How to classify terrain complexity before micrositing

Every siting engineer runs into the same fork in the road on a ridge project: is this simple terrain, where a single met mast and a WAsP run will hold up, or is it complex enough that you need multiple masts, lidar, or a CFD flow model to trust the energy yield number. Getting that call wrong either wastes a mast on terrain that didn't need the extra instrumentation, or ships a resource assessment that falls apart in due diligence. Classifying it is a defined procedure, and most of it can be done from a desk before anyone drives out with a crew.

The IEC 61400-12-1 definition, in practice

IEC 61400-12-1 sets the baseline most developers and lenders default to. The core test looks at the slope of the terrain within a given radius around the turbine or met mast location, typically out to 2 km, measured as the ratio of elevation change to horizontal distance. Cross a slope of roughly 1:3 (about 17 degrees) anywhere in that radius and the site gets flagged as complex. Stay under that everywhere and you're looking at simple terrain for power curve testing purposes.

The slope test alone misses a lot of what trips up a wind flow model: a sharp escarpment edge, a saddle between two peaks, a valley that channels flow at an angle nobody predicted from the regional wind rose. That's why most resource teams pair the IEC slope criterion with a second check before deciding how many masts a ridge needs and where.

RIX: the second check nobody skips

The Ruggedness Index, RIX, comes out of the WAsP methodology and gives you a number instead of a pass/fail. RIX is the percentage of terrain within a defined radius (commonly 3.5 km around the site) where slope exceeds a critical threshold, usually set at 0.3 (about 16.7 degrees), the point past which WAsP's linear flow model starts to lose accuracy. A RIX of 0 to a few percent reads as genuinely simple terrain. Once RIX climbs past roughly 10 to 15 percent, the flow model is working in terrain it wasn't built for, and predicted wind speeds start drifting from what the met mast will actually measure.

The number that matters most for micrositing is delta-RIX: the difference between the RIX at your candidate turbine position and the RIX at the reference site (often where you're planning the met mast, or where a long-term reanalysis point sits). A large positive delta-RIX at a turbine site relative to the mast means the model is extrapolating into rougher terrain than it was calibrated on, which usually shows up as a speed-up or deceleration bias nobody catches until the met mast data comes back and doesn't match the model. A large negative delta-RIX flags the opposite problem: flow separation and turbulence risk that the linear model underestimates entirely.

Put the two checks together and you get a working rule: if slope stays under the IEC complex-terrain threshold across the ridge and delta-RIX between candidate positions and your proposed mast location stays low, you can likely run with the standard approach, one or two masts, a conventional flow model, and reasonable confidence in the yield number. If either check fails anywhere on the ridge, budget for the complex-terrain workflow from the start: additional masts at the ruggedness extremes, lidar to fill gaps a mast can't reach economically, or a CFD run alongside WAsP.

The part worth automating is the desk screen itself. Before any of this gets decided in the field, someone has to pull slope and roughness values across the whole candidate ridge, not just at the handful of points where a mast might go, because the segment that fails the test is rarely the segment anyone guessed. A terrain and obstacle map built from stereo satellite imagery does that screening pass across the full ridge at once, so the slope and obstacle numbers feeding your IEC and RIX checks are sitting in front of you before the crew truck is booked.

If you're about to rule a ridge in or out for a mast, run the terrain screen first and see which segments clear it.