Drain (grip) blocking in the Cairngorms National Park

December 14, 2021

 

Working with the combined wave damming and zipper re-profiling techniques

By Matt Watson (Peatland ACTION Project Officer at Cairngorms National Park)


Wave damming


In early 2016 Scottish Power Renewables reported to the IUCN on a new grip blocking technique they’d developed in conjunction with their consultants SCL Ltd. This wave damming technique used a rapid damming method to produce a high frequency of dams along grip lines that look like ripples or waves. Put simply this approach pulls bulks of peat backwards and bunches it up to form a dome shaped dam taking around 1-2 minutes per feature (see diagram).

 

Diagram

Since 2016 this technique has been used by many practitioners with great success, however there were concerns about the robustness of the technique in more extreme situations, for example where the grips were steep and very active or where the grip is eroded down to the mineral material. Another issue that for both wave damming and conventional damming was that these techniques retain large pools or reaches of water which take a long time to terrestrialise. These pools can be hugely beneficial for wildlife and biodiversity but are sometimes hazardous, especially for livestock and game and they can make access over the site with 4x4 vehicles almost impossible.  Furthermore these water bodies focus passage of herbivores and humans alike over the dam heads which can compromise these and cause failures. 

Image


In 2017 while managing grip blocking under Peatland ACTION for estates in the Monadhliath area SCL further adapted this technique based on practical work and helpful suggestions from a contractor. This resulted in an approach termed zippering (see details below) that effectively closed the void between the dams ensuring easy movement around these sites and negating almost all of the concerns about the robustness of wave damming.  This technique has now been widely used in the Monadhliath area and has been the go-to technique being prescribed by Peatland ACTION Project Officers supporting work in and around the Cairngorms National Park since 2019.


Zippering


The zippering process involves pivoting a bulk of peat and intact turf from the drain margin partially across the drain void as shown in the diagram below.
 

Diagram 2

This bulk ideally remains attached at its downslope edge so remains firmly anchored. A second bulk is then moved from the opposite side and slightly downslope off the first. This inter-locks behind the first. Shallower bulks are pulled into the voids behind these first movements using the teeth of the bucket. The process is then repeated down the drain line to the next wave dam location, with bulks interlocking like teeth in a zipper.  With care this technique leaves very little evidence of the drain line and produces an almost fully vegetated surface between dams. Some voids occasionally remain but these quickly fill with water and with time Sphagnum spp. 


In drain lines with bigger voids this approach may leave a shallow, well vegetated, central depression, hence retaining the dams is key to ensure water is dispersed laterally. Using this combined approach the wave dams can be spaced a bit further apart - typically every 5-8m. Complete infilling removes risks associated with water passing under or through dams and means that drains which are incised down to mineral material can be robustly dealt with. Dam failure becomes almost inconsequential and less prevalent as there is little or no water pressure behind them and there is no longer passage damage over the dam tops - the ‘desire lines’ for passage are now between the dams. See before and after images of drains dammed with the combined approach:

Before and after


Practical application


The combination of these techniques can be taught to skilled operators in just a few hours and after a little further mentoring they can be left to work independently. Typically operators working moderate sized features will achieve an average work rate of c. 300m/60 dams per day. In easier areas this can go up to as c. 600m/day, occasionally with very large features rates can dip back to c. 200m/day.  The work is best carried out with larger machines >13T as smaller machines lack power and their buckets are too small for the work – we typically see 30% less productivity from an 8T machine. This combined method requires no variation in dam spacing, regardless of slope and is a one size fits all approach that attracts strong support from land managers, gamekeepers and plant operators alike!  The Cairngorms Project Officers will continue to promote this as the preferred technique for drain/grip blocking across the Cairngorms National Park are and have adopted this for our recent New Entrants program for contractors  with great success.

Wave dams


Matt Watson is currently a Cairngorms Peatland ACTION Project Officer and was formerly the lead consultant with SCL during the development work with Scottish Power Renewables and in the development of the combined technique.
 

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