Rickmansworth and District Residents' Association

Out of Sight is Out of Mind – Why Does Rickmansworth Flood.

Over the past few months we have become used to water lapping on to the pavements, and occasionally into shops and businesses, in High Street and Church Street. Although most of us can remember the effect of very heavy rain a few years ago, with serious flooding along much of the High Street and out towards Ebury roundabout, the constant low level flooding in the main streets of the town is new. So what is contributing to it, apart from autumn rainfall about 20% higher than the average?

Firstly, and probably most importantly, the public drainage system along and from High Street and Church Street is many years old and its capacity is unknown as, in some places, are its actual routes. Add to that the fact that the County Council is responsible for the gullies (the vertical bits down from the road drains) and the road drains whilst Thames Water is responsible for the public rainwater sewers. The road drains flow into the rainwater sewers and then, whilst some of the rainwater sewers flow into the Town Ditch, others flow downstream along the valley of the Colne. The Town Ditch is, after a change a couple of years ago, the responsibility of the Environment Agency. So a drop of rainwater falling in part of High Street could pass from the County Council’s domain to Thames Waters’ to that of the Environment Agency in the course of a couple of hundred yards travel. Not the easiest system to manage.

Secondly, the flow in the drains and sewers can be quite slow because, from the High Street to the river, the town is on flood plain and almost flat. That slow flow will accelerate the build up of deposits within the drains and sewers, particularly where there are awkward corners and changes of gradient, further slowing the flow in a vicious circle of reducing capacity.

Thirdly, there has been a massive increase in the number of buildings and amount of hard landscaping within the town centre since the drains and sewers were built over 80 years ago. What has been built over those 80 or 90 years? How about Northway and Penn Place, Three Rivers House and the car parks, Watersmeet, The Cloisters, most of Joan of Arc School as we know it now and much, much more. On higher ground some of the rainwater flowing off new buildings and hard landscaping would go into soakaways but it is difficult to have operating soakaways when the water table is about 2 feet below the surface. It will be interesting to see the impact of the massive developments of the past few years, and still underway, when a major rainstorm occurs and the run-off pours into the rainwater sewers and reduces their ability to take the water from the highway drains. So, little by little, the town has generated more and more rainfall run-off yet the core network of drains and sewers has remained largely unchanged and, in all probability, with its capacity reduced.

In towns undergoing major expansion a typical requirement is for road drains to be able to cope with a steady rainfall of 25 mm (1 inch) per hour whereas it is almost certain that 2 or 3 hours of rain like that in Rickmansworth would leave half the shops on the south side of the high street flooded, with ruined floors and any stock on lower shelves destroyed. But what is extraordinary is that, under current legislation, in towns like ours planning permission cannot be made subject to a substantial contribution to work to alleviate the increased risk of flooding. In other words, even a major developer cannot be forced to pay for work to prevent the increased risk of flooding that could be created by his development.

So what should be done? Perhaps we should start by refusing all planning permission for development in the town centre, or immediately beyond it, which could increase the flow of rainwater into the highway drains and rainwater sewers. Next, complete a survey of the drains and sewers so that we know both where they are and their condition. Then clean out the whole length of the critical drains, sewers and the Town Ditch. Finally, if the survey demonstrates that, even with the system at its cleanest, it cannot cope with heavy rainfall, maintain the moratorium on development until the town’s drainage system is upgraded to meet the modern day demand. Remember that the climate experts are forecasting wetter autumns and winters, so the flooding problem can only get worse if nothing is done. In the meantime, please keep the Association’s committee informed about changes to the pattern of flooding.