14.6 Erosion Mechanics, RUSLE & the NPDES/SWPPP Framework

Core Focus: Construction activities remove protective vegetative cover, exposing bare subsoils to raindrop impact and concentrated runoff. Landscape architects bear direct legal and professional responsibility for preventing off-site sediment transport under the Clean Water Act. Mastery of erosion mechanics, the RUSLE equation, NPDES Phase I and II permitting, SWPPP preparation, structural BMP detailing, and inspection protocols is mandatory for LARE Section 4.

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1. Soil Erosion Mechanics & Classification

Erosion is the physical detachment, entrainment, and transport of soil particles by water, wind, gravity, or ice. Sedimentation is the subsequent deposition of detached particles when flow velocity drops below transport thresholds.

+-------------------------------------------------------------------------+
|                    STAGES OF WATER-INDUCED SOIL EROSION                 |
+-------------------------------------------------------------------------+
| 1. RAINDROP SPLASH: Terminal velocity impact breaks soil crust          |
|                     Loosens particles and launches them up to 5'        |
|         │                                                               |
|         ▼                                                               |
| 2. SHEET EROSION:   Shallow, laminar overland flow peels topsoil        |
|                     Uniform, imperceptible loss across large areas      |
|         │                                                               |
|         ▼                                                               |
| 3. RILL EROSION:    Concentrated flow carves small parallel channels    |
|                     Depth < 12 inches; repairable by normal grading     |
|         │                                                               |
|         ▼                                                               |
| 4. GULLY EROSION:   Large, deep, destructive chasms carve into soil     |
|                     Depth > 12 inches; cannot be traversed by mowers    |
|         │                                                               |
|         ▼                                                               |
| 5. STREAM CHANNEL:  Scouring of riverbanks and bedload incision         |
+-------------------------------------------------------------------------+

The Four Sequential Stages of Water Erosion

1. Raindrop Splash Erosion: Raindrops falling at terminal velocity (up to 20 to 30 feet per second) impart immense kinetic energy upon bare soil. The impact shatters bonded soil aggregates, dislodging individual particles and launching them up to 5 feet laterally and 2 feet vertically. Dislodged fine clays wash into surface macropores, sealing the soil surface with an impermeable crust that drastically reduces infiltration and triggers immediate overland runoff.
2. Sheet Erosion: Shallow, unconfined overland runoff moving as a thin, continuous laminar film across the ground plane. Sheet flow peels microscopic soil particles evenly from the landform. Sheet erosion is insidious because it occurs without visible channel cuts, stripping valuable topsoil unnoticed over entire construction seasons.
3. Rill Erosion: As sheet runoff moves down-gradient, micro-topographic irregularities force water to concentrate into distinct micro-channels or rivulets called rills. Velocity and tractive shear stress increase, carving parallel incisions into the soil. Definition: Rills are small enough (depth $< 12\text{ inches} / 30\text{ cm}$) to be erased and smoothed out during standard agricultural tillage or site rough grading.
4. Gully Erosion: When rills converge into major flow paths, the concentrated volume and torrential velocity carve severe, deep chasms into the terrain. Definition: Gullies are channels deeper than 12 inches (1.0 foot / 30 cm) that cannot be smoothed or crossed by standard grading equipment or farm tractors. Gully erosion causes catastrophic sediment yields, undermines foundations, and destroys site infrastructure.
5. Stream Channel Erosion: Occurs when increased volume and peak velocity from upstream urban runoff scours natural creek beds and undercuts vegetated stream banks, causing mass bank collapse.

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2. The Revised Universal Soil Loss Equation (RUSLE)

The Revised Universal Soil Loss Equation (RUSLE) is the standard mathematical model developed by the USDA Agricultural Research Service (ARS) to predict the long-term average annual rate of sheet and rill erosion on a given site.

$A = R \cdot K \cdot LS \cdot C \cdot P$

Parameter Definitions and Landscape Architecture Interventions

• $A$ = Computed Spatial Average Soil Loss (tons per acre per year): The gross mass of soil displaced from the slope.
• $R$ = Rainfall-Runoff Erosivity Factor: Quantifies the kinetic energy and maximum 30-minute intensity ($EI_{30}$) of geographic rainfall. Values range from $< 20$ in the arid Southwest to $> 300$ in the humid Gulf Coast. The landscape architect cannot alter $R$ (it is a fixed climatic property).
• $K$ = Soil Erodibility Factor: Measures the inherent susceptibility of a specific soil type to detachment and transport based on soil texture, organic matter content, soil structure, and permeability. Values range from $0.02$ to $0.69$:
  • High Erodibility ($K > 0.40$): Silt loams, very fine sands, and silts (easily detached and easily transported).
  • Low Erodibility ($K < 0.20$): Coarse sands, gravels (heavy particles resistant to transport), and well-aggregated high-plasticity clays (chemically bonded aggregates resistant to detachment).
• $LS$ = Topographic Factor (Length and Steepness): The ratio of soil loss from a specific field slope relative to a standard test plot (72.6 ft length, 9% slope). Combines slope length ($L$) and slope gradient ($S$):
  • Steeper slopes dramatically accelerate velocity and shear stress.
  • Longer uninterrupted slopes accumulate greater runoff volume and erosive power.
  • Landscape Architecture Intervention: Designers manipulate $LS$ by breaking long slopes into shorter segments using diversion berms, contour terraces, slope benches, and interceptor swales.
• $C$ = Cover Management Factor: The ratio of soil loss under a specified vegetative cover or mulch practice compared to clean-tilled bare fallow ground ($C = 1.0$):
  • Bare disturbed soil: $C = 1.00$
  • Hydroseeded mulch with tackifier: $C = 0.10\text{ to }0.05$
  • Temporary straw mulch ($2\text{ tons/acre}$): $C = 0.05\text{ to }0.02$
  • Erosion Control Blankets (ECBs): $C = 0.02\text{ to }0.005$
  • Dense mature perennial turf / native forest: $C = 0.005\text{ to }0.001$ ($99.9%$ reduction in soil loss).
  • Landscape Architecture Intervention: The $C$ factor is the single most powerful tool under designer control on a construction site.
• $P$ = Support Practice Factor: The ratio of soil loss with a specific structural conservation practice (contouring, terracing, silt fences, sediment basins) to up-and-down slope cultivation ($P = 1.0$).

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3. Regulatory Framework: Clean Water Act, NPDES & SWPPP Mandates

Under Section 402 of the federal Clean Water Act (CWA), the US Environmental Protection Agency (EPA) established the National Pollutant Discharge Elimination System (NPDES) to control pollutant discharges from point sources into Waters of the United States.

NPDES Permitting Thresholds

• NPDES Phase I (1990): Mandated permit coverage for large construction activities disturbing 5.0 acres or more of total land area.
• NPDES Phase II (1999 / 2003): Expanded regulatory oversight to small construction activities disturbing between 1.0 and 5.0 acres of total land area.
• The Common Plan Rule: Any site disturbance less than 1.0 acre is legally subject to NPDES permitting if the activity is part of a "larger common plan of development or sale" that collectively disturbs 1.0 acre or more (e.g., grading a 0.25-acre outparcel within a 10-acre commercial subdivision).

Construction General Permit (CGP) & SWPPP Workflow

To legally discharge stormwater from a regulated construction site, the owner/operator must secure authorization under the EPA or state-delegated Construction General Permit (CGP):

1. Develop a SWPPP: Author a comprehensive, site-specific Stormwater Pollution Prevention Plan (SWPPP) prior to turning a single shovel of earth.
2. File Notice of Intent (NOI): Submit a formal electronic Notice of Intent (NOI) to the regulatory authority (EPA or state environmental agency) at least 7 to 14 days prior to commencing ground-disturbing construction.
3. Post Public Notice: Post the CGP permit tracking number and site contact info publicly at the construction entrance.
4. Implement Phased BMPs: Install perimeter controls, stabilized entrances, and sediment basins before clearing and grubbing interior areas.
5. Inspect & Maintain: Execute certified site inspections, log maintenance repairs, and update the SWPPP as field conditions evolve.
6. Achieve Final Stabilization & File NOT: When construction is finished and the site reaches statutory final stabilization, submit a Notice of Termination (NOT) to legally extinguish permit liability.

Final Stabilization Criteria for Notice of Termination (NOT)

Under EPA CGP standards, a site has achieved Final Stabilization only when:

• All bare, disturbed soil areas have established a uniform, perennial native vegetative cover with a density of at least 70% of the background natural vegetative cover (or permanent structural equivalent such as asphalt, concrete, or permanent riprap armoring).
• All temporary structural BMPs (silt fences, sandbags, inlet baskets, sediment traps) have been completely dismantled and removed, and any soil disturbed during removal has been stabilized.

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