Author:G Strayton, M Lillis, Pattle Delamore Partners Ltd
Source:Auckland Council Stormwater Unit
Soakage is a well-known method of disposal of stormwater in the basalt areas around the Auckland Region and within the Papakura peat.
The knowledge base with regard to the character of the majority of the Auckland Isthmus basalt aquifers has grown and the potential of the aquifers to accommodate soakage is now better understood. The state of knowledge of the Papakura peats is based on a 2006 review of the available soakage testing that has been carried out in the Papakura area based on property file records of applications for disposal of stormwater to ground.
There is potential for stormwater disposal to ground in other basalt and peat areas identified in this report. Maps setting out probable, potential and inferred soakage areas have been developed. These have also been provided to Auckland Council (AC) in GIS format. The criteria used to generate the maps given in Section 4 provide a useful guide to aid in the interpretation of site investigation data to determine the potential for soakage.
The design methodologies for private and commercial soakage devices are well documented and have been utilised within the legacy Auckland City Council and Papakura District Council areas over the last few decades.
The clogging of a soakage device is related to the land use of the catchment draining to the device. For average residential loadings the modelling undertaken indicates that pre-treatment in the form of a catchpit is adequate to prevent clogging for sediment from residential driveways. However for industrial, commercial and construction sites more comprehensive pre-treatment is required in order to ensure the effective ongoing functioning of the soakage device. In heavy sediment loading industrial areas, a sediment settlement tank or pond is recommended.
Soakage devices, with rockbores, discharge stormwater directly to the underlying aquifer. As the aquifer is considered to be the receiving environment, the stormwater needs to be treated to the relevant standard before entering the soakage devise.
Maintenance of the device is also an important element to minimise the effects of clogging by the entrained sediment within the stormwater.
The design approach for both basalt and peat follows the methodologies given in Appendices A and B. The most important factor in the design for the soakage devices located in the basalt is the site specific investigation to identify the area of most suitable soakage locating the device in the area of most fractured basalt. For the peat devices the site investigation is also important to identify the depth to the peat and the groundwater level within the peat.
Auckland Council technical report, TR2013/040