Hydrology

Hydrology is the science concerned with the distribution of water on the earth, its physical and chemical reactions with other naturally occurring substances, and its relation to life on earth. We are mainly concerned with only a part of this, the effects of rainfall, and predicting the runoff created by storm events. Hydrological Cycle The hydrological cycle is constant circulation of water from land and sea through the biosphere and atmosphere by evaporation, evapotranspiration (loss of water from the soil by evaporation and transpiration by plants), and precipitation and runoff

Natural Water Cycle
Rainwater follows two paths. Surface runoff flows directly into rills and streams and then into the oceans or landlocked bodies of water; the remainder infiltrates into the soil or evaporates. A part of the infiltrated water becomes soil moisture, which may be evaporated directly or move through the roots of vegetation to be transpired from leaves. The remainder percolates downward, accumulating in the 'zone of saturation' to form the groundwater reservoir. The surface of this is known as the water table, which rises and falls with the seasons and weather.

Engineering Hydrology
Much of engineering is a relatively exact science. Predicting the behavior of water in the environment is much more complex. The runoff generated by a rainstorm depends on the soil type and extent of urbanization, surface streams and groundwater flows and the wetness of the soil before the storm, amongst other factors. The severity of the flooding on the River Severn and elsewhere recently was made worse by the saturated state of the catchments.

Rainfall prediction is a matter of statistics. No-one can say what the rainfall and catchment conditions will be in the future, only what the probability may be for a certain event. A storm event may be described as the 'one in 100 year storm', which may happen next week but is expected, in the long term, to occur on average once a century. It is better expressed as a storm with a 1% probability in any given year. With climate change, past records are an unreliable guide to the future, and extreme storm events may be more frequent than expected.

Engineering design is based on a specified probability storm and the likely runoff generated by the catchment.

For many years, runoff prediction in the UK was based on the 'Flood Studies Report' of 1975, which has been regularly updated. Over a period in 1999-2000, the 'Flood Estimation Handbook' was published, which is more site specific and generally predicts greater runoff than the FSR.

For flood prediction studies, such as the flood risk assessments we carry out for developers, the flood studies report is now obsolete, and we only use the FEH in the studies. More information is available on the Centre for Ecology and Hydrology (formerly Institute of Hydrology) website.