Part 4 – Stormwater Drainage
4.1 General
All lots shall be provided with a means of stormwater drainage at or within the lot boundary.
Where the property in question already has a connection to a public stormwater system then that connection can continue to be used, but the discharge through this connection shall be limited to the discharge rates from the pre-developed site and any excess discharge rate (e.g. overland flow) created shall be dealt with by the development to the pre-development disposal rates. Developers should be familiar with the MPDC Stormwater Management Bylaw.
In some cases a new connection to a public stormwater system may be approved by Council. In those cases the design of the new stormwater system shall conform with the MPDC Development Manual requirements.
In all cases where stormwater is to be disposed through on-site soakage then the developer will find that the MPDC Soakage Design Procedures and Guidelines (known as the Soakage Guideline) provides a useful reference guide. This guideline is to assist designers to provide a soakage system that will deal with the calculated stormwater discharge rates. It also allows for convenient maintenance of the soakage system so that there can be confidence that it will continue to work effectively for the long term.
In cases where connection to a public stormwater system is not available and ground soakage is not adequate then an alternative system may be approved. Council opinion is that ‘wet pond’ type detention dams should be avoided
The design intention is to incorporate natural environment-based systems within new works. A natural environment-focused stormwater system may include features such as groundwater recharge, overland flow, open drains, and storm peak mitigation through dams, lake and wetland systems. Key objectives are:
- Optimise the amount of stormwater entering the piped drainage system;
- Facilitate groundwater recharge;
- Cover the immediate needs as well as those of foreseeable future developments;
- Avoid detrimental effects on downstream properties;
- Build infrastructure to minimise lifecycle maintenance costs.
Where stormwater is designed to flow into existing drainage systems, there shall be no increase in peak discharge flow rates from the construction of the subdivision or development.
Where a stormwater system is proposed for the collection and discharge of stormwater within the land being developed, it shall also have capacity to deal with drainage from the entire catchment upstream of the development site.
Stormwater proposals must take into account the requirements of Council’s current stormwater discharge consents from the Regional Council. All proposals must be consistent with the conditions of these consents including requirements for low impact design principles, stormwater management devices and best practicable options.
Where the discharge is to an existing Council pipe network it is Council’s responsibility to assess compliance for all new connections to its pipe network. It will require the same conditions as applying to any new municipal stormwater system diversion or discharge activities.
In particular it will require that the new diversion or discharge does not increase peak discharge rates to receiving waters above that which would occur at the time of the application for Council’s current discharge consents – unless it is demonstrated that there shall be no adverse effects on the environment or downstream properties as a result of such increase. Acceptance will also be subject to compliance with the requirements for connections to the municipal stormwater system.
Where the new stormwater will discharge to other than an existing Council pipe network, the subdivider or developer is required to obtain appropriate resource consents for discharge for the work from the regional council in the Developer’s name. Evidence of this is required before the subdivision can be approved.
If Council is to take over the finished system, it will need to add the discharge to its own consent at the time of transfer. The consent must therefore specifically state that the work will comply with the requirements for a new municipal discharge as set out in Council’s Comprehensive Storm Water Discharge Consent (refer to Section 4.2 below).
Secondary flow paths shall be provided and must be able to cater for a minimum of a 1 in 100 year return period storm. Secondary flow paths within the development must be protected by an easement registered against the titles affected.
Where secondary flow paths are not feasible the piped system must cater for a minimum of a 1 in 100 year return period storm. This shall also ensure that the peak flow rate from the developed site does not exceed the 100 year pre-development peak flow rate.
Where disposal is to ground soakage with no available secondary flow path, the soakage system must cater for a 1 in 100 year return period storm.
