FLooding 101
A key objective of the Inishowen Rivers Trust is to provide opportunities for the community to learn more about water related topics. Flooding is a concern for us all and the Trust has developed a short booklet called 'Flooding 101' which aims to improve our understanding of why and how floods occur, what are the solutions to flooding and what can be done to protect homes and property from flood damage.
The term '101' refers to a basic course. It is a term used at third level educational colleges to mean a course that everyone can do no matter what your previous experience. In essence, this is a beginners guide for everyone.
Flooding 101 has been developed by the Trust with the help of experts from Ireland, the UK and using local expertise and experience. Our understanding of how to tackle floods is changing, particularly in the context of climate change and it is important for us to stay informed.
Flooding 101
Floodplains & Flooding
Flooding can happen for a variety of reasons. There are different types of floods and Inishowen is most at risk from coastal flooding, river flooding, flash flooding, storm surges and areal flooding (ground becomes super saturated with water which accumulates and cannot escape from an area quickly enough).
Low lying areas bordering the river are known as 'floodplains'. Floods are a natural phenomenon and traditionally floodplains held the water until the flood subsided. However, modifications to our river channels and floodplains means water now have less options and flooding is more common. Climate models for Ireland in the future predict heavier rainfalls in autumn and winter and more intense storms (EPA Research Report No. 159).
Flooding terms
Hydrology is the study of water, in particular it's movement in relation to land, and includes the study of flooding. Hydrology is an important subject for students of geography, agriculture and engineering. The more information we collect about rainfall and water flows the more accurate our predictions of flooding. At Tullyarvan Mill in Inishowen, a gauge has collected data on the water levels at this point since 1975. A large flood was recorded in 1987 but the August 2017 flood was half a metre higher.
The severity of a flood is measured using the term Annual Exceedance Probability (AEP). This is a measure of the chances of a flood occurring in a given year. For example, the August 2017 flood in Inishowen was termed a 1% AEP. This means that there is a 1% chance of a flood of this size occuring each year. This flooding event has been described as a 100 year flood but this is not an accurate description as it implies that the flood will only occur once every 100 years. It is possible to have a 1% AEP event several years in a row.
How does flooding occur?
Flooding can occur at any time of year and Inishowen has suffered some of its most severe floods during the summer months. There are many reasons why flooding occurs, some natural and some caused by human interference.
- When there is lots of prolonged rainfall - this saturates the soil and the water will run off when it can.
- When there is intense rainfall over a short period of time – the carrying capacity of the channel is exceeded
- When the shape or nature of the landscape encourages flooding e.g. Steep slopes on valley sides and non-porous clays
- Land and riverine management practices; when human activity exacerbates flood risk.
Humans can cause or exacerbate flooding by:
- Urbanisation - increased hard surfaces – water will run off more quickly
- Poor planning - building on floodplains
- Deforestation - cutting down trees and removing vegetation that would ordinarily intercept rainfall and store water
- Modifications - modifying rivers or carrying out works that may increase the likelihood of flooding. e.g. channel straightening, hard engineering on banks
- Engineering failures – hard engineering that encourages water to move faster causing it to build up at pinch points and flood
- Poor land management practices - such as cattle poaching in-stream (leads to bank erosion and sedimentation), soil compaction (reduces ability of soil to retain water) and crop growing on steep slopes (increase in overland flow).
- Failure to maintain culverts and bridges to avoid obstruction e.g. a blocked culvert can back up and lead to flooding in nearby properties
How do rivers react to flooding?
How a river reacts to flooding depends on many factors. We already know that a heavy rain event can lead to flooding in rivers. If there is excessive rainfall the ground can become saturated with water which it can no longer hold, and this water is released into the channel too.
Some rivers are known as “flashy” systems as it takes a short amount of time for precipitation to reach these rivers and for the river to reach “peak flow/discharge”. Flashy rivers are sometimes the most dangerous as there is a shorter window of time to respond, and they have the potential to be the most destructive.
During a flood the flow of water can widen over the top of the banks and the speed of the water increases. This can cause the river to deepen and scour the river bed, lifting large objects such as boulders and carrying them downstream. Increased speed also leads to bank erosion. Bank erosion in turn can lead to increased sediment such as soil being washed into the river.
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