IAP-24-012

Assessing the eco-hydrological and geomorphic effects of floodplain restoration.

Naturally functioning floodplains are a key part of river corridors and provide a range of important ecosystem services (Tockner and Stanford, 2002). Through hydrological connections to rivers, they store water thus helping to attenuate floods and maintain baseflows during droughts (Acreman and Holden, 2013). They also facilitate biogeochemical processing of nutrients, thus aiding water quality remediation, and carbon sequestration. Despite their relatively small area in a landscape, floodplains support a disproportionately high biodiversity due to dynamic geomorphic and hydrological processes that create a variety of habitats often over relatively short distances (Cluer and Thorne, 2014). Thus, floodplains are a key natural asset, but many have been impacted by centuries of human modification (Entwhistle et al., 2019). The conservation and restoration of healthy floodplains potentially could mitigate the projected increasing frequency of hydrological extremes of flooding and drought associated with climate change and reverse biodiversity losses. The importance of restoring interconnected river-floodplain ecosystems, is supported by the UN Decade on Restoration, and by the DEFRA Plan for Water in England and the River Basin Management Plans in Scotland.

However, understanding of the hydro-ecology of restored floodplain systems in the UK is not well constrained due to a lack of studies and is, mostly confined to lowland rivers (e.g., Clilverd et al., 2022). Upland floodplains remain understudied and differ by having lower nutrient status and higher energy. Restoration of floodplains often involves the removal of flood embankments and riverbank erosion protection. Removing these constraints could lead to increased channel movement and the re-development of complex river and floodplain features. Increased duration and frequency of flooding could influence vegetation communities by modifying the degree of waterlogging, nutrient deposition and seed dispersal. Furthermore, superimposed on these changes, climate change is projected to increase the magnitude and frequency of hydrological extremes. Understanding floodplain responses to these multifaceted drivers of change and their controls will help underpin successful management of floodplains and plan restoration projects elsewhere in the future. By using a case study of a planned upland floodplain restoration site on the upper River Dee in Aberdeenshire, this PhD studentship will help to advance our understanding of floodplain functioning and restoration.

This PhD study will be mainly centred on the Mar floodplain near Braemar in the Cairngorms National Park. The PhD will build on earlier monitoring (Addy and Wilkinson, 2021), through considering a wider area of the floodplain. Repeated detailed topographical surveys using a differential Global Positioning System (dGPS) will be used to quantify changes in geomorphology. Changes in water table pre- and post-restoration will be monitored using the existing network of piezometers distributed across the floodplain and additional piezometers. Additional measurements, for example soil moisture and groundwater oxygen concentrations within the restored floodplain, will also be taken to provide additional insights. New vegetation quadrats will be established in representative vegetation communities to assess the distribution of vascular and non-vascular plants. The vegetation data will then be modelled as a function of piezometer water level observations and other potential environmental controls (e.g., soil type, nutrients, soil moisture and oxygen concentration) to understand current hydro-ecological associations. To extend the range of conditions covered, further floodplains in the region could be assessed in the same way. Changes in vegetation communities in response to hydrological modifications initiated by the restoration are likely to be too slow for the duration of the project. Modelling will, however, enable the simulation of the hydrological impacts of the restoration under both current and projected future climate which will, in turn, permit assessment of potential vegetation responses (e.g., Clilverd et al. 2022; Thompson et al. 2023). The modelling will employ the coupled MIKE SHE – MIKE 11 system following approaches employed previously (e.g., Duranel et al., 2021). Using the approach employed by Clilverd et al. (2022), floodplain-wide estimates of hydro-ecological conditions will be extracted from fully distributed hydrological / hydraulic model results.

Year 1

Familiarisation with existing data, plan additional monitoring and install new floodplain instrumentation. Topographical surveys will also be undertaken to measure changes in river corridor form and record any high flow trash lines. At the same time, the student will undertake a literature review on the hydro-ecological character of floodplains and scientific methods to give a good grounding in key principles needed for a successful study. This will be used to inform a structure of the study and key research questions. The student will undertake vegetation surveys, soil nutrient sampling, and sensor downloading and analyse hydro-geomorphic data collected to date.

Year 2

Development of the MIKE-SHE – MIKE 11 model based on the data collected and continuing field data collection and processing, for model parameterisation, calibration and validation. Further vegetation surveys will be undertaken. Soil sampling and analyses will follow standard protocols. Sensitivity analysis will allow exploration of future changes in flows and their potential impact on the floodplain. Presentation of initial results at a scientific conference will be encouraged.

Year 3

Further collection of field data. The relationships between environmental variables (e.g., hydrology, nutrients, soil characteristics) and floodplain vegetation communities will be assessed. Climate change projections will be developed using the current UK projections (at present UKCP18) and employed to force catchment and floodplain models to establish changes in floodplain hydrology and in turn vegetation community composition under a range of scenarios. The student will begin to write up the thesis. Further dissemination of results at a scientific conference will also be encouraged.

Year 3.5

Carrying out final model runs and an overall assessment of any changes in hydrology, ecology and geomorphology over the last three years. The student will focus on writing up the thesis. Funding ceases at the end of 3.5 years.

Over the duration of the PhD, the student will establish skills in a variety of fieldwork techniques including vegetation surveys, utilising hydrological sensors and topographical surveys. The student will also make advances in using Geographical Information Systems (GIS) to analyse and present spatial data, the application of statistical software and in the application of state-of-the-art hydrological / hydraulic modelling. The student will have the opportunity to attend formal training modules (e.g., statistical analyses, GIS and hydrological/hydraulic modelling).