2.1 Damming, Diking, Diverting & Retention Operations

2.1 Damming, Diking, Diverting & Retention Operations

NFPA 470 Core Standard: Operations-level hazardous materials personnel perform defensive confinement operations to control the flow of liquid spills, restrict environmental migration, and protect sensitive public receptors without contacting the hazardous chemical or entering the immediate hot zone.

When liquid hazardous materials escape their primary containment vessels, defensive operations focus on controlling the movement of the spilled material across land or through waterways. NFPA 470 defines confinement as those procedures taken to keep a material, once released, in a defined local area. Unlike offensive containment tactics (such as plugging or patching a leaking drum), confinement tactics are executed at a safe distance downstream, down-gradient, or along the perimeter of the spill.

Mastering physical confinement requires a precise understanding of physical chemical properties—most notably Specific Gravity ($SpGr$) and Water Solubility—as well as proper construction geometry for physical barriers.

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Physical Properties Governing Liquid Confinement

Before selecting a defensive tactical control, responders must evaluate how the released chemical will behave when it contacts water or ground terrain.

Specific Gravity ($SpGr$)

Specific Gravity is the ratio of the density of a substance compared to the density of pure water at a specified temperature (typically $39.2^\circ\text{F}$ or $4^\circ\text{C}$), where water has a benchmark value of $1.0$.

$$\text{Specific Gravity } (SpGr) = \frac{\text{Density of Liquid (lbs/gal)}}{\text{Density of Water (8.34 lbs/gal)}}$$

• $SpGr < 1.0$ (Floater): The liquid is lighter than water and will float on the surface of rivers, streams, or pooled water (e.g., Gasoline $SpGr = 0.72$ to $0.76$, Diesel $SpGr = 0.85$, Toluene $SpGr = 0.87$).
• $SpGr > 1.0$ (Sinker): The liquid is heavier than water and will sink to the bottom of waterways or streambeds (e.g., Carbon Disulfide $SpGr = 1.26$, Sulfuric Acid $SpGr = 1.84$, Tetrachloroethylene $SpGr = 1.62$).

Water Solubility & Miscibility

Water solubility measures the degree to which a chemical dissolves in water:

• Insoluble / Immiscible Materials: Do not mix with water. They form distinct layers based on Specific Gravity (floating on top or sinking to the bottom). Physical damming techniques are highly effective for insoluble liquids.
• Soluble / Miscible Materials: Dissolve completely in water (e.g., Acetone, Isopropyl Alcohol, Ethyl Glycol). Once dissolved, physical damming cannot separate the chemical from the water column, making physical recovery impossible without advanced chemical processing.

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Damming Operations: Overflow vs. Underflow

Damming is a defensive confinement tactic used in moving waterways (creeks, streams, ditches, runoff channels) to trap insoluble liquid contaminants while maintaining natural stream flow downstream.

OVERFLOW DAM (SpGr > 1.0 - Sinker Product)
   Water Level
~~~~~~~~~~~~~~~~\___________ <-- Clean Water Overflows Spillway
[ Sinker Product ] | Dam Wall |
==================|__________|
  Streambed

UNDERFLOW DAM (SpGr < 1.0 - Floater Product)
[ Floater Product ]
~~~~~~~~~~~~~~~~~~| Dam Wall |
   Clean Water    |===PIPE===|  <-- Submerged Pipe Discharges Clean Water
==================|__________|
  Streambed

1. Overflow Dams ($SpGr > 1.0$ - Sinker Products)

An overflow dam is constructed when the released hazardous liquid is insoluble and heavier than water ($SpGr > 1.0$).

• Mechanics: The heavy hazardous liquid sinks to the stream bed behind the dam structure. The dam height is constructed lower than the top elevation of the streambank, allowing clean water from the top of the water column to flow over a notched center spillway while the heavy product accumulates along the bottom.
• Construction Technique: Sandbags, earth, or timber are placed across the waterway. A central V-notch or spillway is established in the middle of the dam crest to direct clean water over the center, preventing erosion around the outer bank edges. Accumulating sinker product is recovered from the streambed upstream of the dam using suction hoses or vacuum trucks.

2. Underflow Dams ($SpGr < 1.0$ - Floater Products)

An underflow dam is constructed when the released hazardous liquid is insoluble and lighter than water ($SpGr < 1.0$).

• Mechanics: The light hazardous liquid floats on the water surface behind the dam structure. Pipes or conduits are installed through the base of the dam, slanting downward on the upstream side. Hydrostatic pressure forces clean, un-contaminated water from the bottom of the stream through the submerged pipes to discharge downstream, while the floating product is impounded on top.
• Construction & Pipe Placement Guidelines:
  1. Secure one or more inclined pipes (typically $4"$ to $8"$ PVC, corrugated pipe, or hard suction hoses) at the streambed prior to stacking sandbags.
  2. The upstream inlet of the pipe must be positioned well below the surface layer of floating product (submerged in the clean lower water column).
  3. The downstream outlet of the pipe is capped or inclined upward to set the desired upstream pool height.
  4. Ensure total pipe cross-sectional area equals or exceeds incoming stream flow to prevent dam overtopping.

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Diking, Diverting & Retention Tactics

When liquid spills occur on land, terrestrial confinement tactics prevent liquid migration into drainage networks or sensitive environmental zones.

Diking Operations

Diking involves constructing a physical barrier across the path of a liquid spill on flat or sloped terrain to hold back the liquid and create an artificial impoundment.

• Barrier Materials: Sandbags, soil, clay, commercial soil berms, synthetic absorbent booms, or dirt constructed with hand tools (shovels) or heavy equipment (backhoes/graders).
• Geometry: Dikes should be built V-shaped or horseshoe-shaped pointing up-slope to trap the liquid pool safely. Dike walls must be compacted to prevent seepage and tall enough to handle total calculated spill volume plus head pressure.

Diverting Operations

Diverting is the controlled redirection of a liquid spill away from vulnerable areas (such as storm drains, water intakes, or inhabited structures) toward an acceptable containment area.

• Tactics: Digging diagonal trenches across the spill path, building angled sandbag walls, or deploying heavy rubber diversion dikes across pavement.
• Key Rule: Diverting must always lead liquid toward a secondary containment area; never divert liquid into public sewer systems or unlined waterways.

Retention Basins & Excavation

Retention is the physical containment of a liquid spill in a designated depression, excavation pit, or natural low area.

• Lining Requirements: Unlined earth pits allow liquid chemicals to percolate into soil and contaminate groundwater aquifers. Retention basins must be lined with compatible chemical-resistant plastic sheeting (such as $6\text{-mil}$ polyethylene or heavy polypropylene tarps) before product accumulates.

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Sorbent Materials: Absorbents vs. Adsorbents

Sorbents are solid materials used to pick up and contain small liquid spills. NFPA 470 requires Operations responders to distinguish between absorption and adsorption.

ABSORPTION (Bulk Sponge Effect)
   Liquid enters matrix --> [ Sorbent Particle Expands ]

ADSORPTION (Surface Adherence)
   Liquid molecules adhere --> [ Sorbent Particle Surface ] (No Swelling)

Absorbents

• Mechanism: Liquids are drawn into the bulk structure of the sorbent material, similar to a sponge soaking up water. The absorbent swells as it retains the liquid.
• Materials: Sawdust, clay (kitty litter), vermiculite, cellulose fibers, peat moss, synthetic foam polymers.
• Operational Limitations: Absorbents expand in volume, increase total hazardous waste weight, and may release trapped liquids under mechanical pressure. Organic absorbents (sawdust, peat moss) are strictly prohibited on strong oxidizers (such as concentrated Nitric Acid or Hydrogen Peroxide) due to immediate spontaneous combustion risk.

Adsorbents

• Mechanism: Liquid molecules adhere to the outer surface of the sorbent material without penetrating the solid matrix or causing volume swelling.
• Materials: Activated carbon, silica gel, specialized polypropylene particulate, hydrophobic oil-only mats/booms.
• Operational Advantages: Adsorbents do not expand significantly, allow easy mechanical recovery of floating hydrocarbons from water, and minimize hazardous waste volume.