3.4 Wind Erosion, Sediment Transport, Climatic Erosivity, and Resource Impacts

Isohyetal maps and climate interpretation

An isohyet joins locations with equal precipitation depth for a stated storm or averaging period. Read the legend, units, date, and period before interpreting a contour: annual-average inches and a 24-hour design-storm depth answer different questions. A drainage area between the four-inch and five-inch isohyets may be subdivided into bands and assigned representative depths to calculate an area-weighted basin average. For example, 40 acres assigned four inches and 60 acres assigned five inches have an average depth of (40 × 4 + 60 × 5) ÷ 100 = 4.6 inches. This interpolation is an explicit teaching example, not a replacement for the design authority’s rainfall data.

The R rainfall-runoff erosivity factor represents rainfall energy and intensity effects used in erosion prediction. It is not the annual rain depth and is not a soil erodibility measurement. Two regions with similar annual totals can differ substantially in erosivity if one receives frequent gentle rain and the other intense convective storms. Seasonal construction scheduling can reduce exposure to high-erosivity periods, but a predicted dry season does not excuse installed controls or permit inspections.

In cold regions, distinguish snow accumulation from snowmelt discharge. Frozen or saturated soils may admit little infiltration when melt begins, and rain on snow can generate rapid runoff. Inspect access safety, functioning outlets, ice blockage, and the permit’s specific thaw and snowmelt provisions. Record actual weather and rainfall sources representative of the site; a regional map cannot prove that a particular site did or did not meet an inspection trigger.

Wind (Aeolian) Erosion and the Three Modes of Particle Transport

Under SAOP 2 Test Objectives 5 and 7, inspectors must evaluate wind erosion hazards on flat, exposed grading pads, arid and semi-arid projects, coastal sand zones, and unstabilized soil stockpiles. Wind erosion initiates when wind velocity near the ground exceeds the threshold friction velocity (typically $12\text{ to }15\text{ mph}$ measured at $1\text{ foot}$ above smooth, dry, bare soil). Once threshold shear is exceeded, wind transports soil via three distinct physical modes:

1. Saltation ($50\%\text{ to }80\%$ of total wind transport — The Dominant Mechanism):
   • Moves medium-sized particles $0.1\text{ to }0.5\text{ mm}$ in diameter (fine to medium sand and silt-sized aggregates).
   • Aerodynamic lift plucks these grains into the airstream, where they bounce along the surface in short, ballistic trajectories less than $1\text{ meter}$ ($3\text{ feet}$) high (with the majority of mass moving within $12\text{ inches}$ of the ground).
   • When saltating grains slam back into the soil surface at shallow impact angles, they act as abrasive projectiles—shattering dry soil clods, pushing heavier grains forward as surface creep, and splashing microscopic silt and clay into suspension. This self-multiplying chain reaction downwind across a long, unsheltered grading pad is called the avalanching effect.
2. Surface Creep ($5\%\text{ to }25\%$ of total wind transport):
   • Moves coarse sand and fine gravel particles $0.5\text{ to }1.0\text{ mm}$ (up to $2.0\text{ mm}$) in diameter.
   • Too heavy to be lifted into the airstream by wind drag alone, these coarse grains roll and slide slowly along the ground surface as they are bombarded by saltating grains, accumulating in drifts along fence lines, curbs, and haul roads.
3. Suspension ($3\%\text{ to }40\%$ of total wind transport):
   • Moves fine silt and clay particles $< 0.1\text{ mm}$ ($100\text{ }\mu\text{m}$) in diameter, including regulated inhalable fugitive dust ($\text{PM}_{10}$, $\le 10\text{ }\mu\text{m}$) and fine respirable particulates ($\text{PM}_{2.5}$, $\le 2.5\text{ }\mu\text{m}$).
   • Once kicked into the air by saltating sand impacts, upward atmospheric turbulence exceeds their tiny gravitational settling velocity, lifting them high into the atmosphere and carrying them miles offsite as visible dust plumes that violate air quality permits and create highway visibility hazards.

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Predicting Soil Loss: RUSLE, MUSLE, and WEQ

Stormwater designers and CESSWI inspectors use empirical prediction models to evaluate relative erosion risk and verify whether proposed BMPs reduce soil loss to acceptable levels.

The Revised Universal Soil Loss Equation (RUSLE)

Developed by the USDA Agricultural Research Service and NRCS, RUSLE predicts long-term average annual soil detachment by sheet and rill erosion (it does not calculate gully erosion, in-stream channel scour, or mass wasting):

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

• $A$ = Average Annual Soil Loss ($\text{tons/acre/year}$).
• $R$ = Rainfall-Runoff Erosivity Factor: Measures the climatic driving force of local precipitation, calculated as the annual summation of individual storm total kinetic energy ($E$) multiplied by the maximum 30-minute rainfall intensity ($I_{30}$), known as the $EI_{30}$ index.
• $K$ = Soil Erodibility Factor: Measures the intrinsic susceptibility of a soil horizon to detachment and transport per unit of $R$ (highest for silt and very fine sand at $0.40\text{ to }0.65$; lowest for coarse sand and high-clay/high-organic soils at $0.02\text{ to }0.20$).
• $LS$ = Slope Length ($L$) and Slope Steepness ($S$) Topographic Factor: Dimensionless ratio comparing soil loss from a given slope length and gradient to a standard unit plot ($72.6\text{ ft}$ long at $9\%$ slope). Slope length ($L$) is measured from the point where overland flow originates to the point where flow concentrates into a channel or deposition begins. Crucial exam concept: Increasing slope steepness ($S$) increases soil loss much more rapidly (exponentially) than an equivalent proportional increase in slope length ($L$). Installing fiber rolls (wattles) or diversions mid-slope reduces $L$; flattening cut/fill grades reduces $S$.
• $C$ = Cover-Management Factor: Dimensionless ratio of soil loss from a protected surface compared to bare, untended soil ($C = 1.0$). This is the single most powerful lever on a construction site: whereas $R$ and $K$ are fixed by location and native geology, applying straw mulch, hydromulch, or RECPs (or establishing dense turf) immediately drops $C$ from $1.0$ down to $0.01\text{ to }0.05$, cutting sheet and rill erosion by $95\%\text{ to }99\%$.
• $P$ = Support Practice Factor: Dimensionless ratio accounting for mechanical surface roughness and directional orientation—such as contour tracking (driving a tracked bulldozer up and down the slope so the horizontal track cleat impressions run parallel to the contour, $P \approx 0.8\text{ to }0.9$), terracing, or contour furrowing. Conversely, running wheeled equipment up and down a slope creates vertical ruts that channelize runoff ($P > 1.0$).

MUSLE and WEQ

• Modified Universal Soil Loss Equation (MUSLE): Replaces the annual rainfall erosivity factor ($R$) with a single-storm runoff energy factor based on total storm runoff volume ($Q_v$) and peak runoff rate ($q_p$) ($11.8 (Q_v \times q_p)^{0.56}$). Unlike RUSLE, MUSLE directly predicts single-storm sediment yield delivered to a basin or outfall.
• Wind Erosion Equation (WEQ): Expressed as $E = f(I, K, C, L, V)$, where annual wind soil loss ($E$) is a function of the Soil Erodibility Index ($I$, governed by the percentage of non-erodible soil clods $> 0.84\text{ mm}$), Ridge Roughness ($K$), Climatic Factor ($C$, proportional to the cube of wind velocity divided by the square of effective surface moisture), Unsheltered Distance ($L$ along the prevailing wind vector), and Vegetative Residue Cover ($V$).

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Site Climatic Conditions, Rainfall Characteristics, and NRCS Storm Types (SAOP 3)

Under SAOP 3 (Site Climatic Conditions and Rainfall Amounts), an inspector must evaluate four precipitation characteristics—intensity ($\text{in/hr}$), duration ($\text{hours}$), frequency / return period (e.g., 2-year, 10-year, or 25-year 24-hour storm), and seasonal distribution—to anticipate site vulnerability.

To model runoff and size stormwater BMPs across diverse U.S. climates, the USDA Natural Resources Conservation Service (NRCS) defines four 24-hour synthetic rainfall distributions:

Arid/Semi-Arid Climates, Freeze-Thaw Cycles, and Weather Monitoring

• Arid and Semi-Arid Regions ($0\text{ to }20\text{ inches}$ annual rainfall): Characterized by thin, alkaline/sodic soils, protective biological soil crusts, high wind erosion potential, and long droughts interrupted by violent convective flash floods (monsoons). Because low moisture prevents rapid seed establishment without irrigation, inspectors prioritize phased disturbance limitation, non-vegetative surface armor (gravel/rock mulch, soil binders, crimped straw), and perimeter runoff diversions.
• Freeze-Thaw Cycles and Rain-on-Snow Events: In cold climates, repeated freezing and thawing forms needle ice (frost heaving) that detaches surface soil aggregates and leaves the upper $2\text{ to }4\text{ inches}$ of a cut slope saturated and cohesionless atop an impermeable frozen subsoil layer. When spring rain falls on melting snowpack (rain-on-snow), virtually $100\%$ of the water becomes runoff over thawed, soupy surface soil, triggering severe sheet, rill, and translational slope failures.
• Inspector Weather Tracking: Inspectors check National Oceanic and Atmospheric Administration (NOAA) / National Weather Service (NWS) Quantitative Precipitation Forecasts $48\text{ to }72\text{ hours}$ before predicted storms to verify pre-storm BMP readiness, and read calibrated on-site rain gauges (or representative NOAA stations within $1\text{ mile}$) to verify when permit inspection thresholds (such as $0.25\text{ inches}$ under the EPA CGP) are triggered.

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Resource and Community Impacts of Offsite Erosion and Sedimentation

Sediment is the largest pollutant by volume entering surface waters in the United States, and construction sites contribute a disproportionate share when controls fail. Offsite sedimentation causes three interconnected categories of harm:

1. Physical and Hydraulic Infrastructure Impacts: Coarse and medium sediment settles in roadside ditches, storm sewer pipes, catch basins, and navigation channels, reducing hydraulic conveyance capacity and increasing localized upstream flooding. Fine sediment accumulates in downstream lakes, reservoirs, and post-construction stormwater basins, permanently destroying flood storage volume and requiring costly dredging.
2. Biological and Ecological Impacts:
   • Turbidity and Light Attenuation: Suspended silt and clay increase turbidity (measured in Nephelometric Turbidity Units, NTU) and Total Suspended Solids (TSS, $\text{mg/L}$), blocking sunlight penetration into the water column, halting photosynthesis in submerged aquatic vegetation (SAV) and phytoplankton, lowering dissolved oxygen, and raising water temperatures (dark turbid water absorbs more solar radiation).
   • Gill Abrasion and Feeding Disruption: Angular suspended mineral grains clog and abrade the delicate gill filaments of fish and aquatic larvae, while high turbidity blinds sight-feeding game fish.
   • Benthic Smothering and Redd Siltation: When suspended silt settles onto cobble and gravel streambeds (substrate embeddedness), it fills interstitial pore spaces, smothering benthic macroinvertebrates (mayfly, stonefly, and caddisfly larvae at the base of the aquatic food web) and suffocating fish eggs and fry inside salmonid and trout spawning beds (redds) by cutting off oxygenated intragravel water flow.
3. Chemical and Pollutant-Transport Impacts ("Piggyback" Pollutants): Because clay micelles and fine organic silt possess immense specific surface area and negative electrical surface charges, they strongly adsorb chemical pollutants—most notably phosphorus (the primary limiting nutrient driving freshwater cultural eutrophication, harmful algal blooms, and hypoxic dead zones), heavy metals (lead, zinc, copper, cadmium), pesticides, and petroleum hydrocarbons. Retaining fine sediment on site is therefore simultaneously a chemical water-quality control.