Abstract: This paper examines the necessity for innovative drainage solutions in response to the increasing frequency and intensity of rainfall during a time of rapid climate change. Conventional methods used for designing drainage systems are no longer adequate. Traditional gravity-based drainage systems often struggle to cope with extreme weather events, leading to flooding and significant economic, environmental, and societal impacts. The conventional approach of calculating return periods for extreme rainfall events through the study of the Clausius-Clapeyron relationship does not provide a sufficiently robust modification to explain high intensity-short duration events associated with higher temperatures and greater convective downpours. Case studies corroborate the increased occurrence of such events at several locations and indicate that such events are becoming increasingly damaging both to infrastructure and human safety.
Advances in attribution studies and the use of improved forecasting models will assist in projecting future drainage needs, but, in the medium term, technical solutions, such as increased use of siphonic drainage systems, will be necessary to provide resilience against increased rainfall intensities. The paper demonstrates how traditional extreme weather calculation approaches are inadequate for the current rapid climate change and argues that siphonic drainage systems are a superior alternative to traditional gravity drainage systems in buildings, highlighting their efficiency in managing large volumes of water, higher flow rates and greater resilience in extreme weather conditions due to climate change.
Rainwater drainage systems conduct rainwater away from roofs, gutters, and valleys to distribute rainfall to below ground systems efficiently and effectively with the principal purpose to prevent flooding.
Traditionally, roof drainage systems use gravity for water to flow from the roof to a pipe system that ultimately leads to a larger scale network and discharge location. Since the gravity pipe system operates on a partially full basis, design considerations for such systems are intended to prevent capacity being breached due to obstacles around the roof drainage outlet.
Gravity-based drainage systems operate by channelling water away from areas using natural slopes and elevations. While effective under normal weather conditions, they are susceptible to failure during extreme rainfall events, leading to potential flooding, property damage, and environmental degradation.
This paper argues for a shift towards siphonic drainage systems, which utilise the principles of siphonic action to achieve full-bore flow, thereby maximising the efficiency of drainage under high rainfall conditions. A comprehensive understanding of both systems is crucial for developing resilient infrastructure capable of withstanding the impacts of climate change.
Significant evidence has emerged that global climate change is altering the pre-existing magnitude and frequency of extreme rainfall events (IPCC,2021). Traditional gravity-based drainage systems may no longer be appropriate to cope with the new climate regimes emerging.
Often, gravity systems are designed around a very short rainfall return period with minimal design considerations as regards future rainfall conditions. Accordingly, standards for drainage systems require new design considerations that represent an evolution of the traditional return period approach.
The frequency of extreme weather events, particularly intense rainfall, has increased due to climate change and necessitates a re-evaluation of current infrastructure, especially drainage systems, which are critical in managing excess water. Traditional drainage methods, primarily gravity-based systems are increasingly proving inadequate to protect buildings in the face of unprecedented rainfall intensities and volumes.
Climate warming implies a shift to greater convective rainfall as confirmed by recent climate reports. In its sixth Assessment Report, the Intergovernmental Panel on Climate Change reported that global surface temperature over the period 2011-20 was about 1.1oC higher than 1850-1900. Since 1970, global temperatures have increased faster than any other 50-year period over the last 2,000 years (IPCC, 2021).
Warmer oceans facilitate greater evaporation, and warmer air can hold greater amounts of water vapour. This potentially leads to an increase in rainfall in a warmer world. Of the three types of rainfall, it is convective rainfall that is expected to increase most (Dai et al. 2020). Convective rainfall is the primary cause of short-term rainfall extremes.
The Clausius-Clapeyron relationship
The capacity of the atmosphere to hold water vapour increases by a rate of 7% per degree Celsius of warming. This is known as the Clausius-Clapeyron (CC) relationship. In Ireland, annual average temperature for the period 1991-2020 increased by 0.7oC over the corresponding 30-year period 1961-90. This was accompanied by a 7% increase in annual rainfall. In the UK, the corresponding figures were 0.8oC and 7.2%, relatively close to that predicted by the CC relationship.
The conclusion that increases in extreme rainfall intensities above the CC relationship will follow higher temperatures provides further evidence that climate change will produce significant increases in short term (sub daily) rainfall yield on occasion. This further amplifies the need for roof drainage systems to be designed to cope with short term events beyond the thresholds that were applicable in the past.
Attribution of extreme rainfall events
A breakthrough in climate science has occurred in recent years, facilitated by the increased capability of high-speed computers to run multiple climate models. This has provided a capability for quantifying the extent to which a particular extreme event was rendered more probable because of anthropogenic climate change.
From the turn of the century, and especially in the last decade, attribution studies have been performed for different weather events (eg, Christidis et al, 2015). For an extreme occurrence, such as a heatwave or rainfall event, identical climate models are run under two scenarios.
First, multiple runs are made with pre-industrial levels of greenhouse gas loading in the atmosphere, recording how often a particular extreme typically would be observed.
Second, multiple runs are made with realistic levels of greenhouse gas each year added. The frequency of the same event is recorded and compared with the ‘pre-industrial’ frequency. This enables an estimate of how much anthropogenic climate change has shifted the odds in terms of severity and frequency of the extreme event concerned.
Attribution is more robustly achieved with temperature extremes than with rainfall, which is inherently more localised and a response to sometimes quite localised thunderstorm development. Attribution studies have however also been conducted on some extreme rainfall events.
Case studies on extreme rainfall events
Several case studies can be used to analyse and illustrate the increase in frequency and duration of extreme weather events, as well as overall increase in rainfall amounts throughout the world due to climate change.
- In October 2023, Co Cork experienced two days of extreme rainfall from Storm Babet that fell on already saturated soils. Damage costs amounted to approximately €200m. In Midleton, many areas received more than 100mm of rainfall in two days. The attribution study concluded that the probability of a two-day October rainfall at least as high as this has more than doubled in likelihood, and increased in intensity by about 13%, due to global warming since pre-industrial levels (Clarke et al, 2024);
- In July 2021 extreme rainfall across western Germany and the Benelux countries resulted in severe flooding and more than 200 fatalities. Rainfall accumulations exceeded 175mm over a 48-hour period with the greatest falls occurring in an area between Luxemburg and Cologne. Several regional climate models were used to assess the extent to which climate change affected the frequency and severity of the event. It was concluded that for locations across a wide swathe of western Europe, a similar event could be expected every 400 years under current climate conditions (Tradowsky, JS, 2023). Climate change had made such an event 1.2 to nine times more likely with further increases in frequency and intensity projected to occur as warming continued. This event was one of a number of rainfall events between 2002 and 2021 with damage costs exceeding €12.6bn. Berlin, for example, suffered damage to one in seven homes over this interval;
- On September 3, 2023, a cut-off low-pressure system became slow-moving over central Spain and produced extreme rainfall both on the south coast and the Madrid area. In San Rafael, near Madrid, 158mm of rain fell, leading to extensive flooding across the region of Madrid. While it is not clear over how many hours the rainfall occurred, the extreme nature is apparent when the event is compared with an IDF table developed by Casas-Castillo et al (2016) (Table 1) and subsequently used to estimate a value of the one-day extreme for Madrid. 158mm is well in excess of the 500-year event.
Table 1: Estimated maximum rainfall intensity (in mm/h) for every duration t and return period T for Retiro Observatory of Madrid plus PMP values in for every considered duration in minutes.
Deep areas of low pressure in the Mediterranean Sea occasionally acquire characteristics similar to a tropical cyclone. Unlike tropical cyclones they have less warm water to intensify over and so most such features, termed medicanes, seldom last for more than a few days but are capable of producing extreme rainfall and flash floods.
Medicane Daniel originated off the west coast of Greece in early September 2023 and was the deadliest medicane in recorded history, causing approximately 10,000 fatalities, mostly in Libya, but also in Greece, Turkey and Bulgaria (Hewson et al, 2024). In Greece, 1,092mm of rain was recorded in Zagora on September 5, 2023, 55 times the national monthly average.
On the same day 330mm was observed in Strandja, Bulgaria and 125mm in Istanbul, Turkey. Flooding was particularly severe in central Greece and a catastrophic dam failure at Derna in Libya resulted in more than 5,000 fatalities.
Future warming will further intensify rainfall extremes
It is clear that short duration rainfall extremes are becoming more intense (Westra et al. 2014). For example, in Ireland, heavy or extreme rainfall events are contributing significantly more to annual precipitation totals, with rainfall becoming more intense, particularly in the east and southeast of the country (Ryan et al, 2021).
The IPCC Summary for Policymakers indicates that the once in 10-year event for rainfall intensity in preindustrial climate is presently occurring 1.3 times in a decade and will approximate to a one in five-year event should global warming of 2oC be exceeded (Figure 1).
Figure 1: Projected changes in the intensity and frequency of extreme precipitation over land (IPCC, 2021).
Designing for resilience: Strategies for drainage innovation
Rainwater drainage systems exhibit significant variation in design to effectively manage the volume of precipitation within a specified region. These systems are characterised by a multitude of roof outlets, which are determined by the roof's surface area, its pitch, the volume of rainfall, and the required directional routing of the drainage pipes.
Traditional gravity drainage systems necessitate the installation of a downpipe for each outlet and rely on a pronounced gradient to facilitate the movement of water to lower elevations. These pipes must maintain a slope to ensure efficient water transfer and typically contain up to 30% water and 70% air, which restricts the flow rate due to air entrapment. In these systems, the water height (h) on the roof correlates directly with the water flow rate (Q).
Conversely, siphonic drainage systems operate on a fundamentally different principle. At lower flow rates, the siphonic system initially behaves similar to a gravity system. However, during periods of high flow, the system transitions to a bubble flow state, where some air bubbles may be present, yet the pipe remains predominantly filled with at least 80% water. The final flow state reaches full bore and is operationally capable of 100% water at peak storm event.
This final configuration allows siphonic drainage systems to achieve a high maximum flow rate of up to 65 litres per second, compared to the maximum 35 litres per second that gravity has.
The siphonic system outperforms traditional gravity systems due to its high flow rate capability, self-cleaning attributes, and the ability to utilise the full cross-sectional area of the pipe for optimal flow.
The inadequacies of traditional gravity-based systems necessitate innovative approaches to drainage that can better handle extreme rainfall events. Siphonic drainage systems offer several advantages over their gravity counterparts.
Unlike gravity systems, siphonic pipework systems do not rely on slope and can be installed level, making them versatile and easy to plan in heavily serviced environments. The siphonic system uses negative pressure to draw water through pipes, creating a full-bore flow that increases the velocity and capacity of water transport. This allows for a more rapid removal of large volumes of water, reducing the risk of overflow and flooding.
Siphonic systems are designed to function efficiently even under high-intensity rainfall conditions, where gravity systems would typically falter. The key principles of siphonic drainage involve minimising air within the pipes, which prevents the formation of airlocks and maintains continuous water flow.
Additionally, siphonic systems require significantly less raw materials and labour needs resulting in reduced time for installation and maintenance costs compared to gravity systems. By analysing data from regions experiencing extreme rainfall, it is evident that siphonic systems outperform gravity-based systems in both efficiency and reliability.
Environmental impact
Siphonic drainage systems offer significant environmental benefits by enhancing a building’s resilience to climate change. One of the core principles of sustainable drainage design is the reduction of greenhouse gas (GHG) emissions and the promotion of water reuse.
Siphonic systems align with these principles by reducing the need for extensive excavation and raw materials that contribute to GHG emissions. Furthermore, these systems facilitate the integration of green roofs and rainwater harvesting solutions, which can reduce the burden on municipal water supplies.
For example, green roofs combined with siphonic drainage can absorb rainfall, lower peak flow rates, and reduce run-off volumes, contributing to flood mitigation and cooling. Rainwater harvesting systems integrated into drainage networks enable the capture and reuse of stormwater, reducing dependency on potable water supplies and promoting water conservation.
In the future, a greater adoption of rainwater harvesting at a larger scale can help to reduce capacity constraints on municipality drains during extreme weather events. This approach not only enhances the sustainability of building environments but also supports the goals of net-zero emissions by reducing the carbon footprint associated with traditional water management practices.
Societal impact
Beyond environmental benefits, siphonic drainage systems also offer significant societal advantages by fostering community engagement and education on climate change. Innovative drainage solutions can serve as a platform for public awareness, highlighting the importance of sustainable water management in the context of climate adaptation.
Community projects, such as the installation and demonstration of siphonic drainage systems in schools or public buildings, provide educational opportunities to demonstrate the benefits of sustainable drainage practices. These projects can involve clients in the planning and implementation process, fostering a sense of ownership and responsibility towards climate resilience.
Moreover, by reducing the incidence of flooding and associated health risks, siphonic systems contribute to improved public safety and wellbeing. Educational programmes that accompany these installations can further enhance public understanding of climate change impacts and the role of innovative infrastructure in mitigating these effects.
Economic impact
Economically, siphonic drainage systems present a cost-effective alternative to traditional gravity-based systems. The initial cost of installation of siphonic systems is lower than gravity systems due to construction time reduction, reduced excavation and reduced raw material costs. Additionally, the maintenance costs associated with siphonic systems are typically lower, as these systems are less prone to blockage and damage caused by debris and sedimentation from the added leaf filters.
The efficiency of siphonic systems in managing large volumes of water reduces the need for expensive flood mitigation measures, such as retention basins and pump stations, leading to further cost savings.
Moreover, the ability of siphonic systems to function effectively using level pipework and areas with limited space makes them suitable for retrofitting, avoiding the high costs associated with extensive infrastructure modifications. By minimising the need for large-scale excavation and reducing the frequency of system failures, siphonic drainage systems offer long-term economic benefits that justify their adoption in climate-resilient planning.
Conclusion
This paper provides a comprehensive overview of why siphonic drainage systems are better suited for today's climate challenges compared to traditional gravity-based systems.
The increasing intensity and frequency of rainfall events due to climate change necessitate a rethinking of drainage systems. Traditional gravity-based drainage systems, while effective under normal conditions, are often inadequate in managing the extreme rainfall associated with a changing climate. Siphonic drainage systems offer a more resilient and efficient alternative, capable of handling large volumes of water quickly and cost-effectively.
By enhancing environmental sustainability, fostering community engagement, and offering economic savings, siphonic drainage systems represent a comprehensive solution to the challenges posed by climate change. The adoption of these systems in drainage planning can mitigate the adverse effects of increased rainfall, protect infrastructure, and promote sustainable water management practices.
To ensure the effective implementation of siphonic drainage systems, stakeholders – including engineers, architects, and customers must collaborate to develop policies and initiatives that support sustainable infrastructure development. By prioritising innovative drainage solutions, buildings can enhance their resilience to climate change and safeguard their communities against the impacts of extreme weather events.
Designing drainage structures for coping with extreme rainfall events have been classically based on outdated calculations. These require to be customised for specific geographical areas to reflect unique rainfall climatologies.
The reliability of using observational data for return period calculations is highly dependent on the length and quality of the records concerned. In particular, the relatively short record for short period rainfall observations in many parts increases the uncertainty of return period calculations and the resilience of infrastructures based on them.
Classical calculations based on assumptions of stationarity in rainfall records are no longer valid, particularly for short term events and where convective rainfall increases are occurring.
Scaling of return periods using the Clausius-Clapeyron relationship provides some improvement on classical approaches, but less so for short term extreme events where this relationship is less in evidence.
Advances in attribution confirm that significant changes in rainfall intensity are under way due to global climate change and pose an increased hazard that requires to be considered in drainage systems.
Proven intensified rainfall requires more adequate drainage systems to prevent flooding and build up from extreme weather events. Rainwater drainage systems require greater safety margins to accommodate climate change-induced increases in short term convective extreme events.
Siphonic drainage systems that can evacuate rainfall more efficiently and in larger volumes provide more resilience to short term high rainfall events than conventional gravity-fed systems. The reality of designing structures and drainage systems to cope with more intense rainfall events is essential for building integrity and public safety.
Authors: Julia Cavanaugh, sustainable and technical design engineer at Capcon Engineering; and John Sweeney, emeritus professor, Geography Department, Maynooth University.