Analysis

Flood defence – the debate between demountable or permanent solutions

As the UK prepares to defend against the annual flooding season, balancing capex against opex remains a challenge.

Catastrophic flooding seen across the Somerset levels last February plus the extensive and prolonged flooding elsewhere over the UK, underlined the critical need for the UK to invest in flood defences.

And while the political momentum that followed last winter’s inundations bolstered budgets allocated for flood defence projects, the practical delivery of critical schemes remains a huge challenge.

“Capital and maintenance investment in flood management should increase to meet the pressures of climate change and property development. Flood assets that protect other infrastructure networks are being made resilient to severe weather events.” ICE State of the Nation: Infrastructure 2014

Central to this debate is the balance between capital investment and operational cost. 

And as we move towards the flood prone part of the year, the decision being wrestled with is whether to invest in permanent flood defence measures or to rely on semi-permanent demountable or even temporary, modern day sandbags solutions. 

Of course, with post-recession capital budgets still hugely stretched, it is very often now a decision made for, rather than by, engineers. 

Low capital cost flood defence options start with temporary sand bags then progress to more modern options capable of holding back 1m more of flood water. Such devices can be rapidly set up on site, are better performing than sand bags and more cost effective. 

Moving one step beyond we get demountable defences which, still temporary in nature, usually involve permanent foundations and/or structural supports. However, in advance of a flood, these require an operating team to assemble the defence structure having first moved the equipment out of storage and transported it to the site.

Such temporary and semi-permanent demountable systems are now often seen as an attractive, low cost and ultimately deliverable option compared to more permanent defence schemes, which while providing lower operating and maintenance costs through requiring no human intervention or power source to deploy, are harder to get through planning and more expensive to install.

So the question across the industry remains: “is temporary or demountable flood defence a commercially viable mitigation option for the UK’s 21st Century built environment?”

And further, if we do continue to invest in demountable and temporary defence measures “how can lifetime cost be evaluated?”

"It is not a panacea by any stretch of the imagination but just another tool in the armoury of effective flood mitigation.” Keith Clarke

According to the recent ICE State of the Nation Infrastructure report published earlier in the year, the economic case for investment in new infrastructure to reduce flood risk is extremely strong compared to many other infrastructure investments.

In fact, it argues that capital flood management schemes frequently return cost to benefit ratios better than 1:8.

Pressure on the need for action is growing, it added, due to a “growing population and changing weather patterns” placing pressure on networks in both the short and long term.

“Capital and maintenance investment in flood management should increase to meet the pressures of climate change and property development,” the ICE said. “Flood assets that protect other infrastructure networks are being made resilient to severe weather events.”

Keith Clarke, ICE vice president and lead author of the recent ICE State of the Nation Infrastructure report reiterates this point but maintains that a range of tools and solutions are vital in the face of a changing climate.

And when questioned as to whether or not temporary demountable solutions are commercially viable the answer, he says, “is obviously "yes"”.

“But it requires a change in the way we look at capital and operational budgets where the capex side becomes relatively inconsequential compared with the systems infrastructure necessary to operate such temporary measures,” he explains.

“The EA has used them in special places like Iron Bridge and Shrewsbury where historic riverside views were considered vitally important and the only way to overcome planning concerns. On economic viability, use of demountable flood defence is a “maybe” in boutique type situations.” David Wilkes, Arup

“It’s a classic situation where the blurring between capex and opex is fundamental.  It is not a panacea by any stretch of the imagination but just another tool in the armoury of effective flood mitigation.” 

David Wilkes, Associate Director at Arup and Honorary Vice President of CIWEM, emphasises the need to distinguish between the different and growing types of defence on the market but suggests that anything requiring human intervention to install should be considered as a “last resort” and justified because a special view or special access route needs to be maintained.

“The EA has used them in special places like Iron Bridge and Shrewsbury where historic riverside views were considered vitally important and the only way to overcome planning concerns,” he explains. “On economic viability, use of demountable flood defence is a “maybe” in boutique type situations.” 

According to Wilkes the hierarchy of choice preferred by the Environment Agency and other flood authorities is:

  1. Fixed or passive defences which require no attention to make sure they work during the “heat” of a live flood situation
  2. The active solutions, like barriers and pumping stations or gates which need to be closed to complete the protection.
  3. Demountable defences which need to come out of warehouse, be transported to site and deployed – expensive in terms of labour and potentially fragile in overall reliability. 

 “Calculating life time cost will include initial capital costs and repair, maintenance repairing damages costs, training of staff needed for deployment, standby payments for deployment staff, then payment as and when they operate,” he added. “And then also costs of warning and of operating trigger systems.”

Atkins water sector director, Mike Woolgar agrees that while demountable defences can be a viable option he also points out that, as with any potential solution there are pros and cons, and there are circumstances where such defences work better than others. 

Demountables, he says, can of course include conventional flood gates, stop logs, proprietary kits which can be brought to site and erected, or even gates which are stored on site but which rise automatically as the threatening water level rises. 

In fact, Atkins, working for Environment Agency, used such automatic defences for the first time in this country at Cockermouth, he adds.

“The unit costs of demountables may be higher than conventional fixed defences and the long term maintenance costs may also be higher.” Mike Woolgar, Atkins.

“Part of the reason why people choose to live in many riverside towns is the beauty of the townscape and the adjoining river,” said Woolgar. “Often small market towns have period houses representing the architectural vernacular, huddling close to the river’s edge. Defending these against flooding can be difficult not least because the space available to build permanent defences – walls, embankments – is insufficient or because in constructing such defences the visual impact is permanent and damaging.”

However, Woolgar also highlights that the risks associated with demountables for towns and villages including the time needed to raise the defence effectively and time needed to deliver the kit of parts from storage to site. 

Obviously if a long river edge is to be protected in this way the time to erect can be more of a disincentive to proceed than the cost, he says.

“The unit costs of demountables may be higher than conventional fixed defences and the long term maintenance costs may also be higher.” says Woolgar.

“However clear benefit cost assessments are made of all the contributing factors, and making good use of natural or already built features between which the demountables can be inserted can provide an excellent, manageable and cost effective solution for schemes which otherwise could not happen.”

What do you think? Have your say by commenting below?

If you would like to contact Antony Oliver about this, or any other story, please email antony.oliver@infrastructure-intelligence.com.

Comments

If forecasts are good enough, the time needed to take demountables out of storage and erect them is not an issue. If they are not, investing in better forecasts may be more cost effective than building big walls.
It's not about building big walls! It is about using innovative passive systems, requiring no human intervention and Zero operational cost. Common sense solutions delivering long term protection with zero operational cost.