How to Build a Dam on a Farm: A Step-by-Step Guide

Knowing how to build a dam properly is the difference between a storage that holds water for 50 years and one that leaks in its first wet season. Lindsey Hughson has designed and built farm dams across New South Wales, Queensland and Victoria since 2002, and this guide sets out the exact order Big Ditch follows on site. It is the step-by-step companion to our complete guide to farm dam construction costs and timelines, which covers pricing in detail.

How to build a dam: the 10 steps at a glance

1. Check your water rights and council rules before you spend money on design.
2. Calculate how much water you need and whether the catchment can fill it.
3. Choose a site with a gentle fall, a natural gully and no rock near the surface.
4. Dig test pits and confirm the subsoil holds enough clay to build a watertight wall.
5. Design the wall, batters, freeboard and spillway to suit the dam size.
6. Peg out the footprint and strip the topsoil into a separate stockpile.
7. Cut a keyway trench under the wall line into firm, impervious clay.
8. Excavate the basin and compact the wall in thin layers with a padfoot roller.
9. Build the spillway and fit a trickle pipe below spillway level.
10. Respread topsoil, seed the wall, and let the dam fill slowly while you inspect it.

Each step is explained below with the measurements we work to. Where a figure comes from government or university guidance rather than our own practice, it is linked to the source.

Before you dig: rules, water needs and site

Do I need permission to build a dam in NSW?

In NSW, most landholders can build a dam without a water licence if it captures no more than their harvestable right, which is generally 10% of the average regional rainfall runoff from the property, according to the ANU Sustainable Farms planning guide. Dams on rivers, larger storages and dams within 3 km of a Ramsar wetland need further approval. Your local council may also require a development application. Queensland and Victoria use different rules, so read our guide to dam construction regulations in each state before you commit to a design.

The NSW Environment and Heritage guide to building a farm dam recommends seeking professional advice for any dam with a wall higher than 4 metres or a capacity above 5,000 cubic metres (5 ML).

How much water does the catchment need to supply?

Start with demand: stock, irrigation, firefighting and household use, plus a margin for evaporation and drought. Then check that the catchment can fill that volume in an average year.

Catchment yield is the volume of runoff a catchment produces, calculated as area (ha) x rainfall (mm) x runoff (%). As a rule of thumb from the NSW guide, a 5 hectare catchment of moderate slope should fill a small dam of up to 1,000 cubic metres. A dam that is too large for its catchment rarely fills, and one that is too small for its catchment needs a bigger spillway.

Where is the best place to build a farm dam?

The best site is a natural gully or gentle hillside where a short wall stores a lot of water. ANU Sustainable Farms advises a gully floor slope under 2% for gully dams and a hill slope under 5% for hillside dams, because steeper ground needs more earth for less storage. The NSW guide advises avoiding sites steeper than 15%.

Rocky ground and steep country can still work, but they change the design. See our separate guides to building a dam on sloping ground and building a dam on rocky terrain.

Testing the soil before building a dam on your property

Soil decides whether a dam holds water. A wall built from sandy or dispersive soil will seep or tunnel no matter how well it is compacted, so testing comes before any design work.

Big Ditch digs three to five test pits to at least 1.5 m across the basin and wall line, then checks clay content, plasticity and dispersion. ANU Sustainable Farms describes good dam-building soil as roughly 20% to 40% clay, 10% to 20% silt and 40% to 60% sand.

A dispersive clay is a clay that breaks apart and clouds water when wet, which lets water tunnel through a dam wall. The Emerson crumb test is a simple field test that drops a soil crumb into water to see whether it disperses. Dispersive soils can be treated with gypsum, which ANU suggests at about 1 tonne per 750 cubic metres of earth moved.

If the site has no usable clay, a dam can still be built with imported clay or a bentonite liner, but the cost rises. Big Ditch typically applies sodium bentonite at 30 kg per square metre where the native soil is too porous.

Designing the dam wall and spillway

The wall design sets the dam’s safety margin. These are the key dimensions for a typical small to medium earth farm dam, drawn from the NSW and ANU guides and from Big Ditch practice.

Design element	Typical specification	Source
Upstream batter (horizontal:vertical)	3:1 or flatter for deeper dams	ANU Sustainable Farms
Downstream batter (horizontal:vertical)	2.5:1 to 3:1	ANU Sustainable Farms
Crest width (m)	At least 3	ANU Sustainable Farms
Freeboard (m)	At least 1	NSW guide and ANU
Compacted layer thickness (mm)	150 or less	NSW guide
Cut-off trench depth into impervious clay (mm)	At least 300	NSW guide
Trickle pipe below spillway (mm)	About 300	NSW guide and ANU
Largest rock allowed in the wall (mm)	75	NSW guide

Freeboard is the vertical height between full supply level and the top of the wall. It stops waves and flood surges overtopping the crest.

A spillway is a shaped channel cut into natural ground beside the wall that carries flood water safely around the dam. The spillway should be sized for the catchment, not the dam, and must sit on undisturbed ground so water never runs over fill.

How deep should a farm dam be?

Most farm dams Big Ditch builds have walls between 3 and 6 metres high. Deeper dams lose a smaller share of their water to evaporation, because less surface area is exposed per megalitre stored. Once the wall passes 4 metres, the NSW guide recommends professional design advice.

How to build a dam wall that holds water

This is the stage where most failed dams go wrong. The steps below follow the order Big Ditch works in on site.

Why do you strip the topsoil first?

Topsoil is full of organic matter that rots and leaves voids, so it must never go into the wall. Strip it from the wall footprint and basin, typically 150 to 300 mm deep, and stockpile it separately to respread on the finished batters.

What is a keyway in dam construction?

A keyway is a trench cut along the centreline of the wall and backfilled with compacted clay, which locks the wall to the foundation and blocks seepage underneath it. The NSW guide requires it to reach at least 300 mm into impervious material. On a typical 4 metre wall, Big Ditch usually cuts the keyway 1.2 to 1.5 metres deep.

How do you compact a dam wall?

Spread clay in even layers no thicker than 150 mm and compact each one with a padfoot (sheepsfoot) roller before placing the next. Moisture matters: clay that is too dry will not bind, and clay that is too wet will pump and rut. Keep logs, roots and rocks over 75 mm out of the wall. In Big Ditch’s experience, most leaks in new dams trace back to layers placed too thick or too dry.

Spillway, trickle pipe and first fill

Cut the spillway into undisturbed ground at one end of the wall and grass or armour it so it does not erode. If you fit a trickle pipe to carry low flows, the NSW guide sets its inlet about 300 mm below spillway level, with collars to stop water tracking along the pipe.

Respread the stockpiled topsoil over the batters and seed them straight away. Grass cover protects the wall from erosion in the first storms.

Fill the new dam slowly if you can control inflow, and walk the downstream batter weekly during the first fill. Wet patches, cloudy seepage or small sinkholes need attention before the dam is full, when repairs are cheaper and safer.

Can you build a farm dam yourself?

A small soak or stock dam on good clay is within reach of an experienced machine operator. Larger dams are a different job. They need a 20 to 30 tonne excavator or a dozer with rippers, a padfoot roller and the judgement to read clay moisture, and the NSW guide notes that a bulldozer with rippers suits most dams over 250 cubic metres.

A typical Big Ditch dam crew uses this plant:

• An excavator of 20 to 30 tonnes for the keyway, basin and spillway.
• A dozer with rippers for larger dams and hard ground.
• A padfoot roller of 12 to 20 tonnes for compacting each wall layer.
• A water cart for bringing dry clay up to the right moisture.
• A laser level or GPS for setting batters, crest height and spillway level.

Hiring this plant without an operator who can read clay moisture is the most common false economy in DIY dam building. A wall that looks finished can still fail on its first fill if the compaction was wrong.

Most farm dams take 5 to 14 working days on site once approvals and design are complete. If you are weighing up a contractor, Big Ditch offers a $500 (GST inclusive) site inspection to check your dam site and soil before you commit to a full build.

Frequently Asked Questions

How much does it cost to build a dam on a farm?

Big Ditch prices new dams per megalitre stored: $12,000 per ML up to 3 ML, $8,000 per ML for 3 to 5 ML, $7,000 per ML for 5 to 10 ML, $6,000 per ML for 10 to 20 ML and $5,000 per ML above 20 ML, all excluding GST, mobilisation and fuel surcharge. The farm dam construction guide breaks these costs down further.

How long does it take to build a dam?

Earthworks for a typical 5 to 10 ML farm dam take 5 to 14 working days. Approvals, soil testing and design can add several weeks before machinery arrives, and wet weather can stop compaction work entirely.

What is the most common reason a new dam fails?

In Big Ditch’s experience, poor compaction and unsuitable soil cause most new-dam failures, often during the first fill. Layers placed too thick, clay that is too dry, a missing keyway or dispersive soil left untreated all let water find a path through or under the wall.

Planning a new dam and want it done right first time? Book a site inspection with Big Ditch and get expert advice on how to build a dam that lasts.