Detention

Stormwater detention storage is required when the proportion of building roofs, driveways or other hard surfaced areas exceeds approximately 50 per cent of the total site.
In this example, it is clear that the areas of hard surface add up to substantially more than 50 per cent of the 500m2 site:

Storage can be in tanks, above or below ground, or outdoor areas can be landscaped to include an area that allows for short term ponding of water during heavy rainfall.

Council has developed a detention calculator which will help gauge how much storage you need, if any, for your  residential development. You will need to know the total area of the property, the proposed roof area, and the total area of all other proposed hand stand areas, including driveways, paths and decking.

If your calculations show that you need stormwater detention on your residential or city centre site, discuss potential solutions with your architect or project manager. Local contractors and plumbers will be able to install pipework and tanks that meet Building Act and/or Resource Consent requirements.

Storage can be in tanks, above or below ground, or within formed swales, ponds or other areas on the site, for example carparking or landscaped areas designed to perform a detention function during heavy rain. The outlet pipe from the detention area is restricted in size to limit flows into Council’s storm water system (road or pipe).  This reduces storm water flows to more manageable levels during heavy rain.

All commercial and industrial developments require a site-specific stormwater management design that meets Resource Consent and Building Consent requirements. Discuss this with your project manager during the early phase of your planning.

The amount and design of the detention system will depend on the activities that will be carried out on the site, the size of the site and how much of it will be covered with impervious materials, and the commercial/industrial area in which the project is being built, as set out in the District Plan.

Your project manager needs to submit your stormwater detention design to Council (through the Resource Consent/Building Consent process) for approval before you start  your development. Particular attention should be paid to the relevant sections of the District Plan, and to the Hastings District Council Engineering Code of Practice: Section D.

Include the following:

• The sizes and materials of all site elements including driveways, paved areas, footprint and roof sizes of all buildings
• Stormwater and on-site water use calculations
• Site plan including pipe layout and sizes and any tanks (see example below)
• Pipe grades and levels

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.

For further information on engineering theory and requirements, design considerations and examples refer to the following:

• Hastings District Council, Engineering Code of Practice
• Hawke’s Bay Regional Council, Waterway Design Guidelines
• Compliance Document for New Zealand Building Code, Clause E1 Surface Water Loading