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 Although this flood outline has now been superseded by more detailed modelling in many parts of Britain, it remains a useful generalised indication of flood extent.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.