Flooding Case Study: Llanrwst
Background information and causes of flooding
The historical market town of Llanrwst is
located in the Conwy Valley, a few minutes
drive from FSC Rhyd y Creuau.
The town, which has a population of 3,323, is
positioned on the banks of the Afon Conwy.
One of North Wales’ largest rivers, 55km Afon
Conwy is fed by a drainage basin which covers
approximately 590km2 of Eastern Snowdonia,
including several of the regions tallest
mountains such as Moel Siabod.
Llanrwst’s location makes it vulnerable to
flooding and the neighbouring, smaller
settlement of Trefriw has also experienced
flooding.
Causes of flooding
Trefriw
(SH 780 630)
)
Llanrwst
(SH 800 615)
)
Afon Conwy
Location of Llanrwst and Trefriw (Image: Ordnance Survey (OL17)
)1:25,000
Physical factors contributing to the risk of flooding in Llanrwst include the high annual rainfall of 1,500-4,000mm. The
settlement is positioned in a glacial trough, with steep sides and a flat valley floor. This shape, combined with the
local geology of impermeable metamorphic and volcanic rock overlain with thin or saturated soils, contribute to a
short lag time. This means that time between peak rainfall and peak discharge in the river is very short, creating a
sudden or ‘flashy’ response to rainfall. By the time the river reaches Llanrwst, the three main tributaries (the Machno,
Lledr and Llugwy) having already joined. The resulting high discharge, combined with the tidal limit being less than
less than a kilometer downstream of the settlement, can dramatically increase the water levels around Llanrwst.
Land use in the surrounding also affects the risk of flooding in the settlement. Urbanised areas, such as the town
centre, impact levels of surface run-off, and historical clearance of trees for agricultural purposes impacts on the
amount of rainfall stored temporarily on surfaces, such as leaves, before reaching the ground (interception) and on
the amount of water moving down into the soil (infiltration). Reduction in these processes decreases lag time.
Case Study: 2004 flood event
During a period of extreme wet weather in February 2004,
417.8mm of rainfall was recorded in Capel Curig over six days,
equivalent to approximately 60% of London’s annual rainfall. The
resultant flooding affected 86 properties in the Conwy Valley,
including 17 properties in Llanrwst and 20 in Trefriw. Other
impacts included £5 million of damage to the Conwy railway line
and RAF intervention to rescue three motorists trapped in their
vehicle in Llanrwst.
This was compounded by flooding in 2005, which affected many of
the same properties. These floods, two of the most severe in living
memory, resulted in the construction of many of the flood
defences in place today.
The Tu Hwnt i'r Bont Tea Room during the 2004 floods
(Image: FSC Rhyd y Creuau, 2004)
Flooding Case Study: Llanrwst
Examples of Flood Management in Llanrwst
As a result of the 2004 and 2005 floods, a £7 million defence scheme was launched, part funded by the Welsh
Government and the EU Regional Development Fund. Plans were formulated by the Conwy Valley Flood Partnership
Group, made up of representatives including the Conwy County Borough Council, and Natural Resources Wales. The
defence scheme, which was completed in 2010, is designed to protect 96 homes and businesses against floods of up
to a one-in-200 year magnitude.
Locations of flood defences in Llanrwst (Image: Google Earth, 2014)
Floodplain Zonation and Levees
Flood plain zonation aims to utilise the recreational and
agricultural land to the west of the river as a storage area for flood
water, reducing the risk of flooding to the town centre and
residential areas on the east bank.
A series of levees (raised earth embankments) act to direct the
water in the desired direction. Old levees were modified to
complement the flood plain zonation, and new levees have been
constructed to protect priority areas, including a substantial new
earth embankment to protect Trefriw. Fenced, raised areas have
been constructed to provide refuges for livestock in times of flood.
Levees have been altered to facilitate flood plain
zonation (Image: FSC Rhyd y creuau 2009)
Artificial Flood Culvert
The Afon Bach, a small tributary of the Conwy, runs through
Llanrwst’s Central Business District. Despite previous
channelisation and increased bank height, high water levels can
still result in increased flood risk to this high economic value area.
Culvert construction (Image: A.E. Yates Ltd., 2009)
To reduce this, an artificial flood culvert was constructed linking
the Afon Bach to the Afon Conwy via a 720m long underground
channel. In high flows, excess water enters the culvert, allowing it
to bypass the town centre. This culvert, which was finished in
2009, was the most expensive part of the scheme at £2 million.
Flooding Case Study: Llanrwst
Dutchdams and Other Structural
Defences
Dutchdams have also been installed in
Llanrwst at a cost of £750,000.
Dutchdams are temporary flood barriers,
which are stored in-situ in the ground,
and can be assembled in times of high
flood risk to protect priority areas. In
Llanrwst, these barriers have been used
to protect the residential and commercial
areas near the Pont y Fawr.
Evidence of other temporary flood
defences can also be seen in Llanrwst.
The foot bridge over the Afon Bach, near
the Post Office, can be blocked off using a
wooden barrier, and several individual
properties have household flood gates to
block off entrances. New build houses
with raised ground floors can also be seen
new the Pont y Fawr.
(Top) Dutchdam in position across Watling Street (Image: Dutchdam, 2014).
(Left) Household flood barriers protect properties and (right) the foot bridge
over the Afon Bach at full capacity (images: FSC Rhyd y creuau, 2004)
Behavioural Responses
Future Flood Defences
Behavioural responses to flooding include a more
unified development plan, with restricted
development on the east floodplain, and improved
flood prediction. Flood warning systems have been
improved, with Automatic Voice Messaging systems
expanded to include settlements of less than 100
homes, such as Trefriw.
In 2011 Environmental Agency gauging stations recorded
more water in the valley than in any other recorded
incident. No properties experienced flooding on the
heavily protected east floodplain, although some parties
expressed concern over increased flooding elsewhere
A Welsh Government grant of £604,000 for additional
flood alleviation work, targeting housing estates in the
South of Llanrwst was approved in 2013.
Fieldwork: mapping and evaluating
flood risk and management
In addition to visiting flood defences, students can
use G.I.S (Geographic Information Systems) to
explore flood risk in Llanrwst.
The ArcGIS Collector App. is used in the field to map
data such as landuse and defence locations.
This data can then be manipulated to create
detailed choropleth maps of the area, and
compared to other layers of mapped data, enabling
students to evaluate the flood management
strategies first hand.
Map of flood risk created onArcGIS Online (Image: ArcGIS, 2015)