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Let&#;s start with getting an idea of the size of the dam you&#;re looking to construct. The size of your dam should primarily depend on its purpose. How much water do you actually need? If the primary purpose is swimming, your dam probably doesn&#;t need to be very big, but if your dam is going to double up as an irrigation reservoir, you&#;ll need to build something substantially larger.
The size of your dam also depends on how much water the location (see below) is capable of collecting. There is no point building a whopping big dam on the side of a tiny hill as it will never fill. The same with building a tiny dam on the side of a massive hill or gully, you&#;ll likely encounter problems with broken dam walls, excessive erosion, flooded equipment etc. We discuss this more in the watershed section below.
The depth of your dam also depends on its purpose but keep in mind the biggest problem for dam owners is having a dam that is too shallow. Shallow dams generate high temperatures&#;ideal conditions for algae, weeds etc&#;and are difficult and expensive to aerate. We explain why in our Aeration section below.
For these reasons we recommend a minimum depth of 3m, but more if possible. For an irrigation dam we suggest going 8-10m deep with a small surface area (to reduce evaporation). For an ornamental / swimming dam, we recommend 3-5m.
Once you have a rough idea of the size and depth of your dam, move onto the next section to see if your idea holds water. You can adjust the size, depth etc as you move through the steps.
The first big question is do you have site access? i.e., can you get heavy machinery into the area to do the dig? You&#;ll also need to consider what you&#;ll do with the removed dirt. Can the dirt be used in the dam walls, or do you need to transport it out? Keep in mind you can&#;t just dump the dirt on your paddocks as it will kill everything. If you do need to transport the dirt out, we recommend advertising &#;free clean fill&#; and trying to strike a deal with someone where they come in and remove the piles themselves as transport is the most expensive part.
How big is the watershed for the proposed dam site? A watershed boundary is defined by the highest elevations surrounding the dam. Any map showing detailed elevations of the area, or a good study from the ground, will show you the watershed boundary.
Basically, rain that falls within the watershed will run downhill (often underground) and head towards your dam, while rain that falls just outside the watershed will travel in a different direction away from your dam.
Is the watershed large enough to keep your dam topped up most of the year? If not, you could look at building a second smaller dam which you use to pump water into your main dam.
You also need to understand what the land in your watershed is used for. This is not only to make sure the water entering your dam is clean and healthy for people and animal visitors, but also to reduce excessive algae, weed or siltation (dirt build-up on the dam floor).
If any of the below apply to your watershed, we suggest moving your dam elsewhere:
Confident in your watershed? Move to the next step; testing the soil.
Do some test drilling to see what&#;s in the earth where you want the dam to go. You&#;ll need at least 20% clay in your soil to effectively hold water. Soils that are too sandy or shaley tend to be too porous, resulting in a leaky dam.&#;You can still have a dam in these areas, but you&#;ll need to bring in clay or use a liner.
If you&#;re unsure whether you have enough clay in your soil, we recommend getting a soil testing company to do a survey and report for you.
If you&#;re fairly sure there is enough clay in the soil, you can probably skip the paid soil survey and conduct your own jar test. To do this, get several glass jars, say around five, and fill them with dirt from different areas you plan on excavating for your dam. You&#;ll need to dig down at least 20cm (we recommend 50cm), so you get past the topsoil to the main substrate. Fill the jars to the top with clean water. Wait at least a few days, even a week, then look at the jars. You should see 3 main layers on the bottom: sand, clay, and silt. Above these layers you should also see a band of murky water and a floating layer of organic material.
Above: The jar test&#;to measure the ratio of clay in your dirt
Between the bottom 3 layers (ignore the band of water and organic layer), you&#;re hoping to see at least 20% clay. So, for example, if you have 5 cm of height in the bottom 3 layers, you need at least 1 cm of clay (the top layer).
If you have less than 20% clay, we highly recommend using a liner or bringing in bentonite clay or a similar polymer which can be added to soil to increase the clay level. It&#;s a good idea to cost up both options if this is the route you take, as bringing in clay can be costly.
Big trees may look nice near a dam, but leaf litter turns to sludge and becomes fertiliser for algae and weeds. Excessive leaves entering your dam can also stain your water a tannin colour. And these are the least of your problems with trees nearby to your dam. Tree roots crack dam walls and when the trees eventually die, or are killed to stop further damage, their roots shrink, and the holes fill with water causing a lot of damage to your dam&#;s structure. The best thing is to simply keep trees far away. Smaller plants are fine, just always consider what the root structure of a mature plant looks like before adding a new species to nearby your dam.
Areas with a bit of wind action but not too much are ideal for dams. A little wave action on the surface of the water will help naturally aerate your water through what we call the &#;ripple effect&#;. Basically, rippled water has a greater surface area than flat water and therefore facilitates more gas exchange between the water and the air. This &#;gas exchange&#; is where oxygen enters the water and nasty gasses (e.g., methane, hydrogen sulfides) exit the water.
Keep in mind though, a lot of wind can lead to erosion problems through constant wave action. That said, a lot of erosion can be mitigated by adding large rocks and/or vegetation to the banks most battered by waves. Look at council parks or wetlands for ideas as they tend to factor blocks of stone and vegetation into their designs for this very reason.
Avoid long, skinny dams as these are difficult to aerate and keep healthy. The same is true for adding islands to your design. We recommend you don&#;t add islands as the more obstacles you put in the water, the more static and unhealthy the water becomes. The easiest dams to keep healthy are circular bodies of water.
To get an idea of how easy or difficult it is to aerate a dam, see the different shapes below and the common number of aerators/aerator air-stations required to sufficiently aerate the water.
Shape plays a big role in aeration because of what we call the &#;zone of influence&#;, which refers to the section of water aerated by a single aerator/aerator air-station. The zone of influence is set when the radial wave of water being mixed and aerated hits a bank/island which interrupts the flow. The less intrusions, the greater the zone of influence, e.g., a square or circular dam might only need 1 air station because the flow is uninterrupted across the entire body of water.
To reduce weed problems, we recommend you design your dam&#;s banks to get as deep as you can as quickly as you can. A 3:1 slope on dam walls is ideal. In other words, for every 3m across you go 1m down.
If you want native plants in your dam, you can cut out ledges and create different zones of different depths. Native plants can help siphon nutrients out of your water and reduce the chances of pests like weeds and algae showing up. However, this is only true if you maintain the vegetation and remove any dead organic material. Left on their own post-mortem, the opposite will happen, this organic material becomes sludge on the floor of your dam, which is fertiliser for pests. If you&#;re using plants also make sure no plant roots are in the way of swimmers entering the water.
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We recommend designing your dam so livestock can&#;t access the water. Livestock excrement leads to excessive nutrients in your water, which can trigger algal blooms, weeds, odour etc. If your animals need water, we suggest fencing off the dam and setting up a trough. It&#;s a good idea to include at least one easy exit point for any animals (livestock or wild) that fall into the water. If you&#;re using liner (we&#;ll run through whether to go for a liner or not in the next section), it&#;s very important to design an animal escape route. We recommend a gentle slope with a rope ladder that would allow an animal to climb out on its own. If you don&#;t and an animal falls in, the likelihood is they will slip on the material and panic leading to damage to your liner. Not to mention, the death of the animal. A rotting body also means more nutrients in your water&#;meaning more algal blooms and weed outbreaks&#;unless you haul it out.
A good excavator contractor will be all over this, but remember to incorporate a spillway for overflow in your design. This should take into consideration excessive amounts of water, such as once-in-a-decade floods. Basically, you want to make sure the water has somewhere to go once the dam is full, so the walls don&#;t get washed out. Where the water exits the dam it&#;s a good idea to add large rocks/boulders or encourage grasses to grow, so the water doesn&#;t erode the valley below. If you need to slow the water down to reduce erosion, consider adding zigzags to the intake or build a smaller pond further up the hill that can aid in slowing the water.
If you&#;re creating a swimming dam, maybe you want to add a steep edge with a jetty or a large overhanging rock that makes for a good diving area? This will need to be factored into the design pre-excavation so you can cut in a deeper area. You may also want to consider adding stone steps where swimmers enter from the shallows. Or better yet, you could consider bringing in some white beach sand to add to the main swimming entry. Just beware, riverbank sand is cheap, but it may turn your dam muddy, so we&#;d avoid this. We&#;d also avoid making a midnight run to the local sand pit. Road sand is often mixed with salt. Your best bet is to purchase the sand new. To add beach sand, you&#;ll need your excavator contractor to dig out around 10-20cm where you want the sand to be. You&#;ll need to lay down a geotextile underlay to keep the soil and sand from mixing. Lay the sand on top.
If your dirt has less than 20% clay, i.e., it&#;s made predominantly of rock, shale, or sand, and you&#;re not prepared to bring in additional clay, liner is a good option. We recommend EPDM liner (made from high-grade synthetic rubber) over plastic liner for several reasons.
Benefits of EPDM liners
EPDM liner is an upfront investment but it can save you a lot of time, money, and frustration in the long-term. Patching a leaky dam down the line is likely to be a lot more expensive than sealing it from the get-go. Bringing in clay is also typically a lot more expensive than lining your dam.
If you do go the EPDM liner route, a pro tip is to dig your dam hole prior to purchasing your liner. So dig your hole then, like the image below, lay a string or a tape measure down on the ground to get accurate measurements of the width and length of the hole. Be sure to measure the longest and widest sections of the excavation and include 0.5-1m of overhang on all sides. This overhang will be secured in place by a perimeter anchor trench. Now you have the exact size of liner needed.
Above: How to measure the width and length of liner needed for a dam
If you&#;re looking to save a penny wherever you can, you may even want to consider building your dam to a size that fits our rolls of EPDM liner. By limiting wastage, you can certainly reduce the total cost.
Check with your local Council prior to excavation to see if you need a permit.
We recommend doing the excavation during a dry period in Summer when site access is easiest and when the dam is least likely to start to get filled by rain.
To avoid returning home to a nasty surprise, e.g., your dam being half the size or twice the size you expected, we highly recommend being on-site during as much of the excavation process as possible. That way you can chat to the excavator as things unfold. After all, how things look on paper and in real life are very different things. You may just find you pivot your design on-the-fly as the excavation unfolds.
Most excavators will take the topsoil first and put it in separate piles to the deeper dirt. Chat to your excavator first and decide where you want these piles as this dirt is useful for garden beds, spreading across paddocks etc, unlike the deeper soils.
After the dam is dug and compaction has been done to make the clay tight, your excavator should conduct a process sometimes referred to as &#;keying it in&#; where they test the dam to ensure it will hold the force of the water. This needs to be done prior to the dam being filled.
To fill your dam, most will wait until the rain naturally does it in winter. If you&#;re in a rush, you could look at pumping in bore water. Although be careful how much iron content is in this water. You can also talk to water processing companies. Sometimes they need to get rid of water and a deal can be struck. Just be aware there are different grades of water in terms of cleanliness. You want to make sure the water you are bringing in is not contaminated in any way.
Here are some other fun ideas to take your dam design to the next level:
Once your dam has water in it, we recommend adding an aerator to ensure your aquatic ecosystem is as healthy as possible. Aeration simply means adding oxygen to water. Oxygen is the backbone to any healthy aquatic ecosystem&#;keeping plants, animals, and &#;good&#; microbes alive. These &#;good&#; microbes&#;aerobic bacteria who thrive in oxygenated environments&#;are the key to a healthy dam. They feed on the sludge on the floor of your dam and starve out algae and weeds by consuming their nutrients (e.g., nitrogen, phosphorus). They also eradicate smells by outcompeting &#;bad&#; microbes which emit odours.
Aerators add oxygen to dams and ponds by pumping air or water into the waterbody, either at the surface (surface aerators) or the floor (sub-surface aerators). This circulates the water, encouraging oxygenated top-waters to sink to the bottom and deoxygenated bottom-waters to rise to the surface. When the bottom-waters reach the surface, gas exchange occurs, whereby &#;bad&#; gases (e.g. methane, hydrogen sulphides) exit the water and &#;good&#; gases (e.g. oxygen) enter the water.
Above: How surface aerators and sub-surface aerators add oxygen to water
We also recommend you use a probiotic like Biostim on a monthly basis to help grow populations of &#;good&#; (aerobic) bacteria in your dam, reducing problems with algae, weed, odour, sludge etc.
Biostim is a 100% natural biological stimulant made of bacterial cultures, enzymes and nutrients. You can think of it as a probiotic that feeds the &#;good&#; microbes in your dam, much the same way you might eat Greek yoghurt or drink Kombucha to feed the &#;good&#; microbes in your gut. These &#;good&#; bacteria actively produce enzymes, speeding up the breakdown of organic waste and degrading sludge. This reduces the amount of nutrients available for pests (e.g., algae and weeds) to survive.
These bacteria also feed on the sludge on the floor of your dam. Getting rid of sludge also helps reduce algae and weeds because sludge acts as a fertiliser for these pests. Because Biostim consumes organic material, it can also clear water of cloudiness caused by suspended microorganisms and decaying material. This is not to be confused with muddiness from inorganic material; Biostim does not clear muddy water.
That&#;s it. You&#;re all set to go!
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