6.2 Inspecting Active Construction Controls and Diagnosing BMP Failure Modes

The Four Fundamental Diagnostic Questions for Every Installed BMP

When walking an active construction site from the upland perimeter down through graded slopes, conveyances, and discharge outfalls, a CESSWI inspector does not merely check off that a Best Management Practice (BMP) exists. Under SAOP 7 Test Objective 2 (General Principles Review Manual Chapter 8.5), the inspector evaluates every structural and non-structural control by asking four fundamental diagnostic questions in sequence:

1. Is the Specified Control Present at the Location Shown on the Current SWPPP Map?

The inspector cross-references the physical landscape against the current red-lined SWPPP site map:

• Confirm whether every control required for the current grading phase has actually been installed.
• Identify whether grading, trenching, or utility subcontractors have prematurely removed or flattened a perimeter barrier, check dam, or inlet protection device to create an unauthorized haul path without replacing the control or updating the site map.

2. Is the Control Appropriate for the Actual Hydraulic and Soil Conditions at That Location?

Even a meticulously installed BMP will fail if it is mismatched to the site's flow regime, contributing drainage area, slope gradient, or soil texture:

• Sheet Flow vs. Concentrated Flow (Classic CESSWI Exam Trap): A silt fence (or straw bale barrier) is designed exclusively to intercept shallow, low-velocity overland sheet flow along a perimeter or contour (typically limited to $\le 0.25\text{ to }0.50\text{ acres}$ per $100\text{ linear feet}$ of fence and slope lengths of $25\text{ to }100\text{ feet}$). Placing a silt fence across a concentrated-flow drainage ditch, swale, or gully is a severe hydraulic misapplication; concentrated flow rapidly overtops or blows out the geotextile fabric. Concentrated-flow channels require engineered rock check dams, wattles/compost filter socks staked as ditch checks, or turf reinforcement mats (TRMs).
• Soil Particle Size Compatibility: Coarse aggregate filters and standard woven slit-film silt fences retain sand ($0.05\text{ to }2.0\text{ mm}$) and coarse silt by ponding water to allow gravitational settling, but they cannot settle fine colloidal clays ($< 0.002\text{ mm}$) without extended detention storage, floating surface skimmers, and chemical flocculants such as anionic polyacrylamide (PAM).

3. Was the Control Installed According to Standard Specifications and Manufacturer Details?

Improper installation is the leading cause of BMP failure during the first storm event. The CESSWI verifies critical structural details for each practice:

• Silt Fence:
  • Trenching and Compaction: The bottom edge of the geotextile fabric must be buried in a minimum $6\text{-inch deep by }6\text{-inch wide}$ ($6\text{ in.} \times 6\text{ in.}$) trench excavated on the upslope side of the fence line, backfilled with soil, and mechanically compacted.
  • Post Orientation: Steel T-posts or hardwood stakes must be driven on the downslope side of the fabric so impounded water pushes the fabric against the posts rather than ripping staples or zip-ties off the posts.
  • Joining Panels and Terminal J-Hooks: Adjacent fabric sections must be joined by rolling/wrapping the two end stakes together $180^\circ\text{ to }360^\circ$ and driving them into the ground as a single unit (never by simply overlapping loose fabric). The terminal ends of the fence run must be turned upslope at a $45^\circ\text{ to }90^\circ$ angle for $6\text{ to }10\text{ feet}$ to form a J-hook ("smile") that impounds runoff and prevents end-around bypass.
• Rolled Erosion Control Products (RECPs — Erosion Control Blankets and TRMs):
  • Top-of-Slope Anchor Trench: The upper edge must be secured in a $6\text{ in.} \times 6\text{ in.}$ anchor trench at the crest of the slope, stapled in the trench bottom, backfilled, compacted, and unrolled downslope.
  • Intimate Soil Contact (No "Tenting"): The slope face must be graded smooth and free of rocks, dirt clods, and wheel ruts before laying the blanket so the fibers maintain 100% intimate contact with the seedbed. Adjacent rolls must overlap shingle-style ($4\text{ to }6\text{ inches}$) in the direction of water flow and be stapled at the specified density.
• Fiber Rolls (Straw, Coir, or Excelsior Wattles):
  • Installed strictly along the contour (perpendicular to sheet flow) in a shallow $2\text{- to }4\text{-inch}$ concave trench with excavated soil packed tightly against the upslope face.
  • Wooden stakes must be driven through the center of the wattle (with a pilot bar in hard soil) at $3\text{- to }4\text{-foot}$ intervals, leaving $2\text{ to }3\text{ inches}$ of stake projecting above the roll, with adjacent ends tightly abutted or overlapped so water cannot jet between rolls.
• Rock Check Dams:
  • Constructed of well-graded aggregate over a non-woven geotextile filter fabric underlayment keyed into the channel bed and banks.
  • Weir Geometry: The center weir notch must be at least $6\text{ inches}$ lower than the outer abutment edges tied into the channel banks, forcing concentrated flow over the armored center rather than cutting into the earthen banks.
  • Toe-to-Crest Spacing: Spaced along the channel gradient so the elevation of the crest of the center weir of the downstream check dam equals the elevation of the toe of the upstream check dam.

4. Has the Control Been Properly Maintained Before Reaching Its Capacity Threshold?

Every sediment control device has a finite storage volume and structural lifespan:

• Silt Fence: Remove accumulated sediment when deposits reach $1/3\text{ to }1/2$ of the above-ground fabric height (e.g., $8\text{ to }12\text{ inches}$ on a $24\text{-inch}$ fence). Replace torn, slumped, or UV-degraded brittle fabric.
• Wattles and Check Dams: Clean out accumulated sediment when deposits reach $1/2$ of the original barrier or center weir height.
• Sediment Basins and Traps: Excavate trapped sediment when accumulation reaches $50\%$ of the wet/sediment storage volume (marked on the cleanout depth stake). Verify that floating surface skimmers are buoyant, unobstructed by debris, and not resting in bottom mud.
• Stabilized Construction Entrances: Top-dress with clean $2\text{- to }3\text{-inch}$ coarse stone or scarify/re-build whenever mud fills the aggregate void spaces, and sweep any street track-out immediately.

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Diagnosing Common Field BMP Failure Modes and Root Causes

When a CESSWI discovers a failed BMP or off-site sediment plume, the inspection report and Corrective Action Log must diagnose the exact failure mechanism so the contractor corrects the underlying engineering or installation defect rather than repeating the same mistake.

1. Undercutting (Piping / Scouring Underneath)

• Field Signature: Runoff tunnels beneath the bottom of a silt fence, wattle, check dam, or erosion blanket, carving rills or gullies under the device while the barrier remains suspended above the eroded void.
• Root Causes:
  • Silt fence bottom edge laid flat on the ground or stapled to the surface without excavating, backfilling, and mechanically compacting a $6\text{ in.} \times 6\text{ in.}$ upslope toe trench.
  • Fiber roll/wattle placed on rough, cloddy ground without a $2\text{- to }4\text{-inch}$ shallow trench and upslope soil backfill.
  • RECP unrolled over rocks, dirt clods, and existing rills ("tenting" or "bridging"), or installed without a top-of-slope anchor trench, allowing sheet flow to dive underneath the blanket and erode the unarmored soil.
  • Rock check dam placed directly on erodible channel soil without a geotextile underlayment and shallow key trench.

2. End-Around Bypass (Flanking / Bank Scour)

• Field Signature: Turbid water flows around one or both outer ends of a linear sediment barrier or check dam, eroding rills down the adjacent slope or carving gullies into the earthen channel banks.
• Root Causes:
  • Silt fence or wattle run terminated straight along a sloping contour without turning the last $6\text{ to }10\text{ feet}$ upslope into a J-hook ("smile") to impound runoff.
  • Rock check dam or ditch check constructed with a flat top where the center crest elevation is equal to or higher than the outer edges meeting the ditch banks.

3. Overtopping, Slumping, and Structural Blowout

• Field Signature: Support posts bent or snapped downslope, geotextile fabric ripped off wooden stakes, gaps opened between adjacent panels, or muddy water cascading over a flattened barrier.
• Root Causes:
  • Hydraulic Misapplication: Installing a silt fence across a concentrated-flow swale or ditch, or exceeding the maximum allowable contributing slope length without intermediate slope-break barriers.
  • Reversed Post Placement: Driving stakes on the upslope side of the silt fence fabric so hydrostatic water pressure and sediment weight push the fabric away from the posts and pop the staples/ties loose.
  • Improper Panel Splice: Overlapping two silt fence panels without twisting and driving the end posts together, allowing water pressure to force the seam apart.
  • Deferred Maintenance: Allowing trapped sediment to accumulate beyond $1/2$ of the barrier height, eliminating ponding volume and overloading the posts.

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BMP Substitution and Field Modification Protocol

During active construction, site conditions frequently render a SWPPP-specified control infeasible or counterproductive. For example, excavating a $6\text{ in.} \times 6\text{ in.}$ silt fence trench is impossible across frozen winter ground, exposed shallow bedrock, or existing asphalt/concrete pavement, and trenching across the Critical Root Zone (CRZ) of preserved heritage trees will sever shallow feeder roots and kill the trees.

In such cases, substituting a surface-applied alternative—such as a heavy compost filter sock or weighted sediment tube that conforms closely to the ground surface without trenching—is sound practice, provided the team follows the mandatory 4-step field modification protocol:

1. Verify Technical Equivalence: Confirm that the proposed substitute BMP provides equal or greater sediment retention and hydraulic capacity for the specific flow regime, slope, and soil type.
2. Obtain Designer / Operator Approval: Secure concurrence from the SWPPP designer (CPESC/PE) and the permitted operator's duly authorized representative prior to or upon substitution.
3. Install Per Manufacturer Specifications: Install the substitute control properly (e.g., ensuring intimate surface contact, crisscross staking without piercing the sock fabric where specified, and turning terminal ends upslope into J-hooks).
4. Red-Line and Amend the SWPPP Within 7 Calendar Days: Immediately annotate the on-site red-line site map with the date, location, and operator initials, and formally amend the SWPPP narrative and detail sheets within 7 calendar days of completing the change (EPA CGP Part 5.2 / Part 7.4).

Field BMP Diagnostic and Corrective Action Matrix