Design by Nature in a Confined Flood Alleviation Scheme: Analysis of Form-Process Feedbacks and Morphological Evolution

Abstract:

Abstract

Many conventional hard flood risk alleviation schemes have been detrimental to natural geomorphic processes and have damaged fluvial habitats. This is primarily due to the over-riding focus on managing flood risk by dictating channel capacity and hydraulics, which is not always conducive to the promotion of geomorphologically-healthy and diverse conditions that allow and promote natural processes.

This paper explains how the principles of fluvial geomorphology had a large influence on the design, construction and post project monitoring of a flood alleviation scheme in Wales within a heavily confined river corridor that is designated as having special ecological status; without adversely impacting on flood risk.

The challenge was to ensure that the physical habitat required by the important species (including Atlantic Salmon and Ranunculus) were retained and that the surrounding infrastructure and properties were not at risk of being undercut as a result of scour in the confined high energy channel. A geomorphologically-guided soft engineering approach was taken to promote local morphological diversity and flow diversity, utilising information from up and downstream natural river reaches, and general geomorphological principles. The proposed layout was modelled in 1D to understand the effects of the reprofiling on flows, allowing for a basic assessment of coarse sediment transport to be undertaken. A combination of terrestrial laser scanning and contact GPS surveys were used to monitor morphological evolution post construction, and to determine how morphological form adjusted post-construction within the confined channel.

This paper will introduce the guiding principles of process restoration that influenced scheme design, and then report on the morphological evolution of the river channel that occurred as river processes produced and maintained a dynamic, diverse and healthy physical habitat.

Keywords: Process Restoration; Form Process Feedbacks; Fluvial Geomorphology; Flood Risk Management; Terrestrial Laser Scanning; Morphological monitoring,; DTM; SSSI; Salmon; Ranunculus; Environmental Engineering.