3.3 Soakage Systems, Retention & Detention Tanks

Design note: E1/VM1 requires territorial-authority acceptance of soakage suitability and field investigation. Groundwater clearance, setbacks, storage, outlet controls, and maintenance access are site- and authority-specific; worked values are not universal rules.

Soakage Systems, Retention & Detention Tanks

Urban development replaces naturally permeable forests and pasture with impermeable rooftops, concrete driveways, and asphalt roadways. This transformation prevents rain from soaking into the ground, drastically increasing total runoff volume and accelerating peak storm surges. In New Zealand, regional councils and territorial authorities mandate on-site stormwater management to protect receiving streams from channel scour and prevent municipal sewers from surcharging. Certifying Drainlayers frequently install retention tanks, detention attenuation systems, calibrated flow controls, and subsurface soakage fields to manage stormwater at the source.

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1. Engineering Distinction: Retention vs. Detention

Although frequently confused by property owners, retention and detention perform opposite hydraulic functions:

Stormwater Retention (Permanent Storage for Reuse)

• Objective: Water conservation and reduction of total stormwater volume discharging to public infrastructure.
• Operational Mechanism: Stormwater from roofs passes through a primary leaf filter and enters a sealed storage vessel. The water is permanently retained on-site for domestic non-potable uses—such as toilet flushing, washing machine supply, and landscape irrigation—delivered by an automatic pressure pump.
• Water Level: Under normal operating conditions, a retention tank remains full or partially full. It only releases water when domestic fixtures consume it or when sustained storms fill the reserve, forcing excess water out through an upper emergency overflow.

Stormwater Detention (Temporary Peak Flow Attenuation)

• Objective: Flood prevention, network capacity protection, and stream morphology safeguarding.
• Operational Mechanism: The tank is engineered to remain completely dry or empty before a storm. During a heavy downpour, runoff enters the tank much faster than it can escape. The detention volume fills dynamically, buffering the peak flood surge, and then slowly empties over several hours through a precision-calibrated low-level throttling device.
• Hydraulic Target: The post-development peak flow rate ($Q_{post}$) must not exceed the natural pre-development flow rate ($Q_{pre}$): $Q_{peak-post} \le Q_{peak-pre}$

Dual-Purpose (Combo) Retention/Detention Tanks

Under modern planning rules (such as the Auckland Unitary Plan), developers are frequently required to provide both retention volume ($V_r$) and detention volume ($V_d$) in a single tank:

• Lower Chamber (Retention Zone): Located below the low-level discharge orifice. This volume (typically 2,000 to 5,000 Litres) stores water permanently for household reuse.
• Upper Chamber (Detention Zone): Located between the low-level orifice and the top overflow weir. This volume (typically 3,000 to 6,000 Litres) buffers peak rainfall and drains down completely after each storm.

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2. Hydraulic Flow Regulators: Orifices & Vortex Controls

To restrict the outflow of a detention tank to pre-development levels, drainlayers install precision hydraulic throttling devices at the tank base.

Calibrated Orifice Plates

An orifice plate is a circular opening machined into a marine-grade stainless steel or uPVC blanking flange installed over the low-level discharge outlet.

• Orifice Flow Formula: $Q_o = C_d \cdot A_o \cdot \sqrt{2gh}$ Where:

  • $Q_o$ = Orifice discharge rate in cubic metres per second ($\text{m}^3/\text{s}$)
  • $C_d$ = Discharge coefficient (typically 0.60 to 0.62 for a sharp-edged circular orifice)
  • $A_o$ = Cross-sectional area of the orifice ($A_o = \frac{\pi d^2}{4}$, $\text{m}^2$)
  • $g$ = Acceleration due to gravity ($9.81\text{ m/s}^2$)
  • $h$ = Effective hydraulic head (vertical height of water surface above the centreline of the orifice, m)

• Minimum Practical Diameter: Orifices smaller than 25 mm to 30 mm in diameter are highly vulnerable to blockage from small twigs, leaf fragments, and spiders. Under council rules, any orifice under 50 mm must be protected upstream by a removable stainless steel mesh cage (aperture 5 mm to 8 mm) with a total surface area at least 20 times the orifice area.

Vortex Flow Limiters (Hydro-Brakes)

In municipal and commercial detention systems, vortex flow controls provide superior hydraulic performance without moving parts:

• Low-Head Behavior: At the beginning of a storm, water flows tangentially into the internal spiral casing and exits freely through the central outlet with minimal head loss.
• High-Head Vortex Throttle: As the tank fills and hydraulic head ($h$) increases, water enters the spiral chamber at high tangential velocity, creating a rapid swirling vortex. An air-filled core forms in the centre, exerting severe centrifugal back-pressure that chokes the discharge to a nearly constant rate.
• Blockage Resistance: A vortex flow limiter delivers the same flow throttling as an orifice plate while possessing a physical flow aperture 300% to 500% larger, dramatically reducing blockage hazards.

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3. Soakage System Configurations & Materials

Where ground conditions permit, surface water is discharged directly into the ground via subsurface soakage, recharging local volcanic or gravel aquifers.

Traditional Rock-Filled Soak Holes

• Construction: An excavation backfilled with clean, washed basalt or scoria ballast (aggregate size 50 mm to 150 mm).
• Void Ratio ($e$): Only 30% to 40% (i.e. a $10\text{ m}^3$ rock-filled excavation provides only $3.0\text{ to }4.0\text{ m}^3$ of actual water storage volume).
• Geotextile Lining: The entire perimeter (walls and top) must be lined with a non-woven, needle-punched geotextile filter fabric (e.g. Bidim A14). The base is often left unlined or lined with permeable fabric depending on local soil stability.
• Defect Alert: Never use woven polypropylene weed matting. Silt particles blind the woven pores within months, transforming the fabric into an impermeable plastic lining that causes total soakage failure.

Deep Volcanic Boreholes

• Application: Common in Auckland volcanic regions (Mt Eden, Onehunga, Penrose, Epsom) where an impermeable clay overburden sits atop highly fractured, porous basalt lava flows.
• Design: A borehole (DN150 to DN300) is drilled 6 to 15 metres deep into the fractured rock. Perforated uPVC or steel casing is installed, surrounded by pea gravel. Runoff must pass through a surface sediment trap and oil interceptor before discharging down the borehole.

Modular Plastic Drainage Cells (Matrix Crates)

• Construction: Interlocking polypropylene crate modules (e.g. ACO, Atlantis, or Marley cells) assembled into an underground matrix.
• Void Ratio ($e$): 90% to 95% (a $10\text{ m}^3$ excavation provides $9.0\text{ to }9.5\text{ m}^3$ of water storage volume).
• Structural Efficiency: A modular cell system requires less than half the excavation volume of a rock soak hole for identical storage capacity. When installed with proper bedding and minimum cover (typically 300 mm to 500 mm), they support axle loads up to 400–600 kPa (compliant with AS 5100 vehicle loading under residential driveways).

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4. Percolation Testing & Soil Permeability

Before sizing a soakage device, the Certifying Drainlayer or geotechnical engineer must verify the ground's percolation rate under NZBC Clause E1/VM1 or local codes (such as Auckland Council Technical Report TR 2013/040).

The Falling-Head Percolation Test Procedure

1. Borehole Excavation: A test hole (typically 100 mm to 150 mm diameter) is augered to the exact proposed invert depth of the soakage structure.
2. Slotted Pipe & Gravel: A slotted test pipe is inserted and packed with clean pea gravel to prevent sidewall collapse.
3. Mandatory Pre-Soaking (4 Hours Minimum): Clean water must be poured into the hole for a minimum of 4 hours (or filled and left overnight). Pre-soaking saturates the surrounding soil pores (the vadose zone). Performing a percolation test in bone-dry summer soil without pre-soaking yields falsely inflated percolation rates, leading to severely undersized soak holes that overflow during the first winter storm.
4. Measurement: The hole is filled to the design water head ($h_0$), and the water level drop ($\Delta h$) is recorded at fixed time intervals ($\Delta t$).
5. Percolation Rate Calculation: $k_s = \frac{\Delta h}{\Delta t} \cdot \frac{A_{bore}}{A_{wetted}}$

Soil Permeability Guidelines

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5. Clearances, Setbacks & Groundwater Protection

Soakage structures saturate the surrounding ground, posing severe risks to building stability and groundwater quality if installed carelessly.

Vertical Separation to Seasonal High Groundwater Level (SHGWL)

• The 1.0 Metre Rule: The base of any soakage pit, trench, or modular cell field must terminate at least 1.0 metre above the Seasonal High Groundwater Level (SHGWL).
• Failure Consequence: If the soakage base penetrates the groundwater table, the pit becomes a flooded sump with zero hydraulic head. In winter, groundwater back-floods through the pipes, while untreated surface wash-off contaminates the underlying potable aquifer.

Horizontal Separation Setbacks

• Property Boundaries: Minimum 1.5 metres (extended to 3.0 metres in sloping or sensitive soils) to prevent subsurface saturation from destabilizing neighbouring land or boundary retaining walls.
• Building Foundations: Minimum 3.0 metres when located down-slope of foundations, and at least 5.0 metres when located up-slope. Saturating foundation bearing soils under strip footings triggers soil softening, differential settlement, and wall cracking.
• Retaining Walls: Minimum 5.0 metres setback to prevent building excessive hydrostatic head behind the wall face.

Upstream Sediment Filtration

Stormwater must pass through a silt trap with a 300 mm minimum sump and an oil/debris baffle before discharging into soakage cells. Raw runoff rapidly clogs the geotextile filter fabric, resulting in irreversible premature failure.

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6. Secondary Emergency Overflows

Every soakage and detention system must incorporate an overt, hydraulically certified secondary emergency overflow path:

• Exceedance Events: If a 100-year storm exceeds the tank or soak hole capacity, or if the control orifice blinds with debris, the excess flow must discharge harmlessly across an overland path without entering buildings.
• Pipe Sizing: The emergency overflow pipe must equal or exceed the diameter of the primary inlet pipe (e.g. DN100 inlet requires minimum DN100 overflow).

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7. Certifying Drainlayer Trade Traps

• The Dry-Soil Testing Trap: Running a soakage test in dry clay without pre-soaking. The dry fissures drink water instantly, creating a false reading of 80 mm/hr. Six months later, saturated winter clay drops to 2 mm/hr, and the homeowner's driveway drowns in water.
• Wrong Geotextile Specification: Using woven silt fence fabric or black weed matting to wrap scoria. Woven materials block with clay fines within 12 to 24 months. Only needle-punched non-woven geotextile provides long-term filtration.
• Neglecting Groundwater Mounding: Installing multiple soak holes too close together. As water infiltrates, an underground mound of groundwater rises beneath them, drowning the upper chambers.