I am Arup’s global stormwater skills...
By introducing the dimension of time into the management of water, drainage and flooding systems, environmental planners would be better able to identify opportunities to move the timings of peak flows, creating more capacity either all of the time or just when needed.
Many of the systems I encounter when dealing with water, drainage or flooding need to consider time in the calculations, but then they ignore it in the results. I see peak flows, peak volumes, flood heights and flood extents, but these are all frozen in time.
The UK's Environment Agency flood maps show the same water in different places at the same time in order to show the worst case everywhere. In reality, the water cannot be in two different places at the same time, but because a map is a static image, time has to be taken out of the picture.
When Water Companies consider the capacity of their sewerage system, they quote peak flow or pipe full flow figures. These are even more misleading, because although the units used include time i.e. litres per second, these figures are still frozen in time. The flow before the peak and after it will both be lower.
This is an issue because designers sometimes forget that in the real world, things change, flows and floods can vary during an event. At the height of a flood, the water may be a metre deep, but six hours earlier, the flood did not exist. Six hours earlier, we could have put temporary flood barriers up that would stop the flood getting into properties.
Time gives us opportunities to react
Understanding when things will happen, can give us time to do something about it. Knowing that your property can flood to a metre deep is not much use but knowing that rising water in the river means that you have six hours after it gets to point A before it will get to you, could be of use.
Sewerage systems also have variable flows and if we knew that a big storm was coming and due to pass over the north part of the catchment, we could consider (via pumps and gates) diverting flow into the south of the catchment to make extra capacity in the north. This is sometimes called real-time control and although we have known about this approach for many years, very few drainage systems use it.
Storage systems are being built in almost all new developments. These systems delay flow, but we rarely consider if delaying flow actually makes downstream conditions worse than just letting the flow go. We normally assume that holding the flow back is a good thing.
Introducing time into many of these systems will identify opportunities that we currently ignore. We could then choose to shift the time of peak flows to create more capacity all of the time or just when we need it.
Can we stop ignoring time when looking at flooding?