The current Environment Agency Flood Zones indicate areas that are potentially at risk of flooding from rivers or from the sea. These show areas that are currently identified to be at risk of flooding under the present day climate.

Flood Zone 3 shows the High Probability zone indicating areas that might be affected by a fluvial (river) flood with a 1% annual exceedance probability (AEP) (or 1 in 100 year return period of occurrence) or greater chance of happening each year. For tidal areas Flood Zone 3 shows areas that might be affected by a tidal flood with a 0.5% AEP (or 1 in 200 year return period of occurrence) or greater chance of happening each year.

Flood Zone 2 shows the Medium Probability zone indicating areas that might be affected by a fluvial (river) or tidal flood with a 0.1% AEP (or 1 in 1,000 year return period of occurrence) or greater chance of happening each year. 

Areas outside of these Flood Zones are identified as Low Probability risk, and have been identified as having less than 0.1% AEP (1 in 1,000 of flooding from a river or the sea. Flooding due to other sources, such as surface water or groundwater flooding, may still be likely.

Yorkshire Water, the sewerage operator in the region, is required by OFWAT to maintain a register of flooding incidents due to hydraulic capacity problems on the sewerage network. This database identifies locations where flooding occurs on a frequency of 2 incidents in a 10-year period (nominally 1 in 5-year return period) and one incident in a 10-year period (1 in 10 year return period). 

An additional register, for 1 in 20-year return periods, is also maintained. The 1 in 20 register includes records for flooded properties which are under investigation. On conclusion of the investigations the properties may be allocated to either the 2 in 10, 1 in 10 or 1 in 20 registers as appropriate, or be recorded separately as properties where flooding has been caused by blockages or other operational problems in the system, or be shown to be the responsibility of a third party.

Yorkshire Water has a prioritisation process for delivering investment projects. The projects generally provide a 1 in 40-year standard of protection to properties identified at risk of internal flooding on the 2 in 10 and 1 in 10 registers. Currently Yorkshire Water classify storm events greater than a 1 in 40-year return period as extreme and therefore there are likely to be a significantly greater number of properties and areas at risk during a larger magnitude, 50 or 100 year (2% or 1% annual probability) storm.

Flooding to the region’s transport networks has been undertaken at a regional broadscale. The estimated impacts on roads and rail systems have been determined based on the locations where the Environment Agency’s Flood Zones affect road or rail crossings. This has been based on the high probability Flood Zone 2.

This assessment does not take into account where crossings are at a higher level above the river or estuary, and obviously raised bridges will prevent the networks from being affected. However, the sections identified highlight the potential risk of flooding posed to the relevant transport systems. Lower sections either side of the crossing are likely to be affected. The Flood Zones are based on available local topography data, so where these affect areas either side of a crossing it is highly likely that sections of the road or rail networks may flood.

In addition, to the direct impacts, flooding around bridge abutments is likely to cause increased scour and erosion problems.

It should be noted that the Environment Agency’s flood vulnerability report identifies motorways, primary and main roads as high vulnerability, and it is recommended that the individual identification should be based on usage and traffic ‘population’

The Regional Flood Defence Committee is an executive Committee of the Environment Agency. It is tasked with directing the flood risk management work of the Agency and with raising a Local Levy from Local Authorities. This Levy is used to deliver a programme of work centred on flood risk issues, but with climate change as a core priority. The Local Levy has part funded this particular study.

The flood impacts presented within the report have been collated based on property addresses from the National Property Dataset located within the Environment Agency’s Flood Zones 2 and 3 (January 2009).  The resulting properties listed within the Impacts section of the report indicate the number of properties that may potentially be affected by river or tidal flooding during the extreme events.  The figures do not include for locations that may be affected by flooding from other sources, such as groundwater, drainage or surface water runoff.

The assessment of future flood impacts has been undertaken by extrapolating the method applied for the current situation in order to estimate the potential future impacts.  The Environment Agency’s Flood Zones have been expanded to consider the potential likely increase in flood extents in the future due to projected climate changes.  The addresses from the National Property Dataset that fall within the expanded flood extents have then been collated to identify the additional properties that may be affected by the increased flooding, which are presented in the report.

The estimates have been derived independently and are not based on data acquired from other documents.  The estimates may therefore differ from those presented in other reports (such as local Catchment Flood Management Plans) as these are based on different methods of collation and may use different extreme flood extents and/or base property datasets.