This story has had to be worked out from Google maps and the Internet as it is too far to visit. The ideas may be completely wrong but someone on the ground should be able to work it out.
The aerial picture from Mercury showing the flooded valley may be the key. Comparing this with the views from Google and Bing show the flood water stretching down to Rhuddlan and then getting back into the river bed beyond.
The old bridge in St Asaph appears to be 70m wide, less the bridge piers, and this flood width needs to be maintained down to the sea to allow the water to get away. The river 80m above the dual carriageway appears to be 31m wide between the paths on the flood banks - the river is less.
The bungalows that flooded in St Asaph itself appear to be in the lowest point behind the flood banks ( see heights on Google image). Once the flood gets behind the banks it cannot get away. Perhaps walls and flood banks interfere with water flow and cause the gravel, trees and soil to accumulate thus blocking the river. Removing the gravel etc from a river and lowering the bed to itís original level would be a better way. This applies in many rivers I have seen.
The river itself appears to have gravel deposits all the way down the valley as usual. If the flood water could get away faster it would not back up and keep the necessary fall to drain the valley. The narrow point below the caravan park above Rhuddlan appears to be only 17m wide. The river between the bridges appears to be full of gravel, perhaps the flood banks concentrate the accumulation by restricting the flow. Clearing this would increase the river flow.
At Ruddlan the flood water has been split into two channels by slightly raised ground and the flood bank above Rhuddlan bridge. The west flood channel has recently been blocked by the new road forming a pond showing how effective a dam has been created. With the river narrowed and without the flood channel where does the water go?
Solving this flood problem will be difficult due to building (including roads) in the flood plain. Opening out the river to the flood banks, which may have to be breached to allow flood water into the river, would certainly help, but opening the flood channels is virtually impossible.
A flood channel was blocked by a new roundabout in Workington (see home page) in a similar manner maybe causing the collapse of their bridge. This also killed someone.
Please stop building flood banks and walls - you cannot push water, you can only allow it to go itís own way!
If someone from the BBC or any other reporter is reading this site would it be possible to sequence photograph or video any river valley in flood for the area downstream of a flooded town until the river returns to it's bed. This will show where the blockage may be. Publishing this on the net would be greatly appreciated. The Environment Agency has these aerial photos on file for many floods but they are under copyright and I cannot access same.