Impact of Large Woody Debris used for Flood Mitigation on In-Stream Nutrient Cycling.

Numerous flooding events in the UK over recent decades have highlighted the need for a range of flood mitigation measures. These include both large scale engineering solutions and smaller scale natural attenuation methods such as the introduction of large woody debris (LWD) into streams. The latter have received an increasing amount of attention due to their low cost and the fact that they are more visually appealing than most engineered structures. Whilst research has shown that LWD can potentially be effective in preventing flooding (Dixon et al, 2016; Thomas & Nisbet, 2012), little work has been undertaken on its wider environmental impact, despite the fact that it has potential to alter habitats and nutrient cycling (Krause et al, 2014).

This PhD project will look at the impact of LWD on in-stream nutrient cycling processes. Understanding the fate of nutrients (N and P) in river systems is particularly important because they receive diffuse and point source inputs from agricultural run-off and sewage treatment works respectively. Excess nutrient loads can lead to eutrophication and associated degradation of the aquatic environment. Bed sediments are often the sites of most intensive nutrient cycling, and they can provide some natural capacity for rivers to self-cleanse (Palmer-Felgate et al, 2009). Addition of LWD changes the hydrology of a stream, altering the sediment accumulation, and modifying nutrient cycling.

Sampling will be undertaken at sites established on the stream network at Nottingham Trent University’s Brackenhurst campus as part of an ongoing project with the Environment Agency and Southwell Flood Forum to look at the effectiveness of LWD for flood prevention in the local agricultural catchment. Diffusive Equilibrium in Thin films (DET) gel probe sampling, and conventional water sampling and measurement will be used to understand the cycling of nutrients (N, P) and related chemical species (Fe, S, C) within the bed sediments and water column respectively. Sites immediately upstream and downstream of LWD will be compared to analogous control sites in reaches without LWD to establish the impact of LWD on nutrient dynamics.

The results will inform future catchment management options by providing a detailed understanding of the impact of LWD on nutrient cycling. Furthermore, taken together with information from the existing project on flood mitigation, the results will provide the first integrated summary of the impact of LWD on rivers and streams prone to flooding. This is an important step change in knowledge that will allow policy makers and catchment managers to deploy LWD in future with a much better understanding of the likely consequences.

Specific qualifications/subject areas required of the applicants for this project (e.g. First degree in specific subject area):

UK 1st Class/2:1 Bachelor’s degree (or UK equivalent according to NARIC) (essential); UK Masters degree (or UK equivalent according to NARIC) (desirable) both in relevant subject (geochemistry; geology; geography; environmental science; chemistry; microbiology).