Sunday, July 15, 2012

Surface Water Flows and Flooding

The recent ASC report on flooding and water scarcity makes some interesting points about surface water flows, particularly those associated with flood hazard in urban areas. The report states that ‘Every millimetre of rainfall deposits a litre of water on a square metre of land’. Water falling onto a paved, impermeable will not infiltrate into the ground and so the volume has to move somewhere. The amount of paved surface is increasing as the report noted that green spaces in urban areas have been paved over and so surface water flows in urban areas are increasing even before the more intense rainfall associated with climate change is considered. The figures cited are that the proportion of paved gardens has increased from 20%in 2001 to 48% in 2011 of the total garden area of 340,000 hectares.

To combat this increase in paved area contributing to runoff the report suggest that urban creep should be minimized, sustainable urban drainage (SUDS) (SUDS) (SUDS-EA) should be improve to slow down water flows and store water above ground, and that conventional sewers should be maintained or upgraded: all good ideas. Recent floods in urban areas have highlighted the importance of such measures. Paved surfaces permit no storage of water, runoff is almost immediate and, with intense rainfall, the volumes of runoff involved can be huge within a short period of time. Overwhelmed urban drainage systems mean that the water moves rapidly across impermeable surfaces and flows through streets and roadways using them like predefined river channels. Similarly, when a river bursts its banks the water tends to use the paved, impermeable surface as a routeway for movement. The urban road network provides a convenient substitute for natural channels providing water with a rapid means of moving across an urban area.
A great deal of the potential damage from a flood and even flash floods could be mapped using a detailed digital elevation model (DEM) and a knowledge of past events in an urban area. This will help map out previous routeways that surface flows have used. Future events may be harder to predict, as the urban infrastructure changes and precautions are taken by planners to block or re-route surface flow, then the microtopogaphy of the urban area may be a guide to patterns of surface flow but other factors will also affect the detailed routes the water takes. The local detail is a bugger for modelling flow patterns. It will be interesting to see what, if any, use is made of the information about flood damage from the recent floods. There is a great deal of information online from Twitter, as well as local blogs and newspapers accounts that could provide a great deal of information about how surface water moved through urban areas. The potential for ‘citizen science’, for ordinary people (a horrible term that seems to imply scientists and planners are extraordinary) to contribute to the scientific investigation of flooding is immense. Co-ordination of this type of information, the mere exercise of collecting and collating information, or judging its quality and usefulness fro modelling and understanding urban surface flow is immense. Time, expertise and, potential funds, are needed for these activities but by who is unclear. Once the aftermath of the floods disappears from public view, the chances of funding such work drops dramatically. The need for people, the public (rather than the ordinary – anyone got a better term that isn’t condescending?) to be involved is important, however, if some of the recommendations of the ASC report are put into practice. In particular, the emphasis on households undertaking property-level flood protection measures might be enhanced if they were also actively involved in monitoring and in the feedback loop from modelling studies of their local areas. This would not only mean they were better informed about the risks of flooding but also more likely to act in the manner hoped for by planners if they felt they were an active part of preventing flood damage rather than passive victims in urban flooding.

1 comment:

  1. there is a disturbing increase in the number of disasters for the last few years. i am glad that measures are being taken.