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Stormwater drainage

Stormwater management is an integral part of building construction and all buildings are required to have roof water directed to an approved stormwater disposal system. The National Construction Code includes provisions for stormwater disposal but the disposal method may vary depending on the site conditions.

General requirements

The National Construction Code requires that surface water resulting from a 1 in 20 year frequency storm event be disposed of in a way that avoids the likelihood of damage or nuisance to any other property. Common system components include soakwells, sumps and local drainage networks depending on the site conditions. The building permit will include conditions confirming the approved disposal method for a building. Greater capacity may be required as part of the development approval for commercial or high density developments following assessment of a site geotechnical report.

The NCC also requires that a 1 in 100 year frequency event not enter into a building. This is generally achieved by ensuring the dwelling is elevated slightly above the surrounding ground and that landscaping, driveways, paving and the like are graded such that water does not flood or pond against the dwelling.

Soakwells for sandy sites

Soakwells are underground tanks that allow water to gently seep into the surrounding soil. They are generally suitable for sandy sites with no underlying clay or silt content. Soakwell sizes can be varied to achieve the required capacity, however in some districts soakwell depth may be restricted due to a high ground water table. Soakwells should be interconnected to provide relief if one tank reaches capacity, and in some areas smaller lots may be provided with a connection to the local drainage network to help the tanks to empty after heavy rainfall.

Clay sites / silty sand soils

Soakwells are generally not suitable for sites with silty sand or underlying clay content as such sites do not allow water to be effectively dispersed. Furthermore, clay is reactive and will shrink and swell based on its water content, resulting in differential movement which can cause structural damage to buildings. To ensure stormwater disposal is effective and complies with the NCC, lots in these areas will generally require interconnected sumps connected to the local drainage network.

Sumps are solid underground tanks that hold water instead of allowing it to seep into the surrounding reactive soil. They should be interconnected at the base and must be connected into a local drainage network to enable the tanks to empty. In most new developments silt pits and lot connections are provided at subdivision, but in some older areas a new connection may be required to accommodate infill development (e.g. units). If necessary, a new connection will be installed by the City at the applicant’s cost. Connections are costed on an individual basis due to the varying circumstances on each site.

Most of the City’s drainage network is not sufficient to accommodate a 1 in 20 year event. As such, direct lot connections without sumps will generally not be approved as they do not satisfy the NCC and may result in damage to buildings.

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