Overland flow paths

​​
​​

Overland flow paths are the predicted routes stormwater takes on its way to streams, stormwater networks or the sea during an usually large rain event.

We've added information about overland flow paths to Land Information Memorandums (LIMs) for properties in our district.

We've produced an overland flow path map which indicates where water is likely to flow and accumulate during an unusually large rain event when natural or man-made storm water networks are overloaded. 

The LIMs will also identify depression areas which may be prone to accumulating water during such an event.

 Media release - Stormwater flow paths included on LIMS

Before you take a look at this map, there are a few things we want you to know.

Disclaimer: guidance note to overland flow paths​

The overland flow path represents the predicted path of storm water associated with an unusually large rain event and has been derived with all above ground structures stripped from the LiDAR*.

Some of those structures (e.g. fences and buildings) may divert the overland flow path, producing delineations that differ to those presented in the information. Any rain event that exceeds the capacity of the Council's stormwater infrastructure will cause varying levels of inundation.

Assu​mptions

As no two rainfall events are the same, a theoretical uniform rain event was assumed for the analysis in order to simulate a rainfall event that would constitute an unusually large rain event.

The stormwater network would be unavailable during the event owing to inundation.

The above assumptions may not reflect the catchment behaviour during an actual rain event of a similar magnitude owing to rainfall duration and concentration; e.g. some areas may flood in a specific event but not in a subsequent large event and vice versa.

Accordingly, while realistic in some areas, it may be over-conservative in other areas depending on the interaction between the above ground conditions and the stormwater network. The full effect of an unusually large rain event has not yet been analysed.

The presence of an overland flow path and an extreme rain event does not necessarily equate to a flooding event which is defeined as an instance when an overflow of stormwater from the Council's stormwater system that enters a habitable floor. Depression areas may only partially fill during an unusually large rain event.

In order to understand flooding events the Council is using the overland flow path information to do further analysis on water levels and flow velocities to ascertain the number of habitable floors that are at risk; that information is not yet available.

Recommendation

Council recommends further overland flow path information enquiry be made with the Council's Infrastructure team or with a qualified professional.

*LIDAR, which stands for Light Detection and Ranging, is a remote sensing method that uses light in the form of a pulsed laser to measure ranges (variable distances) to the Earth.

These light pulses, combined with other data recorded by the airborne system, generate precise, three‐dimensional information about the shape of the Earth and its surface characteristics.

Page reviewed: 11 Aug 2016 1:22pm