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Natural Flood Storage and Extreme Flood
Events Final Report
8 CASE STUDY 4 - River Clyde
8.1 Background
The River Clyde catchment draining to the tidal limit
has an area of approximately 1,970km
2. Significant amounts of the catchment drain
through rural areas upstream of the major
Glasgow-Hamilton-Motherwell conurbation with reasonable
floodplain areas. The built-up area, mostly located at the
downstream end of the catchment, only represents 7.5% of
the catchment. The major land cover in the catchment is
grassland (improved or rough), which covers about 55% of
the area.
Widespread flooding occurred in the Clyde catchment in
1977, 1985 and 1994. A River Clyde Flood Management
Strategy is currently being undertaken by Halcrow-
Fairhurst JV and JBA Consulting for Glasgow City Council.
Barrages, hard defences and flood storage areas upstream of
the main urban areas are being investigated.
Owing to delays in the early part of this project, there
has not been time to complete modelling on the Clyde and an
alternative approach has been used. However, it has been
retained in the analysis of land cover and environmental
considerations to provide a fourth set of data for
context.
8.2 Flood volume and outline
Time constraints have limited the modeling undertaken on
the Clyde catchment. Instead, we have worked with more
limited data to illustrate how the methods could be applied
in such cases.
A flood management study on the Clyde is currently
underway, including consideration of storage areas at a
number of locations within the catchment. As this work is
in progress it is not quoted in detail here. However, the
study included frequency analysis of flood volumes at flow
gauging stations. We have used the volume frequency data
for Daldowie GS (OS NGR NS672616), draining 1,903km
2, to set the hypothetical required storage
volume. This was defined as the difference between the
volumes of a 100 year event and a 5 year event and equals
101 million m
3. An event duration of 72 hours was used in
this calculation.
In the absence of 2-D model results, we did not have
access to detailed digital flood outlines. The Institute of
Hydrology Report 130 (IH130, Morris and Flavin, 1996)
outline was used instead (Figure 8-1). Although this flood
outline has now been superseded by more detailed modelling
in many parts of Britain, it remains a useful generalised
indication of flood extent. We have used a 200 year outline
to define an assumed 'natural' floodplain for the other
three example catchments. The IH130 outline corresponds to
a 100 year flow. A proportional buffer zone was therefore
added to enlarge the area to that estimated to be covered
by the 200 year flow. (A scaling factor of 1.1 was chosen
for this illustrative example).
Figure 8-1: Clyde
catchment
The steep section of the distance-area curve (Figure
8-2) at 30km indicates a location where there is a
potential to hold water back on the 'natural' floodplain.
However, flood storage is most effective if it is located
immediately upstream of the flood risk location, which
cannot be found it seems on the modelled Clyde floodplain
Also, at this location (30km upstream) the average flood
depth is modelled to be approximately 8m (Figure 8-3),
which would require significantly heavy engineering to
achieve.
Figure 8-2: Clyde -
Distance-area curves for natural flood extents
Figure 8-3: Clyde -
Distance-depth curves for natural flood
extents
8.3 Environmental assessments
The Clyde catchment does contain numerous SSSIs and
Scheduled Ancient Monuments. In the more rural area on the
main Clyde watercourse between Motherwell and Lanark, where
opportunities for enhanced flood attention on the
floodplain exist, few of these are located within the 100
year flood extent (Figure 8-4). However, more sites would
require consideration if a greater area of floodplain were
to be used to provide attenuation.
Figure 8-4: Clyde - SSSIs
and Scheduled Ancient Monuments
The Clyde floodplain also contains many man-made assets
that could be affected by enhanced inundation. For example,
Scottish Power have provided the project with a GIS dataset
showing the locations of all their electricity sub-stations
within the catchment. Figure 8-5 indicates that a number of
these assets are located within the 100 year flood extent
and would require a more detailed assessment, together with
consultation, should any proposed scheme to enhance natural
floodplain attenuation be taken forward.
Figure 8-5: Clyde -
Electricity assets
8.4 Agricultural economic assessment
8.4.1 MDSF-based
The estimated total cost of the approximate 200 year
event margin (based on the 'buffered' IH130 flood extent
for the 100 year event) using the MDSF methodology is shown
in Table 8-1. The cost figures may be compared with an
equivalent value of £123,338 for one of the proposed flood
management options, a combination of three storage ponds
with a total top-level footprint of 4.4km
2.
Table 8-1: Clyde - Economic
cost of flooding on agricultural land, based on
MDSF
Flood return period | Cost
(£) |
|---|
Approx. 200 year | 650,370 |
8.4.2 Single flood compensation payment
based
Table 8-2 provides a summary of the overall cost of
permitting the land to be flooded for the approximate 200
year event (based on the 'buffered' IH130 flood extent for
the 100 year event) using the single payment for all land
cover classes (excluding land cover classes 1, 2 and
3).
Table 8-2: Clyde - Potential
annual compensation costs, based on single
value
Flood return period | Area inundated
(km
2) | Cost (@ £300/ha)
(£) |
|---|
Approx. 200 year | 40 | 1,088,040 |
The Clyde floodplain area is quite different to that of
the other three case study floodplains in that there is a
significant area of wood/forest and moorland within the
floodplain and a relatively small proportion of the very
high value arable (non-cereals)/horticulture land cover
class. As a result the total single payment cost is higher
than that derived from the MDSF method.
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