4.3 Channel, Stream Bank, Shoreline & Wind Erosion

4.3 Channel, Stream Bank, Shoreline & Wind Erosion

Quick Summary: Beyond upland rainfall erosion, civil and environmental engineers must manage concentrated hydraulic scour in channels, geotechnical failures along stream banks, wave-driven shoreline degradation, and wind erosion across arid or denuded plains. Open channel stability is governed by tractive force theory, where applied boundary shear stress ($\tau = \gamma R S$) must not exceed the channel lining's critical shear stress. Stream banks degrade through a combination of hydraulic toe scour (undercutting by high-velocity currents) and geotechnical mass failure (rotational slumping induced by high pore water pressures during rapid flood hydrograph drawdown). Wind erosion transports soil via three distinct modes: saltation (bouncing particles, 50%–70%), surface creep (rolling coarse grains, 5%–25%), and suspension (airborne fine dust, 3%–15%), with saltation serving as the dynamic engine driving all aeolian transport.

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Concentrated Flow Channel Hydraulics & Tractive Force Theory

When surface runoff is intercepted by roadside ditches, diversion swales, storm sewer outfalls, or natural stream channels, water transitions into concentrated open channel flow. Designing stable, non-erodible conveyances requires calculating the hydrodynamic shear forces exerted by moving water on the channel boundary.

The Tractive Force Equation (Boundary Shear Stress)

In modern erosion engineering, the tractive force (shear stress) method has superseded older maximum permissible velocity approaches. Applied tractive force represents the tangential drag force exerted by flowing water per unit wetted area of the channel perimeter:

$\tau = \gamma R S$

Where:

• $\tau$ = Boundary shear stress exerted on the channel bed and banks ($\text{lb/ft}^2$ or $\text{N/m}^2$ [Pascals])
• $\gamma$ = Unit weight of water ($62.4\text{ lb/ft}^3$ for fresh water, or $9,810\text{ N/m}^3$)
• $R$ = Hydraulic radius of the flow cross-section, defined as $R = \frac{A}{P}$ (where $A$ is cross-sectional flow area in $\text{ft}^2$ or $\text{m}^2$, and $P$ is the wetted perimeter in $\text{ft}$ or $\text{m}$)
• $S$ = Energy slope of the flow, represented by the channel bed slope for uniform steady flow ($\text{ft/ft}$ or $\text{m/m}$)

Maximum Shear Stress Distribution in Channels

In open trapezoidal or parabolic channels, shear stress is not distributed uniformly along the wetted perimeter:

• Channel Bed Maximum: Maximum shear stress on the channel bottom occurs along the centerline, approximated by $\tau_{bed} = \gamma d_{max} S$, where $d_{max}$ is maximum flow depth.
• Channel Bank Maximum: Maximum shear stress on channel side slopes typically reaches 75% to 80% of the bed maximum ($\tau_{bank} \approx 0.76 \gamma d_{max} S$). However, because soil grains on steep side slopes also experience downslope gravitational sliding forces, side banks are frequently more vulnerable to failure than the channel bed.

Permissible Shear Stress Design Criterion

For a channel lining (whether bare soil, vegetative turf, turf reinforcement mat [TRM], or rock riprap) to remain stable, the applied shear stress must not exceed the critical permissible shear stress ($\tau_c$) of the lining material:

$\tau_{\text{applied}} \le \tau_{\text{allowable}}$

If $\tau_{\text{applied}} > \tau_c$, hydraulic drag dislodges the protective lining, tears up root networks, and initiates channel bed incision.

Channel Incision & Knickpoint Migration

When an unlined or inadequately protected channel experiences shear stresses far exceeding $\tau_c$, the channel bed downcuts, initiating channel incision. Incision frequently manifests as an upstream-migrating vertical bed step known as a knickpoint. As concentrated water cascades over the knickpoint, plunge-pool scour undermines the upper bed, causing the knickpoint to migrate up-gradient. This downcutting lowers the local base level, transforming stable tributary swales into suspended, hanging gullies.

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Stream Bank Erosion Mechanisms

Stream bank degradation is rarely a pure hydraulic scouring event; it is almost universally a coupled hydraulic-geotechnical failure involving two distinct, interacting physical mechanisms:

Step 1: Fluvial Hydraulic Toe Scour  ➔  Undercuts Bank Foundation
Step 2: Rapid Hydrograph Drawdown   ➔  Triggers Geotechnical Rotational Slumping

1. Hydraulic Toe Scour (Fluvial Undercutting)

• Helical Secondary Currents on Meanders: As streamflow navigates a channel bend, centrifugal force pushes high-velocity surface water toward the outer (concave) bank. Near the outer bank, water plunges downward, creating a powerful helical (spiral) secondary current that sweeps along the channel bed back toward the inner bank.
• Toe Undercutting: This helical current concentrates maximum velocity and shear stress directly against the toe of the bank (the zone where the bank slope meets the channel bed). Fluvial forces pluck particles from the toe, carving a deep, horizontal notch or undercut bench.
• Loss of Basal Support: Toe scour removes the foundational counter-weight that supports the entire overlying bank mass, transforming a stable, sloping bank into a precarious, over-steepened, or cantilevered cliff.

2. Geotechnical Bank Failure & Rapid Drawdown Slumping

While toe scour prepares the bank for collapse, catastrophic structural bank failure most frequently occurs not during the peak of a flood, but immediately after the flood crest has passed—during rapid hydrograph drawdown:

1. Saturation During Flood Crest: During high-stage flood flows, water infiltrates deeply into the stream bank, completely saturating bank soils and filling pore spaces. At peak river stage, the outward hydrostatic pressure of the floodwaters in the river channel acts as a stabilizing structural buttress, holding the saturated soil mass in place against the bank face.
2. Rapid Hydrograph Recession (Drawdown): Following the storm, the river water surface elevation drops rapidly. However, within fine-grained bank soils (silts and cohesive clays with low hydraulic conductivity, $K$), groundwater cannot drain quickly enough to keep pace with the falling river.
3. Elevated Pore Water Pressure & Loss of Effective Stress: The removal of the river's stabilizing hydrostatic head creates a severe lateral hydraulic gradient. Simultaneously, residual positive pore water pressures ($u$) remain trapped inside the bank. According to Terzaghi's effective stress principle: $\sigma' = \sigma - u$ High pore water pressure ($u$) drastically reduces effective normal stress ($\sigma'$), driving the soil's internal shear strength ($\tau_f = c' + \sigma' \tan\phi'$) to near zero.
4. Rotational and Planar Slumping: Surcharged by the weight of trapped water and deprived of lateral confining support, the entire upper bank shears along a curved failure surface (rotational slump) or planar slip plane, tumbling into the river channel as a massive earthen slide.

3. Subaerial Weathering & Freeze-Thaw Loosening

Exposed bank faces are subjected to severe subaerial weathering. In cold climates, capillary moisture within bank pores freezes into thin, vertical ice columns known as needle ice. Needle ice heaves soil particles outward perpendicular to the bank face; when the ice melts, the loosened, un-cohesive soil sloughs directly down into the riverbed, priming the bank for hydraulic washout during subsequent spring flows.

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Shoreline and Lacustrine Erosion Mechanics

Shoreline erosion along lakes, coastal bays, and large detention impoundments is driven by the dynamic energy of surface water waves rather than unidirectional channel flow.

Wind-Generated Wave Dynamics

When wind blows across an open body of water, frictional shear drag transfers atmospheric kinetic energy into the water column, generating surface gravity waves. Wave height ($H$), period ($T$), and total energy ($E = \frac{1}{8} \rho g H^2$) are governed by three primary environmental variables:

1. Wind Velocity: Higher wind speeds impart exponentially greater kinetic energy.
2. Wind Duration: The continuous duration of time that wind blows from a constant direction.
3. Fetch Length: The uninterrupted horizontal distance of open water over which wind blows without encountering land barriers. On large lakes or reservoirs with long fetches (> 2 to 5 miles), storm winds generate high-amplitude, high-energy wave trains capable of extensive shoreline destruction.

Nearshore Wave Shoaling & Breaking Mechanics

In deep water, wave energy moves via circular orbital particle motions that do not transport mass. However, as waves propagate into shallow nearshore waters, water depth decreases to less than half the wavelength ($y < \frac{1}{2} L$):

• The circular orbits interact with the lakebed (shoaling), inducing bottom friction that slows wave speed and compresses wavelength.
• Wave height surges until the wave crest becomes unstable and breaks violently in the surf zone, dissipating concentrated kinetic energy against the shoreline.

Swash, Backwash & Littoral Drift

• Swash: The turbulent sheet of water that surges up the beach face following wave breaking. Swash exerts high shear stress, carrying sediment up the slope.
• Backwash: The gravitational retreat of water flowing back down the beach face into the lake. Backwash pulls sand and silt lakeward, eroding the beach profile.
• Littoral Drift (Longshore Transport): When waves strike the shoreline at an oblique angle, the swash drives sand grains diagonally up the beach, while the backwash pulls them straight down perpendicular to the contour under gravity. This zig-zag movement produces a net horizontal sediment transport vector parallel to the shoreline known as littoral drift. If upstream sediment supply is blocked (e.g., by groins or jetties), down-drift shorelines experience severe sediment starvation and rapid bluff recession.

Boat Wake Turbulence

On inland lakes, reservoirs, and recreational rivers, wake turbulence generated by motorized recreational vessels (especially deep-draft wake-surfing boats and cabin cruisers) frequently exceeds natural wind-wave energy. Boat wakes generate steep, short-period waves that slam into sheltered, un-armored wetland fringes and earthen bluffs, eroding shoreline vegetation and causing severe bank undercutting.

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Wind Erosion Mechanics & Transport Modes

Wind (aeolian) erosion occurs when atmospheric wind shear exceeds the critical threshold required to detach and move dry, bare, un-aggregated soil particles. Wind erosion degrades agricultural productivity, strips valuable topsoil, and generates hazardous airborne particulate matter ($PM_{10}$ and $PM_{2.5}$) that violates Clean Air Act National Ambient Air Quality Standards (NAAQS).

Aerodynamic Forces: Threshold Friction Velocity & Roughness

Air flowing over a land surface establishes a boundary layer profile governed by the Prandtl-von Kármán logarithmic wind equation. For wind erosion to initiate, the wind shear velocity ($u_*$) must exceed the *threshold friction velocity ($u_{t}$) of the soil:

$u_{*t} = A \sqrt{\frac{\rho_s - \rho_a}{\rho_a} g d}$

Where $\rho_s$ is particle density, $\rho_a$ is air density, $d$ is particle diameter, and $A$ is an empirical coefficient. Fine-to-medium sand grains (0.1 to 0.15 mm) exhibit the lowest threshold friction velocity (approximately 8 to 12 mph / 3.5 to 5.5 m/s measured at standard anemometer heights); they are the easiest particles for wind to detach. Conversely, coarse gravels are too heavy, while undisturbed fine clays are bound by strong cohesive and electrostatic forces, requiring higher wind velocities to dislodge.

The Three Modes of Wind Particle Movement

Once the threshold friction velocity is surpassed, wind moves soil through three distinct physical transport modes, partitioned strictly by particle diameter:

[Suspension: < 0.05 mm]    ◄── 3% to 15% (Lofted high into atmospheric turbulence)
          ▲
          │ (Impact Bombardment)
[Saltation: 0.05 - 0.5 mm] ◄── 50% to 70% (Bouncing grains: The Central Engine)
          │ (Kinetic Push)
          ▼
[Surface Creep: 0.5 - 2.0 mm] ◄── 5% to 25% (Rolling and sliding along bed)

1. Saltation (0.05 mm to 0.5 mm) — The Central Engine of Wind Erosion

• Transport Share: Accounts for 50% to 70% of total wind erosion by mass.
• Particle Fraction: Very fine, fine, and medium sand grains.
• Physical Mechanics: Grains are dislodged by wind lift and drag, rising into the air in a low, steep trajectory (typically rising a few inches to 1–2 feet above the surface). Downwind air velocity accelerates the grain horizontally before gravity pulls it back to the surface along a shallow descent angle (6° to 12°).
• The Avalanche Effect (Saltation Bombardment): Saltation is the primary physical driver of all wind erosion. When a saltating grain strikes the ground at high speed, its kinetic energy shatters dry soil aggregates and delivers two simultaneous mechanical impacts:
  1. It kicks coarse sand grains forward into surface creep;
  2. It shatters clay/silt clods, projecting microscopic dust particles upward into atmospheric suspension. Without saltation, neither significant surface creep nor massive dust suspension can occur.

2. Surface Creep (0.5 mm to 2.0 mm)

• Transport Share: Accounts for 5% to 25% of total wind erosion by mass.
• Particle Fraction: Coarse sand grains and fine soil aggregates.
• Physical Mechanics: Particles in this size class are too massive to be lofted into the air by wind forces alone. Instead, they roll, slide, and creep along the ground surface. Surface creep is sustained partly by direct wind drag, but is powered predominantly by the continuous physical bombardment of descending saltating grains transferring momentum upon impact.

3. Suspension (< 0.05 mm)

• Transport Share: Accounts for 3% to 15% of total wind erosion by mass.
• Particle Fraction: Fine silts, clays, and organic colloidal humus.
• Physical Mechanics: Once projected into the air by saltation impacts, the terminal gravitational settling velocity of these tiny particles ($d < 0.05\text{ mm}$) is substantially lower than the upward velocity of turbulent atmospheric thermal eddies. Consequently, suspended particles are swept thousands of feet high into the troposphere, traveling hundreds or thousands of miles downwind before settling out via wet or dry atmospheric deposition.
• Air Quality & Health Hazards: Suspended dust constitutes respirable particulate matter ($PM_{10}$ and $PM_{2.5}$), causing severe respiratory illness, reducing roadway visibility to near-zero (brownout conditions), and stripping the most biologically fertile organic nutrients from the source soil.

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Wind Erosion Factors & Construction Dust Control BMPs

The susceptibility of a bare site to wind erosion is governed by the classical Wind Erosion Equation (WEQ) variables: soil erodibility index ($I$), surface roughness ($K$), climate factor ($C$, incorporating wind speed and precipitation), unsheltered distance / field length ($L$), and vegetative cover ($V$).

Critical Environmental Factors

• Soil Moisture: Capillary water films between soil grains create powerful meniscus surface tension forces that bind particles together. When soil moisture exceeds field capacity, wind detachment is virtually eliminated. Wind erosion occurs almost exclusively on dry, desiccated surfaces.
• Surface Crusts & Aggregation: Non-erodible soil clods ($> 0.84\text{ mm}$ diameter) and intact structural crusts resist wind detachment. Disking or tracking heavy machinery across dry ground pulverizes clods into erodible saltation-sized grains.
• Vegetative Windbreaks (Shelterbelts): Semipermeable barriers (such as tree rows or 50% porosity wind fencing) absorb atmospheric wind momentum, lifting the boundary layer. A functional windbreak provides downwind wind velocity reduction over a distance equal to 10 to 30 times the barrier height ($10H$ to $30H$).

Construction Dust Control BMPs

1. Water Application (Sprinkling): Applying water via dedicated water trucks is the most common temporary dust control measure. Water temporarily restores inter-particle capillary cohesion. However, in hot, arid climates, evaporation is rapid, requiring water trucks to cycle every 1 to 2 hours, making water spraying water-intensive and labor-expensive.
2. Chemical Soil Stabilizers & Tackifiers: Applying liquid soil binding agents creates an artificial surface crust that cements loose grains:
   • Calcium Chloride ($\text{CaCl}_2$) & Magnesium Chloride ($\text{MgCl}_2$): Hygroscopic salts that extract moisture directly from ambient humidity to keep roads damp.
   • Synthetic Polyacrylamide Emulsions (PAM): Anionic polymer sprays that bond mineral particles without altering soil pH.
   • Lignin Sulfonate & Bituminous Emulsions: Organic tree resin and asphaltic binders for heavy haul roads.
3. Physical Wind Barriers & Porous Silt Fencing: Installing 4- to 6-foot high porous wind fences oriented perpendicular to prevailing winds spaced at intervals of $10H$ to $15H$ to disrupt saltation avalanching.
4. Surface Roughening (Emergency Tillage): Operating a chisel plow or furrower perpendicular to prevailing winds to bring non-erodible cohesive clods ($> 0.84\text{ mm}$) to the surface, creating furrows that trap saltating grains.

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Reference Table: Wind Erosion Transport Modes

The following engineering matrix summarizes the particle mechanics, mass fractions, physical trajectories, and primary control interventions for the three modes of wind erosion.

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The Full Menu of Wind Erosion Control Measures

The body of knowledge enumerates ten distinct wind-erosion management practices. Each attacks one of the three WEQ levers — reduce the wind velocity at the soil surface, increase the size or cohesion of surface aggregates, or shorten the unsheltered distance over which saltation can accelerate.

WEQ and WEPS: The Two Prediction Models

The classical Wind Erosion Equation (WEQ), developed by Woodruff and Siddoway (1965), is the wind analogue of USLE:

$E = f(I, K, C, L, V)$

where $I$ is the soil erodibility index, $K$ the surface roughness factor, $C$ the climatic factor, $L$ the unsheltered field length, and $V$ the vegetative cover equivalent. WEQ is deliberately not a simple product — the terms interact through lookup charts and tables rather than by multiplication, which is a favorite distractor on exam items that try to present it as "$E = I \times K \times C \times L \times V$."

The Wind Erosion Prediction System (WEPS) is its process-based replacement, released by USDA-ARS and adopted by NRCS. WEPS runs a daily time-step simulation with submodels for weather, hydrology, soil surface state, crop growth, residue decomposition, tillage management, and erosion, and it reports saltation/creep, suspension, and PM10 losses separately rather than as a single lumped tonnage. NRCS uses WEPS for conservation planning today; WEQ survives in older manuals and in exam questions. Know that both exist and that WEPS is the current model.