Peatland ACTION - Technical Compendium - Restoration - 6 Flow Management

General guide 

The type of intervention required and material to use is often very site specific, with key indicators being the extent of vegetation or depth of peat in the gully base.  

Vegetated and Peat Filled Gullies

•    It is necessary to deal with water flow that is further eroding the bottom edge, or eroding the sides, of the gully. This is achieved by creating small pools of water within the gully, where water is diverted into these pools.    
•    It is important to avoid damaging stable gully bases.  
•    Where possible, consider methods to stabilise and revegetate areas of eroding peat. 

Mineral or Very Shallow Peat in base of gullies 

•    Where there is insufficient peat at the base of gullies, there are several options available. Mineral gullies may need stone dams or other specialist solutions to slow the flow. It may be possible to create a new channel in the middle of the gully, allowing water to flow down this, rather than allowing it to run down the bottom edge of gully sides.  
•    Be aware that not every mineral base will generate the right material that can be used to slow the flow. Work should be carried out on a site by site, and gully by gully, basis.  
 
A range of suggested techniques are described in the following sections (Surface Bunds; Stone Dams)

Aim of the Technique 

Surface bunds are < 50 cm high and typically span a wider feature than dams to slow and break up the flow of water through a gully system or are applied on areas of bare peat in order to reduce erosion. Bunds can catch a lot of sediment which reduces peat loss and facilitates revegetation, enabling the creation of new peat-forming habitats. Bunds can be used to slow the surface flow of water. This is particularly important in areas of micro-erosion. It also helps reduce pressure on other restoration measures (e.g., reprofiling and geo-textile installation). 

Surface bunds can be made from range of materials such as: 

•    Turves/peat (preferred if good material can be found on site in borrow pits) 
•    Coir logs (can’t be staked in on mineral. Need a peaty substrate).  
•    Wood (untreated round wood, untreated sawn wood) – wooden bunds are similar to wooden leaky dams but are only 2-3 planks high. 
•    Peat filled geo-textile sacks. 
•    Heather bales 
•    Mineral (gravel/finer material found in the gullies) 
•    The use of sheep wool for surface bunds (e.g., inside coir bag) will require special considerations and discussion with SEPA in order to avoid potential contamination of water courses with e.g., sheep dip chemicals  

Best Practice (see also Section 5) 

•    Think about creating a chain of sumps (shallow depressions or hollows that collect water) to slow flow and reduce sediment load in stages.  Care must be taken not to break into substrate in peat-bottomed gullies so that water leaches underground into the mineral layer. 
•    In wide gullies, the amount of material needed to create a bund across the full width of the gully may become difficult to secure into place. The amount of water held behind such wider bunds may be more likely to contribute to bund collapse during high flows (and potential for a chain of such events down the gully). In wide gullies, it may be better to install a staggered series of ‘baffles’ diagonal to the gully sides but that only go part way across the gully. Baffles direct the flow into the centre of the gully but slowing the flow rate by forcing the water to meander around the baffles.  
•    Baffles are often made from timber or stone and generally kept more porous in the upper part to allow some flowthrough and sedimentation behind the baffle (similar to a leaky dam).  
•    If a gully is eroded down to the mineral layer, build a small mineral bund across the gully to 30 cm high, placing peat on top of mineral bund, then turf over. Place bunds approximately 10 m apart. Modify this to suit site conditions: if peat in the gully is >50 cm deep build a peat/turf bund as above every 7- 10 m. An alternative may be to use leaky wooden dams if residual peat depths in the margins of the bare area allow
•    Start at the top of the system and work down, slowing the flow as you go.  
•    Smaller bunds are required where flow is less. Size of bund may be increased down the system. 
•    Bunds should not be too high as you do not want them holding back lots of water. It is preferable to hold back lots of shallow areas of water. 
•    Dams/bunds should be lower in the middle so as to allow overspill in the middle of the gully rather than on the sides, and must be keyed in well to the gully sides so water cannot find a way to erode a new path around the bund.  
•    Mark out where you want the bunds/dams. Ideally a contractor should be able to identify where to put these in.  
•    Do not use turves from the gully bottom – they have grown there and if disturbed could cause further erosion. The exception is in areas that exhibit a significant density of hags/gullies - here the only eligible turf may be in the gully base as the remainder is needed to track upon or be used for reprofiling. In locations where a sump has been created to source material for the bund (or where the upslope side will be perennially wet), the vegetation won from said sump should be used as the cap on the bund.     
•    Bunds can be used as ’deflectors’ (e.g., spur bunds) to move water away from erosion feature (e.g., at the base of a reprofiled hag).  
•    Mineral bunds (created using exposed mineral deposits) can be used to create surface bunds in gullies which have eroded down to the mineral layer. They could also be created in drains which have eroded down to the mineral layer to encourage water flow away from eroding edge of drain (particularly on cross-slope drains).  

Appropriate Machinery  

•    Bund installation can be carried out by hand depending on the material.  
•    Turf/peat bunds can be constructed using a low ground pressure excavator. 
•    Tracked vehicles often less damaging for material delivery. In some cases, when transporting a lot of material, helicopter use can be advisable. 

When most likely to succeed 

•    In gully systems with less water flow, and where the right bund material is selected for the substrate.   
•    When good care has been taken to key bunds well into the base and sides on the gully. 
•    When lots of bunds are installed – reducing water build up and pressure on bunds downstream. 

When most prone to failure 

•    When there is a lot of water moving through the system. 
•    When the bunds are too big, holding back too much water and sediment, or when they are poorly constructed, particularly at their edges. Large turf/peat bunds can hold too much water and erode at the edges or burst. Deep water will prevent revegetation and kill off any turfed areas 
•    When there are high herbivore densities on site.  
•    Where there is very soft, sloping ground. 
•    Bunds made of mineral material will probably wash out, so careful consideration of their longevity in aiding recovery should take place, if used at all. Straw bales do not work as material for bunding.

Points to Remember 

•    A lot of water can flow through these gully systems. Bunds can very quickly silt up and be over-topped. Sites may need re-visiting to increase bund height in the future. 
•    If the whole system cannot be done, then concentrate efforts at the head of gullies and work downstream. 
•    In areas with high water flow it is vital to consider robust spillways to accommodate flow.  
•    Work can be costly and therefore needs to be well planned and executed to have maximum chance of success. 
•    Understand CAR licensing requirements.   

Further Guidance and 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.

Aim of the Technique 

Stone dams are one of the last available options in very deep gullies with high flows of water (max 2 m wide). They can also be used where mineral bunds were found to have washed away. Stone dams are used when gully erosion is down to the mineral layer (i.e., where there is no alternative), or where peat is less than 50 cm deep. Peat or timber dams will not key into the surface and water will scour underneath either of these dams in such situations. The intention is to trap sediment, thus enabling growth of vegetation, slowing the flow of water and reduction of erosive forces, and ultimately raising of the water table in the peat adjacent to the gully.  

Best Practice 

• Consider all available alternatives first. Stone dams can be very expensive as it requires helicopter drops of heavy loads. Although some material may be borrowed nearby in some perhaps rare circumstances.
• Look at the cost versus benefit and monitor the results as this technique is expensive and requires suitable weather for flying. It is not worth the investment for only a few stone dams.   
• Stone dams can be a very effective technique which is not prone to erosion or damage by fire. They can be fast to implement, with 60 to 80 dams possible per day (once helicopter up and running). 
• Marking out the dam locations takes time and must be done by an experienced project officer.  Focus should be on pinch points, confluences, or where gully floor changes from peat to mineral or vice versa.  
• Source stone appropriate to the local geology, 15-40 cm in diameter (a random mix is fine). It depends what you are trying achieve.  Size of stone is important to the intended purpose of the dam.  Use larger cobbles where a leaky dam is required, or smaller mineral where the dam is meant to be watertight. 
• Use a pick-up point for stone by helicopter as close as possible to the site. This reduces flight time and cost. 
• Drop the stone using a modified skip, as this will quickly deposit the stone exactly on the dam location. Once the stone is dropped, each dam will require some movement of stone by hand to get the right shape and profile.  
• Space dams appropriately for the slope and the intended aim (7-8 m is an average, but wider or closer spacing may be required). Dams will be required more frequently on very steep slopes. Each dam to be marked out at appropriate locations by an experienced contractor or the project officer. 
• Dams should be no higher than 1 m, the downhill slope of the dam should be 60° and the uphill slope 45°. The dam should be higher at the sides than in the middle (approx. 60 cm). The average dam is 1 unit of 750 kg stone across a gully less than 1 m wide. If machine access is possible then re-profile and turf the gully sides and key into the dam. 
• Stone dams can backfill with sediment very quickly, so consider if lots of small, leaky stone dams would be better. 

Appropriate Machinery 

• Helicopter able to lift 750 kg of stone. 
• Small digger to fill skips with stone. 
• Fuel bowser for helicopter. 

When most likely to succeed 

• A very robust method in gullies less than 2 m wide.  
• When not installed on peat.

When most prone to failure 

• On very steep slopes if spaced too far apart – space closer together on slopes. 
• If stone is too small (less than 10 cm diameter). 
• When not installed on mineral or rock.

Points to Remember 

• Consider alternatives first.
• Can be used where there is too shallow a substrate to support timber dams.  
• Useful for gullies receiving high flow rates to slow water down.  
• Useful where you can’t access the gully with an excavator, and the gully is eroded to mineral base.  
• Contractors receiving stone on the ground must have experience of dealing with helicopters.  

Further Guidance and 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.  

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.