Stormwater flooding

Groundwater is naturally occurring as water moves underground towards waterways. Where this groundwater may rise to the land surface, it is known as seepage.

Seepage issues that occur will generally be where natural ground exists such as where natural rock is close to the surface. Groundwater movement and quantity may change over time due to a variety of conditions such as periods of drought or extended rainfall.

Numerous flooding occurrences are often caused by natural overland flow. Properties situated downhill from bushland, parks, and road reserves may encounter natural overland flow during periods of rain. This flow volume can escalate when the ground is already saturated and incapable of absorbing additional water.

Private approved drainage systems including roof water drainage, stormwater pipes, gutters downpipes and pits should be constructed and maintained in good condition to comply with Council requirements. Council has limited ability to intervene in drainage issues between neighbours. It is recommended and encouraged that neighbours discuss to seek an appropriate solution.

Council’s stormwater network is designed to safely handle rain events up to a specific rain event. Current design standards where possible are the 20% Annual Exceedance Probability (AEP) and 1% AEP for the minor and major networks respectively. Standards have evolved across many years so the network will be of varied design capacity. If a rain event surpasses the network's capacity, some properties may consequently experience flooding.

If the stormwater network experiences structural failure or blockage, it will ultimately lead to reduced performance, potentially resulting in flooding for some properties.

River flooding occurs when extensive and prolonged rainfall affects the catchment area of a river. When river channels fill to capacity excess water spills over its banks, resulting in flooding. This is generally not experienced in Willoughby LGA.

To report a stormwater drainage problem on private property, you can lodge a request. Including the following details will assist in Council responding appropriately to the request:

• Describe the issue.
• When it started and how often it happens.
• If you have reported it to Council before.
• Where the stormwater is coming from - for example, from neighbouring property.
• What damage it is causing to your land or building and include any reports from a qualified person.
• Whether you have spoken to a professional about the cause.
• Photos of the issue while it’s happening.

• Drainage issues relating to seepage, natural overland flow and inter-allotment drainage.
• Stormwater outlets from property to Council kerb. Their maintenance is the responsibility of the property owner.
• Surface water run-off that occurs only during periods of heavy rain.
• Surface water directed as natural run-off from the property or properties due to the natural topography and is not being redirected in any manner. This includes surface water as a result of overflows from stormwater absorption pits where the lack of access prevents direct connection of a building’s roof water to the council’s drainage system.
• Surface water is flowing down or across existing hard surface areas such as driveways, tennis courts, concrete slabs or paved areas.
• The location of a dwelling or other structures on property impacts on surface run-off
• Run-off is from new development work that is the subject of a development consent and has been constructed in accordance with that consent.
• Drainage issues through discharges from defective or blocked private inter-allotment drainage easement infrastructure such as pipes and drainage pits.