Flood / by Kristian Reay

Here in the UK we love to talk about the weather, but over the last few years flooding has become a more and more frequent occurrence across the country. This winter The British Isle was struck by 12 major storms and with some areas receiving over double the local annual average rainfall, over 6,500 homes and businesses were flooded. Understandably, flooding ranks high in many peoples list of concerns and as the topic gains more public attention politicians are coming under pressure to offer reassurance to the victims of flooding that their lives wont be disrupted like this again.

Although the total volume of water remains roughly the same level globally, it is constantly flowing around the planet in a cycle of evaporation and precipitation, and it is this localised change that leads to drought and flood.

  • 96% of all water is held in the world’s oceans
  • 1.6% is stored as ice in the poles and glaciers
  • 0.8% is locked in the rock as ground water
  • 0.002% flows through the rivers and lakes
  • 0.001% is in the air as water vapor
  • The rest is stored within plant and animal life or bottled in a vending machine

However it is not just an increase in the volume of water that causes flooding. During a storm pockets of low pressure form and as air pressure decreases sea levels rise, coupled with a strong wind and high tide this variation can lead to disasters such as the Great north sea flood of 1953, which struck the east coast of Britain as well as the coasts of Belgium and the Netherlands. The Storm caused a great deal of damage and in the UK over 300 people lost their lives.

As a result the Thames Barrier was built in Woolwich Reach to protect London and the east coast from future disasters. The barrier is made up of 8 gates; each as long as Tower Bridge and capable of holding back over 9,000 tonnes of water. Since opening in 1984 the Thames Barrier has closed a total 174 times, yet this winter it was closed on 48 occasions, with 28 closes in the month of February. The Environment Agency estimate that without the Thames Barrier over 125km2 of land would be at risk of flooding including over 400 schools, 16 hospitals and many of London’s famous landmarks.

THE THAMES BARRIER

THE THAMES BARRIER

As sea levels continue to rise over the coming years many of our towns and cities are going to look towards large scale engineering infrastructure to keep them safe, yet equally important are the many small scale interventions, that cumulatively can have a large impact to the risk of flooding. In recent years there has been an increase in the amount of hard landscaping in our cities as people pave their gardens seeking to reduce maintenance and add room for additional parking. In London it is estimated that an area seven times the size of Hyde Park has been covered in the last 10 years. This increase in hard landscaping has lead to a urban environment where run off is filtered into the drainage system much more quickly with less water being stored and absorbed into the ground, adding pressure to our aging infrastructure. Ultimately any local interventions where water is directed of site at speed and in volume can only lead to further problems and need for interventions downstream. A sustainable water management system in the urban environment must look to address these changes in volume and flow (that increase the risk of ground flooding and can exacerbate river flooding). SUSDRAIN outline the following key SUDS principles.

  • Store runoff and release it slowly (Attenuation)
  • Allow water to soak into the ground (Infiltration)
  • Slowly transport water on the surface
  • Filter out pollutants
  • Allow sediment to settle by controlling the flow of water

Despite the pressing need for improved water management in the urban realm much of the flooding that the UK has experienced in the past year has been in the more rural parts of the country, notably the Summerset Levels. The Levels are a natural flood plain at risk of flooding from both ground water and the sea. Flood plains, as the name suggests, flood and are part of the natural river cycle yet over the last decade we have seen a 12% increase in the rate of building on flood plains across the UK.

In Summerset there has been a public outcry to dredge the rivers as many local residents believe that increasing the river capacity will alleviate flooding however dredging alone cannot solve the problem as the volume of excess water during flood is usually far greater than the extra capacity gained from dredging. Additionally as dredging increases the carrying capacity of the rivers it is likely to lead to a increased rate of flow carrying large volumes of water downstream, and with it debris and pollutants, which if not properly managed may lead to flooding in other towns and villages.

It is important to look at the whole of the river catchment when designing flood management systems, interventions should look to store water upstream of ‘at risk’ areas, converting areas of farmland with particularly high run off rates to woodland and incorporating natural debris dams to slow the flow of water during flood.

To solve the issues of flooding within the UK and globally we will need a range of natural and engineering lead solutions and we will need to think holistically taking responsibility for the changes we have made to the climate and landscape. We need to develop a long-term strategy and with advances in technology we can better predict the weather and direct our interventions. Rather than promoting schemes that try to tackle natural process’ head on I believe that a flexible management system that works with nature, absorbing and mitigating excess water during flood and retaining it for periods of drought will provide a more sustainable, scalable, affordable solution.

NATURAL DEBRIS DAM

NATURAL DEBRIS DAM