Peatland Action - Guidance for land managers - installing peat and plastic dams

Published: 2019

This guidance note – aimed at land managers and contractors – is divided into three sections.

Section 1 outlines step-by-step best practice techniques for installing peat dams, including the kinds of conditions where they will and will not work, advice on machinery and much more.

Section 2 outlines step-by-step best practice techniques for installing plastic piling dams including tips on the kinds of situations where plastic piling dams are more appropriate and where they will not work, and what to look for one month after installation, monitoring and repairs.

Section 3 covers best practice methods used in the construction of large dams (where the ditch width is greater than 1.5meters), and dam reinforcement.

Note: Before carrying out any ditch-blocking activities, the potential requirement for a SEPA CAR (controlled activities regulations) license must be considered. Further details on this are included in the guidance note.

Section 1 - Installing peat dams

Please see Annex 1 for legislative requirements in relation to ditch-blocking.

Peat dams should be considered the default technique to block ditches or moor grips. Blocking erosion gullies may also benefit the peatland, but this usually requires other techniques to be used. They are much cheaper to install than dams made of other materials, especially where a large number of dams are required. The results blend in with the landscape and the excavator can be used to profile dams and ditches to prevent deep, steep-sided pools being left behind after ditch blocking that might be a hazard to stock. The aim of the dams is not only to hold back water in the ditches, but also to direct water away from the ditch line and back onto the bog.

Circumstances where peat dams do not work

1. Drains on shallow peat

In order to effectively key-in a peat dam, sufficient depth of good quality peat (see peat condition below) is required at the base of the drain. Peat dams are generally unsuitable where the depth of solid peat at the base of the drain is ≤50cm.

2. Sloping ditches or grips

Peat dams are usually only effective on a shallow slope (<12° gradient), although the method can be adapted to steeper slopes e.g. in high flow ditches on slopes, turves should be placed on top of dams at a 45° angle to the flow of water to help prevent erosion. However, if the gradient is too steep, the water flow can cause erosion over the top or round the sides of dams of the dam, resulting in failure and/or increasing the risk of peat slide. Where dams need to be installed on the sloping edges of lowland bogs, we advise that plastic piling is used.

3. On wide drains/ditches

Hand built peat dams can only be made in ditches no larger than 70cm wide and 60cm deep. Machine built dams can be in ditches no larger than 150cm wide and 120cm deep, depending on the peat composition. Above these dimensions, the peat dam structure can become unstable particularly if flow rates are high.

A good site for peat dams with open land and hills — A good site for peat dams

Peat condition

The peat used for damming must be saturated, but not sloppy, and should have a darker appearance with a putty-like consistency. Peat that has been dried and exposed to air will not form a watertight dam. Only peat taken from the bottom and wet sides of the ditch (or nearby borrow pits) should be used. The original ditch spoil is not suitable; as with other dried peat, its structure and water holding properties will have been damaged by exposure to air and weathering.

Availability of suitable machinery

Smaller low ground pressure excavators are capable of constructing peat dams on drier firmer peats, but wetter peats will require wider tracks. Low ground pressure excavators used to be hard to obtain away from peat extraction sites (mostly in the Central Belt and Aberdeenshire). Many more contractors now have wide tracked excavators as part of their fleet, due to their use in wind farm construction.

Low ground pressure excavator digging on land — Low ground pressure excavator

Selecting locations of dams

It is advisable to check aerial photographs for linear changes in vegetation that may indicate hidden ditches before going on site. Ditches are not always apparent on the ground. Vegetation may grow across the top of a narrow ditch, whilst water actively flows underneath.
Ensure there is sufficient depth of solid peat at the base of the drain (≥50cm) to create an effective key for each dam.
It is advisable to walk the line of each drain in advance, and probe the base of each drain at regular intervals in order to determine the depth of available peat, and suitability for peat dam installation.
The intensity of damming required depends on the flow of water in the ditch, and the amount of water held back by individual dams.
Generally dams are installed at 7.5 metre to 20 metre intervals on flat ground, however, dam frequency should increase to between 5 metre and 7 metre intervals on steeper, sloping ground. It is expected that the final water level from the lower dam will rise half way up the next upstream dam. The final water level should be at the peat surface or no more than 20cm below the surface.
Appropriate locations for peat dams can be marked using bamboo canes with plastic marker tape or cut up carrier bags attached to the tops. Select a location where the ditch narrows to form a pinch point. This provides a firmer footing for the dam and uses less material.
Check for small depressions and channels at the edges of the ditch that could effectively divert or restrict the amount of water contained by the dam.
Cracks running parallel to the ditch may need blocking as well.
Where ditches are running parallel, stagger the dam positions, to increase the wetting effect on the surrounding peat. The wetting influence of each dam will extend sideways into the peat for up to 15 metres.

Where not to site dams

Avoid undercut banks, tree roots and thick grass tussocks. If unavoidable and with care, tree roots and grass tussocks can be cut with a chainsaw.
Avoid dry peat banks that will slump when wetted up.
Avoid peat pipes, cracks and fissures where water may suddenly disappear
Avoid dams in the dry edge of the moss as these will inevitably leak due to cracks in the peat. Trench bunding may work better in these places.
Ensure that the dam location is not too close to the junctions with tributary drains, as these points have unstable peat. Two to three metres downstream of these junctions is ideal, as the peat is firm, but there is the added benefit that two ditches are blocked by one dam.
Ensure dams are clear of timber/brash previously placed in the ditch. Even small amounts of timber/brash can make it difficult to install the dam properly.

Method

Installing peat dams using an excavator

Careful planning is important for a machine operation, including planning locations for delivery, access and re-fuelling of machines. Machines that work on bogs need to be low ground pressure and excavators will require a moderate bucket reach to reduce movements. It is important that operators are experienced at working on deep peat and are made aware of the specific risks you may have on your site. It is a good idea for the operator to walk the site before bringing the machine on. On wet parts of the bog, the excavator may need to travel on bog mats.

Method (also see methodology for hand-built peat dams below)

Where possible, use consolidated peat from an in-ditch borrow pit located upstream to create the dam. Avoid leaving deep, steep-sided borrow pits as these can be dangerous to stock. Where the quality of in-ditch peat is unsuitable for creating a solid peat dam, use peat with the right degree of plasticity from a nearby borrow-pit, located near to but not too close to the ditch, and upstream/upslope of the dam.

Remove the vegetated turfs from the surface of the borrow pit and the dam location, ensuring that the root structure remains intact, and place to the side.
Clean out/push away from dam location the unconsolidated peat and debris.
Create a key for the dam into each side of the ditch, with a 0.5 to 1 metre notch on both sides. Carry out a visual inspection of the peat in the vicinity of the key, checking for any obvious cracks or pipes that could compromise the efficacy of the dam. If either is present, select another location within 2 metres and repeat steps 1-3.
Roll the back of the machine bucket along the key to compact the peat and remove any cracks or voids.
On sloping ground, shallow swales that extend out (or on one side) from behind the dam can be added to re-direct water from the ditch line. Wider dams can also be useful on slopes to help encourage water out of the ditch-line and allow it to shed across a wider area downslope.
Regularly compact the peat in the dam with the back of the excavator bucket to ensure an effective watertight seal.
Reprofile the sides of the drain behind the dam to a shallow slope, to enable animals to enter and exit the pool created by the dam.
When the dam is 50cm above ground level place the vegetated turfs across the top of the dam (and in swale if present), ensuring that the top and face of the peat dam is covered to prevent oxidation of the bare peat. Use the machine bucket to tamp down the turfs to ensure good contact with the peat. Vegetated turfs obtained from a nearby borrow-pit can also be used to ‘dress’ the dam.

For further guidance also see the NatureScot ditch-blocking demonstration video.

Creating successful peat dams

Creating good peat dams requires a skilled machine operator who understands what result is required. If possible, get supervision for the creation of the first few dams by someone who has seen then made before.
Dam thickness will depend on the ditch width and will increase as the ditch widens to counter the increased water pressure.
Make sure that access can be gained whilst maintaining stock proof fences and any public access points.
Care should be taken to avoid creating deep open water pools where stock might drown when obtaining peat from in-ditch borrow pits. The sides of the borrow pit should be sloped to allow animals to climb out.
Any borrow pits created beside the ditch should be restored by reprofiling the sides to a shallow slope and reinstating turfs and/or stretching the surrounding vegetation to cover up any bare, exposed peat where turfs have been removed.
Borrow pits located beside the ditch should be staggered and should not be selected in a systematic manner, to avoid creating a line of depressions that could form a new drain line.
The excavated material should be flattened down to almost level with the overall bog surface (allowing for the peat to slump/settle slightly). Large hummocks of spoil should not be left, as they are a prime location for invasive scrub to germinate, particularly on lowland bogs. Instead, the spoil should be placed back into the channel upstream of the newly installed dam.
Get the contractor to distinguish between time spent on site and time spent transporting machinery when quoting.
It can be difficult for machine operators to get their bearings and interpret maps on unfamiliar open ground.
Double-bunded cells should be used for storing fuel. Refuelling points should be located off the wetland site and at least 50m from any watercourse.
Sharp or stationary turns should be avoided as they churn up the peat and dislodge surface vegetation. If this occurs, replace the turves in their original position.

turves that have been dislodged from the ground — If dislodged by turning, replace turves in original position

An alternative method for slopes and wider ditches: re-profiling ditches

On sloping sites or where ditches are wider, better results may be obtained by damming and re-profiling the ditch. This is done by installing peat dams close together, e.g. every 5-7 metres, then using the machine bucket to roll back vegetated turves on either side of the drain in consecutive strips. The sides of the ditch are then re-profiled to a shallower gradient by pulling down peat from the banks and compacting. Water can be directed away from the ditch line using swales behind each dam. The turves are then rolled back over the re-profiled sides and compacted using the bucket. The swales and any remaining areas of bare peat need to be carefully patched with turves, so that the vegetation recovers and peat is protected from erosion. Re-profiling drains also helps make them safer for livestock and other wildlife as it results in a much shallower channel with less severe banks, thereby offering a better chance of escape should anything fall in. Re-profiling requires training and/or experienced supervision to achieve a good result.

Questions that you should ask potential contractors are:

Where will the excavator be delivered and unloaded?

Excavators are transported on large low loader lorries that will require good roads on which to drive and turn.

Where will the re-fuelling point be?

This should usually be off the peatland and the contractor should be prepared with a spill kit, should a spillage occur.

Will the excavator fit through our existing gates?

Often bog excavators have extra wide tracks that mean they will not always fit through standard gates.

What route will the excavator take onto across and off the site?

Work should start at the furthest point and work back to the exit. If this is not done the excavator could become trapped by the rising water levels it is creating by damming. The route should minimise ditch crossing points and very wet areas.

How will machines access the site if further work or maintenance of dams is necessary?

Hand-built peat dams

Generally we do not recommend building peat dams by hand, although they can be worthwhile where machine access is not possible. Building dams by hand is exhausting and it is difficult to achieve a good, long-lasting result without the level of compaction offered by an excavator bucket. It is also more difficult to successfully reinstate smaller hand-cut turfs compared to larger machine-cut turfs. Modern low-ground pressure machines can be taken onto most sites without causing any long-term harm.

A two person team with two spades or a rutter and a spade can successfully make small dams. Each dam can take two people up to 2 hours to complete. In addition, two grabs, that are basically four-pronged forks with the tines bent at right angles, are required to move the cut block of peat. Blocks of wet peat are cut to an approximate size of 20cm x 30cm x 20cm. The principle of the exercise is to create a notch to ensure the dam is keyed into the surrounding vegetation. Then to create the dam using peat from a borrow pit located in the ditch upstream of the dam.

Full description provided next — 1. Remove all unconsolidated peat and debris from where the dam is to be constructed

2. Place any turfs to one side for later use.

3. Cut a notch on both sides of the ditch 1 metre wide, 20cm thick and 20cm deep. Remove the sod.

4. Continue cutting down the back of the sod, and fold into the ditch leaving a hinge at the bottom of the sod. Press and compact each sod with your feet.

Installation of peat dams - Points to remember

Peat dams alone may not be suitable on very large active drains. Machine built peat dams should only be used in ditches ≤150cm wide and ≤120cm deep.
Peat dams are unsuitable on drains (or sections of drain) that are incised/eroded down to the mineral layer.
The type of dam required may vary across a site, or even along the length of a drain, depending on the drain depth/width, flow rate, and/or peat depth at the bottom of the drain.
Even small, seemingly infilled, drains can still be very active so dams should always be made well and spaced adequately in order to ensure their efficacy.
Damming a drain can result in large volumes of water being held back. Drains should be blocked up their entire length where possible to reduce the amount of water being held back by any one dam.
Behind the dam, water needs to be encouraged to seep laterally across the bog on flatter ground, and to shed out of the drain and down the slope on steeper ground. It is not desirable to create large pools of water behind dams.
The use of swales and/or wider dams can be useful on sloping ground to help encourage water out of the ditch-line and allow it to shed across a wider area downslope.
On sloping sites or where ditches are wider, better results may be obtained by a combination of damming and re-profiling the ditch.
When working on slopes, drains should be blocked first at the top of the slope then progressively working downstream.
When working on flatter ground, e.g. lowland raised bogs, drains should be blocked first near the centre then progressively working outwards.
Water flow can vary significantly under different conditions. Where possible the “winter flow” should be planned for.
Where possible, install peat dams at “pinch points” in the drain.
The frequency of dam spacing should be between 7.5m and 20m on flatter ground, however, this should increase to between 5m and 7m on steeper, sloping ground. Dam spacing may also depend on drain size or if dams are supplemented with other material in the channel e.g. stone, bales, brash, felled woody material/debris.
Take good care to tamp down the dam with the excavator bucket to ensure that the peat used to create the dam is well compacted and consolidated.
If turf or peat is required from a new borrow pit adjacent to the ditch, the borrow pit should be located upstream /upslope of the dam, taking care not to create a new drain line via a systematic, linear series of pits.
It is essential that there is a minimum 50cm depth of solid peat at the base of the ditch in order to effectively key-in a peat dam and ensure a good watertight seal.
Hand dug peat dams can be worthwhile on smaller drains (no larger than 70cm wide and 60cm deep) and where machine access is not possible, however, they can be time-consuming and less robust compared to dams installed by machine.

Appropriate Machinery

Using low ground pressure excavator
The optimal size of machine bucket used will depend on the size of drain and size of dam required (e.g. if a wider dam is desirable for a stock crossing point). However, a 4-foot wide, toothed, general purpose bucket tends to offer the most practical option for this technique.

Section 2 - Installation of plastic piling dams

Please see Annex 1 for legislative requirements in relation to ditch-blocking.

This section of the guidance is intended for contractors and land managers who wish to install plastic piling dams on ditches less than 1.5m wide. Plastic piling is light, versatile and slots together on site. If properly installed, plastic piling dams can form a good watertight seal that will last for decades. They are not usually feasible on large, remote upland sites, due to the cost and difficulties of transporting piling and moving it around site.

Plastic piling dams are useful in the following situations:

Where only a small number of dams are required and it is not cost effective to get an excavator on site to make peat dams.
On ground that is too soft and/or unstable to allow safe machine access.
On small, extremely remote sites that are inaccessible by machine.
On active drains that are partially infilled with vegetation, yet hold too much water to allow effective installation of peat dams.