Overland Flow Paths Explained for Home Buyers

Overland flow paths are a lesser-known but increasingly important flood risk for new home builds in Australia. As urban development increases and the underground drainage system ages, more properties are being identified as being in overland flow paths — areas where stormwater flows across the surface during heavy rain because the underground drains cannot cope.

If your block is in an overland flow path, the council will impose conditions on how the building must be designed and constructed. Understanding these conditions and their cost implications is essential if you are buying land in a low-lying, sloping, or urban infill area.

What Is an Overland Flow Path?

An overland flow path is the route that stormwater takes when it flows across the surface of the land during heavy rain. In a natural environment, rainwater soaks into the ground or flows along natural depressions and gullies. In an urban environment, the ground is covered by hard surfaces (roofs, roads, driveways) and the rainwater is collected by underground drains.

During extreme rainfall, the underground drainage system may not have enough capacity to carry all the water. When this happens, the excess water flows across the surface — following the natural contours of the land, flowing through gaps between buildings, along roads, and through properties. This surface flow is called overland flow, and the route it takes is the overland flow path.

Overland flow paths are different from riverine flooding (which comes from a creek or river overflowing its banks) and from coastal flooding (which comes from the sea). Overland flow is typically shallow (a few centimetres to a few hundred millimetres deep) but can be fast-moving and can cause significant damage.

How Overland Flow Paths Are Identified

Councils identify overland flow paths through flood modelling and drainage studies. The flow paths are shown on the council's planning scheme maps, typically under a 'Special Building Overlay' (SBO) or equivalent. The overlay identifies areas where overland flow is expected during the 1% AEP (1-in-100-year) storm event.

The council's flood modelling also provides information about the expected depth and velocity of the overland flow at each point along the flow path. This information is used to determine the construction requirements for buildings in the flow path.

How Overland Flow Paths Affect Building

If your block is in an overland flow path, the council will impose conditions on the building design to ensure the building does not obstruct the flow of water. The specific requirements depend on the depth and velocity of the flow at the site:

Design Requirements for Overland Flow

The council may require one or more of the following design measures:

• **Raised finished floor level** — The building's floor level is set above the overland flow level plus freeboard, so water flows under or around the building without entering.
• **Open subfloor** — The subfloor is left open (no enclosed walls) so water can flow through freely. This is common for buildings on piers or posts.
• **Flood openings** — If the subfloor is enclosed, flood openings (vents) are installed to allow water to enter and exit the subfloor, equalising the water pressure on both sides of the walls.
• **Flow path maintenance** — The flow path is maintained as an open channel, swale, or landscaped depression through the property, with no buildings or structures obstructing it.
• **Building setback** — The building is positioned to one side of the flow path, leaving the flow path clear.
• **Flood-resistant materials** — Materials below the flow level are flood-resistant (e.g. concrete, brick, water-resistant plasterboard) to minimise damage if water does enter.

Overland Flow vs Riverine Flood

It is important to understand the difference between overland flow and riverine flooding, as they have different causes, different overlays, and different construction requirements.

How to Check If Your Block Is in an Overland Flow Path

1. Check the council's planning scheme maps for a Special Building Overlay (SBO) or equivalent overland flow overlay.
2. Contact the council's drainage or engineering department and ask if the block is in an identified overland flow path.
3. Look for natural depressions, gullies, or low points on the block that water would naturally follow during heavy rain.
4. Check if neighbouring properties are higher than the block — water will flow from higher ground to lower ground.
5. Ask the builder or a surveyor to assess the site for overland flow risk.
6. If the block is in an overland flow path, obtain the expected flow depth and velocity from the council.

What to Check Before Buying Land

Common Mistakes and Hidden Costs

• Not checking whether the block is in an overland flow path before buying.
• Assuming that because the block is not near a creek, it is not at risk of flooding — overland flow can affect properties far from any watercourse.
• Obstructing an overland flow path with a wall, fill, or enclosed subfloor — this can cause flooding on neighbouring properties and may result in council enforcement action.
• Not budgeting for the cost of raising the building or designing the subfloor for overland flow.
• Forgetting that overland flow paths can change as upstream development increases runoff — a block that is not currently in a flow path may become one in the future.
• Not checking whether the builder has included overland flow-related costs in the contract.

Frequently Asked Questions