Cam IncollIf you're designing swales on a property, the single most important thing you need to understand is where water flows. And the single best tool for understanding water flow is a contour map.
This guide walks through how to read contour lines practically — not as a geography exercise, but as a design tool for placing swales, dams, and water harvesting infrastructure on real land.
What contour lines actually tell you
A contour line connects points of equal elevation. Every point on a single contour line is the same height above sea level. Water doesn't flow along contour lines — it flows perpendicular to them, from higher ground to lower ground.
This simple principle is the foundation of all water management design.
When contour lines are close together, the slope is steep. When they're far apart, the ground is relatively flat. Where they form concentric rings closing inward, you're looking at a hilltop. Where they form V-shapes pointing uphill, you're looking at a valley or drainage line.
Why contour interval matters
Contour interval is the vertical distance between adjacent contour lines. A 1-metre interval shows more detail than a 5-metre interval, but also creates a busier, harder-to-read map.
For swale design, the right interval depends on your scale. A kitchen garden on a gentle slope might need 0.5m intervals to see subtle grade changes. A 50-acre paddock might work well at 2m or 5m intervals where you're looking for major landscape patterns.
The key insight: you're not trying to see every bump. You're trying to see where water wants to go.
Reading water flow from contours
Water flows downhill, perpendicular to contour lines. To trace the path water would take across your property:
Step 1: Find the highest point on your contour map.
Step 2: Draw an imaginary line that crosses each contour at right angles, always moving toward lower elevation.
Step 3: Where multiple water paths converge, you've found a natural drainage line — a creek, gully, or seasonal waterway.
These drainage lines are critical for swale placement. You generally don't put swales across drainage lines (that creates a dam, not a swale). Instead, swales go across the slope on the ridges between drainage lines, intercepting sheet flow before it concentrates.
Placing swales using contour lines
A swale is a level trench dug along a contour line. Because the trench follows the contour, water that enters it doesn't flow to one end — it sits evenly along the entire length, soaking into the soil.
The design process:
Identify your contour lines. You need accurate contour data for your site. This can come from a surveyor, LiDAR data, or a tool like Fieldwalker that generates contour lines from elevation data.
Choose your swale locations. Look for areas where sheet water flows across relatively even slopes — the spaces between drainage lines. The contour lines in these areas will be roughly parallel and evenly spaced.
Follow the contour. Your swale follows one contour line. The excavated soil goes on the downhill side to form a berm (mound). The berm itself becomes a planting area — fruit trees, perennial crops, or revegetation species.
Space your swales. The vertical distance between swales depends on rainfall, soil type, and slope. In high-rainfall areas with clay soils, closer spacing (1-2m vertical drop between swales) catches more water. In low-rainfall sandy soils, wider spacing lets water infiltrate naturally between swales.
The keyline connection
Keyline design, developed by P.A. Yeomans in Australia, takes contour-based water management further. Rather than following contours exactly, keyline cultivation runs slightly off-contour — parallel to a "keyline" that starts at the "keypoint" (the point where a valley transitions from steep to gentle slope).
The effect is to move water from valleys (where it concentrates) toward ridges (where it's scarce), evening out moisture distribution across the landscape. Understanding contour lines is the prerequisite for understanding keyline — you need to see the valley and ridge patterns before you can design across them.
Where to get contour data
The quality of your contour data directly affects the quality of your swale design. Options range from free global datasets to high-resolution local surveys.
Global elevation data (SRTM, Copernicus) — free, covers the whole planet, but resolution is typically 30m. Useful for broad landscape patterns, too coarse for swale placement on small properties.
National LiDAR data — in Australia, Geoscience Australia provides 5-metre resolution LiDAR-derived elevation data through their Web Coverage Service. This is survey-grade data captured by aerial LiDAR scanning. Resolution is high enough for meaningful swale design on properties down to a few acres.
Site-specific surveys — a professional surveyor with RTK GPS or a drone with LiDAR can produce sub-metre accuracy. Expensive but necessary for construction-grade earthworks.
Design tools — Fieldwalker generates contour lines from Geoscience Australia's 5m LiDAR data automatically, rendering them directly onto your satellite imagery. You set the interval, colours, and opacity, and immediately see the shape of your land.
Practical tips
Start with the big picture. Use a wide contour interval (5m) to see the major landscape patterns — ridges, valleys, drainage lines. Then zoom in with a tighter interval (1-2m) for detailed swale placement.
Walk the contour. After designing on paper or screen, walk the actual contour line on your property. Bring a dumpy level or a phone app with a spirit level. The ground truth always surprises you.
Start small. Build one swale first and observe it through a full rain season before committing to a whole system. Watch where water overflows, where it sits longest, and how the soil responds.
Respect the drainage lines. Swales across drainage lines become dams — sometimes intentionally, but often disastrously. If you need to cross a drainage line, use a spillway or culvert, not a continuous swale.
This article was written by the Fieldwalker team. Fieldwalker is a design tool for permaculture designers and regenerative farmers that generates contour lines from real LiDAR elevation data. Try it free →