The Department of Infrastructure Flood Management Division is to update its river and sea flood maps.
It says the update of the maps is “following industry good practice methods and taking account of the most up to date information”.
These new flood maps will be available on the public flood viewer on www.iomfloodhub.im in the near future.
Flood Maps
The DoI says the “primary purpose of these maps is to increase awareness of flood risk among the general public, government agencies and other stakeholders”.
In a statement to Manx.News a spokesperson for the DoI said: “The maps outline the likelihood of an area experiencing flooding to assess risk and encourage actions to be taken to increase resilience and awareness of flooding.
“The flood maps also inform land use planning, and support emergency planning so that emergency services and asset owners are aware of vulnerable areas.”
How Are They Produced?
The flood risk maps cover flooding from the sea, rivers and surface water.
River flood maps have been produced using hydrodynamic 1D/2D modelling software. Sea and surface water flood risk maps are produced using 2D hydrodynamic modelling software.
These models are developed using a combination of:
- Detailed topographic surveys of river channels, bridges, culverts, weirs and flood structures.
- Island wide LiDAR (light detection and ranging) derived digital terrain models, which provide ground height data for floodplains.
- Hydrology estimates of rainfall profiles, river flows, sea conditions and catchment conditions are produced following industry good practice techniques.
- Model results are calibrated and validated using locally observed river level, tide level and rainfall data. Historic flood records and anecdotal evidence is also used to assess the validity of model outputs.
- Small scale topographic features and localised sub surface drainage networks are not included in the models. National flood risk maps are not sufficiently detailed to assess individual property risk.
‘Raise Awareness’
The DoI says that its maps are developed to “raise awareness of the potential risk in rare and high magnitude flood events”.
A spokesperson said: “Climate change is predicted to increase extreme rainfall events and increase the rate of sea level rise.
“Climate change allowances are included in the river and sea flood risk maps to build resilience into our assessment of flood risk across the Island, ensure future development is sustainable and informed decisions can be made.
“For these reasons the maps depict flood extents that may not have occurred, or have not occurred for many years.”
The current maps highlight the following risk areas:
- for river flooding, the maps indicate the extent of a flood with a 1% (1 in 100) chance of happening in any year
- in the case of flooding from the sea, the maps indicate the extent of a flood with a 0.5% (1 in 200) chance of happening in any year
For surface water flooding, three risk areas are mapped:
- Extent of a flood with a 3% (1 in 30) chance of happening in any year
- Extent of a flood with a 1% (1 in 100) chance of happening in any year
- Extent of a flood with a <1% (1 in 1000) chance of happening in any year
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