3.2 Stormwater Pits, Sumps, Silt Traps & Inlets

Design note: E1/AS1 requires inlet gratings with at least 35% open area and a maximum smaller opening dimension of 35 mm, plus capacity below the outlet for sediment. It does not prescribe one 300 mm sump depth, one 50% blockage allowance, or one threshold-channel size for every site.

Stormwater Pits, Sumps, Silt Traps & Inlets

Surface water cannot enter an underground pipe network without properly configured surface inlet structures. In New Zealand drainage practice, these structures capture surface runoff from roofs, driveways, car parks, and landscaped ground, filter out destructive silt and floating rubbish, and convey water safely into the reticulated network. For a Certifying Drainlayer, the correct specification, positioning, and installation of catchment pits, yard sumps, silt traps, trench drains, and grates is critical to prevent site ponding, protect downstream soakage or public infrastructure from choking, and maintain code compliance.

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1. Classification & Functions of Stormwater Inlets

Stormwater inlets are classified based on their location, catchment characteristics, and structural loading requirements:

Field Gullies

• Location: Installed in soft landscape areas, grassed swales, playing fields, and natural overland depressions.
• Configuration: Precast concrete or modular polypropylene chamber fitted with a dished surround. In vegetated or mulched zones, field gullies should feature a raised dome or atrium grate rather than a flat grate to prevent fallen leaves from sealing the intake.

Yard Sumps

• Location: Installed in paved residential courtyards, commercial hardstands, driveways, and concrete patios.
• Configuration: Watertight chamber fitted with a flush, heavy-duty ductile iron or galvanized steel grate. Sized to intercept concentrated sheet flow from impervious surfaces before it can pond against building foundations.

Silt Traps (Mud Sumps)

• Function: Specialized primary sediment collection chambers placed at the upstream transition of a stormwater system, immediately preceding retention tanks, modular soakage fields, or public connections.
• Sediment Retention: Traps heavy particulates (sand, decomposed aggregate, gravel, and brake-pad dust) that would otherwise form hard deposits inside pipelines or blind soakage geotextile fabric.

Crucial Code Distinction: Sump vs. Gully Trap

A Certifying Drainlayer must never confuse a stormwater sump with a foul-water gully trap:

• Foul-Water Gully Trap (NZBC G13/AS2): Must possess a minimum 65 mm permanent water seal, maintain a 25 mm lip above paved surfaces (75 mm above unpaved surfaces) to exclude surface water, and discharges solely to the sanitary sewer.
• Stormwater Sump (NZBC E1/AS1): Features an open grating flush with the paving, incorporates an unsealed silt drop, has no sanitary water seal, and discharges strictly to the stormwater system.
• Cross-Connection Offence: Discharging surface runoff into a sanitary gully trap, or connecting a foul fixture into a stormwater sump, violates Section 114 of the Plumbers, Gasfitters, and Drainlayers Act 2006 and constitutes a serious disciplinary offence punishable by substantial Board fines and licence suspension.

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2. Silt Traps & Mud Sumps: The Mandatory 300 mm Rule

Under NZBC Acceptable Solution E1/AS1 Paragraph 3.4.1 and AS/NZS 3500.3 Section 8, all catchment pits and yard sumps designed to capture surface runoff from unpaved or trafficable ground must incorporate sediment storage capacity.

The 300 mm Sump Drop Dimension

The statutory standard dictates that the bottom (floor) of the chamber must sit at least 300 mm below the invert of the lowest outgoing drain pipe:

• Dead Storage Reservoir: This 300 mm deep permanent water basin acts as a hydraulic dead zone. As turbulent surface water pours into the pit, its velocity drops abruptly.
• Settling Mechanics: According to Stokes' Law, coarse particles (grain diameter $> 0.2\text{ mm}$) rapidly decelerate and settle by gravity into the 300 mm silt basin, remaining trapped beneath the laminar exit stream.
• Pipe Invert Alignment: If a branch inlet pipe enters the sump, its invert should ideally terminate at least 50 mm above the outgoing pipe crown, preventing backwater deposits from migrating into the upstream lateral.

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3. Grate Hydraulics & Blockage Allowances

Water entering a flat grate on the ground behaves as a weir under shallow depths ($h$) and transitions into an orifice once the grate becomes fully submerged ($h > 100\text{ mm}$):

• Weir Flow ($Q_w$): Governed by the clear perimeter of the grate ($P$): $Q_w = C_w \cdot P \cdot h^{1.5}$.
• Orifice Flow ($Q_o$): Governed by the clear open area of the grate ($A_o$): $Q_o = C_d \cdot A_o \cdot \sqrt{2gh}$.

The Mandatory 50% Blockage Factor

In real storm events, wind and surface runoff wash autumn leaves, pine needles, bark mulch, and plastic rubbish directly across grates. If an engineer or drainlayer sizes a grate based on 100% clean aperture capacity, the pit will inevitably surcharge during peak deluges.

Under AS/NZS 3500.3 Table 8.3.2, the following clogging factors must be applied during hydraulic verification:

• Flat On-Grade Grates in Sags / Low Points: Must apply a 50% blockage factor (i.e. design capacity = 0.50 * theoretical clean open area).
• Continuous Grade Kerb Inlets: Must apply a 20% blockage factor.
• Side Entry Kerb Pockets: Must apply a 10% to 20% blockage factor.

Specialized Grate Profiles

Certifying drainlayers must match grate profiles to site traffic and safety risks:

• Heel-Proof Grates: In public pedestrian precincts, courtyards, schoolyards, and retail entrances, high-heel shoes and walking aids create puncture and tripping hazards. Heel-proof grates must restrict aperture slots to a maximum width of 8 mm, with bars oriented perpendicular to the dominant direction of pedestrian travel (complying with NZBC Clause D1 Access Routes and AS 1428.1).
• Bicycle-Safe Grates: Standard parallel-bar grates allow narrow bicycle tyres (23 mm to 28 mm wide) to drop between bars, pitching the cyclist forward. Bicycle-safe grates feature transverse cross-rods, diagonal bars, or honeycomb grids to support bicycle wheels across all approach angles.
• Domed / Atrium Grates: In mulched garden beds and turf swales, floating organic mulch mats against the bottom rim of a flat grate. A domed grate provides an elevated hemisphere of open bars, allowing water to enter freely through the top and sides even if 100 mm of leaves blanket the base.

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4. Trench Drains & Level-Entry Threshold Drainage

Where surface water flows across broad, sloping hardstands—such as garage aprons, commercial vehicle crossings, and basement car park ramps—point-source yard sumps are ineffective because fast-moving sheet flow bypasses them. Drainlayers install linear trench drains (modular polymer concrete or high-density polyethylene channels with lockable grates).

Interception on Sloped Driveways

On driveways steeper than 1:10 (10%), sheet runoff accelerates to high velocity. The trench drain channel must possess sufficient width (minimum 150 mm) and grate intake area to capture the hydraulic sheet before water shoots across the channel lip.

NZBC Clause E2/AS1 Level-Entry Threshold Drainage

Modern architectural residential designs frequently feature level-entry (barrier-free) ranch sliders where the interior timber floor or concrete slab connects seamlessly to an exterior tiled patio or timber deck, eliminating the traditional 100 mm to 150 mm step-down.

To satisfy NZBC Acceptable Solution E2/AS1 (Section 7.3.2), the drainlayer must protect the internal timber framing and floor slab against catastrophic exterior water ingress:

• Continuous Open Channel: A linear drainage channel must span the full length of the exterior door opening.
• Physical Channel Dimensions: The channel must be at least 150 mm wide and 150 mm deep, laid to an active fall (minimum 1:100) draining directly to a stormwater sump or collector pipe.
• Proprietary Slot Drains: Alternatively, an approved proprietary stainless steel level-entry slot drain may be installed, featuring a minimum continuous 12 mm slot gap located directly beneath the door joinery overhang.
• Perimeter Fall: Paved patios or decks immediately outside the channel must fall away from the building at not less than 1:100 (1%) for a minimum distance of 1.5 metres.
• Removable Grates: Threshold channels must feature easily removable grates or access ports to permit routine cleaning of silt and spiders.

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5. Submerged Outlets, Dip Pipes & Scum Baffles

In commercial parking lots, driveways, and industrial sites, surface runoff carries floating debris, pine needles, organic leaves, and petroleum hydrocarbons (dripped motor oil and diesel fuel). If allowed to enter the drainage pipes, floating debris creates snags at pipe joints, while oil coats soakage rocks and contaminates groundwater.

The Submerged Dip Pipe

A simple, robust method to trap floating debris and oils is the installation of a submerged 90° downturned elbow (dip pipe) on the outgoing pipe inside the sump:

• Submergence Depth: The vertical leg of the 90° bend extends downwards into the water basin, terminating at least 100 mm to 150 mm below the standing water level.
• Hydraulic Operation: Water enters the drain from the calm mid-depth layer of the sump. Floating scum, leaves, and oil remain trapped on the surface, while heavy sand settles in the 300 mm sump below.
• Anti-Siphon Air Hole: The top crown of the dip pipe elbow must feature a 15 mm to 20 mm ventilation hole (air relief orifice). Without this hole, an energetic storm surge will prime the bend as a siphon, drawing the floating oil and leaf mat straight down the pipe.

Scum Baffle Plates

In larger precast catchpits, a vertical concrete or stainless steel scum board is mounted across the outlet chamber. The baffle terminates 150 mm below the permanent water line and projects at least 100 mm above the pipe crown, acting as a physical dam that retains floating detritus.

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6. Access, Sizing, and Maintenance Standards

Catchment pits must provide sufficient internal volume for tool and suction hose access based on overall depth:

Drainlayer Maintenance Handover

A Certifying Drainlayer must instruct the asset owner that silt traps are active maintenance devices. The 300 mm silt basin must be vacuumed out or manually shoveled clean at least twice annually, and immediately following major storm deluges. When a silt trap fills to the pipe invert, 100% of incoming sediment bypasses the trap and enters the drainage reticulation.

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7. Trade Traps & Practical Failure Modes

• Omitting the Sump Drop (Flat-Bottom Pits): Setting the outgoing pipe directly on the concrete floor of the pit. Without the 300 mm dead storage basin, coarse grit shoots directly into downstream pipes, silting up detention tank orifices or cementing modular soakage cells.
• Unsealed Pits Adjacent to Foundations: Installing unsealed masonry sumps or dry-stacked block pits within 2.0 metres of a house foundation. Infiltration from the unsealed base saturates foundation bearing soils, triggering foundation settlement and floor slab cracking.
• Flush Pits in Turf Areas: Installing a flat metal grate flush with soft lawn. Grass clippings and lawn topsoil quickly grow over the perimeter, hiding the pit and forming an impermeable sod dam that induces deep garden ponding.