On the tools STORMWATER DRAINAGE
TECHNICAL THEME
THE PROPER
CHANNELS 5 things to consider when creating stormwater drainage systems. AUTHOR: CHRIS PATERSON, ALLPROOF
T
he primary objective of NZBC Clause E1 Surface water is to “safeguard people from injury or illness, and other property from damage”. There are many elements that contribute towards this objective. Outlined here are five key factors to consider when selecting the appropriate products for the job.
1. Application
Ensure you select the correct type of product for the job. Is a channel required or will a sump suffice? Is this a level entry into a habitable space, a channel drain in front of a drive/garage, or a collection point before going into the local authority’s stormwater system? Each application has different requirements set out in NZBC and applicable AS/NZ Standards. A level entry scenario requires a 200mm x 150mm channel and a 12mm drip line along the joinery (plus other various details), so a standard three-sided channel is not applicable in this application. Each drainage system has been designed for a specific application. If you are unsure, it is worth discussing your project with Allproof to avoid costly delays and work involved in removing a system that isn’t compliant.
2. Performance
Ensure that the selected drainage product’s hydraulic performance meets the needs of the application. This may be a combination of the channel depth/width and grate open area or even just how the product is set up. Understanding the topography of the area will also have a large part to play. Installing a slot drain with a 15mm opening at the bottom of a steep drive means that during a heavy downpour surface stormwater will likely bridge the
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nzplumber
Polymer Concrete channel, 75mm depth with 316 stainless steel wedge wire grate (load rating: A, slip resistance: R11, open area: 47%), installed at South Wharf, Melbourne.
gap and the drain will not perform to the desired level, whilst a wider grate with more open area (eg, Allproof PC with cast iron grate) would allow the water to drain. A channel slot drain would work well in a courtyard with a fall towards it from both sides. Best practice is to seek advice from a specialist hydraulic consultant in this area.
The majority of applications will require a load rating between A and D, which would cover commercial vehicles in areas such as loading bays. Allproof have developed in-house testing to ensure their products meet the specifications laid out by AS 3996. This information is available in Allproof’s literature to aid product selection.
3. Load Rating
4. Environment
AS 3996:2019 Access covers and grates outlines the requirements for grate load class ratings. Grates must perform well under subjected loads appropriate to their application once installed. The standard provides a rating of A to G, with A being “areas (including footways) accessible only to pedestrians and pedal cyclists and closed to other traffic (extralight duty)” with a nominal wheel load of 330kg, and G for “docks and aircraft pavements subject to very high wheel loads (extra heavy duty)” with a nominal wheel load of 30,000kg.
The environment plays a large factor in material selection when choosing a product. Stormwater drainage products are exposed to large amounts of moisture by the nature of the application. It is important to understand the chemical and physical characteristics of materials in each environment. Cast iron is an excellent material for strength with a long-life expectancy and economic price point, and its bituminous paint coating provides a degree of protection from the elements. However, unless regularly re-painted it will wear off, exposing the cast iron, and oxidation will naturally occur.
