A five step strategy for mitigating the effects of flooding.
Planning for flood risks is a priority around the world as the effects of climate change, increasing urbanisation and construction on floodplains grow. Changing weather patterns have exacerbated localised flooding, and the increasing construction of impermeable areas means flood peaks are sharper and the catchment response time shorter, and therefore less easy to mitigate.
Flood damage is costly: reinsurers Munich Re report that the costs of natural disasters reached $175 billion in 2016, with devastating floods in China costing $20 billion alone. Munich Re also highlighted the ‘exceptional’ number of flood events, including river flooding and flash floods, which accounted for 34% of overall losses without considering the substantial personal trauma.
The following five steps provide a strategy for mitigating the effects of flooding that considers costs, water storage, time to site and the current state of the local environment.
Participation in flood alleviation projects requires consultants to consider a number of key issues, notably determining the suitability of solutions and products for at-risk areas. Each location has its own unique, possibly complex causes for flooding and, consequently, requires careful assessment to identify sustainable and appropriate remedies.
Considerations include isolating pluvial (surface) and fluvial (riverine) causes of flooding as well as potential groundwater contributions and, in coastal areas, a range of issues such as erosion, storm surges, high tides and storm damage to coastal defence structures. All of the above issues may interact in a catchment, so surveys and modelling are essential to determine appropriate solutions.
Assessing the surrounding environment of at-risk areas enables identification of whether and how it can be employed as part of a risk mitigation strategy. For example, fluvial flooding provides opportunities for natural flood management upstream, such as planting, improved soil management, restoration of wetlands, and the use of fallen trees as dams.
Pluvial flooding, which occurs when the capacity of the drainage system is exceeded, is closely related to the growth of the total area of impermeable paving. In urban areas, sustainable drainage solutions (SuDS) - known as Best Management Practices (BMPs) and Low Impact Development (LID) in the US, and Water-Sensitive Urban Design (WSUD) in Australia and the Middle East - are increasingly being used to slow the speed of runoff, divert collected rainfall back into the natural environment and ease system capacity constraints.
These embrace solutions such as swales and retention ponds, as well as permeable paving teamed with underground storage systems. UK research body CIRIA has produced a SuDS manual that outlines cost-effective planning, design, construction and maintenance for these systems. The US EPA has similar guidance for US-based projects.
Time to site is an important consideration, as is ease of build, both of which must be balanced against the overall costs of the project. To achieve this balance requires collaboration with clients and regulators (sometimes one and the same body) and key members of the supply chain from the earliest possible stages of a project, where feedback on feasibility and the acceptability of potential solutions are critical.
Technologies that achieve the principles of sustainable drainage can also play a significant role in diverting and attenuating flooding. Vortex flow control systems have been employed extensively as a large-scale protection solution at watercourse level, holding back excess floodwaters in temporary flood storage areas, as well as within drainage projects to provide control and balancing of flow rates and volumes across a catchment.
Engineered options include embankments and levees, and for urban areas demountable barriers and grass floodwalls, the latter offering a less visually intrusive solution. Below-ground storage has been widely deployed as a solution, with a further option being in-pipe storage in oversized collection systems.
Smart monitoring can provide vital hydrometric data for early warning of flood events, with wireless connectivity now providing web-based outputs.
There are potential pitfalls when mitigating effects of flooding – for example, measures to hold water upstream of an urban area may increase the incidence of upstream flooding, and urban barriers may increase the water speed and amount of downstream erosion.
Advance optioneering is crucial to ensuring potential knock-on effects are identified and eliminated – for example, by running computer simulations with the proposed solutions in situ to determine all onward effects.
Design engineers and contractors alike need to deliver a project that fundamentally mitigates the effect of flooding. With other responsibilities and concerns, such as time to market, budget allocations and even climate change, it is crucial to stay one step ahead. Putting a strategy in place, that is built around effectiveness and reliability, will help to plan for flooding while remaining compliant with environmental and regulatory requirements all at the same time.
When planning a strategy to mitigate the risks of flooding, design engineers need to scope out potential risks that their environments pose, identify potential causes of flooding and determine what products or systems would negate these risks best.
Whenever a new project comes about, the important balance between time, costs and ease of build must be met. Any system created to mitigate the effects of flooding must be within reason, leaving finances spare for crucial maintenance and operations.
When planning your strategy, reach out for advice from the experts who can help you to run computer simulations of your chosen solutions in situ so you can determine potential pitfalls, as well as successes.
Mark Goodger explains how we help engineers and contractors to design and deliver more effective flood mitigation projects: