Large cobblestone dams for natural flood management in wide peatland gullies

Peatland restoration is increasingly used for natural flood management (NFM). In the restoration of heavily degraded blanket peatland sites, erosional gully blocking is used to capture sediment and rewet peat in smaller peat gullies. There is limited prospect for restoration of wider late-stage erosional gullies, however these have potential for large water storage capacities for NFM barriers. A new type of peatland gully block, Large Cobblestone Dams (LCDs), designed to create large temporary in-storm storage volumes are evaluated here. Different modifications to LCD design, including dam-face modification and drainage outlets are tested. Observations of dam pond levels and input discharge are used to evaluate storage functioning. LCDs achieve temporary in-storm storage volumes to the order of 10–100 m 3 . Bare cobblestones were too permeable, with peat turfed dam faces improving storage utilisation. A drainage outlet is essential for inter-storm drainage to enable storage availability for following storm events. Opening of drainage holes and blocking of the outlet was observed immediately following peat turfing, however this stabilised following 1 year. With an optimised design, LCDs can provide relatively large dynamic storage volumes in peatland gullies which are not restoration priorities which with targeted design could provide NFM benefits to downstream communities. • Wide late-stage erosional gullies are not priorities for peatland restoration. • Large cobblestone dams provide large storage volumes relative to traditional gully blocks. • Appropriate dam design is essential for effectiveness storage utilisation. • In-storm storage behind these dams has potential for Natural Flood Management benefits with targeted drainage outlet design.