Assessing the potential of channel re-meandering for flood attenuation
: a detailed case study of the restoration of the Eddleston Water, Scotland

  • Isabelle Costaz-Puyou

    Student thesis: Doctoral ThesisDoctor of Philosophy

    Abstract

    Natural Flood Management (NFM) focusses on the use of natural catchment characteristics and processes to complement the traditional approach based on hard structures in adapting to the increasing flood risk. The Eddleston Water Project began in 2009 and has led to the implementation of a large number of NFM measures within this 69 km² rural catchment of the Scottish Borders. One type of measure is the re-meandering of a 1.6 km long reach (final length) of the Eddleston Water in its central part. This PhD thesis assesses the impact from this remeandered reach on flood attenuation using a combination of three methodologies.

    First, the observed hydrologic time series provided by the dense local network were analysed before and after the re-meandering, five metrics related to peak flow hydrograph characteristics were selected, and statistical analyses were performed on their median values. Second, the geomorphic changes of the study reach over the period April 2018 - May 2020 were estimated by repeat topographic surveys. A sediment budget was estimated and combined with the changes in geomorphic units, geomorphic mechanisms at play during that period were identified. Third, 2D hydrodynamic models (HEC-RAS) were built to model design flow events of increasing magnitudes with return periods from 2 to 50 years (Q2y to Q50y) and compare flood dynamics, extents and peak attenuation from three channel morphologies: the pre-restoration straightened reach, the re-meandered reach in April 2018 and in May 2020.

    The results showed a limited increase in flood attenuation with a maximum of 1.2% of peak flow attenuation and 15 minutes of peak travel time delay for a Q5y event. Topographic features within the floodplain play a major role in interacting with the flow dynamics and limiting the space available for the flood to expand and as a consequence limited flood attenuation. The channel incision identified by the repeat topographic surveys was found to have reduced the expected flood attenuation by 0.5% for small flood events (Q2y). The combined use of these three methodologies has proved to be complementary. The limited amount of flood attenuation from this re-meandering was first identified from analysing the observed hydrological time series, then the topographic datasets identified channel incision that opposes flood attenuation and lastly the 2D modelling helped to understand further reasons behind this limited flood attenuation.
    Date of Award2022
    Original languageEnglish
    SupervisorAndrew Black (Supervisor), Chris Spray (Supervisor) & Richard D. Williams (Supervisor)

    Keywords

    • Flood risk reduction
    • Geomorphology
    • Re-meandering
    • Restoration
    • Monitoring

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