Most of us are aware of how devastating a flood can be if it rains hard enough. It seems universal and simple. It must rain with sufficient intensity and volume. The rain hits the ground and flows downhill as it always wants to move and follow a flow pathway. Flow pathways converge to form bigger flow pathways which eventually spill from their channel and spread outwardly across the landscape. The pathway may remain confined but could rise well above its normal water height and flow fast within a ravine or canyon.

Where does the rain come from?

To understand how enough rain can be generated to cause flooding we must understand the hydrological cycle and how the sun’s energy drives this cycle. We all know how water, in all its states, moves around our planet. How it rises into the atmosphere is transported and then falls as rain, hail or snow on our oceans or on our earth. Water has moved around our planet in this way largely unchanged since our world was in its teenage years - one great ocean long before the first signs of life. Water vapour heated by the sun will lift to a very great height. Turbulent, unstable thunderclouds can form pushing condensed and frozen water into our stratosphere far above our wet troposphere. Have you ever been in a jet liner cruising smoothly at 35,000ft only to climb suddenly and sharply to gain altitude above 40,000ft in an attempt by the pilot to fly above a rapidly formed thundercloud (the classic ‘anvil’ shaped cloud)?  

The ability of the sun’s energy to evaporate huge volumes of water from the ocean and then transport this water by global circulation across our earth (generally from the equator to the Polar Regions) is mind boggling. It makes sense that at the equator there is a greater capacity for water to rise rapidly then cool and condense and return to earth as rainfall.

Planning for more severe flooding due to climate change

So the stage is set anywhere in the world (apart from the obvious dry regions) for severe flooding to occur - a natural hazard with catastrophic consequences for our most vulnerable communities.  

We know flooding will increase in severity and intensity due to climate change, but we don’t know exactly when and where and how we will respond.

This means there is a big unknown surrounding our vulnerability in the future. It makes sense that we must do all we can to limit our vulnerability by assessing flood risk to people and property at all stages of planning and land development. We can work with urban designers to develop communities outside floodplains. We can recognise the cultural and amenity values of freshwater. We can design our communities to live with water by restoring natural drainage systems to manage flood risk and enhance ecosystems. Simply put, as engineers, scientists and designers we have a great opportunity to adopt a regenerative approach to community development. Restore natural systems, create green space and build highly desirable places to live where people will be resilient and protected in the face of a changing climate.

This thought leadership article by Mike Chapman, a senior hydrologist in our Water Resources team, is intended to provide you with insights and relevant information about flood risks and planning. Our thought leadership articles on topical and specialist issues are designed to present the key points in an easy to digest and interesting manner.