Sandy site for a new 0.5 ha carp pond
A farmer in Solapur district wants to build a 0.5 ha rectangular pond on sandy loam. Soil testing shows 68% sand, 12% clay and 8% silt. A percolation test in a 1 m deep trial pit shows a water loss of 5 cm/day. The water table is 2.4 m below ground. A canal runs along the higher side of the plot, which has a 1.2% slope. The farmer plans a pond 1.8 m deep with 1 m high embankments and no lining, and will fill it from the canal. His budget is limited. A neighbouring farmer abandoned a similar pond because it would not hold water. The new pond is meant for composite carp culture with a target depth of at least 1.5 m.
1. Assess whether the site and proposed design are suitable. Support your answer with a seepage calculation.
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The site is poor for an unlined pond. A loss of 5 cm/day over 0.5 ha is:
- 0.5 ha = 5,000 m²
- 5 cm = 0.05 m
- Seepage = 5,000 × 0.05 = 250 m³/day
That is about 250,000 litres lost every day, before evaporation. The pond cannot be kept full from a canal without continuous pumping, and the farmer's budget does not cover that. The soil has only 12% clay and 68% sand, so it lacks the clay content needed to seal naturally. The neighbouring abandoned pond is evidence of the same problem.
The proposed 1.8 m depth with only 1 m high embankments is also inconsistent: with 1.8 m water depth and 0.5 m freeboard, the dyke must be higher than 1 m above ground unless the pond is excavated deeply. The site is not suitable as proposed; it needs a liner or a smaller, sealed pond.
2. Recommend a lining and layout plan that fits a limited budget.
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For a limited budget, the first choice is a compacted clay blanket rather than an expensive geomembrane over the whole area. Options:
- Clay blanket: 20–30 cm of good clay soil, placed in layers and compacted at optimum moisture over the pond bottom and inner dyke slopes. This is labour-intensive but uses local material.
- Bentonite: 5–10 kg/m² incorporated into the top 15 cm of soil and compacted. It is useful for smaller areas but can be costly at 0.5 ha.
- Geomembrane: reliable but expensive; use only if subsidised or if a smaller pond is built.
Layout: put the pond on the higher part of the plot with the long axis along the prevailing wind. Use cut-and-fill so the excavated soil forms the dykes. If the budget is tight, build a smaller lined pond first, for example 0.2 ha, and expand later. Do not attempt the full 0.5 ha unlined.
3. What inlet, outlet and freeboard details should be included?
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The pond needs a proper water-control layout:
- Inlet: place at the highest corner, with a screened pipe or channel to keep wild fish and trash out. The canal water must not enter directly without screening.
- Outlet: place at the lowest corner, with a monk or standpipe that can control water level and drain the pond completely. The outlet should lead to a drainage channel, not back into the canal.
- Freeboard: allow at least 0.5 m above the normal water level to prevent overtopping during rain.
- Depth and dykes: keep 1.5–1.8 m water depth at the deepest point, with dyke top width of about 1–1.5 m and inner side slope of about 2:1. Compact the dykes in layers.
- Sealing: line the bottom and inner slopes, not just the bottom, because seepage also occurs through the dyke sides.
With 1.2% slope, the outlet can use gravity drainage, which is an advantage. The critical judgement is that the inlet and outlet are useless unless the seepage problem is solved first.