Flood defence: Get the right product

Climate change is placing a strain on traditional drainage. But the building industry is still wrestling with the implications of having to design and construct flood alleviation systems. Michelle Fleming looks at the issues at stake.

THE UK’S climate is now characterised by long, dry periods interspersed with shorter but intense bursts of rainfall.

In parts of southern England, the rainfall has been nearly a third below average for 23 consecutive months. It’s Britain’s worst drought in nearly a century. Kent, one of the most parched counties in the UK, had 186% of average rainfall – nearly double the norm – in August.

These shorter, heavier bursts of rain are resulting in large volumes of water over a short period of time, volumes that are potentially capable of overwhelming traditional drainage systems.

Climate change is altering rainfall patterns and effectively creating long periods of water shortage interspersed with short and intense episodes of flooding like the catastrophes that affected the Cornish town of Boscastle in 2004 and Carlisle in 2005, both of which were devastated by flash flooding.

Flooding, of the type that affected more than 3,000 properties in Carlisle, was once regarded as a once every 50 years type of event. Unfortunately, flooding on that scale is now occurring every five to ten years.

These shorter, heavier bursts of rain, coupled with the increase in impermeable surfaces, such as parking areas and roads, have combined to make surface water quickly run off into drains and then into nearby streams and rivers.

As a result, traditional drainage systems receiving the run-off have to cope with larger and more intense flows, creating an increased risk of flooding as they become overloaded.

This increased risk of flooding, both in developments and downstream, has led to an increased focus on developing sustainable urban drainage systems (Suds) to promote efficient soak-aways for infiltration, and offer a temporary storage for excess water.

Both Planning Guidance (PPG25) and Building Regulations (Part H/M) now promote the use of Suds with the objective of restricting the run-off to a greenfield site equivalent, while improving the run-off quality and amenity of a location.

In recent years, one of the most popular Suds techniques has been the geocellular flood alleviation box, or crate as they are often known. These systems have the capacity to manage larger and more intense flows of water, and offer a temporary store for run-off water to ensure that traditional drainage systems are not overloaded.

Critically, flood alleviation boxes can be used to infiltrate or for storage. Having collected the stormwater, the boxes can either allow it to gradually empty and seep back into the ground to replenish the natural groundwater and aquifers, or, if ground conditions are not suitable for soak-away, store the water and release it into the traditional drainage system in a controlled manner.

Effective planning

The popularity of modular flood alleviation systems has also been driven by their lightweight, easy-to-install and high-capacity characteristics. This is particularly the case when compared with the concrete alternative, which is neither lightweight nor as easily installed.

There is, though, still a lot of work to be done. There are two areas in which much more work is needed. Firstly, at a macro level, the building industry has to plan much more effectively for the use of flood alleviation systems.

Figures from the Environment Agency suggest that half of all planning applications are currently turned down because the plans do not contain any assessment of flood risk or alleviation system. This is an absurd waste of time and resources.

Secondly, at a micro level, the phenomenal growth of geocellular products as a method of flood control has led to problems when lower load-bearing capacity products have been used in trafficked applications.

There are now more than 15 different geocellular flood products available on the market, many of which originate from the Continent. The load-bearing capacity of each product is different and ranges from 90 tonnes/m2 down to less than 10 tonnes/m2.

This is a crucial difference as products in the lower end of the load-bearing capacity range are in effect only suitable for landscaped applications.

Unfortunately, the distinction between the different flood alleviation products available is often inadequately communicated. Too often it is assumed that all the products available are broadly the same.

This has led to a one-product-fits-all applications culture developing, regardless of application suitability. The result has been a number of system failures in recent years, usually involving lower load-bearing capacity products being used incorrectly below car parks.

The blame for this lies with neither the product nor with specifiers, subcontractors nor groundworkers. It is down to the lack of guidance offered about usage and installation to those using these systems.

Unfortunately, the correct information is rarely made readily available for the subcontractor to make a decision regarding product type and applications.

Decisions on the application of these structures cannot be left solely in the hands of the end user. Manufacturers and specifiers have a responsibility.

Climate change is making flood alleviation an increasingly vital issue for all those involved in the construction industry, from architects and specifiers to subcontractors and groundworkers.

We all need to do a lot more work to ensure that flood alleviation systems are not only included in planning applications but that the different products are used correctly.

Michelle Fleming is head of marketing at Wavin Plastics.

T: 01249 766600.

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