How to Build a Farm Dam: A Quick No-Fail Guide

It’s not without reason that humble farm dams are called the “lifeblood of rural Australia.”

With more than 110,000 farm dams in NSW and around 1,8 million in Australia, they are true assets for Australian farmers.

If you are planning to build a farm dam on your property, know this. The dam is most likely to fail without the right planning and building methods.

So, how to build a dam on a farm correctly?

Time to take a closer look at farm dam construction and find out the answer.

Do You Need a Dam at All?

To start with, you need to assess the annual water needs of your farm based on the demands of irrigation, livestock watering, and other activities. If there are reliable water sources near your property, you may not need a dam. In other words, if you live in an area with enough annual rainfall, rainwater harvesting may just do the trick.

So how do you determine whether you need a dam? First step is to understand how much water your farm needs. Then, compare that with the average annual supply, and it will help you get the answer. This water planning exercise will also help you outline your dam’s capacity and size. 

But a farm dam is not just about making sure there’s enough water when you need it. A private dam will give you the advantage of managing water resources across your entire farm more effectively. You can optimise the water supply based on the seasonal demands and minimise waste.

The cool thing is, the latest research also suggests that dams on farms can be effective sources of renewable energy. According to Dr. Nicholas Gilmore from the UNSW Sydney, it can even compensate for power. “People on the fringes of the electricity network can be more exposed to power outages, and the supply can be less reliable. If there’s a power outage during a bushfire, for example, a pumped hydro system will give you enough energy to last a day, whereas a battery typically lasts around eight hours.” His study showed that over 30,000 farm dams in Australia are already eligible, at least in theory, to turn into hydro systems.

Looks like the potential of a humble farm dam could easily be far greater than just offering your crops and livestock relief from droughts!

How to Design a Farm Dam (+ Permits & Licences)

Let me say this straight: designing a farm dam is not an easy task. It’s best that you seek support from professionals to get this critical process right.

The first step towards building a farm dam is selecting the right site. Study the topography of your farm and identify an area that is ideal for retaining water. A basin or a natural depression is a good choice. The best option is to conduct a topographic survey to find the right spot.

This survey will also help you down the line, while prepping the site for construction!

Once the storage requirement is determined, calculate the critical storage period of the dam. This is the time that the water will last in the dam, considering periods of drought. Consider the evaporation rate in your area to determine the final size of the dam.

Here is the basic formula for dam size calculation.

Dam Size = [Annual Water Consumption] x [Critical Storage Period] x [Evaporation Allowance]

The next step is to ensure that the catchment area is sufficient to fill the dam. This is calculated by taking into account the annual runoff – the percentage of rainfall that is lost in the ground. 

It’s equally important to calculate the flood that the catchment area can generate once the dam is filled. This will help to determine the spillway width for carrying away the flood flow. Beyond that, the structural stability of the embankment has to be considered to minimise risks.

Lastly, check the dam construction regulations in your state and the permit requirements. Farmers in NSW can capture and store water based on their maximum harvestable right dam capacity (MHRDC). To construct a dam with a holding capacity more than your maximum capacity, you will need a water access licence for the additional volume of water. Plus, you will also need approval for the dam construction.

Now, there can be restrictions on building dams in specific locations as well. For example, you cannot build a harvestable dam on or within 3 km upstream of a Ramsar wetland site in NSW.

5-Step Site Preparation & Farm Dam Construction

You need sound planning for each step of the farm dam construction. Also, consider the required material availability. Here are the common steps for building a farm dam.

1. The preparatory work includes using a bulldozer to clear the entire dam area from trees, roots, scrub, timber, and rubbish. Generally, the clearance zone extends a few metres beyond the exact dam limits. Any trees around the site that obstruct the work should be felled.
2. The soil for building the embankment should be excavated from the area marked for water storage. For any additional material needed, dig borrow pits of the necessary dimension within the storage area.
3. The spillway should be cut by removing soil and levelled. The soil can be deposited on the embankment.
4. The embankment should be constructed in layers using earth-moving vehicles. Necessary equipment should be used for consolidating each layer before progressing to the next one. If compaction is being done by using bulldozers, consider an extra height of around 10% to take care of soil settlement.
5. Make sure that no tree branches, roots, or any perishable materials go into the embankment. Once completed, a layer of topsoil has to be spread over the embankment to allow a grass cover to grow.

How Much Does a Farm Dam Cost?

The cost of a farm dam will depend on its water-holding capacity. Beyond that, unfavourable site conditions will raise the excavation and earthwork costs. Smaller dams with capacities under one Megalitre can be built with a bulldozer and cost less. Larger dams with higher capacities can cost over a million dollars.

Beyond the excavation expenses, building a larger dam also involves other costs. These include permit and approval fees, survey, soil investigation and design expenses, and miscellaneous expenses.

5 Tips for Farm Dam Maintenance

Keeping your farm dam in good condition is a must and that requires right maintenance and regular inspections. Here are a few maintenance tips to help you out.

• While maintaining a vegetative cover on the embankment is necessary, ensure that no larger plants grow on it. Fence out the dam to protect the plant cover from the farm animals.
• During times of drought, check for silt deposition. Make arrangements for dredging or desilting to remove the silt.
• Keep the spillway free from obstructions, including tall grass, to ensure smooth passage of water.
• Planting around the dam area can reduce soil erosion and water evaporation. However, avoid planting large trees with deep penetrating roots.
• Check for any cracks along the length of the embankment. If needed, seek professional advice for repairing any cracks or animal burrows.

How to Build a Farm Dam: FAQs

How do you plan a farm dam?

Planning for a farm dam requires calculating your water needs and determining the dam size. The next step is to design the dam components and apply for the necessary permits. Lastly,  plan a step-by-step construction schedule.

What is needed to build a dam?

Farm dams can be constructed from the soil material on your farm. However, evaluating the soil quality through proper sampling is necessary to prevent embankment failures.

How long does it take to build a farm dam?

The time needed depends on the size of the dam and the earthwork challenges. The construction time can range between a few weeks to a few months.