Alternative way of reducing flood damage
Traditional underground drainage systems might not always be the best way to protect us from flooding, writes Dr Alan O'Connor
Recent flood events have given people a renewed sense of their long relationship with rivers. Fertile river plains have been the desired location for human settlements since time immemorial. Though as urban settlements have evolved, rivers have become rather divorced from normal life.
We might even find residents entirely oblivious to smaller rivers in built up areas, conveyed in dark culverts under roads and city buildings.
The intensifying phenomenon of climate change has reminded us of the power of water and how limited our ability is to eliminate flooding. Instead we must find innovative engineering solutions to mitigate its effects.
In this regard, our response must first be to protect human life, then to retain the life-enhancing qualities that make rivers so important to us - the leisure activities, their ecology, flora and fauna.
Significant research focuses on risk analysis, quantification, mitigation and optimisation, and on the provision of appropriate flood mitigation measures.
The focus in risk-based approaches, such as those developed by the pan-European RAIN (Risk Analysis of Infrastructure Networks in Response to Extreme Weather) project - www.rain-project.eu, led by Trinity College Dublin - has been on quantifying the probability and intensity of extreme weather events likely to initiate flooding, and the development of processes to assess the risk to local infrastructure.
A third focus is on developing alternative engineering measures to reduce the risk to an acceptable level.
The result of these is the identification of the best possible measures from both economic and human perspectives.
A central consideration in the application of risk-based approaches is on assessing the impact of alternative engineering measures on flood risk and flood management. Ever-increasing river bank flood defences offer the primary defence to river flooding, but innovative projects such as the CEDR (Conference of the European Directors of Roads)-funded WATCH project (Water Management for Road Authorities in the Face of Climate Change) concentrate on assessing the costs and benefits of innovations in water management being employed by European roads administrations.
Central to the approaches considered in the WATCH project is the concept of "natural floodplain management", espoused by the philosophy of Sustainable Drainage Systems (SuDS).
"The SuDS philosophy is to mimic the natural hydrological cycle," explains John Paul Rooney, an expert in SuDS systems and leader of this topic for the WATCH project.
"SuDS offers a return to nature by promoting the temporary storage of surface water in ponds, infiltration, the harvesting of rainwater at source, evaporation, groundwater recharge and the re-use of stormwater.
"In Ireland we tend to think of SuDS as primarily a flood management technique.
"In countries where it is more firmly established, it is also seen as a way of making our cities and towns more attractive, which helps them to attract greater investment."
The purpose of SuDS is to protect us from flooding, enhance our social amenity, drive economic growth and protect and enhance the quality of our rivers and streams.
Nowadays flooding disasters and water pollution are key drivers of the SuDS policy.
These effects are so detrimental to life and to the integrity of our infrastructure that their mitigation is underpinned by legislation - principally the Water Framework Directive.
This directive encourages the retrofitting of SuDS to improve water quality and to reduce combined sewer overflow discharges in urban areas.
Climate change predictions suggest that some types of extreme events will become more frequent - such as heat waves, flooding caused by extreme rainfall, and drought.
The SuDS approach is more robust and adaptable than the traditional approach of underground piped drainage systems. In shallow, grassed, surface-based systems, water levels rise gradually and visibly. When the capacity of the SuDS feature is exceeded, the excess water can be directed to safe storage zones.
This allows the general public and local authorities to prepare for flood events more effectively. Conversely, flooding from underground piped drainage systems can occur suddenly and rapidly when the design capacity is exceeded.
Shallow, visible surface based systems can also be designed to offer greater flexibility to adapt to climate change. SuDS systems can be enhanced more readily and cheaply than underground drainage systems. Lower river flows caused by drought result in the reduced dilution of pollutants following rain. The treatment of surface water run-off through SuDS helps to protect and enhance the quality of receiving watercourses.
America, Australia, The Netherlands, Germany and parts of the UK, among others, have been implementing SuDS for the past 20 years, supported by legislation across all levels of government.
In these countries, the benefits of SuDS have been recognised and are more visibly supported by legislative, social and political factors. International research has shown that the main drivers of SuDS are environmental outcomes, public health benefits, social amenity and community perceptions. Collaboration between various agencies is seen as fundamental to the successful delivery of SuDS schemes.
Ultimately, as engineers our focus must be on the development of solutions to mitigate the effects of flooding.
These solutions lie in our ability to quantify the risks of flooding, and also to determine engineering measures that reduce risk to acceptable levels while also making them the best systems we can provide for the money invested.
In this regard, we must as a society accept that risk is always present: rather than chasing the impossible task of trying to eliminate it completely, we must define acceptable risk and work to provide measures that operate within that level of risk.
We must also consider system resilience, in other words risk not only at a point in time, but also as a function of time - whereby we consider not only our preparedness for the event, but also at the way we respond to it and the speed of our recovery from it.
Only by adopting an objective quantitative basis in applying these approaches will we maximise the potential success of the significant state investment in flood defence measures to maintain flood risk at or below acceptable levels.
Dr Alan O'Connor FTCD, CEng FIEI, Associate Professor (Civil Engineering), is Director of Undergraduate Teaching & Learning, School of Engineering, Trinity College Dublin.