What is stormwater detention?
Stormwater detention refers to storing or detaining rainwater for short periods. The stored water is released at a reduced rate once the rainfall intensity reduces or stops.
Note: This is different to collecting rainwater for later use in irrigating gardens or other uses, where the water is held in tanks for extended periods.
Why do we need stormwater detention?
Urban development can increase the proportion of stormwater runoff and therefore increase stormwater flows. This could worsen existing flooding problems because Council stormwater systems are generally designed to handle stormwater from existing developments only.
When is storage required?
Stormwater detention storage is required when the proportion of building roof, driveways or other hard surfaced areas exceeds approximately 50% of the total site.
Where do I put this storage?
Above ground tanks can be designed to complement the form of existing or new buildings. Below ground tanks are also available for storage while other options include landscaped garden areas that allow for short term ponding of water during heavy rainfall.
What volume of storage do I need?
Find out how much stormwater detention you require (if any), have the following information ready (in square metres) and download our calculator
Please note that this calculator is
Total property area
Proposed roof area of buildings
Area of proposed driveway and other hard surface areas including patios or decks
Industrial & Commercial Zones
Industrial and commercial land is generally covered with hard surfaces such as roofs, car parks and concrete or paved storage areas. These surfaces have a rapid runoff during rainfall which can lead to flooding in high intensity rain events. Hard surfaces also contribute to the generation of contaminants which can then be conveyed to waterways and streams. Contaminants in stormwater include litter, nutrients, heavy metals, petroleum hydrocarbons and silt.
How does Detention Storage Work?
Storage of rainwater is provided in tanks or within formed swales, ponds or other areas within the site e.g. car-parking areas. The outlet pipe from the detention is restricted in size to limit flows to the off-site Council conveyance (road or pipe). This limits stormwater flows to more manageable levels during heavy rain.
Location of Detention Storage?
Storage can be in shaped hard-stand parking or paved areas at the site or alternatively within landscaped grassed or planted areas on the site. Other options include above or below ground tanks or storage facilities. Storage can be provided in different areas for the 5 year and 50 year events. Ponding of car parks or other hard stand areas is likely to be more practical for storage beyond a 5 year rain event.
Stormwater management approach
Hastings District Council is implementing on-site measures for individual commercial or industrial sites and off-site measures for infrastructure that is installed and maintained by Council on road or reserve areas.
What facilities are required on-site?
Stormwater detention is to be provided within each new or developed industrial or commercial site.
In addition stormwater treatment to remove a proportion of contaminants may be required.
Note: High risk types of industry or commercial operations may need specialist design.
How much detention storage is required?
Controls are required for all rain storms to limit runoff from the site to 6 litres / second (l/s) for each 1,000m2 of land in a 5 year return period event and 12 l/s in a 50 year return period event.
For an industrial site with 100% coverage of impervious hard surfaces (roof areas and sealed parking and hardstand) the required storage, per 1,000m2 of development site, is in the order of 10,000 litres.
On-Site attenuation guide for residential and city centre areas
This guideline is for residential and city centre areas of Hastings District that are proposed to be developed or redeveloped and that may require on-site stormwater attenuation. This guideline is limited to above ground tanks for stormwater attenuation.
Attenuation tanks limitations
An attenuation tank sizing tool and acceptable solution has been developed for individual residential or city centre site developments. Refer to details below. These are generally limited to sites of less than 1500 m2 with above ground tanks and gravity outflow.
The acceptable solution and tank sizing gives the basic hydraulic information to assist with developing a full installation design for an individual site. Specific items that need to be considered for individual sites include:
- Layout of roof and downpipes that can be practically drained and connected to a tank or tanks
- Slope and topography of the site and where stormwater is draining to
- Road level versus property levels and level for base of the tank(s)
- Space and location for the tank(s)
Products and suppliers
There are numerous suppliers of tanks and fittings within New Zealand. Some will manufacture to specific site sizes or shapes required. A list of some of the suppliers is available from Hastings District Council for information only. Local contractors and plumbers will be able to provide installation of the pipework and tanks to meet building act / consent requirements.
Other useful documents
For further information around the engineering theory and requirements, design considerations and examples refer to the following:
- Hastings District Council, Engineering Code of Practice
- Hastings District Council, Subdivision and Infrastructure Development in Hastings District, Best Practice Design Guide (PDF)
- NZWERF – On-Site Stormwater Management Guideline, October 2004
- Hawke’s Bay Regional Council, Waterway Design Guidelines
- Compliance Document for New Zealand Building Code, Clause E1 Surface Water (PDF)
Check that you have provided the information below
Site Plan showing:
- Tank and pipe layout and sizes
- Pipe grades and levels
- Roof area draining to tank
- Make up of the site i.e. driveways, paved areas, house floor/roof plan (to check against inputs to the calculator)
- Completed calculator sheet
This information has been prepared for the benefit of Hastings District Council. No liability is accepted by this company or any employee or sub-consultant of this company with respect to its use by any other person.
This disclaimer shall apply notwithstanding that the report may be made available to Hawke's Bay Regional Council and other persons for an application for permission or approval or to fulfil a legal requirement.