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3.3 Splash, Sheet, Rill, Gully, Channel Erosion, and Slope Movement
Free CESSWI Certified Erosion, Sediment and Stormwater Inspector Study Guide 2026 Exam
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1 Chapter 1: CESSWI Program Overview, Scope of Practice & Professional Ethics
1.1 CESSWI Scope of Practice, Eligibility, and Exam Structure 1.2 Certification Renewal, PDH Requirements, and Audit Policies 1.3 EnviroCert Code of Ethics and Professional Conduct
2 Chapter 2: Federal, State, and Local Stormwater Regulations
2.1 Federal Water-Pollution History and Stormwater Phases 2.2 CWA Sections, Water Quality, and Related Federal Programs 2.3 EPA Construction Permit Coverage and Operational Clocks 2.4 Industrial MSGP and Municipal MS4 Programs 2.5 Regulatory Compliance, Enforcement Mechanisms, and Penalties
3 Chapter 3: Soil Mechanics, Erosion Processes, and Climatic Factors
3.1 Soil Formation, Horizons, and Geomorphology 3.2 Soil Texture, Mechanics, and Revegetation 3.3 Splash, Sheet, Rill, Gully, Channel Erosion, and Slope Movement 3.4 Wind Erosion, Sediment Transport, Climatic Erosivity, and Resource Impacts
4 Chapter 4: Hydrology, Runoff Characteristics, and Drainage Principles
4.1 The Hydrologic Cycle, Watershed Drainage Planning, and Precipitation 4.2 Hydrologic Soil Groups, Disturbed Profiles, and Time Parameters 4.3 Hydrograph Characteristics, Curve Numbers, and Runoff Estimation Equations
5 Chapter 5: Inspection Fundamentals, Duties, and Field Procedures
5.1 Inspector Roles, Scope of Authority, and Core Professional Skills 5.2 Pre-Inspection Document Review and Inspector Support Equipment 5.3 Systematic Field Inspection Sequence, Post-Inspection Meetings, and Follow-Through
6 Chapter 6: Inspection Elements for Active Construction Sites
6.1 Pre-Construction Meetings, Entrance Posting, and On-Site Document Inspections 6.2 Inspecting Active Construction Controls and Diagnosing BMP Failure Modes 6.3 Final Stabilization Verification, Final Inspections, and Notice of Termination
7 Chapter 7: Stormwater Monitoring, Field Sampling, and Chain of Custody
7.1 Permit Monitoring Triggers and Water-Quality Indicators 7.2 Sampling Preparation and Analytical Parameters 7.3 Sampling Locations, Collection Methods, and Field Meter Calibration 7.4 Sample Preservation, Data Recording, Shipping, and Chain of Custody
8 Chapter 8: Inspection Documentation, Reporting, and Map Management
8.1 Inspection Notes and Evidence Procedures 8.2 Objective Findings and Photographic Evidence 8.3 Proper Reporting Techniques and Corrective Action Log Management 8.4 Site Drawing (Map) Management, Compliance Resolution, and Enforcement Records
9 Chapter 9: Professional Communication and Site Safety for Inspectors
9.1 Communication Protocols in Pre-Construction, Post-Inspection, and O&M Meetings 9.2 Dealing with Difficult People, Problematic Sites, and Unpleasant Findings 9.3 Site Hazard Recognition: Physical, Biological, Trenching, and Confined Spaces 9.4 Hazardous Substances, Safety Data Sheets (SDS), Spill Response, and PPE
10 Chapter 10: Stormwater Plan Reading, Map Interpretation, and Site Calculations
10.1 Stormwater Plan Types, Credentials, and Authority 10.2 SWPPP Components and Permit Notifications 10.3 Civil Plan Sets, Legends, Phased Sequencing, and Living Site Maps 10.4 Topography, Contour Lines, Slope Calculations, and Plan Amendments
11 Chapter 11: BMP Framework, Site Planning, and Erosion Control Practices
11.1 Six Functional BMP Categories, Preservation of Vegetation, Phasing, and Tree Protection 11.2 Seeding and Sod Establishment 11.3 Hydroseeding and Hydromulching Inspection 11.4 Mulch and Rolled Erosion-Control Products 11.5 Tackifiers, Soil Binders, Roughening, and Temporary Cover
12 Chapter 12: Perimeter, Linear, and In-Water Sediment Control Practices
12.1 Perimeter & Sheet-Flow Sediment Barriers: Silt Fence, Fiber Rolls, Straw Bales, and Filter Dikes 12.2 Access & Roadside Sediment/Runoff Controls: Stabilized Entrances, Cut-Back Curbs, and Ditch Checks 12.3 In-Water & Dewatering Controls: Temporary Stream Crossings, Turbidity Curtains, and Sediment Bags
13 Chapter 13: Sediment Basins, Traps, and Runoff Conveyance Controls
13.1 Sediment Basins and Traps 13.2 Porous Baffles and Surface Skimmers 13.3 Runoff Conveyances: Diversions, Swales, Pipe Slope Drains, and Rock Check Dams 13.4 Riprap and Gabion Inspection 13.5 Retaining Walls, Outlet Protection, and Wet Ponds
14 Chapter 14: Storm Drain Inlet Protection and Good Housekeeping BMPs
14.1 Storm Drain Inlet Protection: At-Grade, Curb, Block & Gravel, Excavated, Fabric Drop, Gravel Bag, Stone Collar, and Pre-Manufactured 14.2 Good Housekeeping Part 1: Concrete Washout, Equipment Maintenance, Materials Management, and Secondary Containment 14.3 Good Housekeeping Part 2: Stockpiles, Dust Control, Wind Fence, Street Cleaning, Sanitary Waste, and Trash
15 Chapter 15: Post-Construction Stormwater Management BMPs
15.1 Vegetative & Structural Dispersion BMPs: Final Ground Cover, Headwalls, Level Spreaders, Filter Strips, and Riparian Buffers 15.2 Permeable Surfaces and Construction Protection 15.3 Infiltration, Bioretention, and Media Filters 15.4 Detention, Retention & Proprietary BMPs: Dry/Wet Basins, Wetlands, Inline Storage, Catch Basin Inserts, Vaults, and Manufactured Devices
3.3 Splash, Sheet, Rill, Gully, Channel Erosion, and Slope Movement
Key Takeaways Explain This Topic AI
Anthropogenic land disturbance strips protective vegetation and compacts soil, accelerating construction site erosion rates to 10 to 1,000 times above natural geologic background levels.
Overland water erosion advances through four sequential stages: raindrop splash (falling at 6 to 9 m/s and splashing soil 2 to 3 ft high and 5 ft laterally), sheet erosion (< 100 ft overland film), rill erosion, and gully erosion.
The definitive field and exam boundary between rills and gullies is equipment obliteration: rills are shallow micro-channels (typically < 3 to 4 inches deep) that can be obliterated by normal tillage or grading equipment, whereas gullies are incised channels too deep to be smoothed by normal tillage.
In-stream channel scour stems from increased peak discharge (Q), flow velocity (V), and prolonged hydrograph duration, while geotechnical slope failures (rotational slumps, translational slides, debris flows) are triggered by pore-water pressure buildup, toe undercutting, and crest surcharging.
Last updated: October 2026
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 to 1.0 ton/acre/year 0.1\text{ to }1.0\text{ ton/acre/year} 0.1 to 1.0 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 to 500 tons/acre/year 50\text{ to }500\text{ tons/acre/year} 50 to 500 tons/acre/year on unprotected construction slopes.
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 to 6 mm 5\text{ to }6\text{ mm} 5 to 6 mm in diameter reach terminal velocities of 6 to 9 m/s 6\text{ to }9\text{ m/s} 6 to 9 m/s ( 20 to 30 mph 20\text{ to }30\text{ mph} 20 to 30 mph ) . Striking bare soil like miniature kinetic bombs, raindrops release kinetic energy ( E = 1 2 m v 2 E = \frac{1}{2}mv^2 E = 2 1 m v 2 ) that:
Shatters weak soil aggregates (slaking) and detaches individual sand, silt, and clay particles.
Launches detached soil particles 2 to 3 feet 2\text{ to }3\text{ feet} 2 to 3 feet vertically into the air and up to 5 feet 5\text{ feet} 5 feet laterally . On a sloped grade, more than 50 % 50\% 50% of splashed particles land downslope of their starting point, causing net downslope soil movement before runoff even begins.
Drives detached fine silt and clay particles into surface macropores, creating a dense, 1 to 3 mm 1\text{ to }3\text{ mm} 1 to 3 mm thick surface crust (surface seal) that slashes infiltration rates by 50 % to 90 % 50\%\text{ to }90\% 50% 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 to 3 feet 1\text{ to }3\text{ feet} 1 to 3 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 % to 99 % 95\%\text{ to }99\% 95% 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 feet 100\text{ feet} 100 feet or less (and often concentrates within 30 to 50 feet 30\text{ to }50\text{ feet} 30 to 50 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 d d ) and flow velocity ( V V V ) increase rapidly, raising boundary shear stress ( τ 0 = γ R S \tau_0 = \gamma R S τ 0 = γ RS ) past the soil's critical shear threshold. Detachment shifts from raindrop impact to hydraulic scour.
Important
The Exam-Critical Boundary Between Rills and Gullies: On the CESSWI exam, the defining distinction between rill erosion and gully erosion is whether the channels can be smoothed out by standard equipment:
Rills are small, intermittent concentrated-flow channels—typically less than 3 to 4 inches 3\text{ to }4\text{ inches} 3 to 4 inches deep (and up to a maximum of 12 inches 12\text{ inches} 12 inches in some NRCS field definitions)—that can be obliterated by normal tillage or standard agricultural/grading equipment in a single pass.
Gullies are deep, incised concentrated-flow channels that cannot be obliterated by normal tillage equipment and require structural earthwork, compacted fill in lifts, grade control, and channel lining.
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:
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.
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.
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.
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:
Higher Peak Discharge ( Q p Q_p Q p ) and larger total runoff volume.
Higher Flow Velocity ( V V V ) and shorter time of concentration ( T c T_c T c ).
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.
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: τ f = c ′ + ( σ − u ) tan ϕ ′ \tau_f = c' + (\sigma - u) \tan \phi' τ f = c ′ + ( σ − u ) tan ϕ ′
where c ′ c' c ′ is effective cohesion, σ \sigma σ is normal total stress, u u u is pore-water pressure , and ϕ ′ \phi' ϕ ′ is the effective angle of internal friction. When water saturates a slope, pore-water pressure ( u u u ) rises, pushing soil grains apart and reducing effective normal stress ( σ − u \sigma - u σ − 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.
Tip
Four Construction Red Flags for Imminent Slope Failure: During field inspections of steep cut and fill slopes, immediately document and escalate: (1) arcuate tension cracks opening parallel to the crest of a slope; (2) surcharging the slope crest with heavy soil stockpiles or equipment staging; (3) undercutting the slope toe for utility trenches or haul roads; and (4) seepage faces or piping boils emerging near the base of a slope.
Erosion / Movement Type Dominant Physical Mechanism Depth / Scale & Obliteration Threshold Key Field Diagnostic Indicators Primary Preventive & Corrective BMPs
Raindrop Splash Kinetic energy of raindrops ( 6 to 9 m/s 6\text{ to }9\text{ m/s} 6 to 9 m/s ) detaching particles & sealing pores Surface ( 1 to 3 mm 1\text{ to }3\text{ mm} 1 to 3 mm crust); splashes 2 to 3 ft 2\text{ to }3\text{ ft} 2 to 3 ft high, 5 ft 5\text{ ft} 5 ft laterally Soil pedestals under pebbles, surface crusting, mud spatter on stakes/fences Surface cover: mulch, hydromulch, RECPs, preserving canopy
Sheet Erosion Thin, uniform overland film ( < 100 ft < 100\text{ ft} < 100 ft flow path) transporting splashed fines Millimeters per storm across broad slope face Exposed fine roots, subsoil patches on slope shoulders, sediment fans at toe Mulch/RECPs, surface roughening (contour tracking), fiber rolls/wattles
Rill Erosion Concentrated micro-channel hydraulic shear stress Typically < 3 to 4 in. < 3\text{ to }4\text{ in.} < 3 to 4 in. deep; obliterated by normal tillage/grading Parallel shallow grooves down slope face, often starting in vertical tire tracks Regrade/track along contour, shorten slope length ( L L L ) with wattles, apply RECP
Gully Erosion Deep concentrated scour, headcut plunge-pool undercutting, bank sloughing > 1 ft > 1\text{ ft} > 1 ft to 10 + ft 10+\text{ ft} 10 + ft deep; cannot be obliterated by normal tillage Steep upstream headcut, plunge pool, caving V- or U-shaped walls Divert upslope flow, compact structural fill in lifts, install pipe slope drain or rock/TRM swale
Channel Erosion Increased peak discharge ( Q Q Q ), velocity ( V V V ), and duration from impervious areas Full ditch or stream cross-section Outer-bend bank undercutting, exposed utility pipes, incised streambed Check dams, TRMs, rip-rap/gabion armoring, outlet energy dissipators, detention basins
Slope Movement (Mass Wasting) Gravity shear exceeding soil strength due to high pore-water pressure ( u u u ) Shallow planar fill sheet to deep rotational slip circle Crest tension cracks, head scarps, bulging toe, tilted posts, seepage at toe Crown diversion berms, horizontal subdrains, benching/keying fill lifts, retaining walls, toe buttresses