More Rain In the Forecast
Many American visitors to Scotland will doubtless have happy memories of their visit to this country, possibly revolving around golf, sampling the wide range of malts available or seeking family roots. However, many will also remember the weather with perhaps less affection. The concept that it could actually rain more and that this should be the driver for a government invitation to universities to consider how research programs in the built environment should respond may appear somewhat bizarre.
One of the measurable indicators of global warming would be an increase in the Global Mean Surface temperature. This has risen during the last century, and taken together with other macro indicators, implies that we in the U.K., and particularly in its northern regions, should expect heavier rainfall in coming years.
The past three years have seen a marked increase in flooding of residential properties, leading to central questions concerning the planning applications that have been allowed in areas historically defined as flood plains for local rivers. Of course, this has to be seen against a national need to build new housing stock. Also involved are the social pressures in choosing to build out of town on land previously defined as "green belt"--land around a city supposedly held free from housing or other building expansion--or to concentrate on re-development of "brown field" sites. Many of these pressures will be recognizable in the U.S., where the flight from the city to suburbia is now historic. City center living is desirable but may have other downsides. So, there are a number of interlocking issues reinforcing the notion that no decision is purely taken on engineering considerations. The increased flooding may be represented by anecdotal evidence--a colleague from Cambridge was flooded on the same day two years running.
So, how does increased rainfall contribute to setting a research agenda within our remit of building drainage system design and simulation? At first sight, one might conclude that the main thrust of such new work would fall to our colleagues who are involved with the urban drainage scene, and in particular, to the proponents of SUDS (Sustainable Urban Drainage Schemes). Indeed, there will be the need for considerable effort in that area that will involve engineers, city planners and building developers. The concept of rainstorm attenuation is well known, involving softer surroundings for buildings to delay run-off and the possibility of flood acceptance areas that may double as ponds in drier weather. Similarly, storage may be possible. However, these criteria have a building and development cost and may not be readily acceptable on urban in-fill or brown field reclaimed sites. The need to address these issues is real, as evidenced by the substantiated complaints that rural river flooding of out of city housing attracted more publicity and funding than the comparable urban flooding as a result of drain surcharge.
In terms of the building drainage design arena, it is also possible to identify quite quickly areas where current design and practice could be materially affected by increased rainstorm frequency, duration and intensity. The obvious place to start is with rainwater drainage systems.
The traditional open gutter and downpipe schemes used in the U.K. are slowly being challenged by the introduction in commercial and other large building structures, such as sports arenas and out of city center malls, of siphonic rainwater drainage systems. These schemes employ open gutters with specially designed outlets that prohibit the ingress of air into the rainwater downpipes and collection networks once the rainfall intensity, and hence gutter depths, exceed a set level. (An introduction to these systems by Dr. Scott Arthur ran in the March 2000 PME, and can be found in the archives at www.pmengineer.com). This leads to full-bore flow in the collection pipes, and hence siphonic action that increases the flow capacity of the network. No new system is free from challenges, and here the challenges revolve around system sizing, bearing in mind that for any siphonic rainwater system, there is only one storm duration and intensity that will trigger full-bore flow once the system is fully primed. Thus, if the design storm is too severe or has an excessive return period, the system may never function correctly. For a properly designed system, this offers huge advantages to the designer of large building systems. Recent examples include Hong Kong Airport and the Olympic Stadium in Sydney. Thus, increased rainfall will lead to a higher probability that such systems will function satisfactorily, and research to improve our understanding of the relevant design criteria would qualify for consideration.
The second area of research that would benefit from increased certainty of heavier rainfall lies in the rainwater re-use sector. Currently in the U.K., the Water Regulations 1999 encourage innovative use of recycled water. However, there is no recognized standard for that water. Similarly, such water must not be stored for more than 24 hours, and the need to treat greywater for re-use imposes a probably unacceptable load on an individual user, although such systems may be effective where a facilities management capability exists within the building. Rainwater storage provides a simpler route, and heavier rainfall would improve the through-flow, hence removing some of the storage duration concerns.
Finally, the increased use of rainwater for w.c. flushing, as well as other activities, would reduce further the through-flow expected in the building-to-sewer network. Hence, pipe sizing again becomes an issue to prevent waste deposition and enhanced maintenance costs for the building owner. Thus, it may be seen that changes in climatic conditions will affect the design criteria governing building drainage and as such will have a place within new research initiatives set up to inform our approach to global warming.