1 acre clay loam 12% slope mountain NC

RUSLE soil loss with cover-factor comparison on a 1-acre Asheville construction site

A 1-acre commercial site graded into a hillside outside Asheville. 12% average slope. The Erosion and Sedimentation Control Plan needs to defend a cover-practice choice. RUSLE solved four times for the same physical site with different C-factors. The numbers make the decision obvious.

Bare grading: 100.7 tons/acre/year. Straw mulch (C = 0.06): 6.7. Erosion blanket (C = 0.10): 11.2. Temporary seed (C = 0.45): 50.4. The C-factor row dominates the equation on any steep, shallow-soil site — single largest lever an engineer controls.

Site inputs

Parameter	Value	Source
Disturbed area	1.0 ac	Project boundary
Average slope	12%	LiDAR
Slope length, λ	150 ft	Field
Soil texture	clay loam	SSURGO Web Soil Survey
R factor (Asheville)	140	USDA isoerodent map
K factor (clay loam)	0.37	USDA-NRCS K table
P factor	1.00	No support practice

Step 1 — LS factor for a 12% slope

Above 9% slope, RUSLE uses a different S formulation. With m = 0.5 for slopes ≥ 5%:

$L = \left(\frac{\lambda}{72.6}\right)^{0.5} = \left(\frac{150}{72.6}\right)^{0.5} = 2.066^{0.5} = 1.437$

$S = 16.8 \sin(\arctan(s/100)) - 0.50 = 16.8 \cdot \sin(\arctan(0.12)) - 0.50$

$S = 16.8 \cdot 0.1192 - 0.50 = 2.003 - 0.50 = 1.503$

$LS = 1.437 \times 1.503 = 2.160$

Compare: a 4% slope with the same length gives LS ≈ 0.69. Tripling the slope from 4% to 12% multiplied LS by 3.1×. Slope is the single biggest physical driver after rainfall.

Step 2 — RUSLE solved four times for the same site

R, K, LS, P are fixed by the site. C is the engineer's lever. Same 1-acre site, four cover practices:

Cover practice	C factor	A = R·K·LS·C·P (tons/ac/yr)	1-ac total (tons/yr)
Bare soil / active grading	0.90	140 × 0.37 × 2.160 × 0.90 × 1.0 = 100.7	100.7
Temporary seeding	0.45	140 × 0.37 × 2.160 × 0.45 × 1.0 = 50.4	50.4
Erosion blanket	0.10	140 × 0.37 × 2.160 × 0.10 × 1.0 = 11.2	11.2
Straw mulch (2 ton/ac)	0.06	140 × 0.37 × 2.160 × 0.06 × 1.0 = 6.7	6.7
Permanent grass (post-stabilization)	0.01	140 × 0.37 × 2.160 × 0.01 × 1.0 = 1.12	1.12

The numbers make the decision. Going from "bare grading" (100.7 tons/ac/yr) to "straw mulch" (6.7) is a 94% reduction for roughly $400/acre in mulch cost. Going from straw to erosion blanket increases yield 67% (6.7 → 11.2) — blankets are usually chosen for slopes too steep for mulch to stay put, not for sediment performance.

Step 3 — Volume converted to design basin storage

The active-grading scenario produces 100.7 tons/yr. NCDEQ permitting typically assumes ~9 months of active disturbance before stabilization:

$Y_{design} = 100.7 \times \tfrac{9}{12} = 75.5 \text{ tons over construction window}$

At a sediment bulk density of 100 lb/ft³:

$V_{sed} = \frac{75.5 \times 2000}{100} = 1{,}510 \text{ ft}^3$

NCDEQ regulatory minimum (1,800 ft³/ac for skimmer basins) gives:

$V_{min} = 1{,}800 \text{ ft}^3$

The two are close on a 1-acre site. Use 1,800 ft³. If you switch to mulch (6.7 tons/yr), the RUSLE-derived volume falls to ~100 ft³ and the regulatory minimum becomes the controlling number — but the basin still costs the same, so cover practice doesn't reduce basin construction cost on small sites. It reduces effluent sediment load, which is what matters under NPDES.

What changes if you tweak the inputs

If you change…	The result moves…
Asheville → Charlotte (R 140 → 180)	Yield scales by 1.29×; bare-grading site → 130 tons/ac/yr
Slope 12% → 6%	LS drops from 2.16 to ~1.05; yield halves
Clay loam → sandy loam (K 0.37 → 0.27)	Yield drops 27%
Add diversion ditches (P 1.00 → 0.60)	Yield drops 40% on top of any cover practice
Slope length doubles (150 → 300 ft)	L = (300/72.6)^0.5 = 2.03; LS rises 41%

Run the same scenario in HydroComplete

Toggle cover practices, change R/K/LS, watch the basin storage and trap efficiency update live. Built on the same RUSLE engine values used in this worked example.

Open in app 14-day free trial

Sources and further reading

USDA-NRCS. RUSLE: Predicting Soil Erosion by Water (Agriculture Handbook 703). Renard et al., 1997.
USDA-NRCS, NC. Isoerodent map: R = 140 for Asheville/Buncombe County.
NCDEQ. NC Erosion and Sediment Control Planning and Design Manual. Cover practices §5.51.
AASHTO Standard Practice. R_USLE 2 implementation for highway/construction projects.

— Michael Flynn, PE
R, K, C and LS values pulled directly from HydroComplete's SEDCAD4 RUSLE tables. Verify against your local SCS office for site-specific corrections.

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