Stormwater and Erosion on Small Outdoor Jobs

Why this matters

A trench, a regrade or a fence line opens bare soil, and the first hard rain carries it down the driveway, along the gutter and into a storm drain that runs untreated to the nearest creek. The neighbor who photographs a brown stream leaving your job site does not care how small the job was, and neither does the city inspector who follows the stain back to your truck. Most small outdoor jobs sit below the federal permit line, which is exactly why shops assume they are exempt from everything. They usually are not exempt from the local rule, and they are never exempt from the mess.

Where the line actually sits

The federal layer. Under the Clean Water Act, section 402, stormwater from construction activity is a regulated discharge. The trigger for the federal construction stormwater permit (the Construction General Permit, or CGP, issued under the National Pollutant Discharge Elimination System, NPDES) is one acre or more of land disturbed, or less than one acre if the work is part of a "larger common plan of development or sale" that will disturb one acre or more in total. Disturbed means cleared, graded, excavated or stockpiled, not just the footprint of what you build. One acre is 43,560 sq ft.

The state layer. Most states are delegated to run NPDES themselves and issue their own construction general permit. California's is issued by the State Water Resources Control Board and carries its own risk-level and training requirements. The one-acre trigger is the federal floor; a state permit can add conditions but generally keeps that trigger for the permit itself.

The local layer, which is the one that catches small shops. Cities and counties running a municipal separate storm sewer system (an MS4, the public network of inlets, pipes and ditches) must control construction erosion under their own permit, and many write ordinances that start well below an acre: a few thousand square feet is common, some trigger on any work in a stream buffer, a few on any disturbance at all. Look up the grading and erosion-control ordinance for each jurisdiction you work in and keep the number on file.

The rule nobody needs a permit to violate. Most local codes also prohibit an illicit discharge: anything but rainwater entering the storm drain. Sediment washed off your site is one, at any size, which is why "we were under the threshold" is no defense at the inlet.

What actually leaves a small site

• Sheet flow is water moving as a thin layer across bare ground, picking up fines. Controlled at the downslope perimeter.
• Concentrated flow is water that has found a path (a swale, a tire rut, the trench itself). It moves far more soil per minute and defeats perimeter controls built for sheet flow.
• Stockpile erosion. Loose spoil is the most erodible material on site, and an uncovered pile beside the driveway apron is the commonest source of a brown gutter on a residential job.
• Track-out is mud carried onto the street by tires and boots, which the next rain washes to the inlet.

Concrete washout and tool rinse water have their own cards; see related below.

The three controls that do most of the work

Perimeter control: silt fence or fiber roll. Silt fence is a woven or non-woven geotextile on stakes. It works by ponding sheet flow behind it so the sediment settles out; it is not a filter, and water does not pass through it quickly. Three details decide whether it works:

• The toe is buried. The bottom edge of the fabric goes into a trench on the upslope side, commonly about 6 in deep and 6 in wide, with the trench backfilled and compacted. Fabric laid on the surface lets water run underneath, which is the most common silt fence failure there is.
• It runs along the contour, with the ends turned upslope. A fence run straight downhill is a channel. Turning each end up the slope a few feet (a "J-hook" or "smile") stops ponded water from running around the end.
• It never crosses concentrated flow. A swale or a ditch carries more water than the fabric can hold back, and the fence either blows out or gets undercut. Concentrated flow needs a check structure (rock or fiber rolls across the channel) or a diversion, not fence.

A fiber roll (a straw or coir tube, or wattle) does the same job on gentler slopes and around stockpiles, set in a shallow trench and staked at the manufacturer's spacing. Unstaked, it rolls away in the first heavy rain.

Inlet protection. The storm drain inlet nearest the site, usually downhill along the gutter, gets a filter: a fabric insert under the grate for a drop inlet, or gravel-filled bags or a purpose-made curb filter across the opening of a curb inlet. The rule that keeps you out of trouble with the city is that inlet protection slows and filters but does not seal the inlet. A fully blocked curb inlet floods the street in a storm and diverts water onto someone's driveway. Leave an overflow path, and check whether your municipality requires permission to place anything in a public inlet; some do.

Stockpile management. Put the spoil pile on the uphill side of the work, away from the driveway apron and the gutter, ideally on a tarp or plywood if it is going back in the hole. Cover it at the end of every day and before any forecast rain, anchored with sandbags or stakes, not loose bricks that blow off. Ring it with a fiber roll on the downhill side. A covered pile loses almost nothing; an uncovered one is the brown streak in the photo.

Track-out and the street

Keep vehicles on the paved driveway or a stabilized path where you can. If equipment must cross bare ground to reach the street, lay a rock pad, plywood or mats on the exit path. At day's end, sweep or shovel mud off the street and sidewalk, never hose it into the gutter; hosing moves the sediment straight into the inlet you just protected and turns your cleanup into the illicit discharge. A flat shovel and a stiff broom are the whole kit.

Worked example: a French drain and swale regrade on a sloped lot

A landscape and drainage shop is booked for a drainage correction at a house whose backyard slopes toward the street. The scope: a 140 ft perforated-pipe trench, 2 ft wide, from the back corner along the side yard to a pop-up emitter near the front, plus regrading a shallow swale in the backyard. Two days of work; the forecast shows about 1 in of rain arriving on the night of day one.

Step one: measure the disturbed area, not the footprint.

• Trench: 140 ft by 2 ft = 280 sq ft.
• Swale regrade: 30 ft by 50 ft = 1,500 sq ft.
• Spoil stockpile: 15 ft by 20 ft = 300 sq ft.
• Equipment path from the street across the side lawn: 10 ft by 60 ft = 600 sq ft.
• Total: 280 + 1,500 + 300 + 600 = 2,680 sq ft.

Against the federal trigger, 2,680 of 43,560 sq ft is about 6 percent of an acre, and the house is an existing lot, not part of a subdivision still being built, so no CGP. The local ordinance is the question. Say, for illustration, this county requires an erosion-control permit at 2,500 sq ft of disturbance. The job is 180 sq ft over (2,680 minus 2,500). Nobody would have noticed had the office counted only the trench, which is 280 sq ft. Counting the equipment path is what tipped it, and the obvious fix is to stop disturbing it: run the compact track loader on plywood mats across the lawn instead of churning a path, and the disturbed area drops to 2,080 sq ft, under the illustrative trigger. That is a real choice, and it has to be made at the estimate, not on day two. Whichever way it goes, the illicit-discharge rule still applies, so the controls below are needed either way.

Step two: find where the water goes. From the back corner, water runs down the side yard, across the front lawn, onto the driveway apron and 30 ft down the gutter to a curb inlet. Everything gets laid out backwards from that inlet.

Step three: choose controls for each pathway.

• Silt fence across the downslope edge of the swale regrade, 50 ft along the contour with both ends turned upslope, toe trenched. The swale itself carries concentrated flow once regraded, so the fence goes across the bottom of the disturbed area, not across the swale's outlet; the swale outlet gets two staked fiber rolls as checks until the grass is established.
• Spoil pile on a tarp at the back corner (the uphill end), with a 20 ft fiber roll on its downhill side and a cover tarp for overnight.
• A fiber roll along the trench's downhill side in the front yard, where the trench crosses the slope.
• Curb inlet filter bags across the inlet opening, set with a gap at the top for overflow.

Step four: before the rain. At the end of day one, the trench is open for 90 ft. The crew backfills what it can, covers the stockpile and weights it, and walks the fence to confirm the toe is buried along its full length. They sweep the driveway apron and the gutter to the inlet.

Step five: after the rain. On the morning of day two, the lead walks the site before any work starts. Sediment behind the silt fence stands about 3 in deep at its lowest point on a fence with about 18 in of exposed fabric, so roughly a sixth of the height; no removal yet, since state manuals commonly call for cleanout at a third to a half of the exposed height. At the trench's open end, water has cut a small rill around the end of the front-yard fiber roll, because it was not turned upslope. Sediment reached the gutter in a fan about 4 ft long, short of the inlet, and the inlet filter shows a light film. The crew shovels the fan back onto the site, extends the fiber roll and turns its end, and logs the miss.

The failure this avoided. A crew naturally puts the spoil beside the driveway apron, closest to the truck. Uncovered there, that pile drains down the apron to an inlet 30 ft away, and that is the stain the inspector follows back to the lettered truck.

What changes the answer

• A lot inside a subdivision under construction. The "larger common plan" language means a small disturbance on one lot can sit under the developer's CGP, with its own inspection and stabilization requirements, even though your piece is a fraction of an acre. Ask the general contractor or builder whose permit covers the lot and follow its stormwater plan; their inspector will hold you to it.
• Work in or next to a stream, pond or wetland. Placing fill or dredged material in waters of the United States is a separate Clean Water Act program (section 404, administered by the Army Corps of Engineers), and states and localities commonly add stream buffer rules on top. A drainage job that daylights a pipe into a creek bank is not a small-job erosion question any more. Stop at the design stage and call the local permitting office before quoting.
• A steep slope or long run. The state erosion and sediment control manual limits the slope length one fence can serve; past it, controls are stacked or the job needs an engineered plan.

How to tell it held

A sensible shop default: walk the site after any rain of about half an inch or more, and before leaving whenever rain is forecast. Check that sediment sits below a third of fence height, toes are buried and stakes upright, no rills run around fence or roll ends, covers are on, the gutter is clean to the inlet, and the inlet filter is intact. Photograph each with the date; a before-and-after set is the record that answers a complaint.

Controls stay until the ground is stabilized (seeded and mulched, sodded or re-covered), then come out, and the trapped sediment is spread on site, not swept into the gutter.

References

• US EPA, NPDES Construction General Permit and the one-acre construction stormwater threshold under Clean Water Act section 402.
• Your state's erosion and sediment control manual (installation details, cleanout heights, slope limits).
• Your city or county grading and erosion-control ordinance and illicit discharge ordinance (the MS4 program).
• US Army Corps of Engineers, Clean Water Act section 404 permits for work in waters of the United States.
• See related: Wash Water and the Storm Drain Rule; Concrete and Mortar Washout on Residential Jobs.