After the Rain – How to design for and manage increasing flood risk
17th September, 2021

By Partner in Charge of Civil Engineering at Price & Myers, Dimitris Linardatos and fellow Partner, Andy Toohey.

The summer of 2021 has seen extensive and severe flooding in the United Kingdom, Turkey, China, New York City and across Europe. It has been dramatic, expensive and at its worst, deadly. Extreme weather events such as these have been increasing in frequency and intensity. Recent downpours have inundated buildings and streets, old and new, that haven’t flooded in living memory or even in historical record. How can engineers and architects design to help mitigate the risks and lessen the impact of ever-more frequent and severe flooding?

Why do cities flood?
Many cities and towns have been constructed around watercourses such as rivers and lakes, so their populations can have easy access to water. As a result, natural floodplains have been replaced by buildings, roads, and other structures. The floodplain is generally the area of low-lying land adjacent to a watercourse which is always more liable to flood than areas of higher ground. Buildings and impermeable areas have now replaced large swathes of land that made up natural floodplains. The water that used to infiltrate the ground now discharges into the watercourses, as buildings and hardstanding areas prevent infiltration. The total volume of water discharging into rivers has increased significantly when compared to the transfer of water under natural conditions. This in turn has increased the flood water depths within floodplains. Including grass, vegetation and soft landscaped areas in our designs can slow down the overland flows travelling towards watercourses, helping to mitigate the loss of natural floodplains that once dealt with water flow.

Old drainage systems
Traditional surface water drainage systems, based on pipes and manholes, were designed to transfer water from roads, buildings and other structures to watercourses such as rivers and bays, as quickly as possible. The impact of climate change has meant an increasing volume of water is travelling at ever-increasing speed, often overwhelming existing drainage systems. The problem is exacerbated by the increasing storm severity that tears leaves and other foliage from the trees, to be washed away, blocking drainage points.

The gutters, downpipes and flashing details in older buildings are being overwhelmed, having simply never had to deal with the intensity of rainfall. In some cases, the impact of this will be immediately clear – in others the effect may be insidious with damp entering roofs and walls where the impacts may only become apparent over time. Detailed checking of existing capacities needs to become standard practice. Where ornate original cast iron or lead rainwater goods cannot be enlarged or replaced, ‘failsafe’ overflows need to be introduced to ensure that whatever the situation, water does not enter buildings.

But it’s not only older buildings that are vulnerable when the volume of water, and the speed with which it arrives, goes beyond what has been ever been designed for. In recent decades we’ve become ever more ingenious in adding area below existing buildings and building multiple level basements. These low-lying spaces are inherently vulnerable, and typically totally reliant on pumped systems for all their drainage. Ever larger pumps and sub-basement chambers are needed – particularly where low level lightwells are created that receive rainwater.

New developments
As cities and towns expand, the distance between the newly developed areas and the watercourses is increasing. Often it is not financially viable to construct new drainage infrastructure for connecting city and town expansions directly to watercourses and rivers. As a result, surface water from city and town expansions is collected into the existing infrastructure which has been constructed decades or even centuries ago under less rigorous standards, and often without making allowances for future development. The existing drainage infrastructure cannot cope with the volumes and flows of surface water that urbanised areas generate, leading to surface water flooding.

New technology, accessibility, and maintenance
Progress to mitigate flood risk has been made. Design regulations are now in place aiming to promote the use of Sustainable Drainage Systems (SuDS) for surface water drainage from new developments. SuDS are drainage solutions which aim to reduce run-off rates and volumes to watercourses from new developments. Systems such as swales, ponds, soakaways and permeable paving provide storage for surface water and drain into the ground where geological conditions permit infiltration. In soils with poor infiltration properties, SuDS can be used for surface water attenuation. Water can be stored in SuDS during the storm and slowly discharge into the existing infrastructure or a nearby watercourse. Current design standards in the UK aim to achieve greenfield run-off rates from new developments to receiving watercourses that mimic natural conditions. The expected lifetime of a new development should be considered, to assess the impact of climate change on the new drainage system, and according allowances made to the drainage design.

While the drainage systems for recent new developments have been designed based on the latest standards, and SuDS have been incorporated in the design, some new developments have also been affected by the recent flood events. The recorded rainfall that we have recently experienced was not in excess of the rainfall intensities which are used in the drainage design for these developments. 

However, as the old infrastructure floods it is unable to collect even low flows from new developments. In other instances, reverse flows from the old infrastructure entered new drainage systems, flooding new developments which are situated in low lying areas. The topography can direct large volumes of flood water from the old infrastructure into newer properties, which unfortunately cannot be stored into the new drainage system. New developments also have to comply with accessibility regulations that can further complicate flood risk. Level, accessible entrance thresholds are generally a requirement for new buildings, but they are also very vulnerable. Positive and meaningful slopes away from entrances should be standard.

We also need to be realistic about maintenance. Everything we design needs to work for decades in the knowledge that it might receive little or no maintenance. So, while a neat little slot drain might look nice for a while, can we be sure that it will be flowing freely when we really need it? Access points for maintaining drainage need to be accepted as a necessity and designed in – not omitted altogether as is sometimes the case.

Get civil engineers involved early
A new drainage system which is designed based on the latest standards will not necessarily prevent flooding. Towns and cities have been constructed in floodplains and will flood in extreme storm events, which will become more frequent in the future due to climate change. An easy solution to the problem would be to direct new development away from the floodplain and areas with inadequate drainage infrastructure. However, this will direct new development in undeveloped land which is not sustainable. As a result, new development in flood-risk areas is unavoidable. Civil engineers need to get involved in the master-planning, layout and building arrangements for new developments, aiming to direct flood water away from habitable and sensitive areas. Ensuring that the site is ‘perforated’ to direct and allow water to escape or gather in appropriate areas will limit any negative impact. It’s essential that we make space for water. This needs to happen early in the design process, not as an afterthought. Technical input for the layout of the development and building arrangements is as important as the planning/architecture for sites in flood-risk areas.

Strategic thinking
Of course, an engineer’s contribution to flood risk management is limited to their projects. Local authorities’ studies and policies on flood management must be reassessed to tackle flooding at strategic level. Now that people are moving towards more flexible working patterns because of the pandemic, there is an opportunity to reassess local plans and development policies. The government’s investment in infrastructure will help to improve commuting distances and times in the future. There will be less demand for office and commercial space in city centres, and an employer’s address will not necessarily dictate where people decide to live. This creates the opportunities for converting brownfield sites to open spaces and parks in densely urbanised city centres. This approach will allow for open spaces designed to store flood water during extreme storm events, reducing the flood risk at a strategic rather than local level.

Think Different, Be Bold
We need to consider deeply the way we’ve done things in the past and challenge received wisdom and the status quo. As with many aspects of our work that are now impacted by climate change, too often the question has been ‘how can we do this?’ - now the question needs to be ‘should we do this?’. When it comes to creating spaces that are vulnerable to flooding, just putting in more and bigger pumps can’t be the answer when there is nowhere to pump that water. This is not sustainable environmentally, financially or logistically.

Civil engineers, structural engineers, architects, planners, local authorities, and central government need to change the way they think. Standard practice should start from a position of imagining a scenario where the pumps fail, the drains are blocked, and the mains drainage can’t accept any more flow. Where does the water go? How can we be sure it doesn’t come into our buildings, and flood-sensitive areas like homes, schools and hospitals? Most importantly, where can open space be allocated for water to be stored and to flow during a storm? Answer those questions effectively and we will save homes, money, livelihoods, and lives.