A model of perfection

30th January 2018

Angus Middleton assesses the use of tools for natural flood management.

There are many tools out there that help us understand how nature can reduce flooding, but how useful are they? I would suggest not very – not because of problems with the tools, but how we are using them. We are getting lost in the joys of modelling and losing sight of the goal.

The tools can be put into two camps: map analysis (geographic information systems), and mathematical flow calculations (hydrology). The former entails creating maps that show certain characteristics, such as habitats and soil type, to identify locations that possess more or fewer target characteristics. In this way, a location with a steep slope, poor vegetation and clay soil would be identified as unable to store water during heavy rain. This would be given a high opportunity rating for natural flood management (NFM): improving its water-holding abilities, perhaps by planting trees, should lessen local flooding.

But imagine that a large marshy area is at the bottom of the slope. When it rains heavily, the water will indeed pour down the hill almost instantly and deluge the area below. But this area is marshland, so will absorb the water and stop it flooding into the river (for a while). Planting trees on the slope will therefore slow the water entering the marsh, but will have almost no effect on how much enters the stream to cause local flooding. We would have spent considerable time and money on creating a lovely hillside forest, but gain no reduction in flooding.

This may sound simplistic, but it could explain why there are such mixed results from NFM projects, which commonly use GIS alone to identify where to make NFM interventions. Even when hydrology is used to understand the degree of flood reductions that these interventions will make, it is generally GIS that identifies where to place the interventions in the first place.

Hydraulic modelling mathematically calculates how water flows down hills and into rivers, then down those rivers to flood settlements (for instance). The models generally try to say how much water will flow past a given point during a given storm event. This is then combined with other information to determine flood levels in and around the river. It does not show where interventions should be placed, or what they should be. This usually has to be identified using other techniques, then these changes put into the hydraulic model to see what happens.

The equations are complex and require detailed input data: any errors can significantly skew results. There is also a problem with assumptions. Many assessments look at a one-in-100-year storm event, during which it is assumed that rain will fall in ‘these’ locations and ‘these’ volumes over ‘this’ period. It is far from certain how well these relate to real-life weather events.

Hydraulic modelling is sophisticated and accurate, but how meaningful is it given a capricious natural system? Does it make sense to model using ‘these’ assumptions and ‘this’ specific storm? NFM is all about using nature, so the solutions themselves will change over the decades. Surely it is wiser to use models that reflect this and work with the data uncertainties of landscapes, averaging results over the lifetime of the natural solutions rather than suggesting an impossible degree of certainty.

GIS and hydraulic modelling are both excellent tools, but for NFM they should be used in the right ways. GIS works best for broad-brush concepts at large scales – such as where NFM is likely to be valuable – rather than informing local actions. Hydraulic modelling is perfect for scenario analysis – the ‘what ifs’ of specific situations – but is not so good for showing what changes to make to deliver most benefit under real conditions.

The answer is to combine hydraulic and GIS modelling into a single system to prioritise NFM actions across catchments. We should then quantify the expected reduction in flooding as an average over the lifetime of the habitats, since this is where NFM functions most efficiently. Generating more exact results for specific storms will give a false sense of accuracy at unnecessary cost.

This pragmatic approach will make NFM prioritisation modelling sufficiently cheap and accessible for every project to use, and help people understand the likely benefits in meaningful terms.

Angus Middleton is director at Viridian Logic


Subscribe to IEMA's newsletters to receive timely articles, expert opinions, event announcements, and much more, directly in your inbox.

Transform articles

SBTi clarifies that ‘no change has been made’ to its stance on offsetting

The Science Based Targets initiative (SBTi) has issued a statement clarifying that no changes have been made to its stance on offsetting scope 3 emissions following a backlash.

16th April 2024

Read more

While there is no silver bullet for tackling climate change and social injustice, there is one controversial solution: the abolition of the super-rich. Chris Seekings explains more

4th April 2024

Read more

One of the world’s most influential management thinkers, Andrew Winston sees many reasons for hope as pessimism looms large in sustainability. Huw Morris reports

4th April 2024

Read more

Vanessa Champion reveals how biophilic design can help you meet your environmental, social and governance goals

4th April 2024

Read more

Alex Veitch from the British Chambers of Commerce and IEMA’s Ben Goodwin discuss with Chris Seekings how to unlock the potential of UK businesses

4th April 2024

Read more

Regulatory gaps between the EU and UK are beginning to appear, warns Neil Howe in this edition’s environmental legislation round-up

4th April 2024

Read more

A project promoter’s perspective on the environmental challenges facing new subsea power cables

3rd April 2024

Read more

Senior consultant, EcoAct

3rd April 2024

Read more

Media enquires

Looking for an expert to speak at an event or comment on an item in the news?

Find an expert

IEMA Cookie Notice

Clicking the ‘Accept all’ button means you are accepting analytics and third-party cookies. Our website uses necessary cookies which are required in order to make our website work. In addition to these, we use analytics and third-party cookies to optimise site functionality and give you the best possible experience. To control which cookies are set, click ‘Settings’. To learn more about cookies, how we use them on our website and how to change your cookie settings please view our cookie policy.

Manage cookie settings

Our use of cookies

You can learn more detailed information in our cookie policy.

Some cookies are essential, but non-essential cookies help us to improve the experience on our site by providing insights into how the site is being used. To maintain privacy management, this relies on cookie identifiers. Resetting or deleting your browser cookies will reset these preferences.

Essential cookies

These are cookies that are required for the operation of our website. They include, for example, cookies that enable you to log into secure areas of our website.

Analytics cookies

These cookies allow us to recognise and count the number of visitors to our website and to see how visitors move around our website when they are using it. This helps us to improve the way our website works.

Advertising cookies

These cookies allow us to tailor advertising to you based on your interests. If you do not accept these cookies, you will still see adverts, but these will be more generic.

Save and close