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CHAPTER 7: STANDARD OF PROTECTION

General/Methodology

7.99 The standard of protection is the standard (expressed as a return period) that the defences will withstand with a high degree of certainty. For this study it was chosen to define the standard of protection with a freeboard which provides a high degree of confidence that the scheme will withstand the design flood levels and flows.

7.100 The freeboard was calculated for each defence element using the Fluvial Freeboard Guidance Note ¹⁴ which recommends that different allowances must be used for different scheme appraisal options. Each of these cases is shown in diagrammatic form in Figure 7-1, and includes:

• The onset of flooding is defined as the current return period flow that will overtop the defences under the as-surveyed condition (no freeboard applied). This gives a clear indication of the current flood risk, and the performance of the scheme, but does not provide any allowance for natural or modelled uncertainty.
• The assessment of the standard of protection for an existing defence should be the return period at which the water level equals the level of existing defence minus the allowance for physical processes and allowance for uncertainties.
• The assessment of the threshold of flooding for an existing defence should be the return period at which the water level equals the level of existing defence minus allowance for physical processes.

7.101 Wave overtopping, defence settlement, super-elevation and the type of defence are the physical processes or properties that can lead to uncertainty of how defence components will respond to high flows. Uncertainties in hydrological and hydraulic modelling are related to the quality and quantity of the data available and may contribute to the overall confidence in the modelled water levels.

Figure 7-1: Freeboard Definition

Discussion on methodology

7.102 The standard of protection, threshold of flooding and the onset of flooding were derived for all Category B schemes assessed in detail. Typically, the freeboard allowances for the physical processes are very small (less than 100 mm) and thus the threshold of flooding and the onset of flooding are very similar. The onset of flooding provides the highest standard and is most readily understandable. As a result the standard of protection and onset of flooding are the most useful in understanding the current flood risk and scheme performance and are those used in the database. In addition each scheme is declared in terms of flow as perhaps the most consistent method of demonstrating ongoing performance.

7.103 After discussion with the Scottish Executive, the standard of protection was used to determine the scheme standard, flood mapping and economic appraisals.

Comparison of surveyed and designed levels

7.104 A comparison was made between the 'as-surveyed' levels and the 'as-designed' levels to check the defences are at a similar height to the level they were designed. Whilst there is no check between the relative levels used for the original survey and the current survey, the difference for most of the defence components is relatively small.

7.105 Generally there was a small decrease between the designed and current levels. This may represent settlement of the defences, poor maintenance of the scheme (especially for embankments) or poor post construction checks. The reduction in levels does not significantly alter the standard of protection of the scheme.

7.106 The condition assessments identified some defences that had specific faults. These are significant defects within defence components that could significantly reduce the scheme standard of protection. Where these faults are readily rectified they have been assumed to be repaired under the standard of protection case.

Results

7.107 A number of the FPS were designed to a 100 year flood and the results of the modelling have indicated that few schemes currently protect to the designed standard. Whilst this may initially appear alarming this is to be expected. Return periods may change when flood estimation is reassessed and these can change substantially with new methods i.e. the use of the Flood Estimation Handbook or more recent high flows may alter the return period of previous estimates. Throughout this analysis and within each of the scheme reports the standard of protection has been shown as both a return period and a flow or level. The performance of a scheme in relation to a defined flow or level is more robust and less likely to vary (although channel agradation or degradation may effect this). The changes in performance cannot be accredited to climate change.

7.108 Table 7-1 shows the standard of protection of the schemes with sufficient information available. This is split up into the pre-scheme standard (undefended), the designed standard (design), the standard of protection and the as-surveyed standard (onset of flooding).

Table 7-1: Comparison of scheme design as return period

Table 7-2: Comparison of scheme standard of protection

7.109 Table 7-2 illustrates the general improvement as a result of scheme construction compared to the undefended case. However it also shows the range of scheme design standards and indicates that the net number of schemes offering this standard is lower than originally designed.

Climate Change

7.110 Each of the schemes assessed in detail were also modelled with an allowance for climate change. Whilst the effects of environmental variability will vary with catchment and site specific influences, the modelling gives a representation of how the schemes will respond to the increases in flow and tidal levels.

Table 7-3: Comparison of scheme standard of protection under climate change scenario

7.111 Table 7 3 indicates the range of scheme protection standards, together with the range under the climate change scenario. This illustrates the effect of current predictions for climate change on the standard of protection and confirms that, should the predicted increase in flows and tidal levels occur, there will be a substantial decrease in the standard of protection of schemes implemented under the 1961 Act.

Fluvial

7.112 The assessment of climate change has been modelled using the estimated increase in flood flows for the three main areas of Scotland ¹⁵. This generally reduces the standard of protection, with lower return period floods overtopping the defence components in 2080.

7.113 Impacts of climate change are currently predicted to be worst in eastern areas of Scotland where a 25% increase in flows is a significant demand on the defence level offered by each FPS. A scheme designed to protect a 100 year flood needs to be designed to the 200 or 1,000 year flow to satisfy the condition to protect against the current climate change scenario.

7.114 Many of the schemes constructed under the 1961 Act were not designed to incorporate for future changes in river flows. The current scheme analyses have shown that even without the incorporation of climate change, the design flows and standard of protection have varied significantly owing to additional river flow data and revised methodologies. As the number of gauging stations and the length of flow record continues to grow flow estimates will become more refined and possibly change. Existing schemes standard of protection may therefore change although their performance against a defined flow is likely to be maintained.

Tidal

7.115 Schemes influenced by the tide were assessed for future changes in sea level using the POL¹⁶ methodology to derive peak tidal elevations for 2080. The average increase in peak tidal levels is 15 to 20% with the latter value representing the lower return periods. This increase in levels reduces the standard of protection of tidal schemes.

7.116 The current assessment of schemes influenced both by fluvial flows and the tide employed a joint probability analysis. In the case of climate change, the influences have typically been modelled separately to provide an indication of how these could influence the scheme.

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