What is Stormwater Drainage?

Stormwater refers to any rainwater that falls on your property. Roofs and gutters collect and remove the water from buildings.

Stormwater drainage is the system to collect and transport the rainfall to minimise the risk of flooding and water ingress into and around buildings.

The basic design rule in all construction is:

Stormwater must not enter buildings.

Read this overview to learn the basics of Best Practice stormwater drainage design.

Why is stormwater drainage important?

In Western Australia, older less efficient stormwater drainage systems are increasingly unable to cope with changing weather patterns, increased rainfall intensities and durations.

The performance requirements of a stormwater drainage system are simple:

• Prevent flooding of or pooling adjacent to buildings.
• Minimise damage to properties.
• Allow replenishment of groundwater resources.
• Control runoff to minimise damage to infrastructure.

A Stormwater Management Plan is an engineered Stormwater drainage solution that allows for the management (control) of stormwater and property runoff.

Stormwater drainage is used to ensure people, property and adjacent areas are not affected by the accumulation of surface rainwater.  From municipal stormwater networks to basic residential soak wells all these systems must be sized correctly to maintain the performance requirements.

How does stormwater drainage work?

Stormwater drainage systems have three basic design components:

• Collection of rainwater (e.g. gutters, gullies, channels, and sumps).
• Transportation (drains, pits, and channels).
• Disposal (offsite disposal using detention tanks and; and onsite disposal through Sumps, basins soakwells and drainage cells).

Over the past decade, Water Sensitive Urban Design (WSUD) has been referenced by Local Authorities in their Planning review of developments.

WSUD is an environmentally friendly design process by which assessment of potential pollutants is undertaken and stormwater pre-treatments are adopted prior to the Disposal of stormwater.

Design

Each design component needs to be engineered to ensure the system can cater for the most critical storm events as well as operate to the best efficiency.

a)    A stormwater engineer or hydraulic services engineer will consider aspects of your development such as: Depth of groundwater;

b)    The nature and type of soil;

c)    Site topography and surface obstructions;

d)    Potential pollutants; and

e)    Current rainfall data.

Current rainfall data must include the full range of storm events to ensure the Critical Storm Eventis used as the basis for stormwater drainage design.

The Critical Storm Eventis simply explained as the storm during which the greatest amount of system storage is required for a storm duration.

Storm InflowVolume over a duration- Disposal Volume over the same duration

= Volume to be stored in the system.

The issue in design is the number of variations that need to be reviewed:

• Permeability of soil;
• Storm recurrence / exceedance period (1, 2, 5, 20, 50, or 100 years / 100%, 50%, 20%, 5% or 1%);
• Storm duration ranges from minutes to days.

Stormwater Management and design experts

PJ Wright & Associates can provide practical and cost-effective solutions for all types of sites and conditions, following Best Practice design principles to address your unique requirements.

Our stormwater drainage designs comply with the National Construction Code of Australia, local authority requirements and other legislation.

We perform a thorough critical storm risk analysis for every individual site to ensure safety in design.

To talk to our hydraulic design engineers in Perth, call PJ Wright & Associates today on +61 8 9443 3466 or request an online quote.