12.3 In-Water & Dewatering Controls: Temporary Stream Crossings, Turbidity Curtains, and Sediment Bags

Temporary Stream Crossings the approved practice detail

Allowing earthmoving scrapers, dump trucks, or excavators to drive directly through an unprotected stream channel crushes aquatic habitat, breaks down streambanks, and sends massive plumes of suspended sediment downstream. Whenever construction equipment must cross a stream, creek, or wetland corridor, an engineered temporary stream crossing is mandatory.

Regulatory Permitting Prerequisites

Because constructing a crossing in Waters of the United States (WOTUS) or state waters involves work below the Ordinary High-Water Mark (OHWM), the owner/operator must obtain prior regulatory authorization before any in-stream work begins:

• Clean Water Act Section 404 Permit from the U.S. Army Corps of Engineers (USACE) (often under Nationwide Permit 14 for Linear Transportation Projects or NWP 33 for Temporary Construction, Access, and Dewatering);
• Clean Water Act Section 401 Water Quality Certification from the state environmental agency; and
• State Stream-Alteration / Fish and Game Streambed Permits (which frequently restrict in-water construction to dry-season work windows and prohibit blocking fish migration).

Three Structural Crossing Types (Ranked Least to Greatest Impact)

Critical Siting, Approach Drainage, and Restoration Rules

1. Perpendicular Orientation at a Straight Reach: Position the crossing at a $90^\circ$ perpendicular angle across a narrow, straight stream reach with stable banks—never on an eroding meander bend.
2. No Earthen Fill in Culvert Crossings: Temporary culvert crossings must be backfilled from the geotextile bed to the driving surface exclusively with clean, washed coarse aggregate and riprap (topped with smaller crushed stone for traction). Never use native earthen soil as culvert backfill, as overtopping storm flows will wash the entire dirt fill embankment downstream.
3. Approach Road Cutoff Drainage: Dirty haul-road runoff must never run down the approach ramps into the creek. Install water bars or diversion swales on both approach roads $25\text{ to }50\text{ feet}$ back from the streambanks to divert road runoff into a vegetated filter strip or sediment trap before it reaches the crossing.
4. Complete Removal and Bank Stabilization: Immediately upon completion of construction access, remove all bridge mats, pipes, riprap fill, and geotextile during low-flow conditions, restore the original streambed contours, and stabilize disturbed banks with erosion control blankets and native riparian plantings.

---

Turbidity Curtains / Barriers the approved practice detail

A turbidity curtain (floating silt barrier) is a flexible, vertical underwater barrier suspended from floating surface buoys and weighted at the bottom by a ballast chain. It is deployed in lakes, reservoirs, ponds, rivers, or tidal estuaries to enclose shoreline or in-water soil-disturbing activities such as bridge pier cofferdam construction, dredging, bank riprap placement, or storm outfall installation.

Structural Anatomy and Type I, II, and III Classifications

Every manufactured turbidity curtain includes four core structural assemblies:

1. Top Flotation Elements: Sealed expanded polystyrene or urethane foam logs wrapped in bright high-visibility yellow/orange PVC fabric, sized to provide at least $200\%$ buoyancy (capable of supporting twice the submerged weight of the skirt and ballast).
2. Top Load-Bearing Tension Cable: In moving water (Types II and III), a galvanized steel wire rope ($5/16\text{ in.}$ or larger) is sewn into a reinforced webbing pocket just below the floats so anchor forces pull on the steel cable rather than tearing the vinyl skirt.
3. Vertical Skirt: Impermeable PVC/vinyl geomembrane or heavy woven geotextile fabric joined in $50\text{- to }100\text{-foot}$ sections using reinforced grommeted, laced, or bolted seams.
4. Bottom Ballast Chain and Anchors: A continuous galvanized steel chain pocket at the bottom hem keeps the skirt hanging vertically against current lift, supplemented by Danforth/fluke bottom anchors and visible mooring buoys every $25\text{ to }100\text{ feet}$.

Mandatory Installation, Inspection, and Removal Rules

• Maintain a $1\text{- to }2\text{-Foot}$ Clearance Above the Bottom Bed: The vertical skirt depth must be sized so the bottom ballast chain hangs $1\text{ to }2\text{ feet}$ ABOVE the streambed or lake bottom during normal low water. If the skirt is too long and rests in the bottom mud, wind waves and currents cause the heavy ballast chain to drag back and forth like a broom across the benthic sediment, resuspending more turbidity than the curtain prevents (and burying the skirt in silt so it cannot be retrieved without tearing).
• Never Install Perpendicular Across a Flowing River: Installing a curtain at $90^\circ$ across a moving river forces the entire river discharge against the impermeable skirt; hydrodynamic drag will immediately drag the anchors, submerge the floats ("planing/diving"), or snap the tension cable. Always deploy the curtain parallel to the main river current or angled gently outward from the shoreline to deflect flow smoothly around the work cell.
• Tidal Dual-Anchoring: In tidal estuaries where flow reverses direction twice daily, anchor the curtain on both the inside and outside so the skirt does not flip into the work zone at slack tide.
• Removal Protocol: Never unshackle or drag a turbidity curtain out of the water immediately after dredging or bank grading stops! The curtain must remain stationary in place until all suspended turbidity inside the enclosure has settled back to ambient background levels; then carefully unpin and fold the curtain onto the bank or work barge without agitating settled bottom sediment.

---

Sediment Bags / Dewatering Filter Bags the approved practice detail

Excavating utility trenches, bridge footings, or foundations below the water table—or drawing down a flooded excavation after a storm—requires pumping sediment-laden water. Pumping muddy trench or basin water directly into a storm drain inlet, roadside ditch, or stream is strictly illegal.

A sediment bag (dewatering filter bag) is a large, factory-sewn rectangular sack (typically $10\text{ ft} \times 15\text{ ft}$ or $15\text{ ft} \times 15\text{ ft}$) manufactured from heavy-duty, non-woven, needle-punched polypropylene geotextile. The pump discharge hose ($2\text{- to }6\text{-inch}$ diameter) is inserted into a sewn spout sleeve and clamped tightly with a steel band clamp or heavy web strap so pumped water is forced through the three-dimensional geotextile pores under pressure.

Siting, Secondary Containment, and Regulatory Compliance

1. Siting on a Permeable, Non-Erodible Pad: Place the sediment bag on a flat or gently sloping well-vegetated buffer strip, a $6\text{-inch}$ bed of clean crushed aggregate, or an elevated platform of straw bales / timber pallets. Increasing under-bag drainage area prevents the bottom panel from blinding against mud and increases flow capacity.
2. Prohibited Siting: NEVER place a dewatering sediment bag on a steep or bare earthen slope! Water weeping rapidly out of all four sides of the bag will immediately coalesce and scour severe erosion gullies down the bare slope.
3. Secondary Perimeter Containment: Surround the downgradient side of the sediment bag pad with a secondary linear barrier (such as a silt fence J-hook or a compost filter sock) so filtered effluent spreads into shallow sheet flow across vegetation before reaching any conveyance.
4. 2022 EPA CGP Part 3.3 Dewatering Inspection & 50 NTU Turbidity Benchmark:
   • Inspect dewatering operations at least once every day on which dewatering water is discharged.
   • Where the 2022 EPA Construction General Permit requires turbidity benchmark monitoring (for dewatering discharges to sediment-impaired or high-quality Tier 2/2.5/3 waters), collect daily grab samples of the effluent after the sediment bag and compare against the $50\text{ NTU}$ weekly average benchmark.
   • If the excavation contains visible oil sheen, floating grease, or petroleum contamination, a standard geotextile bag is insufficient—insert an oil-absorbent hydrophobic boom/pad inside or around the bag, or route water through an oil-water separator / granular activated carbon treatment system.
   • If clays and fine silts remain above $50\text{ NTU}$, route the pump through a portable weir tank (frac tank) or approved anionic polymer flocculant dosing system prior to filtration.
5. Replacement at $1/2$ Capacity: Shut off the pump immediately if the hose neck leaks, if seams split, or when the bag becomes $1/2\text{ full}$ of trapped sediment (or when pore blinding causes excessive ballooning/backpressure). Fold the spent bag closed, excavate or lift it with equipment straps, and dispose of or stabilize the dewatered spoil upland.

Loading diagram...