13.3 Runoff Conveyances: Diversions, Swales, Pipe Slope Drains, and Rock Check Dams

Free CESSWI Certified Erosion, Sediment and Stormwater Inspector Study Guide 2026 Exam

Track your progress as you read

Finished sections get a check mark.

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

13.3 Runoff Conveyances: Diversions, Swales, Pipe Slope Drains, and Rock Check Dams

Key Takeaways Explain This Topic AI

Diversions (12.5.3) installed upslope of disturbed areas intercept clean off-site run-on and route it to stable outlets, whereas mid-slope or toe-of-slope diversions collect dirty runoff and must discharge into a sediment trap or basin—never directly into a receiving waterbody.

Swales (12.5.9) must be lined based on design flow velocity: bare earth scours above 1.5 to 2.5 ft/s, established turfgrass handles 3 to 5 ft/s, temporary Erosion Control Blankets (ECBs) handle 5 to 8 ft/s, and permanent Turf Reinforcement Mats (TRMs) or rip-rap withstand 8 to 15+ ft/s.

Pipe Slope Drains (12.5.6) require four critical inspection checkpoints: a compacted inlet berm with a watertight entrance collar (where uncompacted backfill causes a common piping failure mode), watertight pipe couplings, slope anchor stakes every 10 to 20 ft, and a flared riprap outlet apron or sediment trap at the toe.

Rock Check Dams (12.5.1) are built 2 to 3 ft high over non-woven geotextile with a 4- to 8-inch angular riprap core faced upstream with 1- to 2-inch washed filter stone, a center weir depressed at least 6 inches below the outer bank wings, and spacing of L = H / S so the upstream toe equals the downstream crest elevation.

Last updated: October 2026

Note

Using dimensions in this section: Construction dimensions, material sizes, slopes, cleanout fractions, and drawdown ranges below describe example practice details. Verify the approved design, manufacturer instructions, and applicable permit before using a number as an inspection acceptance criterion. Regulatory thresholds identified explicitly as EPA/OSHA requirements remain distinct.

Principles of Stormwater Runoff Conveyance and Slope Protection

Once rainfall exceeds soil infiltration capacity, surface runoff begins moving downslope as overland sheet flow. If allowed to travel unchecked across long or steep disturbed slopes, sheet flow rapidly concentrates into high-velocity rills and gullies whose erosive shear stress increases exponentially with flow depth and slope gradient. Under SAOP 11: Management Practices , runoff conveyance BMPs perform three hydraulic functions:

Keep Clean Water Clean: Intercept off-site upslope run-on before it enters active grading zones;

Break Effective Slope Length ( L S LS L S ): Intercept dirty sheet flow at regular intervals on disturbed slopes and convey it safely downslope inside non-erodible conduits or armored channels; and

Reduce Channel Velocity and Shear Stress: Slow concentrated ditch flows and trap coarse bedload before runoff reaches the project's sediment basin.

This section covers four high-frequency runoff conveyance BMPs from Chapter 12.5 of the CESSWI General Principles Review Manual : Diversions ( 12.5.3 ) , Swales ( 12.5.9 ) , Pipe Slope Drains ( 12.5.6 ) , and Check Dams: Rock ( 12.5.1 ) .

1. Diversions ( 12.5.3 )

A Diversion ( 12.5.3 ) (also referred to as a diversion dike, earth berm, interceptor dike, or diversion channel) is a temporary or permanent compacted earthen ridge, excavated channel, or combination ridge-and-channel constructed across a slope with a controlled positive longitudinal grade to intercept overland sheet flow and redirect it safely to a stabilized outlet or sediment treatment facility.

Top-of-Slope Clean-Water Diversion vs. Toe/Mid-Slope Dirty-Water Diversion

On the CESSWI examination, an inspector must immediately identify whether a diversion is functioning as a clean-water interceptor or a dirty-water collector , because their required discharge outlets are completely different:

Diversion Classification Location on Site Water Quality Conveyed Mandatory Outlet Destination

Upslope (Clean Run-On) Diversion Installed above the top crest of a disturbed cut slope or along the upgradient site boundary Clean off-site run-on from undisturbed woods, pastures, or adjacent parcels Discharges directly to a stabilized undisturbed area, level spreader, or natural watercourse —bypassing on-site sediment basins so clean run-on does not hydraulically overload them

Mid-Slope / Toe-of-Slope (Dirty Runoff) Diversion Installed horizontally across long disturbed slopes (to break slope length L S LS L S ) or along the base of unstabilized fill slopes Sediment-laden ("dirty") construction runoff Must route into a Pipe Slope Drain ( 12.5.6 ), Sediment Trap ( 12.4.5 ), or Sediment Basin ( 12.4.4 ) — NEVER directly into a stream, wetland, or storm sewer!

Installation and Inspection Checkpoints for Diversions

Machine Compaction of Earthen Dike Lifts: Earthen diversion dikes must be constructed in lifts and thoroughly machine-compacted (typically ≥ 90 % \ge 90\% ≥ 90% Standard Proctor density or tracked repeatedly with heavy equipment) with side slopes of 2 H : 1 V 2\text{H}:1\text{V} 2 H : 1 V or flatter ( 3 H : 1 V 3\text{H}:1\text{V} 3 H : 1 V preferred for equipment crossing) and a minimum top width of 2.0 feet 2.0\text{ feet} 2.0 feet . Loose, uncompacted windrows of bulldozed dirt slump and breach during the very first rainstorm.

Positive Longitudinal Grade & Freeboard: The diversion channel must maintain a continuous, uniform positive grade (without low spots where water ponds and overtops the ridge) and provide at least 0.5 to 1.0 foot 0.5\text{ to }1.0\text{ foot} 0.5 to 1.0 foot of freeboard above the peak design flow depth (typically the 2-year to 10-year, 24-hour storm).

Immediate Surface Stabilization: Because diversions carry concentrated lateral flow, the excavated channel and compacted earthen ridge must be stabilized immediately after grading using seed and mulch, sod, Rolled Erosion Control Products (RECPs), or stone depending on design velocity.

Field Inspection: Walk the entire length of every diversion after rain events to check for ridge overtopping , desiccation cracking or settlement , wheel-rut tracking from haul trucks , channel bed scour , and sediment deposition reducing cross-sectional capacity by more than 25 % 25\% 25% .

2. Swale ( 12.5.9 )

A Swale ( 12.5.9 ) (or grassed waterway / lined drainage channel) is a broad, shallow engineered open channel constructed with a parabolic, trapezoidal, or V-shaped cross-section and gentle side slopes (typically ≤ 3 H : 1 V \le 3\text{H}:1\text{V} ≤ 3 H : 1 V or 4 H : 1 V 4\text{H}:1\text{V} 4 H : 1 V ) designed to convey concentrated stormwater runoff at non-erosive velocities without gullying or bank sloughing.

Swale Lining Selection by Permissible Flow Velocity

As channel slope ( S S S ) or drainage area increases, the velocity ( V = 1.486 n R 2 / 3 S 1 / 2 V = \frac{1.486}{n} R^{2/3} S^{1/2} V = n 1.486 ​ R 2/3 S 1/2 by Manning's Equation) and tractive shear stress ( τ 0 = γ R S \tau_0 = \gamma R S τ 0 ​ = γ RS ) acting on the channel bed rise rapidly. A CESSWI must verify that the installed swale lining matches the hydraulic velocity regime:

Swale Lining Type Permissible Flow Velocity Range Primary Application & Field Inspection Focus

Bare Unprotected Soil < 1.5 to 2.5 ft/s < 1.5\text{ to }2.5\text{ ft/s} < 1.5 to 2.5 ft/s Highly vulnerable to downcutting and headcutting; requires immediate temporary lining or check dams

Established Turfgrass / Sod 3.0 to 5.0 ft/s 3.0\text{ to }5.0\text{ ft/s} 3.0 to 5.0 ft/s (up to 6 ft/s 6\text{ ft/s} 6 ft/s for dense sod-forming grasses) Blades bend flat ("shingle") during storms to shield soil; inspect for bare spots and sediment burial > 1 / 3 > 1/3 > 1/3 blade height

Temporary Erosion Control Blanket (ECB) 5.0 to 8.0 ft/s 5.0\text{ to }8.0\text{ ft/s} 5.0 to 8.0 ft/s Degradable straw/coir blanket protects seeded swale bed until turf roots establish; requires upstream check slot and centerline staking

Permanent Turf Reinforcement Mat (TRM) or Rip-Rap 8.0 to 15.0 + ft/s 8.0\text{ to }15.0+\text{ ft/s} 8.0 to 15.0 + ft/s Non-degradable 3D synthetic matrix filled with topsoil/seed, or angular rock armor over non-woven geotextile for steep, high-velocity channels

Field Inspection Checkpoints for Swales

Channel Centerline & Thalweg: Inspect the lowest flow path ( thalweg ) for rilling, headcutting (vertical waterfall steps migrating upstream), or exposed tree/turf roots indicating bed degradation.

RECP / Sod Installation Errors: Verify that sod strips or RECP rolls in swales are anchored with an upstream key trench ( 6 to 12 in. 6\text{ to }12\text{ in.} 6 to 12 in. deep), overlapped in the shingled downstream direction , and stapled tightly into the bottom center of the channel so water cannot flow underneath a tented mat.

Sediment Burial of Vegetation: Remove deposited sediment and re-establish turf whenever sediment buries more than 1 / 3 1/3 1/3 of the grass height or reduces design channel depth by > 25 % > 25\% > 25% .

3. Pipe Slope Drain ( 12.5.6 )

A Pipe Slope Drain ( 12.5.6 ) is a temporary flexible corrugated high-density polyethylene (HDPE) pipe or corrugated metal pipe (CMP) extending from the top crest of a steep disturbed cut or fill slope all the way down to the toe of the slope. Its purpose is to intercept concentrated surface runoff at the top of the slope and carry it safely down the slope face inside an enclosed watertight conduit so water never touches or carves gullies into the bare erodible soil slope.

Part B Photo Exam Favorite — The 4 Mandatory Pipe Slope Drain Checkpoints: Whenever a Part B Section 2 question shows a photograph or schematic of a Pipe Slope Drain ( 12.5.6 ), evaluate the structure sequentially from top to bottom across these four failure points:

Top Inlet Berm & Watertight Entrance Collar (#1 Failure Mode!): A compacted earthen diversion dike ( ≥ 1.0 ft \ge 1.0\text{ ft} ≥ 1.0 ft higher than the top of the inlet pipe) must funnel runoff into a prefabricated flared metal entrance section or tee-riser. The soil backfill around the entrance pipe must be thoroughly hand-tamped in lifts and sealed with a watertight anti-seep collar or clay plug . If the contractor dumps loose, uncompacted dirt around the inlet pipe, ponded water will pipe underneath or around the outside of the pipe entrance—blowing out the top berm and eroding a massive canyon right down the slope underneath the hanging pipe!

Watertight Pipe Couplings: All pipe segments down the slope face must be connected with gasketed, watertight split-couplings, bands, or mechanical fasteners . A loose or unbanded friction joint will blow apart under hydraulic pressure and jet concentrated water onto the middle of the steep slope.

Slope Anchor Stakes and Hold-Down Straps: Flexible corrugated HDPE pipe becomes extremely heavy when flowing full of water and experiences strong hydrodynamic thrust. The pipe must be anchored to the slope face using driven steel T-posts or wooden stakes and hold-down straps spaced every 10 to 20 feet 10\text{ to }20\text{ feet} 10 to 20 feet (and at every coupling joint) so the pipe cannot snake, whip, buckle, or pull out of the top entrance berm.

Stabilized Outlet Energy Dissipator at the Toe: Water exits the bottom of a steep pipe slope drain as a high-velocity supercritical jet. The pipe outlet must discharge onto a flared riprap outlet velocity protection apron (over non-woven geotextile) or directly into a sediment trap or sediment basin —never onto bare unprotected soil!

4. Check Dams: Rock ( 12.5.1 )

Check Dams: Rock ( 12.5.1 ) are small, temporary graded-stone barriers constructed across concentrated-flow swales, roadside ditches, or drainage channels (typically serving small tributary drainage areas of ≤ 2 to 10 acres \le 2\text{ to }10\text{ acres} ≤ 2 to 10 acres ) to reduce flow velocity, decrease the effective hydraulic energy slope, trap coarse bedload sediment, and prevent channel bed downcutting while permanent vegetation establishes.

Regulatory Prohibition: Temporary rock check dams are designed strictly for engineered construction swales and man-made ditches. Never install rock check dams inside a perennial live stream, natural creek, or jurisdictional watercourse unless specifically authorized by a federal/state Clean Water Act Section 404 / Section 401 permit.

Example Engineering Details and Inspection Criteria for Rock Check Dams

Maximum Structural Height: The total height of a temporary rock check dam from the channel bed to the center weir crest must not exceed 2.0 to 3.0 feet 2.0\text{ to }3.0\text{ feet} 2.0 to 3.0 feet (with upstream and downstream rock side slopes of 2 H : 1 V 2\text{H}:1\text{V} 2 H : 1 V to 3 H : 1 V 3\text{H}:1\text{V} 3 H : 1 V so the structure cannot topple downstream).

Center Weir Notch Depression ( ≥ 6 inches \ge 6\text{ inches} ≥ 6 inches Lower Than Outer Wings):

This is the single most common installation error tested on the CESSWI exam. The center weir notch of the rock check dam must be depressed at least 6 inches 6\text{ inches} 6 inches ( 0.5 ft 0.5\text{ ft} 0.5 ft ) lower than the outer wings (abutments) , and the outer wings must be keyed 1.5 to 2.0 feet 1.5\text{ to }2.0\text{ feet} 1.5 to 2.0 feet into the channel banks .

Why? A depressed center weir forces storm flow to stay in the armored center of the channel as it cascades over the dam. If a contractor dumps a mound of rock that is flat across the top—or higher in the middle than at the edges—stormwater is deflected outward against the bare soil channel banks, causing end-around flanking (bank washout) .

Non-Woven Geotextile Underlayment & Two-Stage Stone Gradation:

Excavate a shallow 6 -inch 6\text{-inch} 6 -inch key trench across the channel bed and up the side banks, and line the footprint with non-woven geotextile filter fabric so turbulent overflow does not scour out the soil underneath the rocks.

Construct the structural body (core and downstream face) using coarse, angular 4 - to 8 -inch 4\text{- to }8\text{-inch} 4 - to 8 -inch (or up to 12 -inch 12\text{-inch} 12 -inch ) riprap , and face the upstream side with a 6 - to 12 -inch 6\text{- to }12\text{-inch} 6 - to 12 -inch layer of smaller 1 - to 2 -inch 1\text{- to }2\text{-inch} 1 - to 2 -inch washed filter aggregate to choke the large rock voids, slow seepage, and create a backwater settling pool.

Spacing Formula ( L = H S L = \frac{H}{S} L = S H ​ ) — Toe-to-Crest Rule:

Consecutive check dams along a sloping channel must be spaced so that the toe of the upstream check dam is at the exact same elevation as the center weir crest of the downstream check dam . This creates a continuous staircase of flat backwater pools that eliminates steep channel bed exposure.

The horizontal spacing distance L L L (in feet) is calculated as: L = H S L = \frac{H}{S} L = S H ​ where H H H is the effective vertical height of the check dam center weir crest (in feet) and S S S is the longitudinal channel slope expressed as a decimal ( ft/ft \text{ft/ft} ft/ft , i.e., Slope % 100 \frac{\text{Slope \%}}{100} 100 Slope % ​ ).

Sediment Cleanout & Final Removal:

Excavate accumulated sediment behind a rock check dam whenever sediment depth reaches 1 / 2 1/2 1/2 ( 50 % 50\% 50% ) of the original center weir height .

When temporary rock check dams are removed after final channel stabilization, remove both the stone and the underlying geotextile, and immediately sod or seed and blanket the bare channel footprints.

Worked Numeric Example: Rock Check Dam Spacing Calculation

Scenario: An erosion and sediment control plan specifies temporary rock check dams along a 360 -foot 360\text{-foot} 360 -foot -long roadside swale graded at a uniform longitudinal slope of 5 % 5\% 5% ( 0.05 ft/ft 0.05\text{ ft/ft} 0.05 ft/ft ) . Each rock check dam has a total bank-wing height of 2.5 ft 2.5\text{ ft} 2.5 ft and a depressed center weir height of 2.0 ft 2.0\text{ ft} 2.0 ft . What is the maximum center-to-center horizontal spacing ( L L L ) between check dams, and how many check dams are required?

Step-by-Step Calculation:

Identify Effective Weir Height ( H H H ): Always use the center weir height ( H = 2.0 ft H = 2.0\text{ ft} H = 2.0 ft ) —not the outer wing height ( 2.5 ft 2.5\text{ ft} 2.5 ft )—because water ponds only up to the elevation of the center overflow notch.

Convert Slope Percentage to Decimal ( S S S ): S = 5 % 100 = 0.05 ft/ft S = \frac{5\%}{100} = 0.05\text{ ft/ft} S = 100 5% ​ = 0.05 ft/ft

Calculate Horizontal Spacing ( L L L ): L = H S = 2.0 ft 0.05 ft/ft = 40 feet L = \frac{H}{S} = \frac{2.0\text{ ft}}{0.05\text{ ft/ft}} = 40\text{ feet} L = S H ​ = 0.05 ft/ft 2.0 ft ​ = 40 feet

Calculate Number of Check Dams for the 360 -ft 360\text{-ft} 360 -ft Swale: Number of Dams = 360 ft 40 ft = 9 check dams \text{Number of Dams} = \frac{360\text{ ft}}{40\text{ ft}} = 9\text{ check dams} Number of Dams = 40 ft 360 ft ​ = 9 check dams