Monday, July 30, 2007
Urban flooding - 15 things to do
'Flash flooding' has played a big role in the recent misery - this arises from very intense downpours overwhelming urban drainage systems creating dramatic surface water flows downhill into river basins. This creates immediate flooding wherever water can collect, fed by these surface flows. The pathways may be quite unpredictable - depending on building, roads, and even small obstructions like walls, and the 'flashiness' will depend on how absorbent the surface is and how much rain runs off over the surface and into drains and sewers. Eventually, water reaches the rivers and the whole catchment drains into the river causing the more predictable type of 'fluvial' (river) flooding, whereby river levels rise and overwhelm the river banks and may 'overtop' flood defences. The faster the catchment drains, the more severe the peak river level is likely to be. If the river level exceeds the banks or defences, water flows into the floodplain, gradually moving up the land contours until all the flow volume can be accommodated. Other types of floods come from the sea and groundwater (also see graphic at the end of this posting).
The basic strategy for dealing with flood risks is as follows:
- Characterise the risks and how they will change over time - producing flood risk maps and guiding investment strategy
- Avoid development in high flood risk areas
- Build defences to get the highest value for money in terms of risk reduction with the available funds
- Go with natural processes as far as possible to slow the movement of water, create buffering and flood storage
- Improve resilience - for example by having a warning system that enables all involved - householders, emergency services, infrastructure owners - to respond.
No comments:
Post a Comment