A well-sited dam is a 30-year asset that quietly underwrites everything else you want to do on the property. stock water, garden irrigation, fire defence, the occasional summer swim. A badly sited dam leaks, silts up, washes out in the first big rain, or sits empty because the catchment was never big enough to fill it.
The siting decision is mostly geography, soil, and slope. three things you can investigate from satellite imagery before you commit to an excavator. This guide walks through how to think about each, and how to use PLOT to model alternatives before signing the earthworks quote.
Step 1. Find the catchment
A dam fills from the area uphill that drains toward it. Bigger catchment, faster fill, more reliable water. The first thing to identify on your property is the catchment for each potential dam site.
Open the editor at your address and zoom to your property. Look for where the land naturally collects water. ephemeral drainage lines on the satellite imagery, darker vegetation following gully bottoms, the convex shapes between ridges. Use PLOT's line tool to trace the drainage lines you can see. The proposed dam wants to sit at or just below the point where two or more drainage lines converge, with a substantial slope above it.
A rough rule for stock dams: catchment area should be at least 10 times the dam's water surface area. In high-rainfall country (>800mm/year) you can go lower; in semi-arid country (<400mm/year) much more.
Step 2. Check the soil
Sandy or gravelly soils don't hold water. the dam will leak out the bottom and the back wall. Clay soils with a high clay-fraction (>30%) hold water naturally and make the best dam sites. Loam is in-between and may need a clay liner or geomembrane to be reliable.
You can't tell soil composition from satellite imagery. The practical step is to dig a test hole or auger sample where you're proposing to put the dam. If you're already on the property, look at the colour of any existing erosion. orange-brown subsoil is usually clay-rich, grey-tan is often sandy. If you have neighbours with existing dams, ask them what they did and whether their dams hold water.
Step 3. Calculate volume from area and depth
Dam volume is approximately surface area * average depth * 0.4 (the 0.4 accounts for the sloped walls). A dam 30m * 20m * 3m average depth holds about 720 cubic metres. 720,000 litres, enough for 50 cattle for 100 days at 150L/day each.
Use PLOT's polygon tool to draw the proposed dam outline; the live area readout gives you the surface area. Estimate average depth from the slope. a dam built into a gully with a 3m wall typically averages 2.5m. Multiply.
Step 4. Respect the regulators
Dam construction is regulated everywhere. Headline rules:
- Australia: state water-rights legislation. Queensland allows on-property catchment dams under stock-and-domestic rights up to a per-property volume cap; larger dams need a Water Act licence. NSW and Victoria similar. Check with your local NRM or DPI office.
- Canada: provincial water rights. Most provinces require a permit for any dam impounding more than a small volume.
- United States: state water rights, plus federal Section 404 if the dam impounds a navigable waterway. Western states ('prior appropriation' jurisdictions) treat water rights very strictly.
- United Kingdom: Reservoirs Act 1975 covers dams >25,000 cubic metres; Environment Agency abstraction licensing covers fill water.
- EU: Water Framework Directive principles apply at member-state level.
Small in-paddock catchment dams for stock-and-domestic use are usually the least-regulated category. Anything in a watercourse, or sized to impound significant volume, will need approvals. and the approval process takes months.
Step 5. Build the spillway
Every dam needs a spillway. a designed low point in the wall that overflow water exits without eroding the wall. The spillway is the single most-failed component of farm dams; cheap dams have inadequate spillways and the next big rain washes the wall.
Spillway should be sized for the catchment's likely peak flow during a major rain event. For domestic-scale dams, your local agricultural department or extension service usually has spillway-sizing tables. For larger dams, engage a designer.
Step 6. Consider what's downstream
If your dam fails, where does the water go? Neighbour's house? Public road? Another waterway? The risk-and-liability conversation has to be had at design time, not after. A dam upstream of any built infrastructure should be over-engineered with a margin.
Modelling alternatives in PLOT
The way to use PLOT for dam siting is to draw three or four candidate dam locations as polygons, sketch the catchment for each, estimate volumes, and compare. The Pro tier's multiple-concept-plans feature is designed for this. three concepts named 'dam. north gully', 'dam. south flat', 'dam. west boundary' can sit side-by-side.
The decision is rarely about which option is best on a single criterion. The right dam location balances catchment, soil, downstream risk, distance to where you want to use the water (every metre of poly pipe costs money), and how it fits with the rest of the paddock plan.
Talk to a contractor
Same advice as the septic guide: a one-day site visit from an experienced earthworks contractor or a soil-conservation extension officer will tell you things satellite imagery can't. PLOT's job is to let you arrive at that visit with a draft plan and informed questions, not to replace it.