7.2 Temporary Diversion Dikes, Berms & Swales

7.2 Temporary Diversion Dikes, Berms & Swales

Quick Reference: The first principle of construction site stormwater management is the strict segregation of clean and dirty water. Clean water diversions intercept undisturbed upland run-on above mass grading operations and route it safely around the project footprint, preventing excessive hydraulic loading and avoiding sediment basin blowouts. Dirty water conveyances collect sediment-laden runoff from cleared surfaces and direct it into sediment retention BMPs. Temporary diversion dikes and berms require a compacted earthen ridge with a minimum top width of 2 feet, a minimum settled height of 18 inches, and side slopes of 2:1 or flatter. Channels must maintain a minimum 6 inches of vertical freeboard above the 10-year, 24-hour storm water surface elevation and terminate into stabilized outfalls to prevent catastrophic breach.

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Runoff Conveyance Principles: Clean vs. Dirty Water Segregation

On any active land development or linear construction project, precipitation generates two fundamentally different streams of surface water. The success of an Erosion and Sediment Control (ESC) plan hinges on the rigorous physical separation of these two streams:

                    ┌─── CLEAN WATER : Intercepted Upland Run-On ───► Route Around Site ──► Stable Natural Outfall
Precipitation Stream │
                    └─── DIRTY WATER : Disturbed Construction Runoff ──► Route to BMPs ─────► Sediment Basin/Trap

1. Clean Water Diversion

• Definition: Surface runoff originating from undisturbed, vegetated watersheds located topographically upslope of the construction limit of disturbance (LOD).
• Management Goal: Intercept this runoff at the upper site perimeter before it contacts exposed soils, and convey it around or through the active work zone via non-erosive diversions, bypass swales, or slope pipes.
• Discharge Protocol: Clean water may be discharged directly into undisturbed downstream natural watercourses, stabilized ravines, or municipal storm sewer infrastructure without detention or sediment filtration.
• Engineering Benefits:
  • Dramatically Reduces Required BMP Storage Volume: Federal and state Construction General Permits (CGPs) mandate sizing sediment basins for $3,600\text{ cubic feet of storage per acre}$ of contributing drainage area. Diverting 15 acres of clean upland run-on eliminates the need for $54,000\text{ ft}^3$ ($1.24\text{ acre-feet}$) of basin volume, saving tens of thousands of dollars in earthwork.
  • Prevents Basin Washout: Sudden storm surges from untreated upland catchments routinely overwhelm internal sediment traps, causing structural embankment washouts and discharging thousands of tons of captured sediment downstream.

2. Dirty Water Collection

• Definition: Surface runoff generated from cleared, grubbed, excavated, graded, or stockpiled areas within the active construction envelope, carrying high suspended sediment concentrations and turbidity.
• Management Goal: Contain and collect all sediment-laden runoff using internal collection swales, perimeter berms, and temporary drainage ditches.
• Discharge Protocol: Dirty water must never be discharged directly off-site or into clean bypass channels. It must be directed exclusively into approved sediment control BMPs—such as temporary sediment traps ($A < 5\text{ acres}$), sediment basins ($A \ge 5\text{ acres}$), or active chemical treatment systems—prior to authorized outfall release.

Core Compliance Rule: Mixing clean upland run-on with dirty site runoff is a major engineering and regulatory failure. Once clean water contacts exposed construction soil, it is legally classified as construction stormwater and must undergo full sediment treatment before discharge.

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Temporary Diversion Dikes and Berms: Design & Construction

A temporary diversion dike (or earthen berm) is a compacted ridge of soil constructed along a designed contour or gradient immediately upslope of cut or fill slopes, around the site perimeter, or across active grading parcels.

Standard Geometric Design Criteria

State erosion and sediment control design manuals and CPESC standards mandate strict dimensional thresholds for temporary earthen dikes:

1. Top Width: Minimum 2.0 feet (0.6 meters). A narrow or peaked crest is structurally vulnerable to sloughing, desiccation cracking, and hydraulic piping.
2. Total Settled Height: Minimum 18 inches (0.45 meters), measured vertically from the upstream channel invert/flow line to the top of the berm crest. During placement, an additional 10% to 15% settlement allowance must be constructed to account for post-construction soil consolidation.
3. Side Slopes: 2:1 (horizontal to vertical) or flatter. In locations where maintenance mowing, equipment crossings, or light vehicle traffic are expected, side slopes must be flattened to 3:1 or 4:1.
4. Flow Line Gradient: The flow channel immediately behind the dike should maintain a positive longitudinal slope (typically $0.5%\text{ to }1.0%$) toward a designated stabilized outfall. Grades under $0.5%$ cause standing water and sediment siltation, while grades exceeding $2.0%$ require artificial armor lining to prevent rilling.

Earthwork & Compaction Specifications

• Subgrade Stripping: The footprint of the diversion dike must be stripped of all vegetation, loose organic duff, large roots, and topsoil. Constructing an earthen berm over unstripped organic duff creates a preferential seepage plane, leading to sliding shear failures under hydrostatic loading.
• Material Quality: The embankment must consist of clean mineral soil (such as sandy clay, clayey gravel, or loam) free of roots, construction debris, large rocks ($> 4\text{ inches}$), and frozen lumps.
• Placement & Compaction in Lifts: Soil must be placed in uniform horizontal lifts not exceeding $6\text{ to }8\text{ inches}$ in uncompacted thickness. Each lift must be thoroughly compacted using rubber-tired earthmoving machinery, sheepsfoot rollers, or excavator track-walking to achieve at least 90% Standard Proctor density (ASTM D698).
• Immediate Stabilization: Within 24 to 48 hours of construction, the entire exposed surface of the diversion dike and its flow channel must be temporarily stabilized with seed and straw mulch, hydraulically applied matrix, or rolled erosion control blankets to prevent rainfall detachment and surface rilling.

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Temporary Drainage Swales and Excavated Ditches

A temporary drainage swale is an excavated linear channel designed to convey concentrated runoff safely along designed grades to a specified point of disposal.

Cross-Sectional Geometry Comparison

Drainage swales are constructed in one of three primary cross-sectional configurations:

Longitudinal Slope Design Ranges

The longitudinal slope ($S$) of a drainage swale directly dictates flow velocity ($V \propto S^{1/2}$) and tractive shear stress ($\tau \propto S$):

• Minimum Longitudinal Slope ($S \ge 0.5%$): Swales must maintain at least a $0.5%$ grade ($0.005\text{ ft/ft}$) to ensure positive drainage. Channels constructed flatter than $0.5%$ suffer from stagnant water ponding, fine sediment aggradation that chokes flow capacity, and mosquito vector breeding.
• Gentle Slope Range ($0.5% \le S \le 2.0%$): Typical operational range for unlined or seed-and-straw stabilized vegetated swales. On non-erosive cohesive soils, velocities generally remain below permissible thresholds ($V < 2.5\text{–}3.5\text{ ft/s}$).
• Moderate Slope Range ($2.0% < S \le 5.0%$): Flow velocities rapidly exceed the critical detachment threshold of bare and establishing soils. Unlined channels will experience severe bed incision and bank rilling. Mandatory installation of Rolled Erosion Control Blankets (RECBs), Turf Reinforcement Mats (TRMs), or temporary rock check dams is required.
• Steep Slope Range ($S > 5.0%$): Concentrated flows enter high-velocity supercritical regimes. Earthen channels must transition to heavy rock riprap armor, articulated concrete blocks, paved flumes, or enclosed pipe slope drains.

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Channel Lining Selection: Permissible Velocity & Shear Stress Thresholds

Selecting the appropriate lining for a diversion swale requires comparing the calculated hydraulic parameters ($V$ and $\tau_{max}$) against the permissible physical limits of the liner material. The following engineering reference table synthesizes design criteria from FHWA HEC-15, USDA NRCS, and leading state ESC manuals:

The Geotextile Underlayment Rule: Whenever rock riprap is used to line a diversion swale or ditch, the stone must be underlain by a non-woven geotextile filter fabric (or a properly graded granular filter blanket). Placing rock directly on native soil allows turbulent water to swirl between the stones, fluidizing and washing out the underlying soil particles. This process, termed piping, hollows out the subgrade, causing the riprap to sink and collapsing the channel banks.

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Freeboard Requirements & Preventing Overtopping Failure

Freeboard is the vertical clearance between the maximum computed water surface elevation (WSEL) during the design storm and the lowest point of the channel bank or diversion berm crest:

$\text{Total Required Bank Height} = y_{\text{design}} + \text{Freeboard}$

The Mandatory 6-Inch Rule

Under CPESC standards, temporary diversion dikes, swales, and drainage ditches must maintain a minimum of 6 inches (0.15 meters) of vertical freeboard above the 10-year, 24-hour design storm water surface elevation. For permanent conveyances or structures protecting occupied dwellings, freeboard requirements typically increase to 12 inches (0.30 meters).

Why Freeboard is Essential

Freeboard is not an arbitrary safety factor; it accounts for real-world physical dynamics that cannot be fully captured in standard 1D hydraulic modeling:

1. Wind Wave Action: Gusting storm winds generate surface waves that slosh against earthen berms.
2. Vegetative Retardance Shifts: Dense summer vegetation retards flow, increasing normal depth ($y_n$) significantly above bare-soil model predictions.
3. Sediment Aggradation: Sediment settling along the channel invert raises the effective bed elevation, elevating the water surface.
4. Hydraulic Transitions: Minor transitions, curvature, and standing waves near inlets or check dams cause localized water surface super-elevation.

Anatomy of an Overtopping Breach

When a diversion berm fails due to insufficient freeboard or crest settlement, catastrophic failure occurs almost instantaneously:

1. Runoff rises above the low point of the berm crest, spilling over the downstream face.
2. Flow accelerates down the unprotected, steep outer slope (often $2:1$ or $3:1$).
3. Rapid shear stress detachment cuts a localized rill into the fill.
4. The rill rapidly deepens into a retreating headcut that slices back through the crest.
5. Once the crest is breached, the entire upstream volume concentrated behind the dike rushes through the opening, washing out hundreds of cubic yards of soil and sending an avalanche of sediment-choked water directly onto unprotected work areas or downslope properties.

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Siting, Outfalls, and Field Inspection Standards

Proper Siting Protocols

• Follow Natural Contours: Diversion dikes should follow natural topographic contours as closely as possible to maintain uniform longitudinal gradients and minimize extensive cut-and-fill earthwork.
• Protect Cut-and-Fill Slopes: Always construct an interceptor dike or swale along the top of all engineered cut and fill slopes before commencing slope excavation. This prevents sheet runoff from cascading down the vulnerable raw slope face.

Stabilized Outfall Requirements

A diversion conveyance is only as good as its discharge point. Concentrating runoff without providing a stabilized outfall merely relocates and intensifies erosion. Diversion conveyances must discharge into:

1. A Stabilized Natural Waterway: With demonstrated hydraulic capacity and non-erosive boundaries;
2. A Rock Riprap Plunge Pool / Level Spreader: Designed to dissipate kinetic energy and transition concentrated flow back into non-erosive sheet flow;
3. A Formal Sediment Basin or Trap: For all dirty water conveyances.

Under no circumstances may a diversion dike or swale terminate onto an unprotected fill slope or an un-stabilized property boundary.

CPESC Inspection & Maintenance Checklist

• Inspection Frequency: Inspect temporary diversions weekly and within 24 hours of any rainfall event producing $\ge 0.5\text{ inches}$ of precipitation.
• Sediment Accumulation: Inspect the channel invert. Remove sediment deposits when they exceed 25% of the design flow depth to maintain required capacity and freeboard.
• Crest Settlement: Measure the berm crest. Any localized dips or tire ruts must be immediately built back up to design height with compacted soil.
• Liner Integrity: Check for undermining, staple pullout, or tearing of erosion control blankets. Verify that rock riprap has not experienced piping or displacement.

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The Rest of the Diversion Family: Silt Ditch, Water Bar, Bypass Pipe and Downdrain

The blueprint lists seven diversion measures. Dikes, perimeter dikes, and temporary swales are covered above; the remaining four are distinct devices with distinct failure modes.

The Clean-Water Bypass Principle

The bypass pipe deserves special emphasis because it is the purest expression of the clean/dirty water rule. If a stream or a clean upland drainage must cross an active work zone, the correct answer is almost never "run it through the site and treat it." It is to isolate the flow and carry it past the disturbance — by pipe, flume, or dam-and-pump — so that the only water needing treatment is the runoff actually generated on disturbed ground. Sizing a bypass to the wrong storm is how in-stream projects turn into enforcement actions: when the bypass surcharges, the entire cofferdammed excavation floods and discharges directly to the receiving water.