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Natural Flood Storage and Extreme Flood Events Final Report

4 Environmental and ECONOMIC assessments

4.1 Ecosystems and historical assessments

Within the four case study catchments there are few ecological constraints as the majority of the floodplains are subject to agricultural uses, which have very little ecological benefit. In extreme return period flood events the low frequency of inundation would be unlikely to change the agricultural use (even cereal production can cope with greater than 1 in 5 return period floods) so there will be very little ecosystem impact from inundation. If the frequency of inundation increased for lower return period events then the potential for ecosystem change also increases significantly which would require investigation and consultation with all the appropriate stakeholders.

The presence of designated sites/areas of conservation value within the floodplain would need consideration, if their maintenance in the natural landscape requires at particular hydrological regime.

Some of the case study floodplain areas also contain sites of historical value, which could be affected by a change in the flooding regime. These include scheduled ancient monuments (SAMs) and listed buildings (LBs), for which records are maintained by Historic Scotland. There is the potential threat of erosion by flood water undermining buildings and monuments. Local velocities and the duration of any inundation are likely to be significant factors in this regard. Historic sites also often comprise remains that survive buried beneath the ground surface, as well as upstanding structures. The impacts on these buried remains also need to be considered as sites of national importance are present within some floodplain areas. One significant effect of flooding and its aftermath is a sudden rise in the height of the watertable (i.e. saturated soil conditions) with a consequent potential re-invigoration of chemical and biological processes which are likely to produce adverse conditions for the preservation of certain buried remains.

4.2 Rural land management

The impact of flood inundation on a particular rural land cover is dependant on the timing, extent, frequency and duration of flood events. All these factors will be affected by the current climate change predictions for Scotland. Grass swards and tree cover can typically withstand and recover from prolonged flood inundation events far better than arable crops. For arable crops the timing of the inundation event within the particular crop growth stage and the duration of the flood are critical factors in determining the loss of yield at harvest, and hence the selling price for the crop. Intensive grassland production would only be significantly affected if prolonged flooding occurred during the spring period when the grass sward should be growing rapidly following its winter dormancy period.

The economic implication of a flood in the rural environment is dependent on the actual land cover that is inundated and its value (measured by price for the produce and the prevailing price of the land). Arable crops inherently have a greater value than grass crops. For example, the average value of equipped land with vacant possession in Scotland in July 2004 was £6,763/ha for arable, £6,113/ha for dairy and £4,241/ha for mixed use (VOA, 2004). Clearly, there will be substantial regional differences in rural land value across Scotland, based on a number of interrelated factors. In some areas the grass sward may be particularly important for silage or hay production, or as grazing cover for commercial livestock units (whether for meat or milk production). The loss of the grass sward could ultimately affect the quality and therefore price of the livestock produce, if the lost feed is not replaced.

4.2.1 Flood damages based on MDSF methods

We have evaluated the potential impact of increased flooding on the floodplain in terms of the agricultural economics. This evaluation was based on an assessment of the appropriate land cover datasets for the floodplain area and generic land classification/crop value datasets for these land cover classes. This type of evaluation is similar to that developed for the modelling and decision support framework (MDSF), which is an integral component of for the development of Catchment Flood Management Plans (CFMP) in England and Wales (EA, 2004).

The calculation of agricultural flood damages incorporated into the MDSF software is based on the crop loss in the year that a flood of a given magnitude occurs and the Agricultural Land Classification (ALC) maps. The ALC system is based on the long-term physical limitations of land for agricultural use. Factors affecting the ALC grade are climate, site and soil characteristics, and the important interactions between them. In Scotland, the ALC data (known as the Land Capability Classification for Agriculture) are maintained by the Macaulay Land Use Research Institute (MLURI) in Aberdeen. Unlike the situation in England and Wales, Macaulay do not receive any central funds to support the maintenance of their soils and land capability datasets, so they have to pass this cost onto customers buying or leasing the data. Due to this cost it was decided to make use of the Land Cover Map 2000 (LCM2000) data as a surrogate for ALC data.

LCM2000 data is based on digital interpretation, with ground truth validation, from Landsat satellite imagery. The raster (gridded) data are based on 25m x 25m grid squares, though the minimum area mapped as a single polygon is 0.5ha (0.005km ² or 8 grid squares). There will clearly be uncertainties in the digital data interpretation (i.e. individual land cover class recognition) when looking at the land cover classifications at a detailed scale, but at the catchment scale these uncertainties are probably less important. The LCM2000 data are also assumed to be the current land cover in the case study catchments as there has unlikely to have been any major shifts in the proportions of the various land cover classes in the last four years when viewed at a catchment scale.

Within MDSF the agricultural flood damage assessment has a number of key assumptions:

1. The flood is single flood event in any one year lasting about one week in duration, which can happen in any month of the year with equal probability.
2. The flood destroys any arable crop which is occupying a field at the time of the flood.
3. The estimates assume a complete loss of crop, less savings on uncommitted variable costs and uncommitted machinery costs, plus clean up costs.
4. Arable cropping assumes typical rotations.
5. Flooding on grassland is assumed to reduce energy from grass, which requires substitution by bought feed.
6. Grassland assumes a mix of grazing and forage conservation, with allowances where relevant for relocation of grazing animals in the event of a flood, savings in forage conservation costs, and clean up costs.
7. Grassland is classed as either intensive or extensive reflecting improved and unimproved grassland, with or low stocking densities respectively.

An area weighted average flood damage cost (in £/ha) is assigned to each ALC class (or grade), based on 2001 values for agricultural commodities in England and Wales. Each ALC class is assumed to have a typical mix of land cover classes for that particular grade of land. For example, ALC grade 1 land (excellent quality) is assumed to have 5% coverage by horticulture, 85% by intensive arable and 10% by extensive arable. At the other end of the grading scale, ALC grade 5 land (very poor quality) is assumed to have 100% extensive grass coverage.

The 22 detailed land cover classes on inland and estuarine areas within the LCM2000 datasets were reclassified into 10 major land cover classes. The original spatially referenced polygon data in LCM2000 were also converted to 25m x 25m grid data within the GIS. The 10 land cover classes were:

1. Water
2. Bare land/rock
3. Built-up (inc. urban, suburban, towns, villages)
4. Wood/forest
5. Short grass (improved and intensive)
6. Long grass/rough vegetation (unimproved and extensive)
7. Bog
8. Moorland
9. Arable (cereals)
10. Arable (non-cereals)/Horticulture (inc. potatoes, brassicas, carrots, sugarbeet, salad crops)

Within this project the agricultural flood damage analysis made use of the major land cover classes 5, 6, 9 and 10 listed above which are the dominant land cover classes on many natural floodplains and for which typical annual flood damage costs existed from MDSF. The proportion of the 200 year floodplain that is under agricultural land cover (classes 5, 6, 9 and 10) in each of the case study catchments is given in Table 4-1. This indicates that productive agricultural land is a key feature of all the case study floodplains.

Unfortunately, a typical damage cost was not available from MDSF for forest/wood land cover, which is probably more prevalent on Scottish floodplains than it is for floodplains in England and Wales. For each of these four classes a typical flood damage cost was calculated, based on average cost specified by MDSF for the combination of each land cover class and each ALC cover. The typical flood damage costs were calculated to be:

• Arable (non-cereals)/Horticulture = £1,500/ha/year
• Arable (cereals) = £500/ha/year
• Intensive grass = £50/ha/year
• Extensive grass = £20/ha/year

The figures have been based on the England and Wales values and have not been revised to reflect Scottish conditions. Therefore the economic analysis described later in this report should only be viewed as providing indicative costs. A revision of the above costs to reflect the prevailing Scottish agricultural economic conditions would be a useful development of this work.

The arable (non-cereals)/horticulture cost is substantially higher than the arable (cereals) cost because this land cover class contains a mixture of some very high value crops, such as potatoes, sugarbeet, brassicas and salad crops together with lower value crops such as peas and beans.

The estimated agricultural damage cost associated with a specified flood event was assessed by overlaying the predicted flood outline for that flood event onto the gridded land cover data within the GIS framework. The spatial analysis tools were then used to calculate the area of each land cover class that was inundated for that flood. The inundated areas calculated for each of the four major land cover classes were then multiplied by the appropriate flood cost and all the sub-totals finally summed together to give the overall annual flood damage cost for that particular flood extent.

In reality, if specific floodplain areas were targeted for enhanced flood attenuation then any arable and horticultural land that currently exist on the potentially inundated land would potentially cease to be a viable agricultural production choice, given an increased risk of flooding. However, the decision on whether or not to convert from arable/horticulture to grassland will be determined by the estimated frequency of any enhanced flooding. Agricultural land owners, unless suitably compensated, would probably not cease their arable/horticultural production on the floodplain if the estimated frequency of enhanced flooding in a proposed scheme was greater than about 1 in 10 years (Morris et al, 2004), as they would still believe that this would be a risk worth taking for the possible return on the produce in the years when the land does not flood. The cost of converting any arable or horticultural businesses to grass based businesses would also require consideration in any proposed scheme.

4.2.2 Single flood compensation payment

A simpler method for evaluation the economic impact of enhanced flooding on the floodplain to the landowners affected was also applied. This method involved calculating the potential annual cost that might be needed to compensate all the affected landowners to have their land actively utilised for temporary flood storage when a flood of a given magnitude occurs and was based on typical flood compensation costs mentioned in some of the recent publications on the subject.

For example, a recent discussion on flood and coastal defence funding options in England and Wales by the Oxford Economic Research Associates (OXERA, 2001) indicated that under current levels of agricultural subsidy a farmer would have to be offered £100-£300/ha/year to gain any financial benefit from the creation of a flood storage area, which would generally be under a fairly extensive grassland management regime rather than a more intensive agricultural system with an inherent higher value. English Nature have identified that additional environmental benefits may match or exceed this value for the best environmental sites (Risk & Policy Analysts, 2001). Morris (2002) has suggested that a change in land cover to extensive grassland for the purposes of washland creation would probably reduce annual financial returns by £200-£300/ha, so this could be used as the basis for compensation payments.

Farmers in the Somerset Levels and Moors Environmentally Sensitive Area, for example, already receive annual payments of £125/ha to retain permanent grassland and £200-£430/ha to maintain wet grassland on the floodplains, primarily for environmental enhancement objectives. Based on these figures a conservative estimate of £300/ha/year has been used for the whole inundated area for all land cover classes (excluding classes 1,2 and 3 - water, bare land/rock and built-up) to compare with the more detailed method based on costs for individual agricultural land cover classes. It should be stressed that this value has been based on agricultural values for England and so will only give an indication of potential costs in Scotland. One useful development of this economic analysis would be to revise this value to make it more appropriate for the agricultural economic conditions in Scotland. The value should also be assessed in relation to other agri-environment payments currently available to Scottish farmers, such as the Rural Stewardship Scheme.

This £300/ha/year figure was probably based on the assumption that the floodplain affected was covered by grassland, rather than any higher value arable and horticultural land. Clearly, there would be an extra cost for converting any arable and horticultural fields to grassland (in terms of farm business reorganisation costs), which would require consideration if any proposed scheme to enhance the flooding of agricultural land were to be taken forward. The cost is therefore the total potential compensation cost if the entire natural floodplain to the extent of the modelled flood outline is inundated, for example, if the 100 year flood is spread over the 200 year floodplain area to a shallower depth. The compensation payment has not been varied according the severity of the flood and hence its spatial extent.

4.3 Built-up areas

The amount of built-up area (i.e. urban, suburban, towns, villages) within the case study floodplains will affect the assessment of what 'natural flood storage is potentially available for flood mitigation purposes. Figure 4-1 gives an indication of how the proportion of built-up area (land cover class 3) within the 200 year flood outline varies with distance upstream of the flood risk area, for the White Cart and Tay catchments. In the White Cart the 200 year flood outline includes substantial built-up areas (>70% built-up) 1-2km upstream from Overlee gauging station. The area approximately 3 to 5 km upstream from Overlee also contains built-up area in the range 20-50%. In the Tay catchment the proportion of built-up area within the 200 year flood outline is much smaller than in the White Cart catchment. Upstream of Perth city there are built-up areas (with 10-25% coverage) at distances of approximately 40km and 65km. Any enhanced natural floodplain storage scheme would have to be designed to avoid flooding these built-up areas.

4.4 Floodplain assets

The use of new flood storage areas on river floodplains could potentially affect some rural houses/farms/settlements, or the operation of various utility structures, for example pumping stations, pipelines, electricity sub-stations, cables, transportation routes (roads, railways) and communications (e.g. masts, cables). Where any potential new flood storage areas are identified it would be necessary to assess whether or not the flooding might affect any of the overground or underground assets in the area. Most of the man-made assets on the floodplain will now have been mapped in a GIS format by the Ordnance Survey for their base mapping or by the organisation responsible for its construction, installation and/or maintenance. However, it was not possible, for the purposes of this project, to acquire spatial data in a GIS format for all the man-made assets in the case study river floodplains because some organisations were unwilling to provide the data, did not reply to data requests or proposed charges that were too costly for the project budget.

4.5 Development and flood management plans

The potential siting of new flood storage areas on river floodplains could also have a direct impact on local development plans in the catchment or existing/planned flood management plans for the river system in question. If a potential new floodplain storage area was identified then all the appropriate Local Authorities (e.g. planning and flood defence departments) would have to be consulted to determine what, if any, plans exist for each of the case study rivers, how the potential new flood storage areas might affect them and whether they had any concerns on the concept of enhancing the natural floodplain storage in some way.

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