Conserving Bogs
Methods and techniques for Management
If the peat used in construction is derived on-site, it should be excavated from the zone in front (bog-side) of the bund. The borrow pit may act as a reservoir for water storage and help maintain moisture levels within the peat bund (Rowell, 1988) preventing shrinkage and cracking. In an effort to deter erosion from waveaction against the bund face, the borrow pit should not directly abut the bund face, but should be inset to leave a wave 'shelf'. This platform may also provide a convenient working location for the excavator. (Figure 5.39) and act as a platform for rafting (see 5.2.5) vegetation (Wheeler and Shaw, 1995).
5.1
As the bund is built up, peat should be well compacted Figure 5.38 An unnatural edge (ii) causes cracking but not 'overworked'. The gradients of the bund edges and shrinkage in contrast to an undisturbed edge are determined by the properties of the peat but as a (i) which generally has shallower gradients and a guideline, 30-40 covers most situations. Where lagg fen. slopes are too steep, the bund may be prone to slumping as moisture content changes through wetting and drying; with shallow gradients, a larger volume of material is required to attain the same height. Streefkerk and Zandstra (1994) offer guidelines for bund dimensions based on experimentation at the Bargerveen (Table 5.3).
Figure 5.39 Bund construction showing the layout of the platform and borrow pit. Table 5.3 Bund dimensions. Water depth at inner side of bund (m)
Height of bund (m)
Width at top of bund (m)
0.50 – 0.75 0.75 – 1.00 1.00 – 1.50
1.50 2.00 2.50
3.00 4.00 5.00
Bunds be constructed allow forofshrinkage if the peat is poorly humified or It may should be necessary to extendtothe sides the sluiceand so itsettlement is keyed further into the peat banks. This very (80% + moisture content). helpswet to prevent seepage and erosion around the sides of the sluice. To stop over-topping and erosion of the bund, provision for outflows should be made at convenient locations. Overflow levels can be altered by using adjustable 90 angle pipes on the inner face of the bund (see 5.1.3.2). Peat sods (containing vegetation) or poorly humified peat should be layered over the finished construction to stimulate vegetation colonisation and deter erosion of the bare peat surface. Scrub should be prevented from establishing on bunds as it promotes drying and cracking. 137