13.1 Sediment Basins and Traps

Role of Impoundment Sediment Controls in SAOP 11

When construction grading disturbs multiple acres of soil, perimeter linear barriers such as silt fence and fiber rolls cannot handle the resulting concentrated runoff volumes or detain fine particles long enough for gravitational settling. Under Specific Area of Practice (SAOP) 11: Management Practices (representing 32% to 39% of the Part B examination), a Certified Erosion, Sediment and Stormwater Inspector (CESSWI) must evaluate four core impoundment technologies from the CESSWI General Principles Review Manual: Sediment Traps (12.4.5), Sediment Basins (12.4.4), Porous Baffles (12.4.2), and Skimmers (12.4.7).

All four practices rely on Stokes' Law, which governs the terminal settling velocity of suspended soil particles in quiescent water. Coarse sand grains settle within seconds to minutes, medium silt particles require several hours, and fine silt and colloidal clay particles require days of calm retention—or chemical flocculation—to settle out of suspension. Consequently, an inspector must verify not only that an impoundment holds sufficient storage volume, but also that its internal hydraulic flow path and surface outlet structure prevent turbulence and premature bottom-water discharge.

Inspection vs. Engineering Design Boundary: A CESSWI is certified to verify impoundment dimensions against approved Stormwater Pollution Prevention Plan (SWPPP) drawings, read sediment staff gauges, inspect spillway integrity, and document deficiencies. A CESSWI must never resize a principal spillway, recalculate dam embankment stability, or modify orifice diameters—any structural redesign must be referred to the project's Licensed Professional Engineer (PE) or Certified Professional in Erosion and Sediment Control (CPESC).

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1. Sediment Trap (12.4.5) vs. Sediment Basin (12.4.4)

One of the most frequently tested distinctions on both Section 1 and Section 2 of the CESSWI Part B exam is how to differentiate a Sediment Trap (12.4.5) from a Sediment Basin (12.4.4) by tributary drainage area, outlet mechanics, and dam safety features.

Drainage Area Thresholds

• Sediment Trap (12.4.5): A smaller, temporary settling impoundment formed by excavating a depression or constructing a low compacted earthen embankment across a swale or low-lying drainageway. Sediment traps are strictly limited to tributary drainage areas of $\le 5\text{ acres}$. They are typically installed for short-to-moderate construction durations ($6\text{ to }18\text{ months}$) where full spillway engineering is unnecessary.
• Sediment Basin (12.4.4): A larger, hydraulically engineered impoundment constructed with a compacted earthen dam embankment and multi-component spillway system serving tributary drainage areas of $> 5\text{ acres}$ up to $100\text{ acres}$. Under the U.S. EPA Construction General Permit (CGP) Part 2.2.12, a temporary or permanent sediment basin is mandatory whenever common drainage from $\ge 10\text{ disturbed acres}$ at one time flows to a single discharge location.

Outlet Structure Differences

The physical dewatering outlet is the fastest visual way to distinguish a sediment trap from a sediment basin in Part B field photographs:

1. Sediment Trap Stone Embankment Weir (12.4.5):

   • A sediment trap does not typically use a vertical riser and barrel pipe. Instead, it dewaters through a stabilized coarse riprap / crushed stone embankment weir built into the downstream earth dike.
   • The stone outlet weir is constructed $1.5\text{ to }3.0\text{ feet}$ high (typically $4\text{ to }10\text{ feet}$ wide across the weir crest) using coarse $6\text{- to }12\text{-inch}$ angular riprap placed over a non-woven geotextile filter fabric keyed into the foundation soil and side abutments.
   • Critical Crest Depression Rule: The horizontal crest of the stone outlet weir must be depressed at least $1.0\text{ foot}$ below the top elevation of the earthen embankment so that storm overflows pass exclusively over the armored rock weir and never overtop or breach the earth dike.
   • Upstream Washed Filter Aggregate Layer: Because large $6\text{- to }12\text{-inch}$ riprap has large void spaces that would allow muddy water to rush straight through without ponding, the upstream face of the coarse riprap weir must be blanketed with a $12\text{-inch}$ thick layer of smaller $3/4\text{- to }2\text{-inch}$ washed filter aggregate (stone choke layer). This two-stage rock gradation slows seepage velocity, ponds runoff in the trap, and captures suspended sediment.

2. Sediment Basin Engineered Spillway System (12.4.4):

   • Principal Spillway (Riser and Barrel Pipe): Consists of a vertical corrugated metal, HDPE, or reinforced concrete riser pipe welded or gasket-sealed to a horizontal barrel pipe that penetrates the base of the compacted earthen dam embankment and discharges onto a stabilized riprap outlet apron at the downstream toe.
   • Anti-Flotation Base: Because an empty or partially dewatered vertical pipe experiences strong upward buoyancy forces when submerged, the base of the riser must be anchored in a poured concrete anti-flotation block.
   • Anti-Vortex Plate and Trash Rack: The top rim of the vertical riser is fitted with a metal trash rack (to prevent floating limbs and construction debris from plugging the pipe) and an anti-vortex baffle plate (cylinder or flat plate that prevents rotational whirlpool vortices, which entrain air and drastically reduce the hydraulic discharge capacity of the pipe).
   • Anti-Seep Collars or Sand Filter Diaphragms: Pressurized pond water naturally seeks to travel along the smooth outside boundary between the horizontal barrel pipe and the surrounding soil backfill. To prevent this internal erosion ("piping" failure), watertight metal or concrete anti-seep collars (sized to increase the seepage path length along the pipe by $10\%\text{ to }15\%$) or engineered sand/gravel filter diaphragms must be installed around the barrel pipe inside the dam core, with fill hand-compacted in $4\text{- to }6\text{-inch}$ lifts.
   • Emergency Spillway: A separate, broad open-channel trapezoidal weir excavated into undisturbed native soil around one abutment of the dam (and lined with sod, permanent TRM, or angular riprap) designed to safely pass extreme storm events (10-year to 25-year, 24-hour storm) while maintaining at least $1.0\text{ foot}$ of embankment freeboard above the maximum emergency design water surface. Crucial Exam Trap: Never excavate an emergency spillway over uncompacted fill or directly over the principal spillway barrel pipe!

Storage Volume Sizing, Geometry ($L:W$ Ratio), and Cleanout Threshold

Under the EPA CGP and the CESSWI General Principles Review Manual, both sediment basins and sediment traps are sized using one of two hydraulic standards:

• Design Storm Volume: Storage capacity sufficient to detain the calculated runoff volume from the local 2-year, 24-hour storm event; OR
• Standard Volumetric Rule: At least $3,600\text{ ft}^3\text{ per acre}$ ($134\text{ yd}^3/\text{acre}$) of total contributing drainage area.

In many state and municipal specifications tested on the CESSWI exam, the $3,600\text{ ft}^3/\text{acre}$ ($134\text{ yd}^3/\text{acre}$) total requirement is divided into two distinct vertical zones:

1. Wet ("Dead") Sediment Storage Zone: $1,800\text{ ft}^3/\text{acre}$ ($67\text{ yd}^3/\text{acre}$) in the bottom of the basin reserved for permanent standing water and trapped sediment accumulation below the skimmer rest elevation.
2. Dry ("Live") Dewatering Storage Zone: $1,800\text{ ft}^3/\text{acre}$ ($67\text{ yd}^3/\text{acre}$) above the wet pool up to the principal spillway crest, which fills during a storm and slowly dewaters via the floating skimmer over $2\text{ to }7\text{ days}$.

Hydraulic Geometry ($L:W$ Ratio) and Short-Circuiting Prevention

Even a basin with sufficient total cubic footage will fail to settle fine silt if the stormwater inlet pipe is located right next to the principal spillway riser. When inflow jets directly from the inlet to the outlet without mixing across the pond, the condition is called hydraulic short-circuiting, which slashes actual retention time from days to minutes.

To prevent hydraulic short-circuiting and maximize effective settling time:

• The effective flow-path length-to-width ($L:W$) ratio must be at least $2:1$, and ideally $3:1\text{ to }4:1$ (wedge-shaped or rectangular, widening toward the dam).
• If site grading forces a storm drain inflow pipe to enter near the riser, an internal compacted earthen diversion berm or a series of porous baffles must be installed to force runoff to travel the full perimeter length of the basin.

Cleanout Threshold & Staff Gauge Inspection

Every sediment basin and sediment trap must contain a permanent, vertical graduated depth marker / staff gauge painted or marked with the design cleanout elevation. The permittee must excavate and remove trapped sediment whenever accumulation reaches $50\%$ ($1/2$) of the wet sediment storage volume (or $50\%$ of the design sediment depth). Dredged sediment must be dewatered and spoiled in an upland location stabilized behind perimeter controls—never side-cast onto the outer dam embankment slope or placed where runoff can wash it back into the basin.

Worked Numeric Example: Sediment Basin Volume, Cleanout, and $L:W$ Verification

Scenario: A CESSWI inspects an active $18\text{-acre}$ drainage sub-basin that routes all runoff into a rectangular temporary sediment basin measuring $210\text{ ft}$ in effective flow length by $60\text{ ft}$ in average width. The SWPPP specifies the standard $3,600\text{ ft}^3/\text{acre}$ total storage rule split equally between wet dead storage and dry live storage, with an initial wet storage depth of $3.5\text{ ft}$ marked on the staff gauge.

Step-by-Step Verification:

1. Calculate Required Total Storage Volume: $\text{Total Volume} = 18\text{ acres} \times 3,600\text{ ft}^3/\text{acre} = 64,800\text{ ft}^3$ $\text{Total Volume in Cubic Yards} = \frac{64,800\text{ ft}^3}{27\text{ ft}^3/\text{yd}^3} = 2,400\text{ yd}^3$
2. Calculate Wet (Dead) Storage and Dry (Live) Storage: $\text{Wet Storage} = 18\text{ acres} \times 1,800\text{ ft}^3/\text{acre} = 32,400\text{ ft}^3\text{ } (1,200\text{ yd}^3)$ $\text{Dry Storage} = 18\text{ acres} \times 1,800\text{ ft}^3/\text{acre} = 32,400\text{ ft}^3\text{ } (1,200\text{ yd}^3)$
3. Verify Flow-Path Length-to-Width ($L:W$) Ratio: $L:W = \frac{210\text{ ft}}{60\text{ ft}} = 3.5:1$ Because $3.5:1 \ge 2:1$ (and falls within the ideal $3:1\text{ to }4:1$ range), the basin geometry meets hydraulic standards.
4. Determine Mandatory Sediment Cleanout Depth: $\text{Cleanout Trigger Depth} = 50\% \times 3.5\text{ ft wet depth} = 1.75\text{ ft } (21\text{ inches})$ When deposited sediment reaches the $1.75\text{-ft}$ mark on the staff gauge (leaving only $1.75\text{ ft}$ of wet pool depth below the skimmer pad), cleanout must be initiated.

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