Bioengineered riverbanks

Posted on March 23, 2016

Streamside riparian zones support diverse and specialized plant communities and provide animal species with habitat, shelter and migratory opportunities. However, human pressures are such that measures are frequently needed to counter the problem of riverbank erosion. While bank protection has often been approached from a hard civil engineering perspective, there are ecological and aesthetic advantages to be gained from bioengineering techniques that encourage the growth of vegetation and assist the recovery of native plant species. To compare plant communities associated with different methods of bankside restoration, French researchers sampled 38 river embankments in the foothills of the Alps. The study sites included banks with (1) riprap (rock armour) protection, (2) mixed protection (bioengineering techniques with riprap at the foot of the bank), (3) bioengineering using vegetalised open-web retaining walls, (4) bioengineering using willow brushwood bundles at the lower part of the bank, and (5) natural riparian willow stands. The taxonomic and functional diversity of each plant community was measured, functional diversity being assessed by reference to biological traits related to colonization and plant succession. In total 177 plant species were recorded. Taxonomic diversity was significantly lower on riprap than at the other sites. In terms of functional diversity, all five types of site differed significantly from one another, with riprap being least diverse. Species richness and functional diversity were highest on mixed and natural banks, but very low for riprap banks even after 3-7 years of colonization. Because levels of total plant cover were low at the mixed sites, the degree of revegetation wasn’t the best indicator of plant diversity. Rather, diversity seemed to be more directly related to habitat heterogeneity, with structural complexity and the range of potential niches being highest on the mixed and natural banks and lowest on riprap. These results highlight ecological similarities between bioengineered habitats and unmanaged riverbanks.

Reference: Cavaillé, P. et al. 2015. Functional and taxonomic plant diversity for riverbank protection works: bioengineering techniques close to natural banks and beyond hard engineering. Journal of Environmental Management 151, 65-75.