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Stormwater Runoff Coefficients: rational C
res large lot to streets gravel
Rational Method runoff coefficients (C values) by land use/surface type and return period, plus SCS/NRCS curve numbers (CN) by hydrologic soil group (A/B/C/D) and land cover/condition — from ASCE MOP 77, FHWA HEC-22, USDA-NRCS TR-55, and NEH-630
| method type | record id | C max (dimensionless) | C min (dimensionless) | C typical (dimensionless) | description | land use | return period yr (yr) | standard | surface type | unit |
|---|---|---|---|---|---|---|---|---|---|---|
| rational C | res large lot | 0.4 | 0.25 | 0.3 | Single family residential, lot > 1/2 acre | residential | 10 | ASCE MOP 77 Table 2-2; FHWA HEC-22 Table 3-1 | large lot | dimensionless |
| rational C | res large lot 100yr | 0.46 | 0.28 | 0.34 | Residential large lot — 100-year return period | residential | 100 | ASCE MOP 77 Table 2-3 | large lot 100yr | dimensionless |
| rational C | res medium lot | 0.45 | 0.3 | 0.35 | Single family residential, 1/4–1/2 acre lot | residential | 10 | ASCE MOP 77 Table 2-2; FHWA HEC-22 Table 3-1 | medium lot | dimensionless |
| rational C | res multiunit attached | 0.8 | 0.6 | 0.7 | Multi-unit attached residential (apartments) | residential | 10 | ASCE MOP 77 Table 2-2; local SW manuals | multi unit attached | dimensionless |
| rational C | res multiunit detached | 0.6 | 0.4 | 0.5 | Multi-unit detached residential | residential | 10 | ASCE MOP 77 Table 2-2 | multi unit detached | dimensionless |
| rational C | res small lot | 0.6 | 0.4 | 0.5 | Single family residential, lot < 1/4 acre (suburban) | residential | 10 | ASCE MOP 77 Table 2-2; FHWA HEC-22 Table 3-1 | small lot | dimensionless |
| rational C | res townhouse | 0.65 | 0.4 | 0.55 | Townhouse / row house | residential | 10 | ASCE MOP 77 Table 2-2 | townhouse | dimensionless |
| rational C | roofs | 0.95 | 0.75 | 0.85 | Rooftops — flat or pitched | buildings | 10 | ASCE MOP 77 Table 2-2; FHWA HEC-22 Table 3-1 | rooftop | dimensionless |
| rational C | streets asphalt | 0.95 | 0.7 | 0.85 | Streets and roads — asphalt or concrete | transportation | 10 | ASCE MOP 77 Table 2-2; FHWA HEC-22 Table 3-1 | asphalt pavement | dimensionless |
| rational C | streets gravel | 0.7 | 0.35 | 0.5 | Streets and roads — gravel or macadam | transportation | 10 | ASCE MOP 77 Table 2-2 | gravel road | dimensionless |
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