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Natural Flood Storage and Extreme Flood Events Final Report

7 case study 3 - Tay

7.1 Background

The River Tay catchment draining through Perth has an area of approximately 5,000km ². It is almost entirely rural upstream of Perth, though the steep topography limits the extent of floodplain in many areas. Also, the River Tay is well regulated through a number of reservoirs built for hydroelectric power (HEP) supply purposes. With appropriate control operations this can help to reduce the magnitude of flood flows for lower return periods, but not for the more extreme floods. Scottish and Southern Energy Group utilise storage capacity in Lochs Errochty, Ericht, Rannoch and Tummel to delay the river Tummel flood peak, whilst allowing the flood peak on the River Garry to pass through (J. Anderson, SEPA, pers comm.). The River Tummel is then released on the River Garry's recession. However, this flood attenuation method only works for floods up to the 1 in 10 year flood.

Only 0.5% of the Tay catchment is built-up. The dominant land covers are moorland (about 45%), rough grass (about 19%) and forest/wood (about 18%). The city of Perth suffered severe flooding in 1990 and 1993. Since then approximately £26 million has been spent on flood defences in the city to protect the main urban area from extreme floods (both fluvial and tidal).

7.2 Modelling

The first step in the modeling of the Tay was to estimate peak flow magnitudes for flooding at the downstream risk location (Table 7-1).

For the Tay, there is no obvious 'threshold' downstream flow for modelling purposes. However, past flooding on the Tay has caused agricultural damages. There are some embankments in the lower Tay that are thought to offer around a 5 year standard of protection. We have adopted this value as a target downstream peak flow in at Ballathie gauging station, in addition to the 100 year event.

The outline of a Muskingum routing model built for this study for the Tay catchment is shown in Figure 7-1. The routing parameters were chosen based on typical values for a catchment of this scale and topography. This model was calibrated such that the peak flows agreed (at least to a close approximation) with the values given in Table 7-1 for the corresponding return period.

The Tay routing model boundaries were set downstream of major lochs on the main tributaries. McInally et al. (1995) suggested that even during the largest floods on record in the Tay (1990 and 1993), the majority of flood runoff was generated from uncontrolled catchments, with lochs controlled for hydroelectric generation having little influence above about a 10-year event. We have therefore treated the FEH inflows to the routing model as natural catchments, with the attenuating impact of lochs being accounted for through the FEH parameter FARL.

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 7-2). 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 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 7-2. Only the main stream branch has been included in this analysis.

Steep sections of the graph (e.g. between 20 and 40km) indicate locations along the river where there are large differences between the inundation extents for the two return periods. These locations may therefore offer the greatest potential to hold water back during the larger event within the 'natural' flood extent. However, at >20km upstream of the flood risk location could reduce the potential effectiveness of these areas for significant flood attenuation purposes.

We have made the simple assumption that the volume of storage required at the downstream location can be divided by the modelled flooded area to give a notional average storage depth (Table 7-3). 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 (expressed as a function of distance upstream from the flood risk location), we have calculated the average depth of water that would be required on the floodplain to achieve the required volume of storage (Figure 7-3). 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 100 year and 200 year extents.

In general, storage is most effective if it is located immediately upstream of the risk location. If the average storage depth falls to a practically-attainable value within a few kilometres of the downstream location, then there may be scope to use land in the natural 200 year extent to provide the required storage. However, for the Tay, an average depth of around 2m would be needed over the area stretching 20km upstream. For the Tay the target of reducing the 100 year event to the 5 year event downstream is a very ambitious one. The results are dependent on this and the modelled extents. However, areas of this floodplain do flood every few years, suggesting that the required volume for an extreme event could not be found without impoundments of the order of several metres in height.

7.3 Environmental assessment

The Tay catchment contains numerous sites of important conservation and historic value. Quite a number of these are present within the floodplain area (Figure 7-4). These include Meikleour SSSI and Thistle Brig SSSI (near Stanley). Dunkeld Cathedral and the remains of the Roman forts at Cargill and Bertha (near Almondbank) are located within the modelled 200 year flood extent. All these sites would require a more detailed assessment if any scheme to enhance the flood attenuation on the Tay floodplain was proposed.

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

7.4 Agricultural economic assessment

7.4.1 MDSF-based

The estimated total cost of the 5 year, 100 year and 200 year events using the MDSF methodology are shown in Table 7-4. The 100 year and 200 year totals are not significantly different as the modelled total inundated area for these land cover classes was not very different for the two events.

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 from the flood risk location in Figure 7-5. For the Tay the higher agricultural costs, associated with arable and horticultural land cover classes exist in the area 20-50km upstream from the Perth city limit.

7.4.2 Single flood compensation payment based

Table 7-5 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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