Peatland ACTION - Technical Compendium - Restoration - 4 Artificial drains

General Guide

•    Check first whether a CAR Licence is required from SEPA (Section 3.1). Only block man-made drains, not natural water courses. 
•    Peat should be used wherever possible, it works well, is cost effective, and requires only a low ground pressure excavator. 
•    Very large dams may require bracing.  
•    Artificial drainage patterns can often be easily identified from aerial images. However, individual drains can be difficult to find on the ground as they may be covered by overgrowing vegetation. Lines of heather can often be a clue in the landscape. It may be necessary to provide GIS information to contractors and/or mark drains, and in some instances dam locations if the site is complex or there are key points where a dam is required. 
•    Drains can also be missed through aerial mapping alone. Contractors should be made aware that they should strive to block all drains they come across when tracking across a site.  
•    Water flow can vary significantly under different conditions in drains aligned with the slope: where possible the peak flow should be planned for.  
•    Contractors need to ensure that they have appropriate information on drain condition, type and variability across the site given the range of restoration approaches. The type of dam required may vary across a site, or even along the length of a drain and can depend on the drain depth/width, flow rates, the peat depth in drain bottom, whether drains have eroded to mineral, and whether they are actively eroding (e.g., the drains have turned into a key-hole shape below the surface and are starting to form gullies). 
•    Damming a drain can result in large volumes of water being held back. Treat the entire length of the drain where possible, to reduce the amount of water being held back by any one dam. The key aim is to displace water out of the drainage line.    
•    Unless using a technique that removes drains altogether, such as wave/zip damming, install dams where drains are at their narrowest. 
•    Drains should be blocked first at the top, then working downstream. 
•    Drain blocking works best when dams are created in conjunction with reprofiling. Areas behind standard peat dams should be reprofiled to create shallow depressions rather than deep pools, unless partially revegetated.  

Dam placement and spacing – general principles 

•    Dams should shed water rather than just create pools. Even small, seemingly infilled, drains can be very active, so, unless wave dammed/ zipped, dams should always be made well and spaced adequately. 
•    Dam spacing depends on the slope of the drain and flow rates. The steeper the slope the closer dams should be spaced. The water should back up to the toe of the previous dam. You could consider the lookup table here (Table 2).  Dams work best on average slopes of 6o or less, but with careful planning can work on steeper slopes. On steeper slopes, consider the risks to contractor health and safety (Section 3.10) and peat instability (See Section 3.3. and Annex 2). It can be very useful to look at slope and slope angle using map contour lines or Digital Terrain Models in the planning phases.
•    The water pressure behind the dam needs to be considered, and therefore more and shallower dams are preferable to fewer tall dams, because the failure of one dam may lead to overwhelming of the next dam downstream.  The more dams are installed the less overall risk of failure across the site.  
•    Dams work best when there is peat in the base of a drain, so a good seal can be created. Dams that are not keyed in are more likely to fail. If your drain has eroded down to the mineral layer consider reprofiling and blocking at another point. (Some drain blocking techniques, however, do not rely on such well consolidated structures; for example, wave dams and zipping rely more on the frequency or intervention and higher proportion of drain line being manipulated, see relevant Chapters for further information). 
•    Always consider what the water will do, try to restore natural flow pathways and displace water over the widest possible area to ‘slow the flow’ and avoid focusing flow into new pathways that may erode out.   

4.1    Peat Dams

Aim of the Technique

To block standard artificial drains (usually less than 1.5 m wide and 1.2 m deep) to help retain water and raise the water table on a peatland where there is still sufficient peat at the bottom of the drain. 

Best Practice 

• Where possible peat dams should be created in conjunction with drain reprofiling (see Section 4.4).  An alternative type of damming with reprofiling could be installation of wave dams and zipping (see relevant sections), however these two methods have not yet been in use for long enough to assess whether they produce effective measures in the long term. Standard peat dams as described here, with reprofiling, have been shown to work across the suite of Peatland ACTION projects over the last decade. 
• Install peat dams where drains are at their narrowest. A general rule is that dams need to be spaced at that the water always reaches the toe of the previous dam. Drains on steeper slopes need dams at short intervals. On sites with very shallow gradients (<1 degree), dams can be spaced more widely with a maximum spacing of 15 m (See Table 2 in the general introduction section on Restoration of Artificial Drains).  
• Spacing may also depend on drain size and whether drain reprofiling is also being undertaken.
• Dams must be a sufficient width to ensure water cannot flow around the dam edge and re-join the same drain line. If the drain is aligned to the slope this is a significant risk. Where possible, construct dams with a bit of a longer spur on the downhill side to encourage spreading out flow laterally. Behind the dam, water needs to be encouraged to seep across the peatland, rather than creating large pools of water behind the dam.
• The height of the dam should be slightly higher than the fall line of the drain so that water cannot flow or seep across the top of the dam.  
• The dams must be formed so they span the full width of the oxidised cross section of the drain i.e., the zone that has sunken due to the drain on each side. On some sites this can be very pronounced and extend up to 2 m either side of the drain. If this is not blocked then water will still be trapped in this swale and will simply flow round each dam shoulder and continue down the drain route. 
• If there is a spoil line adjacent to the drain (typically on the downslope side) this must be broken through or squashed adjacent and immediately upstream of the dam to ensure water can disperse laterally.  Spoil heaps are usually highly oxidised (becoming crumbly) so should not be used for creating dams or infilling when reprofiling. 

• Take good care to tamp down the dam with the excavator bucket to make sure it is well consolidated.  The depth of peat is important; there should be a minimum of 50 cm peat in drain bottom to get a good seal with a peat dam. 
• Donor material of turf required for the top of the dam should be gathered from a donor site that should be upstream and upslope of the dam. Turves should be taken from shallow borrow pits adjacent to (but not too close to) drains. Do not use vegetation from the drains themselves - leave this intact. Borrow pits must be reinstated (for further information please see section 7.6). 
• Where flow rates are high and/or drains are greater than 1 m wide and/or deep, it may be worth considering more substantial “composite” dams (peat used with wood or plastic piling), or plastic piling dams (see relevant sections for further information).  
• Do not use peat dams alone if drain has incised to the mineral layer. Wave dams and zipping may be a more appropriate technique if this scenario is common across the site. Alternatively, if wave dams or zipping techniques cannot be applied, turf-capped mineral core dams, or composite dams with mineral soil instead of peat could be used in some situations to slow the flow of water (see relevant sections for more detail). 

Appropriate Machinery  

• Use a low ground pressure excavator.  
• Bucket size may depend on size of drain and size of dam required.  
• A tilting bucket is useful for dressing of the dams and in some cases a swivel head bucket can be very useful.  
• Hand dug peat dams can be done but can take up to 2 hours per dam and often lead to failure due to poor compression and small turves. 

When most likely to succeed

•    On standard drains which have not incised down to the mineral layer. 
•    On less steep sites (more engineering may be required on slopes).

When most prone to failure

• If drains are close together and run across the slope, then water may be shed from one drain into another because of the dams. Dam locations must be carefully considered. 

• On drains which experience very high flow rates and where peat dams alone may not be sufficient, or on steep slopes. 
• Where borrow pits for turfs have been systematically created alongside drain (particularly when downslope). This risks creating a new drain line, leading to water being directed downslope (see also section on reinstating borrow pits). 
• If dams are constructed by hand, these are often not consolidated enough to hold back water or remain stable over time.  
• If site is affected by peat pipes the internal flow of water may not be stopped by peat dams in drains alone. 
• Where there are “double drains”. If a drain channel has been cut parallel to an older drain which has naturally infilled then larger dams, reprofiling or bunding may be required so water does not flow zigzag like downslope being shed by the dams into adjacent drain line.  

• Very high numbers of deer/sheep/goats or cattle may start using dams as crossing points which could destabilise and erode them. If possible, reprofile the drain sides to prevent dams being the only crossing points, or see whether dams can coincide with existing deer or sheep tracks (perhaps doubling the width or consider composite dams). 
• Where animal crossings are expected, it might be useful to have a large number of dams so animals aren’t focused on a small number of dams, and to build dams high enough so they are not very wet and soft, which would mean they would be particularly prone to trampling damage.
• On previously afforested or heavy scrub sites; tree roots and piping can be routes of internal water flow and make it hard for dams to form good seals. 

Points to Remember

• Peat dams alone may not be suitable on deep and/or wide drains or drains which frequently extend down to the underlying mineral soils.  
• Even small, seemingly infilled, drains can be very active so it is important to assess these drains for restoration too.  

• Peat dam installation can be undertaken in most weathers but snow may obscure drain lines (frozen peat is also less good at making well consolidated dams). 

Further Guidance & Support

Peatland ACTION Project Officers are available to help you with your project. Please contact us as at [email protected] to be put in touch with your local Officer.  

Guidance for land managers on installing peat and plastic dams

Watch video showing ditch blocking with standard peat dams (0:00 to 1 minute 47 seconds)

4.2 Plastic Piling Dams 

Aim of the Technique 

To block drains in order to raise the water table but where peat dams may not be appropriate due to the drain size or slope, where the volume/pressure of water is likely to exceed the shear strength of peat dams, or on key drain exit points on lowland raised bogs (where the peat may be too shallow and/or too oxidised to form good peat dams).  

Larger plastic piling structures can have a visual impact. In addition, land managers and owners may not want to use plastic on their peatland, due to the long-term pollution risk that it creates (e.g., microplastics arise as the material is degraded by UV light, or potential for fumes and leaching of plastics components if the site were affected by wildfire). 

Using plastic piling, and wood to reinforce a structure, is also much more expensive than using peat dams, and the cost and time to transport materials on and off site has to be considered. On the other hand, plastic piling dams can be created by hand, in locations where machinery cannot gain access. 

Best Practice  

•    Piling will only create a good seal if driven into at least 25 centimetres of solid peat or clay. Solid peat is usually found below the 50 centimetres of soft peat in the base of the drain. (i.e., 75 cm into the peat/clay below the drain base). On more porous mineral soils, plastic piling dams may not create a good seal. 
•    The dam wings must extend well into the banks either side of the drain, as wide as the oxidised section of peat extends, so that the water cannot find a way ‘around’ the dam and re-join the original drain line lower down. As a rule of thumb, the width of each one of these extensions into the bank should equal the width of the drain. Just as with peat dams, the idea is to spread water flow across the surface of the bog, rather than holding back large volumes. 
•    If there is likely to be a lot of water coming over the overflow during extremely heavy rain, place branches, heather sods or tree trunks in the flow to stop undercutting of the dam. 
•    Reinforced plastic piling dams can be installed at the end of long runs of peat dams if there are any doubts over whether peat dams can withstand any remaining water pressure. 

Appropriate Machinery

•    Can be done by hand on sites where it is not effective or possible to get an excavator on site to make peat dams. Materials can be carried onto a site if machine access is not possible. 
•    An excavator can be useful to drive in plastic piling, but will break it unless a channel or push board is made to protect the leading edge. 
•    Consider the type of machinery being used to transport materials to site (tracked vehicles can cause less damage). 

When most likely to succeed 

•    On drains greater than 1.5 m wide and/or 1.2 m deep where there is sufficient peat depth in base (>75 cm) or clay soil below the remaining peat. 
•    When dams are well keyed and plastic piling is sufficiently braced (usually with timber).  
•    On flatter sites.  

When most prone to failure 

•    If not reinforced well enough. Very wide plastic dams constructed without additional support tend to belly out in the middle under water pressure and can collapse. 
•    If seal is not sufficient (often sides and base not properly keyed in).  Drying events have resulted in failure due to peat shrinkage around the dams. 
•    When installed in drains with very shallow or no peat in base.  
•    On previously afforested sites. Roots make it very difficult to sink the plastic piling and get a good seal. (Also consider historic tree remnants in deeper layers of peat).  
•    In drains with high flow rates (water can overflow, or splashback can erode base).  
•    Where there is extensive peat piping. 

Points to Remember 

•    Plastic piling dams will need reinforcement on steeper slopes and where drains are wider than 1.5 m. 
•    Dams must extend sufficiently into the bank on both sides of the drain.   
•    Can be very effective and are capable of holding back a lot of water. Consider using different profiles based on flow/dam width. 
•    Plastic dams can catch fire (i.e., arising from vandalism or wildfire incidents) and may leach chemicals over time.  
•    Plastic can become brittle over time and damaged easily. Plastic piling dams are not suitable as crossing points. In some circumstances timber bracing, if substantial, can be used for crossing.  

Further Guidance & Support

Peatland ACTION Project Officers are available to help you with your project. Please contact us at [email protected] to be put in touch with your local Officer.  

4.3 Composite Dams 

Aim of the Technique 

Composite dams are used in place of other types of dam when drains are larger and/or when water flow is higher.  They are often used on wide terminal drains on lowland raised bogs or in small incised gullies. Composite dams could be made from a combination of different materials such as plastic piling, wood, or other materials and are combined with peat.  In special situations, composite dams can also be made using wood and mineral soils. Substantial crossing points can be created with this technique, which could improve access for people and livestock.  

Best Practice  

• Composite dams are generally installed on drains greater than 1.5 m wide and/or 1.2 m deep where other methods may sag, burst or form a poor seal. 

• Composite dams can be very effective and are capable of holding back a lot of water. You may have to control water levels if the drain is very incised (e.g., using pipes or a notch/channel at top of dam) but ideally you plan the runoff to flow back onto the wider bog surface.

• If at all possible with the expected water flows, a sandwiched peat dam is useful to include between composite materials. Sandwiched peat dams can make strong crossing points. Backfilling the downslope face of a timber or plastic piling dam with peat and turf reduces visual impact and reduces the chance of the dam from 'bowing' over time. Back filling the upper slope of a timber dam can improve waterproofing – especially if natural logs or other irregularly shaped timbers are used. If plastic piling is to be used, a peat and turf upslope liner can prevent gapping around the dam when dry peat banks shrink, by keeping the area more moist and vegetated in periods of drought.

• For composite dams with timber, the type of wood used can dictate how long the structure may last and if there are wider environmental considerations. Treated wood could have serious implications on plants, bryophytes, lichens and invertebrates and could impact on water quality, and should therefore be avoided. Resilient wood should be used; hard woods (oak and elm for example) perform better than soft woods. Sitka spruce is not advised as it rots quickly. Fresh wood that is ripped to planks will tend to warp as it seasons. 
• As with other dams, the dam must extend well into the banks either side of the drain. As a rule of thumb, the width of each one of these extensions into the bank should be 1 m unless the banks are very soft/cracked. For depths into the peat, please see the previous sections on peat dams and plastic piling dams.
• If there is likely to be a lot of water coming over the overflow, include a wood or rock baseplate below to stop undercutting of the dam.  

Appropriate Machinery  

• An excavator is useful to drive in plastic piling or installation of any reinforcing wooden structures, or for digging a trench into which wood is installed. Installing composite dams by hand is possible but very hard work and the necessary seal of the dam keying in and degree of compaction of any sandwiched peat may not be achieved.
• Vehicles may be needed to transport materials to site. Materials can be carried in excavator buckets with a thumb, or in a sled. For exceptionally remote and sensitive sites, helicopter delivery could be considered to reduce ground impacts of getting materials on-site but need to be weighed against the environmental impacts of increased fuel use.

When most likely to succeed 

• As with all other drain blocking techniques, they can be very effective when well-built and well keyed into the drain sides.

When most prone to failure 

• As with all drain blocking techniques, if an adequate seal is not achieved and sides are not keyed in far/well enough.  
• Wood can burn so consider risks in case of wildfire or vandalism. The potential impacts of fire on any wooden elements reduces as soon as the works start to retain more water on the bog.

• Plywood was used extensively on larger dams in the past, but the ply was frequently found to fail and subsequently had to be removed at significant costs. 

Points to Remember 

• The materials used can vary and are often dictated by materials available at a site.   
• Timber can be even more expensive than plastic.

Further Guidance & Support

Peatland ACTION Project Officers are available to help you with your project. Please contact us as at [email protected] to be put in touch with your local Officer.  

Watch video on wood and plastic composite dams at around 6 min 45 seconds

4.4.    Wooden, Metal or Stone Dams 

Aim of the Technique 

Wooden, stone and metal dams can be used where peat or plastic dams are unlikely to survive e.g. in high water flows. Wooden dams are often made of several boards to create ‘leaky’ dams. These can impound large volumes of water and then slowly release it, retaining any suspended peat. This is useful in areas of extensively eroded gullies with a lot of bare peat. The aim should be for the captured peat to stabilize behind the wooden dam, and to be stabilised by vegetation over time. Metal dams (iron or steel shuttering) have been used to hold back significant amounts of water in very wide drains. Stone dams are used where there isn’t enough peat depth in the base of the gully to key in peat or wooden dams. These are leaky dams, which should capture and stabilise suspended peat behind the dam, allowing vegetation to develop over time, which further stabilise both the gully base and the dam. 

Best Practice  

Timber dams

•    The type of wood used can dictate how long the structure may last and if there are wider environmental considerations. Only untreated wood should be used. Hard woods perform better than soft woods. Sitka will rot relatively quickly. Woods such as elm or oak are advised as these have a relatively high durability. 
•    The dam must extend well into the banks either side of the drain or gully. As a rule of thumb, the width of each one of these extensions into the bank should equal the width of the drain (this is impractical for wide gullies, in which planks should extend at least 30 cm into the banks). The planks should be well enough keyed in to ensure that water flows primarily between the planks and does not simply undercut the wooden dam. 
•    There must be enough peat in the base to drive the supporting stakes into, to support the dam, and to sink the lower boards into, to prevent water undercutting the dam. 
•    The lower boards should be placed close together to retain water, with gaps left between higher boards to allow water to pass through. 
•    Overspill plates are important as the dams have a smooth, vertical face so water is not slowed by flowing over a log dam, as with well-constructed peat dams. A notch can be cut in the centre to focus the water flow onto a central splash-plate. Splash-plates may also be required along the entire width of the dam if enough water is flowing between the boards to cause erosion. The offcuts from trimming the boards for the dams to length or locally won stone can be used to create overspill plates. 
•    Membrane could be used to reduce permeability of leaky wooden dams if the throughflow is too high. 
•    Do not use fresh wood that is ripped to planks, as it will tend to warp as it seasons. 
•    Backfilling the downslope face of a timber dam with peat and turf can be carried out to improve the dams’ structural integrity and reduce visual impact.

Metal dams

•    Iron sheet piling has been used on very wide ditches, over about 4m in width. This can be installed by vibrating the sheets into the peat using a digger. A peat dam can be installed behind the metal dam, allowing water to be held in place after the metal corrodes. 

Stone dams

•    Stone dams should be a maximum of ~1 m high, to avoid collapse. 
•    Rocks should be ~20 cm in diameter as smaller rocks are more likely to be washed away by strong water flows. 
•    Dams do not need to be keyed into the gully sides or base. 
•    Dams should be shallower on the upstream side. 

Appropriate Machinery  

•    Timber dams can be installed by hand, and are frequently used in areas which excavators cannot access. An ATV may be used to transport the boards to close to the dam site. A mell or vibrating plate can be used to install the dams.
•    Stone dams usually require helicopters to transport stone to site. The helicopters drop the right amount of stone for each dam into the appropriate location, where the dam is then shaped by hand.
•    Metal plates can be installed by vibrating into place with a digger. 

When most likely to succeed 

•    Leaky timber dams are very effective at trapping large amounts of suspended peat, and so are very useful in heavily eroded gullies with large amounts of mobile peat. 
•    Timber dams can be installed by hand in areas where peat dams can’t be installed effectively as they are inaccessible to excavators. 
•    Stone dams can be installed in areas where peat or timber dams are likely to be undercut as there is no peat remaining in the base of the gullies to key them into. 
•    Metal dams can be used in ditches too wide for other dams to work, as they can hold back very large volumes of water. 
•    Backfilling the downslope face of timber and metal dams with peat and turf can improve the structural integrity and reduce visual impact.

When most prone to failure 

•    As with all drain blocking techniques, if an adequate seal is not achieved and sides are not keyed in far/well enough.  
•    Timber dams are particularly prone to erosion around the edges and under the dam. 
•    Wood can burn so consider risks in case of muirburn, wild fire or vandalism. 
•    When materials are used that will rot or corrode before the gully fully vegetates. 
•    Stone dams may be flattened by high water flows. 

Points to Remember 

•    Timber, stone and metal dams are much more expensive than peat dams.  
•    Timber and metal dams will rot/corrode over time. This should be taken into account in the long term planning of site restoration, e.g. by encouraging vegetation to stabilise the captured peat before the dam loses integrity.
•    For timber dams, if the supporting stobs are left higher than the original dam, the height of wooden dams can be raised over time by adding more planks to raise the height of the dam. This allows smaller volumes of water to be retained at any one time, reducing pressure on the dam which could lead to failure, but over time the base of the gully can be raised closer to the peat surface, reducing the height of the peat column which is drained by the gully in the long term.
  

Further Guidance & Support

Peatland ACTION Project Officers are available to help you with your project. Please contact us as at [email protected] to be put in touch with your local Officer.  
 

4.5   Drain Reprofiling in Combination with Drain Blocking 

Aim of the Technique

In combination with damming, drain reprofiling is used to further reduce water loss down artificial drains, to remove incised drain features which can be dangerous for livestock and game, and to reduce erosion of exposed peat sides of a drain. It makes the profile of the drains shallower which can make them safer for animals.  

Best Practice  

•    Reprofiling should always be carried out in conjunction with damming to reduce water flow down a drain.  
•    There are three general techniques: 
1.    Pushing the edges of the drain into the drain line using the back of the excavator bucket.  
2.    Re-turving the drain line, which is more like hag reprofiling, when turves are stretched from the drain side into the drain channel, with borrowed turves used from either side of the drain if required. 
3.    Use of a rollerball. 

Appropriate machinery  

•    Low ground pressure excavators should always be used. 
•    Sometimes it could be better to use a larger machine for the reach (>10 tonnes). This will be dependent on drain size. 
•    Extensions to the jib can be fitted to smaller excavators – these generally can give up to 3ft of additional reach. Smaller machines tend to have a narrower stance, however, and can on occasion not straddle the drain effectively/safely.  

When most likely to succeed 

•    On flatter sites where water flow is lower and reprofiling has been carried out in conjunction with damming along a drain length.  
•    Reprofiling together with installation of dams is seen as best practice and the most effective way to reduce water flow and loss of water off a peatland and support revegetation of artificial drains.  

When most prone to failure 

•    When water flow is higher and not enough dams have been constructed.  
•    When reprofiled edges have not been well consolidated and water can run between and underneath the turves.

Points to Remember 

•    As with all drain blocking techniques it works better on flatter sites where water flow is lower; it can be more challenging on steeper slopes.  
•    This technique should help shed water out into the peatland and accelerate the revegetation on the drains.  

Further Guidance & Support

Peatland ACTION Project Officers are available to help you with your project. Please contact us as at [email protected] to be put in touch with your local Officer.  

4.6.    Wave Dams 

Aim of the Technique 

This is a relatively new technique, which has been applied successfully on a number of sites across Scotland since 2014 and found to produce good results across 100s of kilometres of drains, however, the long-term outcomes (relative to the tried and tested standard peat dam method) are not yet known. The name of the technique stems from the way the dams are shaped and the high frequency of features, which resembles ripples or small waves in the landscape.

Wave dams can be used to block standard artificial drains (typically less than 1 m wide and 1 m deep) to help restore natural flow pathways, retain water on a peatland, accelerate the revegetation of drains and promote the occlusion of drain voids.  

With a rate of around 1 to 2 minute per dam on an individual drain line, this is a very quick technique which has been found to be effective and resilient in the medium term, due to the very high numbers of blockages created. In the right conditions, relatively high levels of daily output can be achieved using this technique.   

Best Practice  

•    Use on a site where there is sufficient vegetation and turves to create dams with. Work should minimise exposing bare peat. 
•    The high density of dams and pools still focuses animal passage across dam heads and compromises vehicle access across the ground which is not good on sporting estates or where there are high deer or sheep numbers. In this scenario, consider using the wave dam and zipping variant.
•    Survey at each location (see below for spacing) how much of a depression has formed in the sides of the drain through subsidence and peat oxidation. The wave dam that is to be created needs to span the full width of this depression relative to the wider surface of the peatland, in order to be effective. Frequently, this is around 3 buckets (1 m either side of the section containing the drain) width, but can locally be more.  
•    Initially insert the bucket vertically to open the turf and pull slightly backwards. Repeat this across the drain line (3 times if doing a 3 bucket width dam) 
•    Then, if working in wet malleable peat, reverse the buckets and push down and backwards to squeeze the peat and vegetation into a bulk. This should be drifted backwards and slight towards the centreline of the drain.  Repeat this action 3 times. It is important to work the sides first pulling these towards the middle and then pack the middle. Alternatively, the middle can be moved first and then this should be squeezed with the sides. The key aspect is that the central mass should be squeezed into place to seal against any through flow. 
•    Ensure vegetation is not smothered, broken up or flipped as the dam is pulled into place. Dams should remain vegetated. Pull the bucket out gently backwards away from the machine. 
•    Using an inverted bucket, ‘squeeze’ the sections of peat into a wave shape. Ensure that blocks are well knitted together in drain line so water cannot flow through between blocks.  Tamp down the vegetation on top of the dam and dress behind it to create shallow slope (see next point). 
•    A pool will quickly form (within hours to days) behind each dam – these can be moderately deep, so the sides and back of the pool that has just been created upstream should also be dressed using the bucket to a give a shallow slope (ca 30%) so that livestock and animals can access/escape the pool.  The base of the hollow can be left bare as it will soon be filled with water and any turf placed or left in the base will likely be submerged. 
•    Ensure the finished height of the dam is higher than the nearest adjacent ground. Initial form up height should be c, 30cm above ground level. These will settle by c. 30% over time. 
•    Take care not to leave features around the wave dam that could form a new drain line, ensure that water during heavy rain can flow laterally over the surface and spread out over the wider landscape.  
•    Keep wave dam spacing very tight. Typically, the dam will occupy 1.5 m. There should then be an un-disturbed zone of ca. 1 m, then the hollow for the next dam 1.5 m. Therefore, crest to crest spacing should be around 4 m regardless of slope. If using wave dams in conjunction with the zipping technique (see relevant section), this spacing may be changed to something more like 7-8 m.   

Appropriate Machinery 

•    An excavator on low pressure tracks with toothed bucket.
•    This technique can be readily achieved with a straight or tilting hitch.  There is no requirement for a rotating hitch. 
•    A 7-8 tonne excavator lacks the power and weight to do this technique well and machines >10 T are recommended. Some contractors exclusively use 8 T machines for the flexibility of deployment, but this method will take longer to complete. 

When most likely to succeed 

•    On gently sloping ground with good vegetation cover. 
•    On sites with wet malleable peat 
•    Minimizing the amount of bare, exposed peat is also a key concern when installing peat dams. These dams have the benefit of vegetation being an integral part of the peat which is pulled in, rather than being ‘capped’ with an excavated layer of peat from elsewhere.  Peat is packed and squeezed into shape to form these features which and then buttressed by adjacent ground. Therefore, they are more robust than conventional dams which are built up using blocks of peat that have been physically moved into place. 

When most prone to failure 

•    On steep ground > 12 degrees due to risk of machine slippage (H&S consideration). 
•    When the technique is poorly executed by inexperienced contractors.  
•    When drains are too large or when drains have incised down to mineral or bedrock (except when done in conjunction with zipping)  
•    On ground with poor vegetation cover. 
•    When the bulks of peat moved are not carefully pressed into each other and water can pass through the spaces. 

Points to Remember 

•    Minimise the risk of vegetation being smothered or flipped as the dam is pulled into place.  Essentially, all that is required is some dextrous movement of the digger bucket. 
•    This technique does not work as well on dry fibrous peat and the approach will need varying in these situations – the turf should be stripped and the peat then worked as for wave damming or by pulling rather than pushing before re-capping with the turf 
•    Can only be used on fairly standard drains (less than 1 m wide/deep)  
•    A universal tilt bucket is preferable for dexterity. Better results are achieved with a bigger bucket of ca. 900 mm on a >10 T machine. 

Further Guidance & Support

Peatland ACTION Project Officers are available to help you with your project. Please contact us as at [email protected] to be put in touch with your local Officer.  
 
Find out more about wave damming and zipping in this useful online guide by Matt Watson Peatland ACTION Project Officer at the Cairngorms National Park Authority.

Watch training video on combined wave damming and zipping technique (CNPA Peatland ACTION team)

The following video shows a good overview of how damming and zipping works and what the end result looks like: 

4.7.    Zipping / zippering

Aim of the Technique 

Zipping is a form of re-profiling that is typically used in conjunction with wave damming. Dams are formed every 8 m with the remaining drain line between each dam “zipped”. Unlike other forms of re-profiling, this technique completely in-fills the drain void and therefore natural surface flow path ways are instantly restored.   

The combined wave damming and zipping approach is a very cost-effective method allowing between 300-400 m of drain to be treated daily. The combined technique is a one-size-fits-all approach that can be used on any site or slope angle that is safe to work. This technique almost entirely removes the incised drain from the landscape, and allows easy movement across the site in All-Terrain vehicles as there are no longer drain voids to get round. It is thought to be resilient to herbivore passage/grazing so enables work in zones with higher deer or sheep use than on most sites.  

Best Practice 

•    Is best when used on drains <1.5 m wide.  
•    Is best when the vegetation in the drain margins is of good quality and intact. 
•    Start below the wave dam, leaving about a metre of ground so that the wave dam is buttressed by this. 
•    Blocks are pivoted across the drain channel from either side of the drain edge in a staggered pattern, inserting the bucket vertically to the full depth, and ensuring the blocks of peat key in behind each other. 
•    The voids created by moving these blocks are filled by bulking up peat from behind the first blocks to ensure no voids remain. For this step, only insert the bucket to half its depth. 
•    To fill behind the half-depth void behind this infill, move further behind the second blocks and insert just the teeth into the vegetation and roots, and gently stretch the vegetation over the void left by the second blocks. 
•    To ensure the blocks of peat are well integrated, tamp down across entire surface of peat that has been moved. 
•    If there is a ridge of drain spoil from the original drainage, especially on the downstream facing side of the drain, use the teeth of the bucket to tease the drain spoil ridge apart and then gently tamp down the surface can help to avoid obstacles to water movement. 
•    This process is repeated until the next wave dam position is reached.
•    The end result of this process gives a zip-like appearance down the drain line. There should be very little sign of the treatment pattern or original drain in the landscape.
•    It is effective on all types of sites, but a shallow depression will remain when this technique is applied in very large drains. 

Appropriate Machinery 

•    A 7-8 tonne excavator can implement this technique, however progress would be typically 30% slower than with larger machines. Smaller machines may lack the ability to compress the peat effectively. Machines >10T and ideally closer to 14T are optimal to do this work effectively. A larger machine may be able to get better compaction of the dams and move peat faster. We do not yet have enough follow-up of this methodology to be able to state whether the larger machines create better outcomes overall, although the visible outcomes are generally better with larger machines. 
•    This technique can be readily achieved with a straight or tilting hitch.  There is no requirement for a rotating hitch

When most likely to succeed 

•    In almost all situations when used in conjunction with wave dams placed roughly every 8 m.
•    This technique works best on gently sloping sites with ‘typical’ (less than 1 m depth/width) drains and with good vegetation cover where the amount of bare and exposed peat is minimal.
•     This technique also has the benefit of vegetation being an integral part of the peat that is pulled in, rather than where reprofiled ground is covered by an excavated layer of turf.  

When most prone to failure 

•    When drains are too large for the machine to straddle the drain. 
•    When not well understood and executed by the contractor.  
•    If used without dams and there is any shrinkage then water flow may flow down the swale that remains and potentially cause new erosion. 

Points to Remember 

•    The work is best carried out with larger machines >13T, although smaller machines may be suitable for sites with e.g. narrow drains despite their lower efficiency. The choice of machine may therefore be somewhat site dependent. 
•    This method requires no variation in dam spacing, regardless of slope and is a one size fits all approach and has attracted strong support from land managers, gamekeepers and plant operators. 
•    Where drains are less than 75% revegetated, the materials lost cannot be compensated for by zipping technique. As noted above, this results in a shallow depression which is still a drainage feature along the same line, just a different shape. This depression is prone to settlement of the disturbed materials which exaggerates the depression over time. Once this depression is created it will be all but impossible to reverse. 
•    This technique is not suitable for deep drains on cross slopes. It should be borne in mind that sites across Scotland are not comparable and techniques should be adjusted to vegetation, topography and rainfall patterns. 

Further Guidance & Support

Peatland ACTION Project Officers are available to help you with your project. Please contact us as at [email protected] to be put in touch with your local Officer.  
 
Find out more about wave damming and zipping in this useful online guide by Matt Watson Peatland ACTION Project Officer at the Cairngorms National Park Authority.

Watch training video on combined wave damming and zipping technique (CNPA Peatland ACTION team)

The following video shows a good overview of how damming and zipping works and what the end result looks like: 

Further Guidance & Support

See our technical compendium for other techniques in the series.

Peatland ACTION Project Officers are available to help you with your project. Please contact us as at [email protected] to be put in touch with your local Officer.