STORMWATER DRAINAGE DESIGN – NEW OPPORTUNITIES IN NZBC E1 NOVEMBER 2020 UPDATE

26 Nov 2020

November 2020 brought a number of updates to the Building Code, including a new Acceptable Solution in the form of E1/AS2 in its newest version, Amendment 11. Designers and specifiers now have the opportunity to use AS/NZS 3500.3 (with modifications) as an Acceptable Solution for stormwater drainage systems. Both E1/VM1 and E1/AS1 contain a number of sundry corrections and updates, while E1/AS2 is presented as a series of modifications to specific clauses in AS/NZS 3500.3.

Other notable updates to E1/AS1 include replacing the rainfall intensity maps with an extensive table of data for numerous specific locations throughout New Zealand. This gives designers the choice of either using these figures, or data from the relevant Territorial Authority. E1/AS1 also provides a comment that, where discrepancies are found, the most conservative intensity figure should be used. No definition of 'conversative' is given, but it would be prudent to err on the side of caution and assume the higher intensity prevails. This comment is also given in E1/VM1.

The rainfall intensity data is based on information produced by the National Institute for Water and Atmospheric Research (NIWA), and is also available online from their High Intensity Rainfall Design System (HIRDS) which provides estimates for any location in New Zealand. Note that Annual Exceedance Probability (AEP) also now appears in the definitions, to link in with its use in AS/NZS 3500.3 (particularly where a more onerous 2% AEP is to be used for overflows, internal gutters and valley gutters). In addition, internal gutters now have a minimum width of 300mm, and 30mm minimum freeboard, tying in with E2/AS1.

Returning to the new Acceptable Solution E1/AS2, MBIE listed the following goals during the consultation period:

Increase ‘deemed to comply’ options for sizing and designing roof gutters and surface water drainage systems
Introduce new ‘deemed to comply’ design and installation solutions for:
- on-site stormwater detention systems (partial solution)
- pumped stormwater systems
- siphonic roof water drainage systems
Introduce informative installation provisions for subsoil drainage systems
Provide ‘deemed to comply’ design and installation solutions for surface water and roof water drainage systems that fall outside the scope of the current Verification Method and Acceptable Solution
Allow for consenting efficiency when stormwater drainage systems are designed using AS/NZS 3500.3:2018 as the design would no longer need to be treated as an Alternative Solution by Building Consent Authorities.

Of these goals, OSDs, pumped stormwater systems and siphonic roof drainage became informative only, as applicable to New Zealand. Upon publication MBIE stated that the purpose of E1/AS2 is "to cover larger sites and designs that were previously considered Alternative Solutions". E1/AS2 makes a number of amendments to AS/NZS 3500.3, which must be read in conjunction with the Standard in order to comply with this new Acceptable Solution.

The calculation procedures set out in the Standard are more detailed than those in E1/AS1, and can enable deemed-to-comply solutions that were previously only possible as Alternative Solutions. Additionally, as this companion article explains, more accurate sizing of rainwater goods is possible thanks to the ability to use the NZMRM Capacity Calculator, which is closely aligned with the methods set out in AS/NZS 3500.3.

Among the areas that the Standard assists with more accurate designs is roof catchment areas. Whereas E1/AS1 lists a variety of areas and roof pitch ranges (up to between 200m2 and 350m2 depending on pitch), AS/NZS 3500.3 gives multiplication factors for any given roof plan area, and also considers the direction of wind driven rain, as well as multiple roof slopes into a common gutter, to arrive at more accurate sizing for rainwater goods.

Whilst larger buildings can now be designed within the scope of E1/AS2, it should be borne in mind that AS/NZS 3500.3 (as modified by E1/AS2) does in itself have limitations in its scope, namely gutter, outlet and downpipe capacity which are generally limited in range to between 3 and 16 litres per second. Roof drainage designs outside this range should be proven by calculation as an Alternative Solution.

NZBC E1 Amendment 10 continues to be operative until 3rd November 2021, while Amendment 11 became operative on 5th November 2020.

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STORMWATER DRAINAGE DESIGN – NEW OPPORTUNITIES IN NZBC E1 NOVEMBER 2020 UPDATE