Pembrokeshire and Ceredigion Rivers Catchment Flood ...

Current and future flood risk
Overview of the current flood risk What is at risk?
Flood risk is the combination of
two components; the likelihood
(or probability) of a particular
flood event occurring and the
consequence (or impact) that
the flood event would have if
it occurred.
The probability of a flood event
is the likelihood of a flood of that
size occurring within a one year
period. It is described as an annual
exceedance probability (AEP) and
is expressed as a percentage.
For example, a 1% AEP flood
event has a one per cent chance
or 0.01 probability of occurring
in any one year.
Unless otherwise stated, numbers
in this report are based on the 1%
AEP river flood event. More extreme
events can occur at any time. The
likelihood of an extreme event
occurring is small, although the
consequences are potentially very
serious, particularly where defences
could be overtopped.
In the past, many of the catchments
have experienced flooding.
Notable flood events occurred in
1987, 1993, 1997, 2000, 2003
and 2005. Areas affected include
Haverfordwest, Tenby, Narberth,
Llandysul, Llechryd and many
other towns and villages across
Pembrokeshire and Ceredigion.
The sources of flood risk are:
• river flooding is the dominant
source in the CFMP area.
Haverfordwest is at greatest
risk, along with many towns
and villages on the River Teifi
including Llechryd, Lampeter,
Llandysul, Newcastle Emlyn,
Pontrhydfendigaid and Tregaron;
• tidally influenced flooding
occurs at the downstream end
of the river catchments and
affects many coastal towns and
villages in Pembrokeshire and
Ceredigion, such as Aberystwyth,
Haverfordwest, Llanrhystud,
Cardigan, and Aberaeron;
• surface water flooding is difficult
to quantify as many incidents
are masked by the impact of river
flooding but it is believed to be
a problem in some areas, for
example Aberystwyth, Aberaeron,
Cardigan and Haverfordwest.
Surface water flooding poses a
low risk of loss to life but can be
a significant contributing factor
to flooding;
• sewer flooding is caused by
overloading of sewerage systems
and is a risk in Newcastle Emlyn,
Cardigan, Llanybydder, Lampeter
and Haverfordwest;
• groundwater flooding is not well
documented in the CFMP area.
The village of Ludchurch is the
only area identified as being
potentially affected.
6 Environment Agency Wales Pembrokeshire and Ceredigion Rivers Catchment Flood Management Plan
CFMPs assess how flood risk is likely
to change in the next 100 years.
They do this at a strategic level
and not at a detailed, local level.
We used computer models to simulate
river flows and produce indicative
numbers of properties, infrastructure
and environmental features at risk.
These models take into account the
benefit of current flood defences.
Where applicable, tidal influences on
river flows have also been modelled.
Where models are not available we
have based our flood risk estimates
on our Flood Maps, which do not
include flood defences. Numbers
produced are sufficient for the
purposes of the CFMP only.
There are currently approximately
1,100 properties across the CFMP
area at risk from flooding during
a 1% AEP flood event.
Flooding has an impact on
infrastructure, including: roads and
railways, water and sewage treatment
works and electricity sub-stations.
The CFMP area has a rich and varied
environmental resource. There are
22 internationally designated sites
either fully or partially within the
1% AEP flood outline; 74 nationally
designated sites, including 9 sites
of special scientific interest; and
16 scheduled monuments,
including bridges and roman
remains. Around 85 km 2 of
agricultural land is also at risk from
a 1% AEP flood, a large proportion
of this is in the Teifi catchment.