In 2007 large parts of Hull and surrounding areas were flooded. The reasons why are still being debated. Drainage capacity, pumping capacity, pumping failures and poor coordination of various agencies have been blamed in reports.
Most flooding in the UK in the past has been either due to rivers bursting their banks or coastal flooding. Coastal flooding along the east coast is often somewhat predictable as it is often the combination of a spring tide, a deep depression and winds from the north or north west blowing down the North Sea. The low pressure allows the sea level to rise, the wind blows the water into the bottle-neck of the southern North Sea and combined with a spring tide the winds blow waves over sea defences which quickly floods land behind the defences.
Rivers bursting their banks is a bit more obvious. Heavy rainfall runs off land and fills water courses, which may burst their banks downstream and flood surrounding land. What we are beginning to see in Britain are more cases of another kind of flooding: direct flooding from heavy rainfall. Here saturated ground and impermeable ground cannot hold any more rainfall, so water sweeps across the surface of land to a low point where it forms a temporary flood, as it did in Hull in 2007. Britain's existing drainage infrastructure tries to deal with this using ditches and other water courses but they are now unable to cope more and more often.
I think more concentrated, heavier rainfall is causing part of this, some of which is down to climate change no doubt. Some of the flooding is man-made in other ways too. Ditches and other water courses are badly maintained and even being filled in. Building causes more run off from roofs, roads and other impermeable surfaces.
One decision made, I think, in the 1950s is also having a devastating effect and that is where any land run off water goes. In most cases the water boards of the 1950 decided to direct run off into sewers. At the time most areas with access the coast simply dumped untreated sewage into the sea and land run off was seen as a way to dilute it. Now all sewage is supposed to be treated before discharge. I said 'supposed to be treated' because the Water Act allows water companies some discretion to pump untreated sewage into the sea when their infrastructure is overwhelmed in an excessive rainfall event. At Bridlington a new pipe has been built for this purpose.
Adding relatively clean land run off to sewage has two big effects. Land run off water ends up getting treated as sewage in expensive treatment works. This water could reasonably be discharged directly into natural water courses or the sea without much problem, instead sewage works with greatly inflated capacity have to be built and run to deal with it. The second big effect is that when unusually heavy rainfall events overwhelm the drainage system, the bottlenecks are the sewage pipes which overflow, putting not just land run off into streets, gardens and homes but sewage too. Fixing this is not easy and will be very, very expensive and disruptive but as climate change progresses some changes will be needed.
I have been helping Cottingham Flood Action Group to understand the drainage and sewer network for that low-lying village by using their knowledge and surveys to add the ditches, drains, culverts, sewers and manholes to OSM and produce a map to help them understand the issues. You can see the map here. So far only a small part of the sewer network of the village has been added but various people in the group have enough knowledge to add the whole network I think. Then the onward link to Hull's network needs adding, which is a much bigger task. All of the network ends up in a single treatment works at the east of Hull at Salt End.
I hope CFAG find my map useful.