Stormwater – this is not a factor to figure out after the rain starts falling. It’s also a step that doesn’t often grab the spotlight when designing a build. We’ve drilled our expert engineers on what you need to know to get stormwater (SW) and drainage design right, the elements that might apply to your build and how to avoid any stormy missteps that slow projects down.
Stormwater requires a drainage plan
During the concept, design and planning stages of a project, the focus is often on the floorplan, layout and visuals of the build. However, once a roof is added or modified, or a new driveway is built, that’s a new impervious surface that collects rainwater – water that needs a drainage plan to direct it away from the home to a connection port prior to discharge into a stormwater system. It’s a plan that needs to take into consideration all water that flows from the dwelling and property.
From a roofed deck to a single dwelling or multi-unit development, every build needs a drainage plan. That is unless there has been no change to the impervious area (meaning any surface that can not absorb water such as concrete, asphalt, compacted rock and the building footprint).
Most importantly your drainage plan needs to meet the current regulatory requirements (Tasmania Urban Drainage Act) and local council requirements. Where the area exceeds 500sqm, including concrete driveways, this will trigger an additional SW quality requirement prior to discharge into the council system.
Who prepares a stormwater drainage plan?
An experienced building designer will consider drainage as a part of the design, however as a builder, renovator or architect it’s an item to consider in relation to the council you’re working with. It’s civil engineers who apply their expertise following the design phase to calculate the flow from a design rainfall.
Allen Wang, one of the engineers on the HED Consulting team, says that their expertise recognises where a drainage plan needs to be scaled up to a higher level of compliance due to:
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- the volume of water – where a detention tank is needed to slow the flow so the outlet downstream isn’t overwhelmed or
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- the composition of water – where treatment is needed ahead of entering stormwater infrastructure
Wang adds, “We also provide the relevant calculations councils request as part of a drainage plan. We prepare your site plan and stormwater layout, organise a SW treatment system, apply to council for your permits and ensure your build’s plumber has the full scope of work early in the construction phase.”
Stormwater treatment thresholds
Where stormwater contains a percentage of solids that exceed regulations (total suspended solids, phosphorus, nitrogen and gross pollutants), your engineer will let you know that a stormwater treatment system is needed. The quality target parameters are the same across councils in Tasmania and the system needs to meet the compliance standards.
Minimising stormwater risks
Whether your site and build is simple or complex, stormwater is unforgiving if not prepared for and managed with good design. It’s important to invest in the expertise needed to not only comply but also minimise risks.
Wang says that “Those risks arise when stormwater isn’t managed correctly which can lead to flooding, foundation damage or cracking in walls. The culprit is nearly always water.”
Factoring in SW runoff from gutters and downpipes, discharge to a public SW infrastructure or a private absorption trench, your pre-construction planning should apply a rigorous engineering process to each aspect.
Stormwater is more than your roof
Yes, a complicated roof design will require a carefully calculated drainage system but stormwater is more than your roof. If your site has a steep driveway, retaining walls, or twists and turns around the property, these variables are all collection points where both the flow of water and where it will go must be accounted for.
For subdivisions where anticipated dwellings, roads or driveways combine, the civil engineering required may be more extensive. Bigger developments often encounter elongation or alterations to the council’s existing stormwater infrastructure which requires detailed catchment analysis and hydraulic modelling. With the involvement of both engineers and hydraulic consultants, it’s important to allow both time and budget in the project plan for these types of developments.
Whilst complexity may change, there’s one certainty that you can rely on…water always travels downhill. When embarking on your next build remember that this is an important part of the pre-construction process and it’s best to be ahead of your council’s requests for your drainage plan.
Contact HED Consulting to have our expert civil engineers on your project team. View our civil engineers here.
