The small port of Boscastle nestles in a narrow ravine in the parish of Forrabury, North Cornwall. In August 2004, a massive flash flood devastated the town. Fortunately nobody was killed due to a massive successful rescue operation, which included helicopters, but millions of pounds worth of damage was caused. Afterward, residents wanted to know how something like this could happen.
On the days prior to the flood in August 2004, the areas around Boscastle received an abnormal amount of rain. This caused the ground to be overly saturated. This saturated ground coupled with moist ocean winds and high local temperatures caused an abnormally large storm to come in off the ocean and pour another 13 cm of rain on the already saturated ground. This is more rain than the town gets in all of August. This extra amount of rain caused the banks of two nearby rivers that are usually tame to swell and overflow their banks.
The location of Boscastle in particular was a cause of the flooding. Boscastle lies in a steep ravine of the Valency River. The steep moors that surround the town, after the previous days' saturation, did little to stop rainwater from funnelling into the river valley. The steepness actually caused the water to accelerate downward into the valley below. This added volume to the narrow banks of the Valency, and the river spilt out into the surrounding town.
The Valency River had never flooded on such a massive scale, so the people of Boscastle thought there was no reason to prepare for such an isolated and freak incident. In hindsight, there were a few steps that could have been taken to prevent the extent of the damage that occurred. The Valency lacks any flood prevention system like raised banks to slow overflow. The sewerage and drainage systems in Boscastle are antiquated and could only handle a small capacity. The sudden added volume destroyed this system, and it was of little effect. Also, structures that span the river, such as bridges, became clogged with debris. The rainwater went up and around these bridges, flooding the town sooner than necessary.