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Adjusting a Flood Zone to Account for Climate Change Allowance


Perhaps one of the most important parts of compiling a flood risk assessment is the data it relies on. This comes from many sources and can give lots of information of flood depths, water velocity, and condition of flood defenses. All this is and more is sometimes available, and enable you to make the best possible case to the Environment Agency when writing a flood risk assessment.

If you are searching for data in London, or you are close to a Major River chances are you will get the data you need, but there are many circumstances where you will have virtually no data at all.

One such circumstance is creating an up to date flood level from outdated J Flow Data, a modeling technique developed by JBA Consulting.

J Flow data is the data used to construct the flood map for planning. But owing to uncertainties in the model and constantly changing opinions on the severity of climate change impacts the EA now often insist that a "climate change allowance" is added.

Details for climate change allowance are given on the gov.uk website as part of PPG. The allowances aim to account for increases in peak river flow account for climate change.

The most commonly chosen allowance is "Higher Central", which range from a 30% to 45% flow increases. Of course taking an existing outdated flood zone and applying this 30%, 35%, 40% or 45% increase requires some maths. It is not complicated maths, but its is time consuming.

There are various modeling programs that will do all these calculation for you. But you will need to feed the model with some data in order to do this. Using modeling software such as ISIS, you can also remodel the flood zone to account for flow increases but trying to use a complex piece of modeling software is not always that easy!

A relatively easy method is to use a cross section area method. For small channel cross section up to 50 meters, it may be possible to work out cross sectional areas on paper. Graph paper is best, as you can count squares to get area.  But for larger areas you will need to use a CAD program. This is because even for small flood plains the depth can be very small relative to the width, and the shapes are ungainly making manual area calculation very difficult.

cross section


When you have worked out the cross sectional area of the flood scenario, you then add your climate change allowance on top. Original area plus 30%, 35%, 40% or 45% depending on the allowance required. As you can see above this will then give you your buffer, which is "added" on to the existing flood zone.

It is worth noting that in an area with steep topography such as the above cross section the climate change allowance may not make much difference, but in areas where the sides of you channel / valley are more shallow, the adjustment could lead to some unwanted loss of you site to flood plain.

Using this method and other SWEL can redraw flood maps to account for climate change allowances. We can redraw / adjust Flood Zone 1, Flood 2 and Flood Zone 3 Lines. If you would like our help please call for a free chat / complementary consultation.