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Natural Flood Storage and Extreme Flood Events Final Report

6 CASE STUDY 2 - SOUTH ESK (ANGUS)

6.1 Background

The South Esk catchment area draining through Brechin is approximately 500km ² in size. The catchment is almost entirely rural, with steep topography limiting the amount of floodplain area. There are very few reservoirs that regulate the river flow. Only 0.6% of the catchment area is built-up. The dominant land covers are moorland (about 44%), arable/horticulture (about 22%) which is mostly located very close to Brechin and rough grass (about 15%).

Brechin suffered serious flooding in 2002. Since then Angus Council has been undertaking a detailed appraisal of flood alleviation options, including engineered flood storage, a flood relief channel, and demountable/hard flood defences. The engineered storage options investigated were an on-line reservoir just upstream of Brechin (at a cost in excess of £8 million) and two storage areas further upstream - one at Gella Bridge on Prosen Water about 27km upstream of Brechin and one at Gella Bridge on the South Esk about 35km upstream of Brechin (also at a cost in excess of £8 million).

The modelling work on the South Esk (HR Wallingford, 2004) concluded that the use of on-line or off-line storage upstream of Gella Bridge and Prosen Bridge to reduce flood levels at Brechin was not a practicable flood mitigation measure. Two of the reasons given for this were that significant flood events could be generated by a large rainfall event over the lower part of the catchment (i.e., downstream of Prosen Bridge and Gella Bridge), and due to the low level nature of the existing embankments on the main drainage network then most of the floodplain was already being utilised during extreme flood events.

6.2 Modelling

The first step in the modelling of the South Esk was to estimate peak flow magnitudes for flooding of the downstream risk location (Table 6-1).

For the South Esk, the recent Brechin flood study considered storage options to reduce a 200 year event to the current 100 year downstream flow. We have therefore modelled these two events only.

An outline of the routing model available for the South Esk catchment is shown in Figure 6-1. The model is a Muskingum routing model. Routing parameters were derived from data given in a report to Angus District Council by HR Wallingford Ltd., who built a less extensive routing model as part of a study for the Council (HR Wallingford, 2004). The routing model was calibrated such that the peak flows agreed (at least to a close approximation) with the values given in Table 6-1 for the corresponding return period.

The downstream hydrographs for the larger events were then compared with the peak flow for the 'threshold' event, and the volume that would have to be stored was calculated (Table 6-2). These figures can be considered as minimum required volumes of flood storage (strictly speaking assumed to exist immediately upstream of the risk location).

The routing model and JFLOW flood inundation model were run to create flood outlines for the specified return periods. The area of inundation was calculated and plotted as a function of distance upstream from the risk location. We have then calculated the differences between the areas of the larger (200 year) and smaller (100 year) events to represent the 'natural' area in which water could be held back to mitigate the larger event. These results are shown in Figure 6-2. The steep section of the graph at 5-15km upstream of the flood risk location may offer good potential to hold water back during the larger event within the 'natural' flood extent.

We have made a simple assumption that the volume of storage needed at the downstream flood risk location can be divided by the modelled flooded area to give a notional average storage depth (Table 6-3). We have calculated the required average depth using both the total extent of the 200 year flood (i.e. the flood event we have used in this study as the extent of the 'natural' floodplain), and also the marginal extent between the 100 year and 200 year outlines.

Based on these figures and the distribution of floodplain extent (as a function of distance upstream from the flood risk location) we have then calculated the average depth of water that would be required on the floodplain to achieve the required volume of storage (Figure 6-3). The graphs show figures calculated using both the full extent of the 200 year flood as a potential limit for storage, and also the marginal area between the 100 year and 200 year extents.

6.3 Environmental assessment

The South Esk catchment contains a number of sites designated as being of significant conservation and/or historical importance. A map showing the Sites of Special Scientific Interest (SSSIs) and Scheduled Ancient Monuments (SAMs) within the catchment is given in Figure 6-4. No SSSIs are located within or near to the modelled 200 year flood outline, which has been used in this project to define the 'natural' floodplain. However, there are some SAMs in or very near the floodplain area between Inverquharity and Tannadice which would require a more detailed assessment if enhanced natural flood attenuation was proposed for this area. If a more extreme flood outline was modelled then further sites may have to be considered.

The project team were unable to acquire any datasets on the man-made assets within the South Esk floodplain. Clearly these would require due consideration, with all the appropriate consultations, if any scheme to enhance floodplain attenuation was to be developed further.

6.4 Agricultural economic assessment

6.4.1 MDSF-based

The estimated total cost of the 100 year and 200 year events using the MDSF methodology are shown in Table 6-4. Unlike the White Cart the areal extent of the 200 year event is markedly larger than that of the 100 year event, which cause the overall damage costs to be quite different

The results indicate that the damage to the arable (non-cereals) and horticulture land caused by the two extreme floods completely controls the overall economic cost. This is due to the very high value of this land cover in comparison to all the other land covers, including arable (cereals).

To provide a concise summary, the results are also presented as a function of distance upstream in Figure 6-5. For the South Esk the higher agricultural damage costs, associated with arable and horticultural land cover classes exist in the area 5-20km upstream from Brechin. Beyond 20km the land cover is dominated by grassland systems.

6.4.2. Single flood compensation payment based

Table 6-5 provides a summary of the overall costs of permitting the land to be flooded using the single payment for all land cover classes (excluding land cover classes 1, 2 and 3).

The total catchment figures are typically 50% less than those derived from the MDSF methods, again indicating the large impact that the high value arable and horticultural crops have on the MDSF figures. However, these figures provide some indication of the magnitude of the longer term total annual compensation payments that might be required if the full 'natural' floodplain was to be actively managed for natural flood attenuation.

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