Erosion Control on a Cut Slope Above a Creek
A 4.5-acre commercial pad is being graded on a hillside. The cut slope on the north side is 1.5H:1V, 22 ft high, and its toe is 35 ft from the top of bank of a perennial creek. The grading will expose the cut for an estimated 5 months (August through December) before the building and paving are complete. The SWPPP proposes a silt fence at the toe of the cut and a straw-wattle check dam in the swale that discharges toward the creek. The site receives about 18 inches of rain in that period, with the largest events in November and December. The creek is listed as impaired for sediment.
1. Assess whether the proposed perimeter controls are adequate for a 1.5H:1V, 22-ft cut slope discharging near an impaired creek, and identify the most serious deficiency.
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The controls are not adequate. A silt fence at the toe of a 22-ft, 1.5H:1V cut slope is a sediment-trapping measure, not a slope-stabilization measure, and it will be overwhelmed by rill and gully erosion off a steep, bare, 22-ft face during the November–December events. The most serious deficiency is the absence of any erosion control on the slope itself before the wet season. A 1.5H:1V face cannot be left bare for 5 months through the rainy season near an impaired water. Required: (1) immediate stabilization of the cut face - hydromulch with tackifier, erosion control blanket, or temporary seeding - applied as soon as the cut is made, with a bonded fiber matrix or blanket appropriate for the slope steepness; (2) a diversion at the top of the cut to keep offsite runoff from running onto the face; (3) a lined or armored conveyance for concentrated flow at the toe, not just a wattle check dam, because the swale discharges directly toward a sediment-impaired creek; (4) a sediment basin or equivalent sized for the contributing area before any discharge leaves the site; and (5) more frequent inspection during storm events. The 35-ft setback from top of bank is also a concern - a 22-ft cut reduces lateral support and increases the risk of slope failure into the creek, which should be evaluated geotechnically.
2. What sequencing change would most reduce the risk, and what regulatory consequence follows from the creek's impaired status?
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The highest-value sequencing change is to complete the cut and its permanent stabilization during the dry season, or to phase the cut so no more than a small area is exposed at one time and each phase is stabilized before the next begins. Concretely: make the cut in late spring/early summer, immediately install permanent slope protection (blanket, seeding, or structural facing), and defer the building and paving to the fall. If the schedule cannot move, the cut should be minimized or avoided by redesigning the pad elevation or using a retaining wall to reduce the exposed face. Regulatory consequence: because the creek is listed as impaired for sediment, the project is subject to a total maximum daily load (TMDL) and the NPDES construction general permit's impaired-water requirements. The SWPPP must include a sediment-basin or equivalent numeric analysis demonstrating that the discharge will not cause or contribute to the impairment, and the permit may require more stringent BMPs, monitoring, or effluent limits. Local agencies may also require a creek setback or riparian buffer that the 35-ft distance may not satisfy.