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Natural Flood Storage and Extreme Flood
Events Final Report
EXECUTIVE SUMMARY
Flooding is an important process in the natural
environment and cannot be entirely prevented. This research
project was established to investigate and quantify the
potential of natural flood storage within river systems to
cater for extreme rainfall events and increased river flow.
Current climate change predictions suggest that substantial
areas of Scotland will in the future experience a greater
frequency of flooding, due to greater and more intense
rainfall. Natural flood storage is now being considered as
an environmentally sustainable method of flood risk
management, which could be a useful complement to
conventional flood defences in certain catchments.
This project has developed a generic method for
broad-scale assessment of the potential to use natural
floodplain areas to provide attenuation during large
events. Natural flood storage is defined as water held back
on an area of the floodplain that would currently be
inundated during flood events up to some assumed size.
However, a better way to view the functioning of a natural
floodplain is to use the term natural floodplain
attenuation, which can be described as the overall impact
of the floodplain to change the shape of the of river flow
hydrograph (reduce flood peak and increase flood duration)
during out-of-bank events due to a combination of storage
and resistance.
More than one option exists for the determination of
flood extents and natural flood storage volumes. The river
characteristics and data/model availability determine what
methods can be used. The methodology for assessing the
potential to use natural floodplain storage was developed
using datasets and models from four case study river
catchments, namely the White Cart, Clyde, Tay and South Esk
(Angus). The methodology involved the simulation of the
area and volume of inundation for events that would give
rise to a flood flow of specified probability at a
downstream location. The generic modelling approach used
both 1-D routing models and a 2-D grid-based floodplain
model, which makes full use of topographic data in the form
of a Digital Elevation Model (DEM). Recommendations for the
use of the modelling methodology described in the generic
assessment, together with possible future modelling
developments are made.
Other datasets and information were also obtained for
the case study rivers in order to undertake a simple
environmental and economic assessment of enhanced natural
flooding upstream of flood risk locations. All floodplains
contain numerous natural and man-made assets which would
require due consideration, in consultation with all
stakeholders, if any proposals to enhance the natural flood
attenuation were to be taken forward. The report reviews
experience of utilising the natural floodplain in recent
flood alleviation schemes in England and Wales. If new
schemes are to be viable in Scotland, there is a need for
dedicated consultation and negotiation with the floodplain
landowners and other stakeholders.
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