Swim or Surf ? : Flooding Vs Overland Flow

by Justin Eslick

We all know about flooding, but how many are aware of 'overland flow'?

What is Overland Flow?

Flooding and overland flow are two distinctly different things, the only common element is they both involve water.

The easiest way to look at them is  : flooding is water that rises from a water source and overland flow is water that flows down to a water course.

So keeping that in mind, overland flow can happen almost anywhere at any time. Don’t believe me? Go outside during a 5 minute downpour and take a look at the water running down your driveway.

The critical thing is to ensure that your property is not in an area where overland flow concentrates, which are low points and gullies. Water will find its natural course as it runs down a hill and heaviest flows are found where there is a dip in the land. It doesn’t even need to be much of a dip – only 30cm over the course of 100m can concentrate overland flow.

The trouble is, flooding is well publicised and easily researched, but overland flow is not. For most councils there are no straightforward searches you can do for this. In Brisbane the Council has a comprehensive database of flood data and a detailed flood report is available for every property affected by flooding. Only recently have they produced maps that indicate areas of overland flow. This is a recent thing and most other councils do not offer this data at all, let alone in a convenient and easy to read map.

How do you find out if you are in an overland flow path?

Assuming there is no overland flow map available, the first thing you should do is look at the contours of the property and look at the location of stormwater pipes. Read the contour map and try to imagine where water concentrations are likely to be. Also, 'zoom out' so you can get a bigger picture of the local area.

The presence of a stormwater pipe is a pretty good giveaway that you could have overland flow issues. These are almost always located in gullies. If you have a stormwater pipe over or near your property, then further research is a must!

If your research does show some potential issues then you need to speak with a civil or hydraulics engineer. They may be able to pass on some immediate knowledge, but if there are overland flow issues, then a hydraulics report is required and this could cost up to $3000 and take a number of weeks.

What are the consequences? 

Flooding and overland flow have similar consequences in that you need to build up your land or dwellings above the flood level - but it isn’t always as easy as that. You also can’t impede flow or redirect flow in a manner that could affect other properties. Imagine if you built your property up with a retaining wall to get above the flood or overland flow levels, then the water that would usually flow over your property now hits this wall and flows elsewhere, or pools in the one location. This is a big no no in the eyes of Council and Town Planning.

One other consequence people forget about is with insurance. Some insurers do not cover for any form of flooding and then others cover you for flooding, but have selective definitions that may say it will cover you where water rises and enters your property, but not where it passes through your property without having come from a water course (i.e. overland flow). Read your insurance policy carefully!

Still in doubt?  Membership of Investigate Property, the Buyer's Agency for buyer's of the best property, entitles you to consultation with a qualified Town Planner.  Click here to learn more.

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