Silt Fence Drainage-Area and Length Check
Whether a silt fence is long enough for the drainage area behind it.
Example
You enter
- Drainage area behind the fence (acres) 0.5
- Silt fence length provided (ft) 250
- Slope length behind the fence (ft) 60
- AHJ maximum slope length (ft) 100
You get
- Required fence length 200
- Max area ac 0.625
Checked against published worked examples from: Franklin Soil and Water Conservation District (Ohio); Metro Water Services, Nashville and Davidson County.
Details, formula, and sources
ScopeWhether a silt fence is long enough for the drainage area behind it (required length = tributary acres x 400, the quarter-acre-per-100-ft guideline) and whether the slope length stays under the AHJ limit. A 0.5-acre tributary needs 200 ft of fence; a 250 ft run catches up to 0.625 acre. Sheet flow only - a channel needs a check dam.
Formula or tablerequired_fence_len_ft = tributary_area_ac x 400 (a quarter acre of drainage per 100 ft of fence); max_area_ac = fence_length_ft / 400; length_adequate = fence_length_ft >= required_fence_len_ft; slope_ok = slope_length_ft <= max_slope_length_ft.
Source and editionSilt-fence drainage-area guideline by name (quarter acre per 100 ft); first-principles arithmetic; the AHJ slope-length limit is user-entered.
Free accessThe quarter-acre-per-100-ft figure is a generic published guideline; the AHJ maximum slope length is entered, not reproduced from a copyrighted table.
What governsEstimate. AHJ and licensed professional govern.
API field namestributary_area_ac, fence_length_ft, slope_length_ft, max_slope_length_ft, required_fence_len_ft, max_area_ac
- Drainage guideline quarter acre of drainage per 100 ft of fence (required length = acres x 400)generic SWPPP guidance
- Slope length 100 ft AHJ maximum default; the SWPPP spec and permitting AHJ governAHJ / SWPPP