A contractor finishes rough grading on a 12-acre subdivision pad in October and leaves for winter without seeding or blanketing. By March the site has lost four inches of topsoil from the steepest cuts, a sediment plume has reached the creek two hundred yards downslope, and the county issues a stop-work order. The erosion risk was obvious to anyone who looked at the slope, the bare silt-loam surface, and the absence of cover — three variables that predict most of the damage before it happens.
This calculator scores erosion potential on a 0–100 scale using slope steepness, soil erodibility class, and ground-cover condition. The output is a screening index, not a soil-loss prediction — useful for flagging high-risk areas, comparing pre- and post-grading scenarios, and deciding where controls deserve priority before commissioning a full RUSLE2 analysis.
Slope Gradient and Length: The Dominant Driver
Slope speeds up runoff and increases the gravitational component of detachment simultaneously. Double the gradient on a bare hillside and erosion roughly quadruples — closer to a square relationship than a straight line. A 6% slope and a 15% slope sit in different risk tiers entirely.
Slope length amplifies the gradient effect. A short, steep bank sheds water before flow concentrates enough to cut rills. A long hillside at the same gradient accumulates sheet flow until it transitions into rill erosion halfway down, moving soil orders of magnitude faster. RUSLE accounts for this with a combined LS factor; this screening index captures gradient but not length, so treat long uniform slopes — anything over 200 feet without a bench or diversion — as one tier higher than the score suggests.
Concave slopes are self-limiting: flow slows and deposits near the toe. Convex slopes put the steepest segment at the bottom where flow has already concentrated. If the cross-section is convex, use the gradient at the lower third, not the average.
Soil Erodibility: Texture, Structure, and K-Factor
A silt loam with weak granular structure is among the most erodible soils on the planet. Silt particles detach easily but are too large to bond into stable aggregates the way clay can. Add low organic matter — common on subsoil exposed after grading — and the K-factor climbs above 0.40.
Sand seems like it should erode easily, but coarse grains are heavy relative to their surface area. Runoff struggles to carry them far, and sandy soils drain fast, generating less surface flow. That combination gives clean sands a K-factor below 0.10 — low erodibility despite poor structure.
Well-aggregated clay resists raindrop impact when undisturbed. Compact it with heavy equipment, destroy the aggregates, and the surface seals within minutes of the first rain — generating runoff like pavement while loose crumbs wash away. Construction sites on clay often erode worse than the K-factor from the USDA NRCS Web Soil Survey would predict, because the published value assumes undisturbed structure.
Ground Cover and the Seasonal Exposure Window
Cover is the variable a landowner can change fastest. A bare slope scoring 75 drops below 50 with a good stand of grass — vegetation intercepts raindrops, roots bind the top two inches of soil, and stems slow sheet flow enough to prevent rill initiation.
Timing matters as much as type. A winter-wheat field has dense cover November through May but sits as bare stubble through summer thunderstorm season. Running the index once with “good cover” misses the months where all the damage happens. Score the worst-case season separately and size erosion controls for that window.
Mulch and erosion blankets count as cover even without living roots. Two inches of straw at 2 tons per acre reduces splash erosion by 80% or more. Rolled erosion-control blankets bridge the gap between grading and germination — typically 6–12 weeks — keeping the index in the moderate range on slopes that would otherwise read very high.
Variable Cheat-Sheet: Risk Scores by Scenario
Compare your site against these representative scenarios to check whether the output lands in a plausible range:
| Scenario | Slope | Soil | Cover | Typical Score |
|---|---|---|---|---|
| Established lawn on gentle lot | 3% | Moderate | Dense | 25–30 |
| Farm field after harvest | 8% | High | Sparse | 55–65 |
| Graded construction pad | 15% | High | Bare | 75–80 |
| Forest on steep terrain | 25% | Low | Dense | 35–45 |
| Road cut in silt loam | 30%+ | Very High | Bare | 85–95 |
If your result diverges sharply from a comparable scenario, re-check the inputs. The most common error is selecting “moderate” soil by default when the exposed subsoil after grading is actually high or very high erodibility — a distinction that moves the score 10–15 points.
When This Index Underestimates Real Erosion
A three-variable index captures the broad strokes. Several real-world conditions fall outside its reach:
- Concentrated flow paths. The index assumes sheet erosion across a uniform surface. Once runoff converges into a swale, wheel rut, or trench, velocities spike and rill or gully erosion takes over. A site scoring 55 as sheet flow can lose more soil from a single rill than the surrounding hillside combined.
- Freeze-thaw cycling. Repeated freezing lifts soil particles out of the surface and leaves them loose for the next rainfall or snowmelt. A slope stable all summer can lose an inch of topsoil during spring thaw if the freeze-thaw cycle runs daily for weeks.
- Construction-phase vs post-stabilization. The same parcel scores very high during rough grading and low after seeding takes hold. Running the index after hydroseed germinates but before root mass develops produces a false sense of security. Score at peak exposure, not peak optimism.
- Rainfall intensity spikes. The index does not model storm intensity. A moderate-risk slope under drizzle behaves very differently under a 2-inch-per-hour convective cell. If your region sees intense short-duration storms, actual erosion will exceed what the static index predicts.
Mistakes that show up after the first storm: using the soil survey K-factor for undisturbed topsoil when grading exposed the subsoil, scoring cover as “moderate” based on scattered weeds that wash away in the first rain, and ignoring a 400-foot flow path concentrating into one low corner.
Related tools: Stormwater Runoff Volume Estimator to quantify the runoff that drives the erosion your index flags, Retention Pond Size Estimator when eroded sediment needs a settling basin downstream, Watershed Catchment Calculator to delineate the drainage area feeding the erosion zone, and Contour Area Calculator to map the slope geometry that underpins the risk score.
Risk scores from this tool are simplified screening estimates based on three input variables — they do not replace a RUSLE2 soil-loss prediction, a professional erosion-control plan, or compliance review against NPDES or local grading-permit requirements.