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

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:

1. Keep Clean Water Clean: Intercept off-site upslope run-on before it enters active grading zones;
2. Break Effective Slope Length ($LS$): Intercept dirty sheet flow at regular intervals on disturbed slopes and convey it safely downslope inside non-erodible conduits or armored channels; and
3. 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).

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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:

Installation and Inspection Checkpoints for Diversions

1. Machine Compaction of Earthen Dike Lifts: Earthen diversion dikes must be constructed in lifts and thoroughly machine-compacted (typically $\ge 90\%$ Standard Proctor density or tracked repeatedly with heavy equipment) with side slopes of $2\text{H}:1\text{V}$ or flatter ($3\text{H}:1\text{V}$ preferred for equipment crossing) and a minimum top width of $2.0\text{ feet}$. Loose, uncompacted windrows of bulldozed dirt slump and breach during the very first rainstorm.
2. 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 $0.5\text{ to }1.0\text{ foot}$ of freeboard above the peak design flow depth (typically the 2-year to 10-year, 24-hour storm).
3. 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.
4. 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 $25\%$.

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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 $\le 3\text{H}:1\text{V}$ or $4\text{H}:1\text{V}$) 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 ($S$) or drainage area increases, the velocity ($V = \frac{1.486}{n} R^{2/3} S^{1/2}$ by Manning's Equation) and tractive shear stress ($\tau_0 = \gamma R S$) acting on the channel bed rise rapidly. A CESSWI must verify that the installed swale lining matches the hydraulic velocity regime:

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 ($6\text{ to }12\text{ in.}$ 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 $1/3$ of the grass height or reduces design channel depth by $> 25\%$.

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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:

1. Top Inlet Berm & Watertight Entrance Collar (#1 Failure Mode!): A compacted earthen diversion dike ($\ge 1.0\text{ ft}$ 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!
2. 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.
3. 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 $10\text{ to }20\text{ feet}$ (and at every coupling joint) so the pipe cannot snake, whip, buckle, or pull out of the top entrance berm.
4. 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!

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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 $\le 2\text{ to }10\text{ acres}$) 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

1. Maximum Structural Height: The total height of a temporary rock check dam from the channel bed to the center weir crest must not exceed $2.0\text{ to }3.0\text{ feet}$ (with upstream and downstream rock side slopes of $2\text{H}:1\text{V}$ to $3\text{H}:1\text{V}$ so the structure cannot topple downstream).
2. Center Weir Notch Depression ($\ge 6\text{ inches}$ 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 $6\text{ inches}$ ($0.5\text{ ft}$) lower than the outer wings (abutments), and the outer wings must be keyed $1.5\text{ to }2.0\text{ feet}$ 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).
3. Non-Woven Geotextile Underlayment & Two-Stage Stone Gradation:
   • Excavate a shallow $6\text{-inch}$ 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 $4\text{- to }8\text{-inch}$ (or up to $12\text{-inch}$) riprap, and face the upstream side with a $6\text{- to }12\text{-inch}$ layer of smaller $1\text{- to }2\text{-inch}$ washed filter aggregate to choke the large rock voids, slow seepage, and create a backwater settling pool.
4. Spacing Formula ($L = \frac{H}{S}$) — 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 $L$ (in feet) is calculated as: $L = \frac{H}{S}$ where $H$ is the effective vertical height of the check dam center weir crest (in feet) and $S$ is the longitudinal channel slope expressed as a decimal ($\text{ft/ft}$, i.e., $\frac{\text{Slope \%}}{100}$).
5. Sediment Cleanout & Final Removal:
   • Excavate accumulated sediment behind a rock check dam whenever sediment depth reaches $1/2$ ($50\%$) 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 $360\text{-foot}$-long roadside swale graded at a uniform longitudinal slope of $5\%$ ($0.05\text{ ft/ft}$). Each rock check dam has a total bank-wing height of $2.5\text{ ft}$ and a depressed center weir height of $2.0\text{ ft}$. What is the maximum center-to-center horizontal spacing ($L$) between check dams, and how many check dams are required?

Step-by-Step Calculation:

1. Identify Effective Weir Height ($H$): Always use the center weir height ($H = 2.0\text{ ft}$)—not the outer wing height ($2.5\text{ ft}$)—because water ponds only up to the elevation of the center overflow notch.
2. Convert Slope Percentage to Decimal ($S$): $S = \frac{5\%}{100} = 0.05\text{ ft/ft}$
3. Calculate Horizontal Spacing ($L$): $L = \frac{H}{S} = \frac{2.0\text{ ft}}{0.05\text{ ft/ft}} = 40\text{ feet}$
4. Calculate Number of Check Dams for the $360\text{-ft}$ Swale: $\text{Number of Dams} = \frac{360\text{ ft}}{40\text{ ft}} = 9\text{ check dams}$