Silt Fence Drainage-Area and Length Check
Whether 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 and source
required_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.
Silt-fence drainage-area guideline by name (quarter acre per 100 ft); first-principles arithmetic; the AHJ slope-length limit is user-entered.
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