With the steep and rocky nature of the catchment that was burnt on Mulloon Creek Natural Farms, a flash runoff event is likely to result in a significant volume of soil and ash being exported from the site, which carries with it a range of negative water quality and aquatic ecological implications. The team constructed a series of sediment-retention weirs, utilising vegetation that was either killed by the fire or willow that was coppiced along the creek.
(File: 1 brush weirs left) Until groundcover re-establishes, these broad log & brush weirs (sills shown in red) offer a considerable capacity to retain soil and ash on the site (yellow shading).
(File: 2 brush weirs left 2) Another brush weir designed to intercept the flow as it concentrates along the left margin of the valley floor, just downstream from the previous image.
A front and side view of another brush weir, built in an adjacent valley. Construction involves logs being staked and wired to the bed to provide a solid foundation, with a tangle of branches and brush then wired regularly along its length. Note the ‘double-U’ that is utilised to protect the flanks of the structure during high flow conditions (lowest in centre, apex upstream). When these structures are viewed under high flow conditions, the roughness of the brush breaks up the energy of the flow considerably. Clearly these biodegradable structures have a shelf life, so vegetation management provides the long-term mechanism for retaining any captured sediment
Brush layering provides an alternative approach in this more confined section of channel. The cut branches of a dead Acacia have been placed with the butt upstream and branches down, then wired to a central anchor point. Once built, lopper are utilised to ensure that the centre of the structure is the lowest point to minimise the chances of the structure being outflanked by flow around the edges.