3.3 Splash, Sheet, Rill, Gully, Channel Erosion, and Slope Movement

Geologic (Natural) vs. Accelerated Construction Erosion

Under SAOP 2 Test Objectives 4 through 6, CESSWI inspectors must distinguish natural landscape evolution from human-induced erosion and diagnose the exact physical mechanism operating on a disturbed site:

• Geologic (Natural) Erosion: The slow, continuous wearing down of the Earth's surface by water, wind, ice, and gravity under undisturbed native ecosystems. Dense vegetative canopies intercept raindrops, surface litter slows overland flow, and fibrous root networks bind soil aggregates, keeping natural soil loss (typically $0.1\text{ to }1.0\text{ ton/acre/year}$) roughly in equilibrium with pedogenic soil formation.
• Accelerated Erosion: When construction clearing, grubbing, topsoil stripping, and mass grading remove vegetative cover and compact the soil profile, natural defenses vanish simultaneously. Without canopy interception or root reinforcement, and with compacted subgrades shedding runoff rather than infiltrating it, erosion rates accelerate to 10 to 1,000 times natural background levels—frequently exceeding $50\text{ to }500\text{ tons/acre/year}$ on unprotected construction slopes.

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The Four-Stage Progression of Overland Water Erosion

Water erosion on an unprotected slope is a two-part physical process—detachment followed by transport—that progresses downslope through four sequential stages as runoff concentrates:

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Stage 1: Raindrop Splash Erosion

Raindrop splash erosion is the initial—and most energetic—detachment mechanism on bare soil. During an intense storm, raindrops up to $5\text{ to }6\text{ mm}$ in diameter reach terminal velocities of $6\text{ to }9\text{ m/s}$ ($20\text{ to }30\text{ mph}$). Striking bare soil like miniature kinetic bombs, raindrops release kinetic energy ($E = \frac{1}{2}mv^2$) that:

1. Shatters weak soil aggregates (slaking) and detaches individual sand, silt, and clay particles.
2. Launches detached soil particles $2\text{ to }3\text{ feet}$ vertically into the air and up to $5\text{ feet}$ laterally. On a sloped grade, more than $50\%$ of splashed particles land downslope of their starting point, causing net downslope soil movement before runoff even begins.
3. Drives detached fine silt and clay particles into surface macropores, creating a dense, $1\text{ to }3\text{ mm}$ thick surface crust (surface seal) that slashes infiltration rates by $50\%\text{ to }90\%$ and rapidly initiates overland runoff.

• Field Diagnostic Indicators: Soil pedestals (small columns of intact soil capped by a pebble, root fragment, or wood chip that acted as a miniature umbrella while the surrounding bare soil was splashed away), crusted surface skin, and mud spatter marks $1\text{ to }3\text{ feet}$ high on wooden stakes, building foundations, or the upslope face of silt fence fabric.
• Primary Control: Erosion control (source cover)—preserving vegetative canopy, applying straw mulch, hydromulch, compost blankets, or Rolled Erosion Control Products (RECPs)—absorbs $95\%\text{ to }99\%$ of raindrop kinetic energy at the surface.

Stage 2: Sheet Erosion

Once rainfall intensity exceeds the infiltration capacity of the crusted soil, ponded water overflows surface micro-depressions and moves downslope as sheet flow—a thin, relatively uniform film of shallow overland water. Sheet flow typically persists for flow path lengths of $100\text{ feet}$ or less (and often concentrates within $30\text{ to }50\text{ feet}$ on graded construction slopes).

• Why Sheet Erosion Is Insidious: Because soil is stripped away in thin, uniform layers across an entire hillside without carving dramatic trenches, dozens of tons of fertile soil per acre can be lost before untrained personnel notice a problem.
• Field Diagnostic Indicators: Exposed fine plant roots, light-colored B-horizon subsoil patches emerging along upper slope shoulders, a concentration of coarse gravel left behind on the slope surface (erosion pavement), and fresh aprons of sorted silt and sand deposited at the toe of the slope or backed up behind perimeter wattles.

Stage 3: Rill Erosion

As sheet flow travels downslope, grading irregularities, tire ruts, or rocks cause the thin film to converge into small, parallel micro-channels called rills. Once flow concentrates, hydraulic depth ($d$) and flow velocity ($V$) increase rapidly, raising boundary shear stress ($\tau_0 = \gamma R S$) past the soil's critical shear threshold. Detachment shifts from raindrop impact to hydraulic scour.

Stage 4: Gully Erosion

When rills are neglected over multiple storm events, or when an upslope diversion berm, downspouts, or roadway crown discharges concentrated flow onto an unprotected slope, rills coalesce into deep gullies. Gullies expand rapidly through three self-reinforcing mechanisms:

1. Headcut Migration: Water cascades over the steep vertical drop at the upstream lip of the gully (the headcut) and plunges into a scour hole (plunge pool) at the base, eroding the supporting toe of the vertical face until the overhang collapses and the headcut marches rapidly upslope.
2. Bed Scour (Downcutting): High-velocity concentrated flow deepens the channel invert, typically carving a V-shaped profile in homogeneous cohesive or gravelly soils, or a U-shaped vertical-walled profile in friable silts (such as loess) where the subsoil undermines easily.
3. Bank Undercutting and Sloughing: Scour at the base of the gully walls oversteepens the side banks until gravity causes blocks of soil to shear and slough into the flow.

• Why Dumping Loose Soil into a Gully Fails: Contractors frequently attempt to "fix" a gully by pushing loose, uncompacted dirt into the trench with a loader bucket or stuffing it with straw bales. The next storm immediately washes out the loose fill (or pipes underneath the straw bales), doubling the downstream sediment load. Proper gully repair requires diverting upslope runoff away from the headcut, shaping the gully walls, placing moisture-conditioned fill in compacted lifts keyed into the intact bank, and stabilizing the flow line with a pipe slope drain, turf reinforcement mat (TRM), or filter-fabric-backed rip-rap channel with rock check dams.

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Channel Erosion (In-Stream and Conveyance Scour)

Channel erosion occurs within defined conveyance swales, roadside ditches, and permanent or intermittent streams. Urbanization and construction site clearing replace absorptive forests and meadows with impervious roofs, haul roads, and compacted subgrades. This watershed hydromodification alters the storm hydrograph in three ways:

1. Higher Peak Discharge ($Q_p$) and larger total runoff volume.
2. Higher Flow Velocity ($V$) and shorter time of concentration ($T_c$).
3. Prolonged Duration of Bankfull Erosive Flows that exceed the critical shear stress of the channel bed and banks.

Channel erosion manifests as bed degradation (vertical downcutting that lowers the streambed, disconnects the floodplain, and exposes utility crossings or bridge piers), lateral bank scour (concentrated along the outside of channel meanders/bends where centrifugal helical flow impinges on the outer bank while point bars deposit on the inside bend), and outlet scour holes immediately downstream of undersized culvert aprons.

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Slope Movement (Geotechnical Mass Wasting)

Whereas water erosion detaches and transports individual soil grains via fluid shear, slope movement (mass wasting) is the bulk downslope failure of an entire mass of soil or weathered rock under gravity when driving shear stresses exceed the internal shear strength of the soil:

$$\tau_f = c' + (\sigma - u) \tan \phi'$$

where $c'$ is effective cohesion, $\sigma$ is normal total stress, $u$ is pore-water pressure, and $\phi'$ is the effective angle of internal friction. When water saturates a slope, pore-water pressure ($u$) rises, pushing soil grains apart and reducing effective normal stress ($\sigma - u$), while simultaneously adding heavy water weight to the driving mass.

• Soil Creep: Slow, nearly imperceptible seasonal movement of the upper soil mantle downslope; diagnosed by downhill-tilted fenceposts, retaining walls, and J-shaped ("pistol-butt") tree trunks.
• Rotational Slump: Failure of a cohesive soil mass along a concave-upward curved slip circle, leaving a crescent-shaped vertical head scarp at the upper crown, backward-tilted slump blocks where water ponds, and a hummocky bulging toe at the base.
• Translational Slide: A planar sheet of soil or fill sliding rapidly along a shallow, flat plane of weakness—most commonly triggered when a contractor places fill or topsoil over a smooth, compacted subgrade, inclined clay layer, or existing vegetation without stripping, benching (stair-stepping), or keying the fill into the slope.
• Earthflows, Mudflows, and Debris Flows: Rapid, liquefied flows of saturated soil, mud, and rock debris when extreme rainfall or snowmelt raises pore pressure until the soil loses all shear strength.