Small Grips & Gullies

Grips (man-made drains) have been dug across much of Yorkshire’s upland peatlands. Most were cut in the mid-20th century to improve the land for agriculture. However, many have become badly eroded and the impact of drainage combined with other factors such as wildlife and past overgrazing has led to the development of large eroding gullies in many areas.

By blocking these grips, Yorkshire Peat Partnership can help to restore the water table to the peatlands and prevent further peat erosion.

Grip blocking techniques vary depending on the condition of the grip so it is essential that grips are analysed using both aerial photography and surveys on the ground prior to restoration. The size of the grips, the base material and the peat depth are all factors determining the method used to block each grip. The 3 materials Yorkshire Peat Partnership uses to block grips are peat, stone and heather bales.

How do we restore grips?

Peat dams

Peat dams are used on smaller grips where less erosion has occurred, usually those that measure up to and including 1m wide. These grips will ideally have a peat base and a peat depth of 50cm or more.

Peat is gathered from the base of the grip or alongside it and is used to create a wedge-shaped dam that is wider than the grip either side to prevent water flowing around the dam. The dam must be built higher than the surrounding ground level to account for subsidence as the dam ages.

A small, crescent-shaped overspill channel is created on the down slope side of the dam to ensure excess water is dissipates across the moorland without causing surface erosion.

Average spacing of the dams is 7.5m but this is adjusted depending on gradient and vegetation conditions. All bare peat, including the top of the dam, is revegetated using previously set aside vegetation to prevent oxidisation of the peat.

For more details see ‘Technical Specification 1 – Blocking with peat dams’.

Stone dams

Stone dams are used in larger grips measuring over 1m wide and up to and including 2m wise. They are also recommended for use in grips that have eroded down to the mineral or where peat depth is shallow. These are grips for which peat dams would be difficult to construct or less effective.

Moors for the Future and its partners in the Peak District have developed this method and Yorkshire Peat Partnership is now including it in our programme of works. The purpose of stone dams is to reduce the speed of water flowing along the grips to enable the trapping of sediment in the base. This prevents the sides of the grips from drying out, cracking and collapsing.

Yorkshire Peat Partnership identifies a suitable location in the channel for each stone dam. The stone is air-lifted to points close to the dam locations and the dams are built by hand. They are built to between 0.5m and 1m high with a mix of 150mm-400mm sized stone and are higher at each side than in the centre to prevent scouring around the sides. Stone dams have a steep face on the upstream side and a slope of 45o on the downstream face.

It is possible that stone dams may need two or more stages of activity to raise the level of the gully bottom over time. Where natural revegetation does not take place on the captured sediment, reseeding, heather brashing or planting of species such as cotton grasses and Sphagnum spp. may be required.

For more details see ‘Technical Specification 3 – Blocking with stone dams'.

Heather bale dams

Ideally, a single, large bale should be used to span the full width and depth of the grip or gully. However, a small grip or even a relatively small gully may, be too wide for a single bale. Therefore, heather bale sediment traps may need to be constructed from multiple bales.

Using a low ground pressure excavator the sides of the grip should be dug out to create a “notch” to enable the bale to be keyed in to roughly one-third of its length. The base of the gully should also be excavated so that roughly one-third of the depth of the bale is keyed in. Material should be banked up either side of this to create two bunds either side. If more than one bale width is needed the bales must sit tightly side by side. There should be no gaps left below or beside the bales and the base or sides of the gully.

Cut a ‘V’ notch in the middle of the top of each bale 10cm wide and 5cm deep. This prevents water from being forced around the side dam in high flows.

For more details see ‘Technical Specification 2 – Blocking with heather bales'.