed grader
Figure 10. Self propelled scraper
Figure 11. Tractor-drawn scraper
Figure 12. Tractor-drawn scraper
Figure 13. Wheel excavator
Figure 14. Operation diagram of wheel excavator
If maintenance or reconstruction works in the ponds must be carried out in or under water, hydraulic dredges can be applied. They can be equipped with buckets or cutter heads . Some types of dredges have a rotating upper body mounted on floating base. If such a machine is applied the mud can be placed in a floating container fixed to the dredge and it is periodically transported out of the pond.
During the growing season the operating canals are covered with vegetation that increased the roughness of the slopes of the canals. If this vegetation is not removed, the water supplying capacity of canals decreases and mud accumulates on the bottom of the canal.
It has been found that the water supplying capacity of a canal decreases to one sixth when it is covered with dense vegetation. Thus the regular mowing of vegetation is important.
Overgrowing of weeds is basically influenced by the length of operation of canals. If the annual average water depth is 40-60 cm not only rotted plants will grow but floating water vegetation as well.
The hydraulic conditions of canals deteriorate if there is overgrown vegetation, and at the same time the settling of mud is speeded up. In general, mud removal is needed when the cross section of a sediment layer reaches 15Z of the wetted surface of bigger canals and 25% of the wetted surface of smaller canals.
Prior to mechanized maintenance, first the terrain correction of dikes must be carried out in order to ensure the free movement of the machines along the canals.
For this purpose, rubber-tyred or crawler bulldozers and graders can be used, preferably with adjustable blade.
For removal of vegetation in medium size canals bucket mowers can be applied. These mowers cut the weed and remove it from the slope. From an economical point of view the removal of mud must be carried out together with removal of water weeds. This work can be done by special type of dredges.
Removal of mud from the smaller and medium size canals in case of low water depth is carried out by machines moving along the canal and equipped with a hydraulic dredging device, by one-purpose machines or by dredging boats if the water depth is sufficient.
Mud placed on the dike of the canal must be spread after drying. For this purpose simple bulldozers can be used.
Agricultural tractors with 40-60 kW capacity can be used for the maintenance of canals. In Hungarian farms the multi-purpose tractor MTZ-80 is applied with high efficiency for maintenance works if equipped with special attachments. These are the following:
- hydraulically operated booms equipped with small size excavating bucket, dirt cage, or mower, rotary scraper, which can be applied for reshaping of smaller dikes of canals,- dozer blade mounted to the back of the tractor and adjustable hydraulically, and manually. Its working depth is 0-1.50 m, working width is 900 - 24 000 m ,
- two-disc rotary mower mounted to the tractor for mowing the shoulder, the crown and the slopes of the dike. Mowing width is 1.65 m.
The tractor equipped with about 7 m long booms has to be supplied with hydraulic supporting legs for the sake of stability.
The hydraulic tractor in general can be equipped with the following devices:
- dirt cage
- silt pump
- rotary mower
- swinging blade mower
- bucket mower
- sawblade cutter.
These devices can be applied for the following maintenance works:
- a dirt cage: excavating the mud from the bottom of the canal and placing it on the dike of the canal.Its capacity is 10-15 m3/hour .- silt pump: pumping out the silt from the bottom of the canal and sprinkling it on the shoulder of the canal,
- rotary mower: mowing the weeds on the dry slope and chopping them, then spreading on the slope. Its capacity is 1 500-2 000 m2/hour.
- swinging blade mower: it can be equipped with an attachment that collects the cut weeds and removes them to the dikes. Its mowing capacity is 5-9 000 m2 per hour, depending on evenness of the ground .
- bucket mower: it can be applied efficiently in highly overgrown canals. The cut weed can be placed on the top of the dike by the bucket. Its capacity is 120-200 m2/hour.
Hydraulic excavators can be applied efficiently only if they are mounted with special devices that are developed for maintenance work. To the K-406 type wheel excavator , a bucket mower with 4 m working width and a dirt-cage of 2.5m width and 0.25 m3 volume can be mounted. These bucket mowers and dirt cages can be mounted on the K-408 type, rubber-tyred excavator, the working beam of which is 10 m long
Slope mowers have been developed in those countries where intensive forage production is carried out on sloped areas. In Hungary the Austrian made RASANT type two wheeled and four wheeled mowers are commonly used. They work with a cutter bar which consists of rows of blades. Working width of the blade is 1.6-2.5 m.
For mowing vegetation in about 1 m deep canals and ponds mowing boats can be applied with high efficiency. In Hungary the Czechoslovakian made mowers are popular, but recently a Hungarian type has been developed. Its T-shaped blade can be used not only in vertical position but with an angle of 40°. Thus this mower can cut the grass both from the slopes of canals and from dikes of ponds .
For operation of the mower a minimum 0.6 m water depth is needed.
The main technical data of this mower are as follows:
- mowing depth under the water surface is 1.1 m,
- length of the boat is 4.93 m,
- width of the boat is 1.35 m,
- mowing capacity is 2 000-6 000 m2/h
For transportation of the mower a trailed transporting truck has been developed.
Mud removal from bigger sized water supply and drainage canals and fish ponds can be carried out economically by a hydraulic suction dredge . Technical data of the dredge designed in Hungary are the following:
- length: 10.9 m
- width: 2.4 m
- draught in operational state: 0.6 m
- biggest dredging deepness: 2.9 m
- capacity: 25 t/h
- transportation distance: 500 m
- energy requirement: 90 kW.
The suction pipe is situated in the front of the dredge and works hydraulically.
