Hydrological impacts of contrasting gully blocking techniques for peat restoration and natural flood management in a degraded blanket peatland

Erosion gullies are a common feature of degraded blanket peatlands and in recent years gully blocking has been increasingly employed as a restoration approach. The stated aims of gully blocking are typically to stabilise eroding gullies and to rewet the adjacent peatland by raising water tables, but in recent years the potential benefits of gully blocking for natural flood management (NFM) have also been recognised. However, data on the hydrological effects of gully blocking and for different gully blocking techniques are sparse.

We report on a before-after-control-intervention (BACI) experiment of gully blocking in peatland micro-catchments (hectare scale) in the Peak District National Park, UK. Three different gully blocking interventions were made in March 2020: impermeable peat dams, permeable cobble dams, and peat dams with a restricted diameter bypass pipe. The first two interventions represent standard restoration techniques, whereas the piped peat dams are specifically designed to be optimal for natural flood management benefit. The micro-catchments were monitored for one year before and two years after gully blocking for: rainfall, discharge, depth to water table proximate to the gullies (within 2m) and depth to water table distal from the gullies (>10m away). Storm hydrograph data (peak discharges and lag times) were extracted for >120 storms from the rainfall-runoff data. All intervention data were analysed relative to data from a control micro-catchment.

After blocking for all interventions there were significant declines in median depth to water table proximate to the gullies, with the magnitude of the rewetting benefit in the following order: peat dams > piped peat dams > cobble dams. There were no significant changes in depth to water table at the distal locations. Peat dams led to a slight increase in storm peak flows but no change in hydrograph lag times. Stone dams led to no change in peak flows but increases in lag times. Piped peat dams resulted in the greatest changes to storm hydrographs, with substantial declines in peak flows and increases in lag times once the pipe diameter had been optimised to the discharge regime.

Peat dams maximise the rewetting benefits of gully blocking but appear to have limited NFM potential, whereas once optimised, piped peat dams provide maximum NFM benefit and greater water table recovery than stone dams.  These findings are important for restoration practitioners when making decisions on which gully blocking techniques to employ to balance the co-benefits of peatland restoration.