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    How are levels within ‘Inland Extensions’ or ‘estuaries’ derived?


    The inland extensions (from here-on ‘estuaries’) are calculated using the following methods. A shapefile is available from coastal modelling teams to find out which method has been used to derive levels for each point within estuaries:

    1. No data. Where no observed data were available an estuary relationship was established by borrowing a relationship from a similar estuary or assuming that the gradient is flat (Scotland only).
    2. Observed data. Upstream of the downstream boundary levels were derived using an estuary relationship based on observed data expressed as a gradient.
    3. Modelled data. In some cases levels were taken directly from existing coastal modelling studies where these studies were deemed appropriate. Results were taken from the modelling of extreme coastal events only. A background fluvial flow may have been included such as the index flood QMED, but the results do not include any joint probability of extreme fluvial and coastal event modelling. All levels derived using this method are labelled ‘ESTUARY_’ in the Location field of the shapefile CFB_Extreme_Sea_Levels_Estuary_2018.shp available on data.gov.uk.

    The CFB 2018 estuary and tidal river levels are based on the interpolation of modelled levels including defences and so do not necessarily represent the scenario in which there are no flood defences. Flood defences can constrain coastal flood waters, resulting in elevated water levels upstream. Similarly, levels based on gauge analysis at upstream locations represent the scenario in which defences exist.

    As improved modelling becomes available following this 2018 update, the CFB ‘ESTUARY’ levels may be subject to review and further updates. The models in this 2018 update also do not include all models available at the time of this report was written. Detailed models have been included as a priority in regions of particular interest. Further locations will be added in the future.

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