By land, water, and air: an evaluation of the impact of embankment setbacks and two-stage channel design on flood characteristics of the upper River Nith

Van, Breanna (2023) By land, water, and air: an evaluation of the impact of embankment setbacks and two-stage channel design on flood characteristics of the upper River Nith. MRes thesis, University of Glasgow.

Abstract

River channelisation is a method of hard engineering that laterally constrains and straightens a river, with the twofold aims of increasing a river’s hydraulic efficiency and minimising land loss due to floodrelated erosion. However, in catchments with low sediment supply, such as Scotland, rivers typically respond to channelisation with channel incision and bed armouring, which further reduce floodplain connectivity and physical habitat diversity. Recently, two-stage channel design and embankment setbacks have been applied as a restoration method to return sediment mobility to channelised rivers with the goal of improving both biodiversity, bank stability and channel-floodplain connectivity. These techniques include carving out small benches to act as a floodplain and pushing back existing embankments to allow for increased river movement. In late-2019, the Scottish Environmental Protection Agency (SEPA) completed restoration works on an upper section of the River Nith near New Cumnock, Scotland by implementing embankment setbacks and two-stage channel design to two sections of the river with the goal of demonstrating natural flood management approaches for this type of channelised, incised river. This thesis investigated the restoration works completed by SEPA and analysed the difference in flood impacts including inundation extent, flow depth, and shear stress, between the pre-restoration and post-restoration topography using a variety of field survey techniques and numerical modelling software. Specifically, data from previous Airborne light detection and ranging (LiDAR) and bathymetry surveys were used in conjunction with unmanned aerial vehicle (UAV) LiDAR, real-time kinematic Global Navigation Satellite System (RTK-GNSS), and echo-sounding via acoustic Doppler current profiler (ADCP) surveys that were conducted as part of this dissertation in 2022. The digital elevation models (DEMs) created from these surveys were then input into Geomorphic Change Detection (GCD) software and the Hydrologic Engineering Center – River Analysis System (HEC-RAS) flood modelling program to quantify topographic change due to channel-floodplain modification and flood impacts by modelling multiple recurrence interval events. Overall, the findings of this thesis suggest that, for particular design flood events, the embankment setbacks and two-stage channel design have increased flood extent and floodplain connectivity while reducing the amount of flow overtopping embankments and decreasing overall water depth and bed shear stress.

• Item Type: Thesis (MRes)
• Qualification Level: Masters
• Subjects: G Geography. Anthropology. Recreation > GB Physical geography
• Colleges/Schools: College of Science and Engineering > School of Geographical and Earth Sciences
• Supervisor's Name: Williams, Professor Richard and Quick, Dr. Laura
• Date of Award: 2023
• Depositing User: Theses Team
• Unique ID: glathesis:2023-83370
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 18 Jan 2023 12:23
• Last Modified: 19 Jan 2023 14:38
• Thesis DOI: 10.5525/gla.thesis.83370
• URI: https://theses.gla.ac.uk/id/eprint/83370