Drainable fish pond, 400 m²
A 20 × 20 m earthen pond with rammed dikes, a screened inlet and a wooden monk: filled and drained by gravity, manured from the farm, and harvested every six months.
Build
Tier 2 · Bronze and Iron Age
Food
Moderate — the work is earthmoving, but the levels and the outlet must be right before any earth goes on the dike
About a dry season by hand, with the outlet pipe laid first
Crew A family or a work group with spades, baskets and a hand rammer
Home: Aquaculture, Level 2 · Forge, level 1 of 1 in its track. Serves 1 step.
Every build on this site has hazards that injure or kill — quicklime, carbon monoxide, molten metal, collapsing ground. Read the safety notice before starting, and read this build's own failure modes below.
What it is and why this shape
A drainable pond is the difference between fishing and farming fish: the farmer sets the water level, controls what lives in the pond, and takes every fish at harvest by letting the water out. Everything that makes a pond drainable is in the levels — a bottom falling to the outlet, an outlet lower than the bottom, a drain lower than the outlet — so they are set out before any earth is moved. The dimensions are FAO's, for a 400–600 m² pond on good clay soil (FAO, Pond construction; outlet structures). How to run it — lime, manure, stocking, the dawn check and the harvest — is in fish ponds.
The pond in plan
Fig 1
Earth / subsoilTimberGravel / aggregateWaterVoid / air
Section through the outlet dike
Fig 2
Earth / subsoilMasonry / brickTimberLoose fill / grainWaterVoid / air
Keyed parts
| 1 | Dike | Rammed earth, 1.53 m high as built on good clay soil, settling to 1.30 m; base about 6.4 m. |
| 2 | Crest | 1.0 m wide (1.3 m on loamy soil), level all round, grassed. |
| 3 | Cut-off trench | About 0.5 m wide and 0.6–1 m deep under the dike's centre line, filled with rammed clay where the ground lets water through. |
| 4 | Inlet and screen | Pipe or gutter with its floor at least 10 cm above top water level, reaching 0.6–1 m out over the pond; screened against wild fish. |
| 5 | Monk | Three-sided wooden tower at the outlet, 30 × 40 cm inside for a 10–15 cm pipe, at least 20 cm higher than the deepest water; no more than 2 m high in wood. |
| 6 | Boards and screen | Stop-boards in the front groove set the water level and act as the overflow; a screen in the second groove keeps the fish in. |
| 7 | Outlet pipe and collar | 10–15 cm inside, laid under the dike before the dike is built, falling at least 1 % away from the pond, with a concrete anti-seep collar. |
| 8 | Catch basin | A hollow at least 30 cm deep in front of the monk, where the fish gather as the pond drains. |
| 9 | Pond bottom | Falls about 1 %, 20 cm over 20 m, from 0.8 m deep at the inlet to 1.0 m at the outlet. |
| 10 | Fence and gate | Round every side that houses and paths face, with a gate kept shut, so no small child can reach the water. |
| 11 | Rescue pole | A long pole or a throw rope hung at the bank, so a rescuer reaches from the dike instead of wading in. |
Specifications
| Water surface | 20 × 20 m, 400 m² | FAO's starting size for a family pond. |
| Depth | 0.8 m at the inlet, 1.0 m at the outlet | Never under 0.5 m anywhere, or water plants take hold. |
| Volume | about 360 m³ | At 7 l/s it fills in about 14 hours. |
| Dike, as built | 1.53 m high, crest 1.0 m, base 6.4 m | Settles 15 % to 1.30 m: 1.0 m of water and 0.30 m of freeboard. |
| Dike slopes | 2:1 inside, 1.5:1 outside | On fair, loamy soil: 2.5:1 and 2:1, crest 1.3 m, base 8.2 m. |
| Outlet pipe | 10–15 cm inside | FAO's size for 200–400 m²; never under 10 cm behind a monk. |
| Monk inside | 30 cm wide, 40 cm long | 8 cm to the first groove, 8 cm between grooves, 16 cm from the second groove to the back wall, two 4 cm grooves. |
| Water supply | 2–5 l/s per hectare of pond | By gravity if at all possible. |
| Earth | about 500 m³ rammed | If the whole dike stands on level ground: 5.6 m² of section along about 89 m of centre line. Digging the bottom down supplies part of it. |
Bill of materials
| Material | Quantity | Notes and substitutes |
|---|---|---|
| Earth with more than 50 % silt and clay | about 500 m³ | Passes the squeeze and bottle tests in fish ponds; from the pond area and borrow pits. |
| Clay for the cut-off trench | about 36 m³ | 0.5 × 0.8 m along the 89 m centre line, only where the ground under the dike leaks. |
| Outlet pipe, plastic or concrete, 10–15 cm inside | about 8 m | Long enough to discharge clear of the outer toe of the dike. |
| Cement and sand, 1 : 2 | enough for one collar | Anti-seep collar at least 10 cm thick, reaching at least 15 cm beyond the pipe on every side. |
| Heavy, durable, water-resistant timber | monk walls, floor and boards | FAO names iroko or mukulungu; treat with a preservative. |
| Screens | 2 | One for the inlet, one for the monk's second groove; mesh smaller than the fingerlings. |
| Inlet pipe or gutter | one, about 8 m | From the supply channel across the dike. |
| Topsoil and creeping perennial grass | for the crest and both slopes | 10–15 cm of the saved topsoil, planted at once. |
| Fence posts, wire or wattle, a gate | about 130 m of fence | Round the outer toe with room to walk the dike. |
| Rescue pole or throw rope | 1 | Hung where a rescuer would stand. |
Tools: Spades and hoes · Baskets or wheelbarrows · Hand rammer, 4–6 kg on about 150 cm² · Water level (clear tube) and pegs · String lines and a 30 m tape · Saw, chisel and drill for the monk · Secchi disc and thermometer · Seine of 3–3.5 cm mesh
Construction
Fence it in the plan, and test the ground
Plan the fence and gate before anything else, and keep children away from the works once digging starts. Dig test holes to below the planned bottom and test the soil at every depth: it must keep the shape of your hand when squeezed, and settle as more than 50 % silt and clay in the bottle test. Reject any spot with sand, gravel, rock, peat or big roots within the depth to be dug.
Set out the levels
Peg the four corners of the 20 × 20 m water surface and the dike lines outside them. With the water level, peg a level crest all round and a bottom falling 20 cm from inlet to outlet. Mark the outlet's floor at least 10 cm below the lowest point of the pond, and a drain beyond the dike whose bottom lies at least 20 cm below the end of the pipe. If the levels do not work, move the pond; do not dig.
Strip the topsoil and dig the cut-off trench
Strip the topsoil from the whole pond and dike area and stack it for later; take every level from the base of the topsoil. Where the ground under the dike lets water through, dig a cut-off trench about 0.5 m wide and 0.6–1 m deep along the dike's centre line and fill it with clay rammed in layers.
Lay the outlet pipe and build the monk first
The pipe goes in before the dike (FAO). Lay it in a level-bedded trench from the monk's position to beyond the outer toe, falling at least 1 %. Cast the anti-seep collar round it under the dike's centre: 1 cement : 2 sand, at least 10 cm thick, reaching 15 cm beyond the pipe on every side. Build the monk on a firm foundation, sealing the pipe into its back wall, with its top at least 20 cm above the deepest water. Ram the earth well around the pipe.
Build the dikes in rammed layers
Lay the earth in horizontal layers 15–20 cm thick, each wetted until a handful forms a firm ball, and ram each one before the next. Build to 1.53 m high with a 1.0 m crest, 2:1 inside and 1.5:1 outside, allowing 15 % for settlement. Ram the finished slopes smooth. Dig the catch basin, 30 cm deep, in front of the monk.
Fit the inlet and grass the dikes
Lay the inlet at the shallow end with its floor at least 10 cm above top water level, reaching 0.6–1 m out over the water, and fit its screen. Spread 10–15 cm of the saved topsoil on the crest, the outer slope and the inner slope down to the water line, and plant a creeping perennial grass at once. Plant no trees on or near the dikes.
Fill, and test
Slide the boards into the monk's front groove to the top water level and the screen into the second. Fill the pond: about 360 m³, 14 hours at 7 l/s. Shut the inlet and watch the level for several days; a fall much faster than evaporation means a leak — walk the outer slope and the pipe outlet for wet patches. Then pull the boards and check that the pond drains by gravity to the catch basin.
Lime, manure and stock
On the dry bottom, spread 80–160 kg of ground limestone (sandy to heavy clay soil), then fill and manure 10–15 days before stocking. Stock 800 tilapia of 5 cm or more. From here, run the pond by the daily routine in fish ponds: dawn check, Secchi disc, manure and feed to the water, harvest at about 6 months, dry the bottom.
Tolerances that matter
| Dimension | Target | Why it matters |
|---|---|---|
| Crest | level all round, at least 1.30 m above the bottom after settling | The lowest point of the crest is where a flood will cut through. |
| Bottom | falls about 1 % to the outlet, no hollows | Any hollow holds water and fish when the pond is drained. |
| Outlet floor | at least 10 cm below the lowest point of the pond | Otherwise the pond cannot drain completely. |
| Outlet pipe | falls at least 1 %; its end at least 20 cm above the drain bottom | Water backing up from the drain stops the pond draining. |
| Earth layers | 15–20 cm, each moistened and rammed | Thick or dry layers leave voids that become leaks. |
| Monk height | at least 20 cm above the deepest water | So the boards, not the dike, are the overflow. |
How this fails
A sand or gravel layer in the dike or under it, missed in the soil tests: the pond never holds water.
An outlet pipe with no anti-seep collar, or loose earth round it: water creeps along the pipe until the dike washes out.
An outlet set too high, or a drain too shallow: the pond cannot be emptied and the harvest is never complete.
Dikes built in thick, dry layers: they settle unevenly and leak.
Dirty screens: the monk stops overflowing and the water rises over the crest, taking the fish and the dike with it.
An unscreened inlet: wild fish come in and eat the food and the fry.
Trees on the dike, whose roots open leaks.
No fence: a pond is a drowning hazard for every small child nearby.
How you know it worked
VerificationThe pond fills to the top board and holds its level with the inlet shut, the outer slopes and the pipe outlet show no seep, pulling the boards drains the pond by gravity to the catch basin, the crest is still level and at least 1.30 m above the bottom after the first wet season, and no small child can reach the water without passing a shut gate.
Scaling and variants
Bigger ponds need bigger outlets: FAO sizes the pipe at 15–20 cm for 400–1,000 m², 20–25 cm for 1,000–2,000 m² and 25–30 cm for 2,000–5,000 m², with the monk widened to match (33–35 cm inside for a 15–20 cm pipe, up to about 50 cm, beyond which the boards are hard to move). Several ponds share one supply channel and one drain, each with its own inlet and monk, so each can be filled, drained and harvested alone. On loamy soil use FAO's wider crest and flatter slopes: crest 1.3 m, slopes 2.5:1 and 2:1, base 8.2 m.