3.4 Overland Flow Paths & Secondary Flow Protection

Design note: E1/AS1 uses 150 mm finished-floor separation in its stated housing conditions. Flood freeboard and fence openings come from site flood information and approved design; there is no universal 225 mm unpaved-ground or 300 mm fence-gap rule.

Overland Flow Paths & Secondary Flow Protection

No underground stormwater pipe system can be economically or physically sized to convey the catastrophic deluges generated by severe 50-year or 100-year storm events. When underground pipes exceed their hydraulic capacity or become blocked by tree roots and debris, excess surface water must travel across the natural topography of the land. In New Zealand, this surface routing is known as the Overland Flow Path (OLFP) or Secondary Drainage System. For a Certifying Drainlayer, understanding secondary flow dynamics, protecting building Finished Floor Levels (FFL) through mandatory freeboard clearances, and strictly avoiding the unlawful diversion of surface water onto neighbouring property are critical professional competencies.

---

1. The Dual Drainage Concept: Primary vs. Secondary Flow

Urban stormwater infrastructure operates under the Dual Drainage Principle:

Primary Drainage System (Piped Network)

• Design Recurrence: Sized for an Annual Exceedance Probability (AEP) of 10% (10-year ARI) under NZBC Clause E1.3.1.
• Components: Roof gutters, downpipes, field sumps, yard pits, trench drains, underground uPVC/concrete pipes, and detention attenuation tanks.
• Objective: Prevents nuisance surface ponding, waterlogged lawns, and pedestrian disruption during frequent rainfall events.

Secondary Drainage System (Overland Flow Paths)

• Design Recurrence: Engineered to convey flows resulting from storms up to a 1% AEP (100-year ARI), or to manage the total failure (100% blockage) of the primary underground pipe system.
• Components: Roadway corridors, swales, open hollows, contoured driveways, boundary easements, and natural overland valleys.
• Objective: Safeguards human life and ensures that surface water does not enter habitable homes, commercial buildings, or essential electrical sub-stations.

The Drainlayer's Responsibility: When installing on-site drainage, the Certifying Drainlayer is not merely connecting pipes underground; they must inspect the final site contours to confirm that if the underground pipes choke completely, surface water will wash safely past the building without breaching external doors or foundation vents.

---

2. Finished Floor Levels (FFL) & Minimum Freeboard Requirements

Freeboard is the vertical safety buffer between the predicted peak surface water level and the interior Finished Floor Level (FFL) of a building. It accommodates hydraulic wave wash, turbulence, wind setup, and localized floating debris dams.

Statutory Freeboard Clearances under NZBC Clause E1 & E1/AS1

Acceptable Solution E1/AS1 (Section 2.1) sets clear, non-negotiable minimum clearances between the ground surface and finished floor levels:

Why Unpaved Ground Requires 225 mm Freeboard

Unpaved surfaces are dynamic and prone to human alteration:

• Vegetation & Mulch Buildup: Gardeners routinely add 50 mm to 100 mm of bark mulch, topsoil, and compost against external walls over time.
• Soil Creep: Lawn turf thickens and bare soil settles or expands.
• Ponding Waves: Saturated grass creates higher hydraulic friction than smooth concrete, causing water sheets to build up higher depths during heavy downpours.

---

3. Site Contouring, Vegetated Swales, Bunds & Mounding

Site grading directs surface runoff away from foundation perimeters into designated overland channels.

Ground Falls Away from Buildings (NZBC E2/AS1)

The ground surface immediately surrounding a building must slope away from the exterior walls:

• Paved Surfaces: Must fall away at not flatter than 1 in 100 (1%) for a minimum distance of 1.5 to 2.0 metres.
• Unpaved Surfaces: Must fall away at not flatter than 1 in 20 (5%, or 50 mm fall over the first 1.0 m) for at least 1.5 metres to prevent surface water ponding against foundation concrete or timber bearer plates.

Vegetated Swales (Grassed Open Channels)

A swale is a wide, shallow, vegetated drainage channel designed to convey secondary flow while filtering out sediment:

• Side Slopes: Must not be steeper than 1 in 4 (25%) horizontal-to-vertical. Steeper banks cause soil slumping, induce bank erosion, and prevent standard lawn mowing.
• Longitudinal Gradient: Must range between 1 in 100 (1.0%) and 1 in 20 (5.0%).
  • Flatter than 1:100 causes sluggish flow, persistent bogging, and mosquito breeding.
  • Steeper than 1:20 accelerates water velocity beyond $1.5\text{ m/s}$, scouring the grass roots and tearing trenches in the turf.
• Manning's Roughness: For swale hydraulic calculations, Manning's roughness coefficient ($n$) is taken as 0.035 for short maintained lawn and 0.050 for long rough vegetation.

Compacted Earth Bunds & Reverse Ramps

• Basement Garage Ramps: An impermeable driveway that slopes directly downwards into a basement garage is a severe flood hazard. Drainlayers must ensure a reverse-grade hump (bund) is constructed at the driveway crest, rising at least 150 mm above the adjacent street/yard level, coupled with a cross-trench drain at the threshold.
• Deflection Bunds: Mounded compacted clay bunds placed along side boundaries direct upstream hillside runoff safely around dwellings into the council roadway gutter.

---

4. Obstruction of Overland Flow Paths: Prohibitions & Permitted Fences

Designated Overland Flow Paths mapped on council GIS databases (such as Auckland Council GeoMaps) carry strict statutory protections under the Resource Management Act 1991 and the Building Act 2004.

Unlawful Obstructions

A landowner or contractor must never obstruct, alter, or dam a secondary flow path. Prohibited structures include:

• Solid Timber Paling Fences: Solid palings extending to the ground act as a solid dam. During storms, water builds up behind the fence, flooding upstream properties until the fence violently collapses under hydrostatic pressure.
• Concrete Block Walls & Unconsented Retaining Walls: Placing solid masonry across a natural overland depression.
• Garden Sheds & Planter Boxes: Siting garden outbuildings or raised vegetable beds in the flow channel.

Permitted Fence Crossings Across OLFPs

Where a property boundary crosses a designated overland flow path, fencing must be designed to pass the 100-year storm discharge without impounding water:

• Open Cantilever Fencing: Vertical metal pickets (pool-style fencing) with clear spacings of at least 100 mm between bars.
• Raised Bottom Rail: A timber paling fence where the bottom rail and palings terminate with a permanent, unobstructed 300 mm clear vertical gap above ground level across the entire width of the flow corridor.
• Top-Hung Flap Gates: Lightweight panels hinged at the top that hang closed under gravity during dry weather, but swing freely open downstream when rising floodwaters push against them.

---

5. Statutory Liabilities & Certifying Drainlayer Sign-Off

A Certifying Drainlayer is personally liable under civil and statutory law for surface water management decisions.

The Common Law Tort of Nuisance & Rylands v Fletcher

Under New Zealand common law:

• Natural Runoff: A lower land owner must accept the natural, diffuse overland runoff that flows naturally from higher land across undeveloped topography.
• Unlawful Diversion / Concentration: If an upper landowner (or their drainlaying contractor) alters the ground, paves areas, concentrates runoff into a pipe, swale, or trench, and discharges or diverts that concentrated water across a boundary, they commit the tort of private nuisance. The drainlayer and owner are strictly liable for all resulting property damage, scour, and flooding on the lower property.

Building Act 2004 Sections 71 to 73 (Natural Hazards)

• Section 71: A Building Consent Authority (BCA) must refuse to grant a building consent if the land is subject to one or more natural hazards—including flooding, overland flow inundation, erosion, or subsidence—or if the building work will accelerate or worsen the hazard.
• Section 72: The BCA may grant consent only if satisfied that the building work will not accelerate, worsen, or result in the hazard on the household site or adjoining property.
• Section 73: A statutory memorial is registered on the computer freehold register (property title), warning prospective buyers that the land is subject to flooding hazards.

Certifying Documentation: Producer Statements & Records of Work

Upon completing drainage installation, the Certifying Drainlayer must execute statutory documentation:

• Producer Statement PS3 (Construction): Formally certifies to the BCA that all stormwater collection, sumps, trench drains, and swales were installed strictly in accordance with the consented engineering plans and NZBC Clause E1.
• Record of Work (RoW): Statutory memorandum submitted under the Building Act certifying that restricted building work complies with code.
• As-Built Drainage Plan: Accurate dimensional drawing showing invert levels, pipe runs, sump locations, and overland flow paths.

---

6. Certifying Drainlayer Trade Traps

• The Downhill Driveway Flume: Installing a smooth concrete driveway pitching down into a basement garage with only a DN100 pipe connection. During a 50-year storm, leaves blind the grate, and the driveway transforms into a river, filling the garage with a metre of water.
• Post-Drainage Landscaping Tampering: Signing off an as-built drainage plan with 225 mm freeboard, after which the client's landscaper places 100 mm of topsoil and concrete pavers right up to the timber weatherboard sill. The drainlayer must document the exact ground levels on their handover inspection sheet to avoid liability for post-completion code violations.
• Diverting Swales to Side Boundaries: Cutting an earth swale that terminates 500 mm short of a timber boundary fence, discharging concentrated sheet water onto the neighbour's lawn. This is an open-and-shut tort of nuisance that will result in BCA stop-work notices, civil court damages, and PGDB disciplinary proceedings.