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Pond Design Basics for Small-Scale Aquaculture

Capítulo 3

Estimated reading time: 12 minutes

Design goals: buildable, drainable, harvestable

A good small-scale aquaculture pond is designed so you can (1) fill it reliably, (2) hold water safely, (3) drain it completely when needed, and (4) harvest fish without fighting mud, weeds, or uncontrolled overflow. The most practical designs use simple earthworks, straight edges, and water control structures you can inspect and repair.

Practical pond shapes and dimensions

Recommended shapes for manageable construction and harvest:

Rectangular: easiest to stake out, excavate, seine, and grade for drainage.
Square: compact footprint, but corners can trap fish during seining unless you round corners slightly.
Long rectangle (e.g., 2:1 to 4:1 length:width): best for seining and directing fish toward the drain end.

Avoid irregular shorelines, narrow coves, and many inlets—these increase erosion points and make netting difficult.

Pond area and depth targets by climate

Choose an area you can manage with your equipment and labor. Many small operations do well with 0.05–0.5 acre (about 200–2,000 m²) ponds because they are easier to drain, treat, and harvest. Depth is primarily about temperature stability, winter survival (if applicable), and limiting excessive plant growth.

Climate / condition	Typical target depth (water depth)	Notes for small ponds
Warm tropical / low frost	1.0–1.5 m (3–5 ft)	Shallower ponds warm fast; manage algae/weed risk by keeping most areas >0.8 m.
Warm temperate (mild winter)	1.2–1.8 m (4–6 ft)	Provides buffer against summer heat swings; keep a deeper zone near the drain.
Cold temperate (regular frost/ice)	1.8–2.5 m (6–8+ ft)	Include a deep refuge area; avoid very shallow shelves that freeze solid.
Very hot, high evaporation / windy	1.5–2.2 m (5–7 ft)	Extra depth reduces temperature spikes and evaporation impact; strengthen wave-facing dikes.

Rule of thumb for harvestability: keep the pond deep enough for fish health, but not so deep that you cannot drain and seine effectively. For many small ponds, a maximum depth around 2 m is a practical upper limit unless you have strong drainage and equipment.

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Dike/embankment design: slopes, freeboard, compaction

Key terms

Dike/embankment: the raised earth wall holding water.
Freeboard: vertical distance between normal water level and the top of the dike.
Crest: the top width of the dike (often used as a walkway/vehicle path).
Upstream slope: pond side of the dike.
Downstream slope: outside slope.

Recommended slopes and dimensions

Use gentle slopes for stability and easy mowing/inspection.

Upstream slope (pond side): 3:1 (3 horizontal to 1 vertical) is a common, stable choice.
Downstream slope (outside): 2.5:1 to 3:1. Use gentler slopes in sandy soils.
Freeboard: 0.5–0.75 m (20–30 in) minimum; increase if waves are strong or rainfall is intense.
Crest width: 2–3 m (6–10 ft) if you want vehicle access; 1–1.5 m (3–5 ft) for foot access only.

Diagram described in words (cross-section): Imagine slicing through the dike. Starting from the pond water surface, the upstream slope rises at a gentle angle (3:1) up to the flat crest. Above the waterline, the crest sits higher by the freeboard amount. Then the downstream slope falls away at a similar gentle angle (2.5–3:1) to natural ground.

Compaction and core trench (to prevent seepage)

Most dike failures and chronic leaks come from poor compaction or building on topsoil/organic material. The basic approach is: remove unsuitable material, build in thin layers, and compact each layer.

Step-by-step: building a compacted dike

Strip topsoil and organic matter under the dike footprint (store topsoil for later grassing).
Cut a core trench along the centerline of the dike down into firm subsoil (typical: 0.3–0.6 m deep, 0.6–1.0 m wide; adjust to soil and pond size). This “key” ties the dike into the ground.
Backfill the core trench with good clayey soil in 10–20 cm (4–8 in) lifts.
Moisture-condition the soil: it should be damp enough to form a firm ball in your hand without crumbling, but not so wet it pumps underfoot.
Compact each lift using a sheepsfoot roller, plate compactor, or repeated passes with a tractor (best results with proper compaction equipment).
Build the dike up in layers, maintaining the designed slopes and crest width.
Finish and protect: smooth the surface, add topsoil on the outer face if available, then seed/establish grass to prevent erosion.

Practical compaction check: after compaction, your boot should not sink deeply, and a shovel should cut the soil with resistance. Soft, spongy areas are a warning sign—rework them before filling.

Inlet and outlet placement (for clean flow and easy control)

Inlet placement

Place the inlet so incoming water does not erode the bank and so it promotes gentle circulation.

Best location: near one end of the pond, often opposite the drain/outlet end, so water moves across the pond.
Elevation: inlet should enter above normal water level or into a small stilling basin/rock apron to reduce erosion.
Protection: use a short pipe with a splash pad (rocks, concrete, or geotextile + riprap) where water enters.
Screening: install a removable screen to keep wild fish, frogs, and debris out (details below).

Diagram described in words (plan view): Picture a rectangle pond. The inlet is on the upper-left corner, discharging onto a rock apron. The outlet/drain is on the lower-right corner. The pond bottom slopes gently from the inlet end toward the outlet end.

Outlet placement

The outlet should be at the lowest point so the pond can drain completely. Put it where you can access it safely during high water.

Best location: at the deepest end, typically in the dike, connected to a drain channel outside.
Discharge area: protect the downstream side from erosion with riprap or a concrete/stone apron.
Accessibility: ensure you can reach valves/standpipes/monk boards without wading in deep water.

Spillway and overflow protection

A spillway is not optional. It prevents water from overtopping the dike, which is one of the fastest ways to lose the pond.

Types of spillways for small ponds

Emergency earthen spillway: a broad, shallow channel cut into natural ground beside the dike (preferred for low cost and reliability).
Pipe spillway: a fixed pipe through the dike set at the maximum water level (can clog; still keep an emergency spillway).

Emergency spillway basics

Location: in undisturbed soil at one end of the dike (not through the dike fill if possible).
Elevation: set the spillway crest slightly below the top of dike, maintaining freeboard (e.g., spillway crest at normal max water level; dike crest higher by 0.5–0.75 m).
Shape: wide and shallow to reduce flow velocity.
Protection: grass cover is minimum; add geotextile and rock (riprap) if flows can be fast or soils erode easily.

Diagram described in words (spillway profile): Imagine the pond at full level. Water reaches the spillway crest first (a low, wide notch in natural ground). Water then flows out over a grassy, gently sloped channel away from the pond, never touching the top of the dike.

Overflow protection details

Anti-erosion: line high-velocity zones (pipe outlets, sharp turns) with riprap over geotextile.
Debris control: keep vegetation trimmed and remove branches/leaves that could block spillways or screens.
Animal damage: burrowing animals weaken dikes; inspect regularly and repair immediately.

Pond bottom grading for complete drainage

Complete drainage makes harvesting easier, reduces disease carryover, and allows pond drying and repairs. The goal is a bottom that slopes smoothly to a drain point without isolated low pockets.

Target bottom slopes

General bottom slope: 0.5–2% toward the outlet (that is, 0.5–2 cm drop per meter, or 0.5–2 ft per 100 ft).
Harvest basin/sump: create a slightly deeper area near the outlet where fish concentrate as water drops.

Practical example: If your pond is 40 m long and you want a 1% slope, the bottom should drop about 0.4 m from the shallow end to the drain end.

Step-by-step: grading a drainable pond bottom

Set reference stakes around the pond footprint with planned elevations (use a laser level, builder’s level, or water level).
Mark the drain point (lowest elevation) at the outlet structure location.
Rough grade with equipment, keeping the slope consistent toward the drain end.
Shape a harvest sump near the outlet: a shallow bowl or trench leading to the drain so fish move there as water recedes.
Remove pockets: walk the bottom (when dry) and fix any depressions that would hold water.
Compact high-traffic areas (near inlet/outlet) to reduce erosion and rutting.

Diagram described in words (bottom slope): Picture a side view of the pond bottom: it is not flat. It gently tilts downward from the inlet end to the outlet end, where a small deeper sump sits directly in front of the drain opening.

Simple water control structures

Water control structures let you set water level, drain the pond, and protect fish from escaping or predators entering. The simplest systems are standpipes and monk outlets. Choose based on budget, skill, and how precisely you need to control water.

Standpipe system (simple and low-cost)

A standpipe is a vertical pipe connected to a horizontal pipe through the dike. Water level is set by the height of the standpipe opening.

How it works: water flows into the standpipe and out through the pipe. To drain, you lower or remove the standpipe (or use a swivel/hinged standpipe design).
Best for: small ponds where you want simple level control.
Key details: include a trash guard (screen cage) around the standpipe intake to prevent clogging.

Diagram described in words (standpipe): In the pond near the dike, a vertical PVC pipe stands upright like a post. It connects at the bottom to a pipe passing through the dike. Outside the pond, the pipe discharges into a protected channel. A wire mesh cylinder surrounds the vertical pipe to keep leaves and fish away.

Monk outlet (better control, common in fish ponds)

A monk is a vertical structure (often concrete or wood) with removable boards (stoplogs) that set the water level. Water exits through a pipe at the base.

How it works: you add/remove boards to raise/lower water level in small steps; to drain, remove boards gradually.
Advantages: fine control, less risk of sudden drawdown, easier to skim surface water if designed with a top intake.
Considerations: more construction effort than a standpipe; must be sealed well to prevent leaks.

Diagram described in words (monk): At the dike, a rectangular vertical box stands with grooves inside. Wooden boards slide into the grooves like drawers, forming a watertight barrier. Water spills over the top board into the box and then flows out through a pipe at the bottom through the dike.

Screens: keeping fish in and unwanted organisms out

Screens are needed at inlets (to block wild fish, eggs, and debris) and often at outlets (to prevent escape during overflow or draining).

Inlet screen: use a removable mesh basket or box screen. Make it large enough that water flows through without clogging quickly.
Outlet screen/trash guard: a cage around the intake (standpipe/monk) prevents leaves and fish from blocking the opening.
Mesh size: choose based on the smallest fish you need to retain; finer mesh clogs faster, so increase screen area if using fine mesh.
Maintenance: plan for daily checks during heavy flow or leaf fall.

Anti-seep collars and pipe-through-dike basics

Pipes through dikes can leak along the outside of the pipe (seepage path). Reduce risk with:

Anti-seep collars (collars or diaphragms around the pipe embedded in compacted clay).
Good bedding and compaction around the pipe in thin lifts.
Proper pipe material (smooth, durable) and protected outlets to prevent erosion.

Word-described diagrams you can sketch before building

Diagram 1: Pond plan view (top-down)

Draw a rectangle. Label the left short side “inlet end” and the right short side “outlet end.” Put a small square at the inlet corner labeled “inlet + screen + rock apron.” Put a circle near the outlet end labeled “standpipe/monk.” Draw arrows showing water moving from inlet toward outlet. On the outside of the dike near the outlet, draw a channel labeled “drainage ditch.”

Diagram 2: Dike cross-section

Draw a trapezoid. The top flat part is the crest (label width). The left slope is the upstream slope (label 3:1). The right slope is the downstream slope (label 2.5–3:1). Draw a horizontal line for normal water level below the crest and label the vertical gap as freeboard. Under the center of the trapezoid, draw a narrow rectangle down into the ground labeled “core trench (key).”

Diagram 3: Bottom slope to sump

Draw a long shallow line sloping down from left to right. At the right end, draw a small deeper dip labeled “harvest sump.” Above the right end, draw the outlet structure. Add a note: “Bottom slope 0.5–2% toward outlet.”

Low-cost materials list (typical small pond)

Exact quantities depend on pond size, but this list covers common low-cost choices.

Earthwork: suitable clayey soil (on-site if available), topsoil reserved for grass cover.
Compaction: access to a sheepsfoot roller (ideal) or plate compactor for small areas; tractor for repeated passes.
Pipes and fittings: PVC or HDPE pipe for outlet (diameter sized to drainage needs), elbows/couplers, solvent cement (PVC) or fusion fittings (HDPE).
Water control structure: standpipe components (vertical pipe + base connection) or monk materials (treated lumber or concrete blocks/forms, stoplogs/boards, seals).
Screens: galvanized welded wire mesh or stainless mesh (longer life), hose clamps/bolts, frame material (PVC, angle iron, or treated wood).
Erosion protection: geotextile fabric, riprap/rock, gravel for aprons, optional concrete for high-velocity outlets.
Spillway protection: grass seed/sod, erosion control matting (optional), stakes.
Tools: level (laser or builder’s), measuring tape, stakes/string line, shovel, rake, wheelbarrow, hand tamper.

Quick sizing checklist (fill in your numbers)

Pond shape: rectangle (L x W) = ____ m x ____ m  (Area = ____ m²)  Depth target = ____ m  Freeboard = ____ m  Crest width = ____ m  Slopes: upstream ____:1  downstream ____:1  Bottom slope = ____ %  Outlet type: standpipe / monk  Emergency spillway: yes (crest elevation = ____)

Now answer the exercise about the content:

Which pond layout choice best supports easy seining and complete drainage in a small-scale fish pond?

You are right! Congratulations, now go to the next page

You missed! Try again.

Long rectangular ponds are easier to seine, and a smooth bottom slope (about 0.5–2%) toward the lowest-point outlet helps the pond drain fully for harvest and maintenance.