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Slowing the flow – Part 1

On-site stormwater management for new developments is now a requirement in many jurisdictions. In the first of this two-part series, we look at the use of soak pits and pervious paving

New developments place additional demands on existing stormwater systems, which increasingly do not have the capacity to handle further loads. When permeable surfaces like grass are replaced by houses, driveways, roads and decks, rainfall that used to soak through the soil or slowly drain over land runs off the land much faster.

Move to hydraulic neutrality

Both the volume of water and the peak flow are increased as a direct result of development. Many authorities now impose a requirement of hydraulic neutrality on new housing developments. In other words, stormwater must be managed on site to ensure the peak flow rate is the same or less than what it was prior to development.

Common management techniques include soak pits, pervious paving, and bioretention devices such as green roofs and rain gardens. All require active maintenance.

Soak pits

Soak pits collect and retain stormwater, allowing it to slowly soak into the earth. Soak pits are lined with a filter cloth and can be filled with rocks or be a solid chamber with porous sides and base (Figure 1). Soak pit requirements:

  • Water is collected and disposed of on site.

  • Useful in areas without reticulated stormwater and with suitable ground conditions.

  • Require careful design – New Zealand Building Code Verification Method E1/VM1.

  • Plans must be carefully followed when the pit is constructed.

  • Proprietary soak pits are also available.

Establishing the appropriate size is a function of several factors, including catchment area, rainfall intensity and the soakage rate of the ground, which is determined by on-site testing.

Rainfall intensity curves are available for most areas from the building consent authority (BCA), tables in E1/AS1 or online from NIWA.

Soak pit maintenance

Soak pits require maintenance to prevent them becoming blocked with silt, vegetation or other matter that may cause flooding problems. They must be cleaned and maintained annually by a specialist company.

Pervious paving

Pervious paving is a specially constructed hard surface that allows water to pass through to the underlying soil layers (see Figure 2). There are two distinct types of surface:

  • Porous – water travels through the pavers into the underlying components.

  • Permeable – water travels between impervious blocks into the underlying components.

Pervious paving can be used to reduce run-off and flooding and help to replenish groundwater. It is limited to flat and gentle slopes and has specific requirements:

  • The pavers are installed on a permeable base course with a permeable geotextile beneath.

  • Edge restraints are required around all edges to prevent displacement of pavers and geotextile.

  • In soils with low infiltration, include a perforated underdrain in a permeable gravel sublayer connected to the stormwater system.

  • The underdrain must be sized to suit site conditions.

  • A specialised design and approval is required.

A notice on land title may be required to inform the owner that maintenance is required.

Pervious paving maintenance

Pervious paving requires maintenance:

  • After storms – inspect paving to check water drains away.

  • Annually – sweep pavers with wet vacuum sweeper to prevent clogging with sediment, and check joint material and top up as necessary.

  • Ongoing – where areas of paving settle, lift blocks, relevel bedding material and relay blocks.

Avoid using herbicides or high-pressure water blasters on pavers.

Original article by Ann Galloway, FNZIA, Ann Galloway Architect Ltd, Napier. The full article was first published in Issue 183 of BRANZ Build Magazine.


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