Assessing the geomorphological effectiveness of river restoration using multi-stage channels

Channelisation has resulted in a legacy of rivers characterised by uniform channel geometry and river-bed sediment, and a lack of the flow variability required to provide varied ecological habitat. In rural settings, with no constraints, restoration of such rivers to enable river migration and sediment connectivity often involves channel diversion and re-meandering. Such schemes are expensive due to excavation works and land use change compensation. Further, for deeply incised rivers and in many urban areas, realignment is not physically feasible. Alternative strategies to improve the morphological condition of incised, straightened rivers include the creation of "multi-stage" channels, where embankments are set back and banks reprofiled to create floodplain benches, and in-channel structures, such as flow deflectors, are used to vary flow and induce sediment transport. The effectiveness of such approaches to river restoration have yet to be investigated.
This studentship addresses the need for quantitative evidence to assess interactions between sediment, morphology and hydrology in channels that are restored using multi-stage design. The studentship will be supervised collaboratively by the Scottish Environment Protection Agency (SEPA) and the University of Glasgow. The primary hypothesis is that multi-stage channel design produces significant increases in the diversity of morphology, river-bed sediment and flow patterns. The project uses multi-stage channel restoration schemes on the River Nith (Ayrshire), Glazert Water (Dumbarton) and Pow Burn (Angus), a total of 10 km of river. Integrated monitoring and numerical modelling will answer the following questions:
(1) How do the morphology and sedimentology of multi-stage channels evolve after flow events?
(2) What controls fluvial landform assemblages and their evolution?
(3) What is the effectiveness of different flow deflection structures in initiating geomorphic processes?
To address (1), repeat topographic surveys will be undertaken using the most appropriate technique for each site (UAV imagery & SfM photogrammetry, TLS, RTK-GNSS, total station). Automated, field-calibrated, photo sieving and roughness analyses will be used to map river bed and bar sedimentology. Fixed point photos and meso-habitat (flow/substrate) surveys will also be undertaken. Geomorphic change will be calculated and analysed. Analysis of these data will be used to evaluate over what timescales dynamic equilibrium may be re-established following restoration. To investigate (2), Delft3D hydraulic models of flow pattern will be built, using the topographic survey data. Metrics for 3D landform shape and depth/velocity relationships will be used to map landforms and habitats. Geospatial patterns, and associated controls, between landforms will be investigated. (3) will be investigated through SEPA's trial of a range of different flow deflectors at Pow Burn to initiate bank erosion. Repeat, high-resolution aDcp mapping of flow hydraulics will be used to map geomorphic change, and calibrate Delft3D morphodynamic change models to inform adaptive management of deflectors.
The findings from this project will be significant because incised, straightened, and/or embanked rivers make up a considerable proportion of rivers in the UK with poor morphological condition, and thus poor ecological status as defined by the Water Framework Directive. In Scotland, they account for a significant proportion of 4,000 km of rivers with less than good morphological condition, out of a total network length of 27,000 km. River restoration strategies are needed to improve the morphological condition of these channels, recognising the infeasibility of re-meandering. The project's findings will provide quantitative evidence to guide future restoration practice for these rivers. The main impact of this studentship's findings will be for regulatory bodies, environmental advisory bodies and consultancies.