Part 4 – Stormwater Drainage

4.8    Design Requirements

1. The land drainage system shall be capable of serving the entire catchment upstream of the subdivision or development and must take due regard to the effect it may have on downstream waterways and adjoining areas.  It shall be designed within the terms of any approved comprehensive drainage scheme.
   
   
2. The design storms shall be in accordance with Section 4.8.1.1 below. Note the MPDC Soakage Guidelines provide 100 year with climate change rainfall events for each of the four (4) townships and surrounding areas. 
   
   
3. Where open watercourses are to form part of the land drainage system this shall be determined at scheme plan approval stage, and the Developer shall submit sufficient engineering design to enable Council to evaluate the proposals.
   
   
4. The means of stormwater disposal shall be capable of serving the whole of the lot (including upstream) to pre-development conditions. Generally each lot will have a single stormwater connection.
   
   
5. Where further subdivision or development upstream of the one under consideration is provided for in the MPDC District Plan or Structure Plan, any stormwater pipelines proposed under the Development under consideration shall be to the upper limits of the subdivision or development under consideration.  This will allow for the future upstream development to connect into the downstream pipe system.
   
   
6. In new developments the stormwater disposal design shall adopt stormwater control measures that retain the secondary overland flow run-off for the particular development to pre-development conditions. 
   
   
7. Stormwater treatment devices such as stormwater detention areas, rain gardens, vegetated filters and swales are to be landscaped with vegetative cover as set out in this MPDC Development Manual, Part 9 – Landscaping Engineered Stormwater Devices.  Landscape plans shall be accepted by Council prior to planting.  For treatment devices constructed in conjunction with sub-division or land use consents, planting shall be completed and maintained for at least one year prior to vesting the treatment device to Council.
   
   
8. Under no circumstances shall stormwater be led to or permitted to enter a wastewater system.
   
   
9. Stormwater secondary flow paths shall be identified for the following situations:
   • Catchpit blockage.
   • Culvert blockage (or alternatively provide an unobstructed waterway capable of passing the once in 100 year return period rainfall event while maintaining at least 0. 5 m freeboard to building floor levels on upstream property).
   • Rainfall in excess of design levels of service as outlined in section 4.8.1.1.
     
     
10. Stormwater secondary flow paths, including peak flow depths, velocities and flow rates, shall be shown on design plans for pre and post development of the site for a once in 100 years return period.
    
    
11. All stormwater secondary flow paths across private land shall be protected by an easement.  The easement shall cover the full extent of the secondary flow path and shall not be less than 3 m wide.  The easement shall have the effect of preventing alteration of the ground surface and prohibit location of structures that might impede the flow of water across the land.  The easement shall be in favour of the Council. The easement shall be duly granted, reserved and shown on the survey plan.
    
    
12. To ensure that the critical duration storm is utilised for each site (including upstream) the 24 hour duration nested storms with a minimum of 10 minutes duration should be used for the proposed development.
    
    
13. Auckland Council TP10 can be utilised as a guideline for water quantity and quality control design methods.