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 natural events, the only common element is they both involve water.
The easiest way to look at the difference is: flooding is water that rises from a water source and overland flow
is water that flows down to a water course.
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.
It is critical to ensure that your property is not in an area where overland flow concentrates, which are generally 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 council
areas there are no straightforward overland flow searches you can undertake, or mapping to view. Brisbane City Council has a comprehensive
database of flood data and a detailed flood report is available detailing flood depths for every property affected by flooding. However,
whilst we can view indicative overland flow paths on maps, for most properties the depth or severity of the overland flow can’t be provided
so you really have no idea whether it will prevent or effect development.
How do you find out if you are in an overland flow path?
Assuming there is no overland flow map available in your council area, 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' on your search area maps so you can get a bigger picture of the local area.
The presence of a stormwater pipe is a good indication that you could have overland flow issues. These are almost always located in gullies.
If you have a stormwater pipe within or near your property, then further research is critical!
If your research does show some potential issues with overland flow, then you need to speak with a civil or hydraulics engineer. They may be
able to pass on some immediate knowledge, but if overland flow issues are identified, then a hydraulics report will be required and this
could cost in excess of $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 unacceptable in the eyes of Council and Town Planning.
One other consequence of overland flow people often neglect to consider is insurance. Some insurers do
not cover for
any form of flooding and then others may cover you for flooding, but have selective definitions for flooding. For example, insurers may
stipulate that cover is provided 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