Technique of the Week: Bunding
If you’re not sure why restoring peatlands is so important, take a look at our FAQ.
For World Wetlands Day, we’d like to revisit this series with a tricky – but interesting – technique called “Bunding”.
Bunding is a technique used to:
Overcome cracks and pipes in deep layers of peat
Catch and hold rainfall and slow the lateral (internal) movement of water from the bog, particularly one that is persistently dry
Stop the flow of surface water across a peatland area
The terminology around bunding does vary but essentially bunds can be constructed in peat itself (by using compacted peat) to stop the internal flow of water (internal bund), or can be constructed on the peat (again, using compacted peat) to stop the surface flow of water (surface bund). Bunds can also be constructed to do both – this is usually referred to as “trench bunding”. See figure 1:
Trench bunding is a useful way of restoring the hydrology of a bog while limiting the impact on surrounding areas. For example, on lowland raised bogs (often adjacent to agricultural fields or commercial forestry, particularly here in the Southwest) bunds hold water on the restored bog and reduce water loss to surrounding drains.
Bunding can also be useful when dealing with the artificial edge of a bog. Kirkconnel Flow (Dumfries) is a good example of where bunding has been used along an edge of lowland raised bog which had been cut for peat in the past, leading to the long-term drying out of the more intact dome.
It’s important when creating trench bunds not to have the area enclosed, often referred to as cells, too large; this can result in the wind causing too much wave action across any standing water (eroding the surface bund) or putting too much pressure on a single bund feature which makes it at higher risk of bund failure.
We can reduce this risk by using lots of bunds, creating lots of cells, often in a “fish scale” formation around the edge of a bog – see figure 2.
When creating surface bunds it is also important to make them as stable as possible by making sure surface bunds are capped with turves (healthy peat and its vegetation). Often, animals like to use these drier ridges as walkways, and deer in particular can cause erosion.
Bunds can also fail to prevent the flow of water if there’s too much woody debris in the peat; it is important in any technique where peat is moved and used as a restoration feature to ensure it is compacted well and knits together – otherwise water can move through the feature and will, over time, erode and potentially damage the feature to the point where it no longer works. This makes it challenging when using the technique on site which have been previously afforested. However, even in these instances the technique, if done well, can work brilliantly to raise the water table and improve the hydrology and function of a bog. A great example of this is Moss of Cree in Dumfries and Galloway.
This technique can be tricky to get right; it takes great skill and understanding of machine operators. To make sure this technique is appropriate for a site, and we are not ultimately going to cause any stability issues, we now follow a specially developed peat slide risk assessment process.