Different types of flood risk
Different types of flood risk
We show flood risk for three different types of flooding:
• rivers
• the sea
• surface water and small watercourses
The risk levels are high, medium, low or very low.
We do not provide a risk level for flooding from reservoirs. We show the worst-case scenario for the area that could be flooded if a large reservoir were to fail and release the water it holds.
Risk of flooding from rivers
River flooding happens when a river cannot cope with the amount of water draining into it from the surrounding land.
On our flood risk map, a ‘river’ generally drains an area of more than 3km2. This is known as a ‘catchment’.
The risk levels take into account the effect of flood defences that may be in this area. Flood defences reduce, but do not completely stop the chance of flooding as they can be overtopped or fail.
This area might be at risk of other types of flooding.
High risk of flooding from rivers
High risk means that each year, this area has a chance of flooding of greater than 1 in 30 (3.3%).
Medium risk of flooding from rivers
Medium means that each year, this area has a chance of flooding of between 1 in 100 (1%) and 1 in 30 (3.3%).
Low risk of flooding from rivers
Low means that each year, this area has a chance of flooding of between 1 in 1000 (0.1%) and 1 in 100 (1%).
Very low risk of flooding from rivers
The shading on the Flood Risk Assessment Wales map shows the risk of flooding from rivers, the sea and from surface water and small watercourses.
All areas outside the shaded area are categorised as having a very low chance of flooding.
Very low means that each year, this area has a chance of flooding of less than 1 in 1000 (0.1%).
Risk of flooding from the sea
Sea flooding happens when there are high tides and stormy conditions.
The risk levels take into account the effect of flood defences that may be in this area. Flood defences reduce, but do not completely stop the chance of flooding as they can be overtopped or fail.
In most cases, the risk levels do not take into account wave action in coastal areas.
This area may be at risk from other types of flooding.
High risk of flooding from the sea
High means that each year, this area has a chance of flooding of greater than 1 in 30 (3.3%).
Medium risk of flooding from the sea
Medium means that each year, this area has a chance of flooding of between 1 in 200 (0.5%) and 1 in 30 (3.3%).
Low risk of flooding from the sea
Low means that each year, this area has a chance of flooding of between 1 in 1000 (0.1%) and 1 in 200 (0.5%).
Very low risk of flooding from the sea
The shading on the Flood Risk Assessment Wales map shows the risk of flooding from rivers, the sea and from surface water and small watercourses.
All areas outside the shaded area are categorised as having a very low chance of flooding.
Very low means that each year, this area has a chance of flooding of less than 1 in 1000 (0.1%).
Risk of flooding from surface water and small watercourses
Surface water flooding happens when rainwater does not drain away through the normal drainage systems or soak into the ground, but lies on or flows over the ground instead.
A small watercourse is a channel which drains an area less than 3km2. This includes small rivers, streams, brooks, ditches, and drains.
This type of flooding can be difficult to predict, much more so than river or sea flooding, as it's hard to forecast exactly where or how much rain will fall in any storm.
This is based on the best information we have available, such as ground levels and drainage.
This area may be at risk from other types of flooding.
High risk of flooding from surface water and small watercourses
High means that each year, this area has a chance of flooding of greater than 1 in 30 (3.3%).
Medium risk of flooding from surface water and small watercourses
Medium means that each year, this area has a chance of flooding between 1 in 100 (1%) and 1 in 30 (3.3%).
Low risk of flooding from surface water and small watercourses
Low means that each year, this area has a chance of flooding between 1 in 1000 (0.1%) and 1 in 100 (1%).
Very low risk of flooding from rivers from surface water and small watercourses
The shading on the Flood Risk Assessment Wales map shows the risk of flooding from rivers, the sea and from surface water and small watercourses.
All areas outside the shaded area are categorised as having a ‘very low’ chance of flooding.
Very low means that each year, this area has a chance of flooding of less than 1 in 1000 (0.1%).
Risk of flooding from reservoirs
Reservoir flooding is extremely unlikely to happen.
Use the Flood Risk Assessment Wales map to see flooding that could occur from reservoirs.
The shading on the map shows the worst-case scenario for the area that could be flooded if a large reservoir were to fail and release the water it holds. A large reservoir is one that holds over 10,000 cubic metres of water, equivalent to approximately 4 Olympic sized swimming pools.
The reservoir flood maps do not indicate any likelihood of a flood occurring.
- More about flooding from reservoirs
Floods destroy - be prepared
Don’t wait until it’s too late. Prepare for flooding by following some simple steps to reduce the impact of flooding on your home or business.
- What to do before, during and after a flood
- Sign up for free flood warnings
- View the flood warnings currently in force
Further information
Read Understanding your flood risk results for more information about flood risk.
If you have any further questions on how to prepare for flooding, please call Floodline on 0345 988 1188.