13.2 Porous Baffles and Surface Skimmers

2. Porous Baffles (12.4.2)

Porous Baffles (12.4.2) are vertical permeable curtains installed perpendicular to the flow path across the full width and depth of a sediment basin or sediment trap to dissipate inflow turbulence, equalize flow distribution across the entire impoundment width, and maximize fine-particle settling efficiency.

Material Specification & Hydraulic Principle

Porous baffles are fabricated from double-layered coir (coconut fiber) or jute erosion control netting, or heavy-duty three-dimensional porous polypropylene Turf Reinforcement Mat (TRM) mesh.

• Typically, 2 to 3 rows of porous baffles are installed at equal intervals between the basin inlet/forebay and the principal spillway riser, dividing the impoundment into 3 to 4 sequential settling bays (chambers).
• As concentrated, turbulent storm inflow enters the first bay, it strikes the first porous curtain. Instead of deflecting sideways or shooting over the top, the water passes through thousands of uniform small openings across the entire submerged curtain area.
• This head-loss resistance absorbs inflow kinetic energy, reduces horizontal flow velocity by $50\%\text{ to }70\%$, eliminates eddy currents and dead zones, and establishes calm plug flow across the full cross-section of the basin.

High-Yield Exam Trap — Why Porous Baffles Are NEVER Solid Silt Fence: Contractors frequently make the mistake of installing standard woven geotextile silt fence or solid plastic sheeting across a sediment basin as a baffle. Standard woven silt fence must NEVER be used as an internal basin baffle! When submerged in turbid pond water, the microscopic pores of woven silt fence blind (clog) almost instantly with clay and silt. Once clogged, the silt fence acts as a solid impermeable submerged dam or weir—forcing storm inflow to accelerate over the top crest, knock down the posts, or scour around the side slopes, which worsens turbulence and short-circuiting rather than preventing it.

Installation and Inspection Checkpoints for Porous Baffles

1. Post Spacing and Anchoring: Drive steel T-posts at least $2\text{ to }3\text{ feet}$ into the compacted basin floor and up both side slopes at $4\text{- to }8\text{-foot}$ spacing.
2. Top Support Cable & Weir Notch: String a heavy galvanized steel support wire or tension rope across the tops of the T-posts to prevent the saturated fabric from sagging. The top edge of the baffle along the side slopes must extend above the emergency spillway elevation, with a shallow center weir depression located at or slightly below the principal spillway crest so extreme storm flows pass over the center rather than eroding the side slopes.
3. Continuous Bottom Floor Staking: Fold the bottom $6\text{ to }12\text{ inches}$ of the porous baffle mat flat along the basin floor pointing upstream and secure it continuously with heavy metal U-staples or stakes every $1\text{ to }2\text{ feet}$ (or trench it in). If the bottom edge is left unanchored, inflow currents will lift the curtain like a swinging gate and bypass underneath.
4. Field Inspection: Inspect after every storm for torn mesh, leaning or pulled T-posts, sagging top support cables, flow scouring around the side-slope tie-ins, or undercutting beneath the bottom edge.

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3. Skimmers (12.4.7)

Under the federal EPA Effluent Limitations Guidelines (ELGs) for the Construction and Development Point Source Category (40 CFR § 450.21(b)) and EPA CGP Part 2.2.12, operators utilizing sediment basins must "utilize outlet structures that withdraw water from the surface, unless infeasible." The primary BMP engineered to fulfill this surface-withdrawal mandate is the Floating Surface Skimmer (12.4.7).

Why Traditional Perforated Risers Fail Water-Quality Standards

Historically, sediment basins were built with vertical metal riser pipes perforated with $1\text{-inch}$ holes from top to bottom and wrapped in gravel. From a physics standpoint, a bottom-perforated riser operates backward:

• Under Stokes' Law, suspended sediment settles downward, meaning the dirtiest, highest-turbidity water is concentrated at the bottom of the basin, while the cleanest, clarified water is in the top $2\text{ to }6\text{ inches}$ at the water surface.
• Under the Orifice Equation ($Q = C_d A \sqrt{2gh}$), the hydrostatic head ($h$) is greatest at the bottom of the pond. Therefore, a perforated riser discharges the muddiest bottom water first at the highest flow rate, flushing unsettleable clay and silt directly into the receiving stream before gravitational settling can occur.

How a Floating Surface Skimmer Works

A floating skimmer consists of a buoyant float ring or closed-cell foam pontoon assembly that suspends a screened intake head and a calibrated orifice plate just $2\text{ to }6\text{ inches}$ below the water surface:

1. Constant-Head Surface Withdrawal: As the basin fills during a storm, the skimmer rises with the water surface, tethered to the basin floor by a synthetic rope so it cannot float into the riser trash rack. Because the intake orifice remains submerged at a fixed depth ($2\text{ to }6\text{ inches}$) below the water surface regardless of whether the pond is $6\text{ feet}$ deep or $2\text{ feet}$ deep, the hydraulic head ($h$) on the orifice stays constant throughout the entire drawdown cycle.
2. Controlled 2- to 7-Day Dewatering Time: Water enters the screened intake, passes through the sized circular orifice plate (which controls the exact discharge rate $Q$), travels through a flexible reinforced non-collapsible hose attached to the bottom of the principal spillway riser, and exits through the barrel pipe. The orifice is sized to dewater the live storage volume gradually over $2\text{ to }7\text{ days}$ ($48\text{ to }168\text{ hours}$)—giving fine silt and clay maximum residence time to settle downward away from the surface intake.
3. Mud-Support Pad at the Top of Wet Storage: When the dry live storage zone finishes draining, the skimmer settles onto a level gravel, concrete, or timber resting pad (mud support) positioned at the top elevation of the wet sediment storage zone, preventing the intake from resting in or sucking up settled bottom mud.
4. Winter Freezing Exemption: The primary regulatory exception where surface skimmers are considered temporarily "infeasible" under 40 CFR § 450.21(b) is during frozen winter conditions, when surface ice locks the float or freezes the flexible hose.

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Summary Comparison of Impoundment Sediment Control BMPs