Optimising peatland gully block design for Natural Flood Management

Abstract

Blanket peatland in the UK was severely degraded prior to and throughout the twentieth century, characterised by expansive bare peat and erosional gully networks. Through extensive restoration work, the hydrological functioning of many blanket peatlands have been improved. In addition to returning carbon and biodiversity ecosystem services, flood risk reduction benefits have also been evidenced. This has promoted interest in the use and potential optimisation of gully blocks used in peatland restoration for natural flood management (NFM). This thesis investigates the efficacy of standard and NFM-optimised peatland gully blocks as NFM interventions. Microcatchments are commonly used for hydrological studies in peatlands and are delineated using digital elevation models (DEMs). The impact of DEM source, resolution, and error on microcatchment delineation is demonstrated using stochastic simulations. Microcatchment areas are highly sensitive to DEM error, particularly in low gradient peatlands, suggesting these DEM factors should be accounted for, by using DEM error simulations to identify potential sources of uncertainty. An automated method for estimating the water storage volume of gully blocks is developed and tested for 200 different blocks. Underestimation of gully width with the automated method produced a lower total storage volume of 259m³ compared to 1290m³ by manually defining gully edges. Using a before-after-control-intervention (BACI) study with microcatchments, standard peat dams are shown to be unsuitable as NFM interventions, significantly increasing peak discharges. Once optimised to the observed discharge regime, piped-peat dams significantly reduced peak discharge by 26% and increased lag times by 147% relative to the control. Monitoring of individual dam storages illustrated that peak attenuation was achieved through the filling of storage, highlighting the importance of a drainage outlet to free up storage capacity. Both standard and piped-peat dams also provide restoration benefits through raising gully-edge water table and sphagnum regrowth. A 1-dimensonal hydraulic model is adapted to optimise pipe diameters in a chain of piped-peat dams. Additional reductions can be achieved by changing the pipe diameter relative to the current design, with an additional 7.8% peak reduction using unique pipe diameters. Peatland gully blocks can provide effective storage-based flood attenuation with correct block design, specifically by including a drainage outlet. This outlet does not compromise other peatland restoration benefits.

Date of Award	12 Mar 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Martin Evans (Main Supervisor), Tim Allott (Co Supervisor) & Emma Shuttleworth (Co Supervisor)