The River Roding in an inter tidal river and these works were undertaken 100m upstream of the Barking Barrier, Greater London. The works were part of an Environment Agency environmental enhancement project with both flood defence and nature conservation in mind.
Above: Unloading of the brushwood fascines
Intertidal rivers are subject to two daily periods of inundation with some rivers having a intertidal range (water depth) of 5-6 metres. The inter-tidal alluvial soils are subject to rapid drawdown which leaves the riverbank very saturated and results in significantly increased pore water pressure that reduces the shear strength of the soil and shallow surface failures are common. However, in areas with mature strands of vegetation cover, noticeably on the River Roding, with Scirpus maritmus (Sea club rush) & Phragmites australis (common reed) in the upper bank area, the root and rhizomes appear to create drainage paths, reducing pore water pressure, decreasing the bulk weight of the bank and increasing soil shear strength and stability.
It has been noted that where cyclic deposition exceeds erosion and sediment accretion occurs then vegetated establishment is more likely. One key objective for a bioengineering revetment is to establish this positive depositional process. Naturally, seed within the inter-tidal sediments is deposited and colonises. Brushwood Fascines (faggots) or unplanted coir rolls can be used to create sediment traps to build up sediment after each inter tidal event. In the case of the River Roding two techniques using brushwood fascines were used. In the mouth of the inlet to the backwater a brush mattress technique was installed, whereby the entire surface of the riverbank in covered with 300mm of brushwood held in place by a grid of 1.4m long chestnut posts. The posts were driven into the alluvial sediment using an adapted piling hammer on the end one of Salix’s long reach excavators. The second method was used on the upper banks with lower water depths and consisted of parallel strips of brushwood faggots.
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Both Scirpus maritimus (Sea Club Rush) and Phragmites australis (Common Reed) were planted using cell grown plug plants although the main revegetation process appears to have been hrough natural colonisation of the Scirpus maritimus in the silt which is trapped and build up within the brushwood revetment.
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