Flooding from rivers or the sea takes place across natural landforms (floodplains and coastal plains) that have a characteristic geomorphology and geological make-up. Although these landforms are not explicitly identified on geological maps, the associated geological deposits are mapped and can provide important insights into identifying where flooding has occurred in the past and complement other methods for predicting where flooding might occur in the future.
Floodplains and coastal plains can be visualised geologically because flattish deposits of clay, silt and sand left behind by previous inundations reveal their natural extent. The routine portrayal of these deposits on geological maps serves to highlight the distribution of floodplains and coastal plains, both large and small, where flooding is likely to be an issue.
The geological history of lowland floodplains, such as that of the Trent valley in Nottinghamshire, poses a salutary lesson to those seeking to develop them for human occupation. Over the past 400 000 years, the Trent river system has experienced five major episodes of floodplain construction, abandonment and incision. The results of this are the river terraces, which stand above the level of the modern alluvium and are the remnants of previous generations of floodplain.
Geological maps can show why flooding will not necessarily occur in every part of a valley floor. For example some river terraces, being relatively upstanding areas in a floodplain, act as natural obstacles to flooding and their higher parts may remain dry. Alluvium, on the other hand, tends to be low-lying and will at least partially flood during a major event. Even the narrow alluvial tracts of small tributary valleys can be prone to flash flooding. On a catchment scale, the capacity of the ground to absorb water is directly related to the underlying geology. For example, clay-rich rocks and some Quaternary deposits are impermeable, as are the soils developed on them. Such geological terrains will thus be susceptible to rapid run-off, enhancing the potential for serious flooding downstream, during high rainfall events.
Geological and geomorphological studies of floodplains can lead to a better understanding of how rivers will behave, and how sediment will be transported when a flood occurs. Such studies have been made possible through the development of airborne laser-based survey techniques (Light Detection and Ranging - LiDAR), much used by the Environment Agency, which produce surface elevation models of the floodplain accurate to ±15 cm vertically. Subtle height variations within the floodplain alluvium are revealed that can be interpreted as separate fluvial domains. The models derived from this type of landform analysis have an important predictive role to play in the formulation of hazard assessment and floodplain management strategies.
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