Overland flow

Overland flow describes the movement of water running over the ground after rainfall, before it enters a waterway e.g. a creek, river, canal or lake. In built-up areas, overland flow can also occur when drains and stormwater pipes reach capacity, causing water to overflow onto streets, footpaths and surrounding land.

Protecting overland flow paths is important because they help move stormwater safely through the city. If these natural flow paths are blocked, altered or built over, local flooding can become more severe. Overland flow is different from river or creek flooding, which occurs within larger waterway systems.

You can often identify overland flow paths by looking at the shape of the land and nearby drainage features such as gullies, waterways and low-lying areas. Stormwater naturally follows the path of least resistance and flows towards the lowest point in the landscape.

Understanding the risks of overland flow

Overland flow can develop quickly during heavy rainfall and often occurs with little warning. Because many areas of the Toowoomba Region are steep, water can move quickly, accumulate from many areas into large volumes and become dangerous, even when outside of our creek and river systems.

Difference between overland flow and creek flooding

Overland flow is different from creek and riverine flooding as it is rainwater moving across the land towards a creek, river or drainage system. Creek flooding happens when a creek, river or waterway fills and spills over its banks.

Unlike creek flooding, overland flow can occur almost anywhere and often develops very quickly. If you’ve ever noticed water flowing down the street or pooling in your backyard when it’s raining, this is an example of overland flow.

Overland flow

  • Caused by intense local rainfall.
  • Can occur anywhere.
  • Often develops quickly.
  • Follows streets and low points.

Creek flooding

  • Caused by rivers or creeks overflowing.
  • Usually affects floodplains near waterways.
  • Usually develops over several hours or days.
  • Spreads from creeks onto surrounding land.

How to prepare for storms and overland flow

Being prepared well before heavy rainfall strikes is one of the best ways to reduce how overland flow impacts you and your property. Steps to be prepared include:

Overland flow maps

Maps will soon be published to help residents and businesses be aware and prepared for overland flow. The maps will be provided below when available.

How the mapping was developed

The mapping was developed using:

  • detailed survey data
  • aerial imagery
  • rainfall information
  • industry-standard hydraulic modelling.

This information helps engineers understand where water is likely to move and collect during significant rainfall events.

Mapping accuracy

The maps are based on the best available information and industry-standard modelling available at the time of the study.

However, they should be viewed as an indication of possible overland flow behaviour, not a prediction of exactly what will happen during every storm event.

Local factors such as fences, retaining walls, landscaping, drainage infrastructure and blockages can influence water flow during a rainfall event.

Overland Flow Path Study

The Toowoomba Overland Flow Path Study identifies major overland flow paths and areas of risk during significant rainfall. It covers urban Toowoomba, including the Gowrie Creek, Westbrook Creek, Spring Creek, Dry Creek and eastern escarpment creek catchments.

The study focuses on overland flow only.

Read the Overland Flow Path Study(PDF, 13MB).

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