We require to look carefully at the chart showing tide and topographic levels of the different visited sites.
(Plan No 58 001)
The ALAMPUR Farm can be remodelled as follows :
20 grow-out ponds, 1 ha each (80 m × 125 m) disposed along a central supply channel, and with a peripheral drainage channel;
5 pregrowing ponds, 1 200 m2 each (80 m × 15 m);
A water supply storage pond.
The size of the farm, if desired, can easily be extended to 28 grow-out ponds, and 7 pregrowing ponds, along the same lay-out design.
Tide levels : the tidal chart indicates the tide levels for DIGHA, which may be considered very similar. We remind hereunder the level requirements for tide-feeding and gravity drainage feasibility :
Pond water level must be at least 0.80 m below the MHWN level for tide-feeding ;
Pond water level must be at least 0.80 m above the MLWN level for gravity drainage.
For ALAMPUR, we have :
Thus : - tide-feeding is not feasible ;
- gravity drainage is no problem.
Pumping and water storage requirements (for 20 ha G.O. ponds) : pumping will be carried out during higher tide, each tide cycle, i.e. over 6.2 hrs per tide. Water exchange daily need : 25,000 m3
For each tide cycle, water needs over 6.2. hrs will have to be stocked into a water storage pond, which will then release the water into the rearing ponds.
Thus, pump capacity is to be : 2 100 m3/hr.
We recommend having 3 sets of pumps, 1100 m3/hr each, one set acting as safety stand-by. Type of pump should be low lift, high discharge, e.g. axial flow vertical pumps, which are extremely reliable and maintenance free. Please refer to the “Standard prawn farm cross-section” for reference levels.
(Plan No 58 002)
We recommend remodelling the POLEKURRU Farm according to the lay-out plan :
46 grow-out ponds, about 1 ha each ;
24 pregrowing ponds, 1200 m2 each.
Levels of bunds, pond beds, and inlet/outlet sluice gates should be revised according to the standard cross-section.
Reference levels in POLEKURRU
Pond water level is 1.0 m above MHWN;
Pond water level is 1.70 m above MLWN.
Thus : - tide feeding is not possible ;
- gravity drainage is no problem.
Water exchange daily requirement
Total pond area is : grow-out 49.2 ha
pregrowing 2.9 ha
Water daily renewal:
| grow-out | 49.2 ha × 95 % × 10 % | = 46 740 m3 |
| 49.2 ha × 5 % × 50 % | = 12 300 m3 | |
| pregrowing | 2.9 ha × 80 % × 10 % | = 2 320 m3 |
| 2.9 ha × 20 % × 50 % | = 2 900 m3 | |
| 64 260 m3/day |
Considering the tidal mean range of 0.70 m during neap tides, and 1.30 m during spring tides, pumping could probably be carried out over a minimum of 8 hours per tide. In that hypothesis, pumping capacity should be 4000 m3/hr, i.e. 3 sets of pumps, 2000 m3/hr each (one set as safety stand-by).
Would the pumping duration be different, pump capacity should be revised accordingly.
(Plan No 58 003)
The lay-out plan shows proposed remodelling of the farm into :
13 grow-out ponds, about 1 ha each (0.79 ha up to 1.20 ha) ;
3 pregrowing ponds, about 3,000 m2 each.
Reference levels in NARAKKAL
Pond water level is just about the same as MHWN level ;
Pond water level is 0.20 m above MLWN level.
Thus : - tide feeding is not possible,
- gravity drainage is not possible for emptying the
ponds, and will just be enough for daily water
discharge if pond water level is raised by 0.20 m.
Recommendations are :
To close the peripheral canal as indicated on the plan, so as to isolate a feeding channel from a discharge channel (isolation will just affect nearby waters) ;
To make arrangements in order to have an average water depth of 1.20 m in the ponds;
There will be no water storage pond, as water can be pumped at any time of the tide. Pump capacity is to be 1000 m3/hr, in two sets of fixed pumps (pumping station) of 500 m3/hr each.
Those pumps shall be operating 24 hours per day.
The existing triangular nursery ponds should be removed, because no longer required ;
For harvesting the ponds, the farm should be equipped with 4 sets of mobile pump units, 500 m3/hr each, that could operate on any of the ponds, upon request, as well as serve as stand-by units for the fixed pumps ;
Inlet structures : should be open reinforced concrete sluices, with possibility of water admission level adjustment, and screening. Sill level may be 2.50 m ;
Outlet structures : should be monk sluice type (see report drawing) in order to allow dirty bottom water and detritus to be flushed, but culvert sill level may be 2.50 m.
Level sketch (in metres)
(Plan No 58 004)
This pilot farm would be set up according to the “Typical 20 HA Farm lay-out plan” with levels as indicated in the standard prawn farm cross section.
3.4.1. Reference levels in KALBADEVI
(to be adjusted with topographic survey)
Pond water level would be 0.10 m above MHWN level ;
Pond water level would be 0.90 m above MLWN level.
Thus : - tide feeding is not possible,
- gravity drainage and emptying is possible
Pumping requirement
Water exchange daily need : 25,000 m3. Pump capacity must be 2100 m3/hr ; we recommend having 3 sets of 1100 m3/hr each, one set as safety stand-by.
3.4.2. Reference levels in ANSURE (according to CICEF proposed prawn farm design)
Pond water level would be 0.35 m below MHWN level ;
Pond water level would be 0.10 m above MLWN level.
Thus : - tide feeding is not possible,
- gravity drainage is not possible either.
Therefore, we recommend, if ANSURE is to be chosen eventually, that the farm be set up in an elevated location of the site, so as to allow natural gravity drainage and emptying of the ponds.
Same pumping equipment as for KALBADEVI would be required, if same “6 hrs pumping per tide” assumption is retained.
(Plans No 58 005 and 58 006)
3.5.1. Remodelling of the MATWAD farm would be according to the lay-out plan :
8 grow-out ponds of 5000 m2;
1 grow-out pond od 5250 m2;
2 pregrowing ponds of 450 m2 each.
Reference levels in MATWAD
Pond water level is 0.60 m above MHWN level ;
Pond water level is 3.10 m above MLWN level.
Thus : - tide feeding is not possible (unlike
preliminary conclusion due to a mistake
in the original farm plan given to us in
MATWAD) ;
- gravity drainage and emptying is easy.
Pumping and water storage requirements
Water exchange daily need : 5600 m3.
If pumping can be operated over 6 hours each tide, the pump capacity should be 470 m3/hr, i.e. 3 sets of pumps, 250 m3/hr each (one set for safety stand-by).
Water storage pond would have an area of about 4,000 m2 (storage volume = 1400 m3, i.e. water layer above +6.08 m level = 0.35 m). (Bed level = + 4.80 m).
3.5.2. Alternative to type I : ASANGAON
Remodelling of the ASANGAON prawn farm is shown on the plan. The farm would consist of :
39 grow-out ponds, size varying from 0.92 ha to 1.65 ha;
16 pregrowing ponds, size varying from 1200 to 1750 m2;
2 water storage ponds.
Reference levels in ASANGAON
Pond water level is 1.20 m above MHWN level ;
Pond water level is 2.0 m above MLWN level.
Thus : - tide feeding is impossible ;
- gravity drainage and emptying is easy.
Recommendations
Remodelling mainly consists in creating separate feeding and discharge channels, modifying shape, size and number of pregrowing ponds, equipping each pond with opposed inlet and outlet structures ;
Pumping requirements : total pond water area for the farm is about 42 ha.
Daily water exchange requirement is :
| Grow-out | 40.3 ha × 95 % × 10 % | = 38 285 m3 |
| 40.3 ha × 5 % × 50 % | = 10 075 m3 | |
| Pregrowing | 2.1 ha × 80 % × 10 % | = 1 680 m3 |
| 2.1 ha × 20 % × 50 % | = 2 100 m3 | |
| 52 140 m3/day |
Assuming that pumping can be effected over 8 hours at each tide, pumping capacity should be : 3300 m3/hr ; we recommend making provision for 3 sets of pumps, 1650 m3/hr each, one set serving as safety stand-by.
