Field measurements of Phragmites australis root reinforcement and traits along a riparian zone

Abstract

Phragmites australis L., a widespread vegetation in riparian zones such as rivers and canals, is extensively studied for its ecological benefits such as nutrient removal and hydraulic retention. However, its direct contribution to bank stability through root reinforcement, a key factor for its use in soil bioengineering techniques, has received limited attention. This study investigated the root reinforcement provided by P. australis and its root traits at a soil bioengineering test site on a canal bank in the Province of North-Holland in the Netherlands. Direct measurements of root-soil composite strength were performed using a corkscrew extraction technique at two distinct distances from the canal. Concurrently, root distribution parameters, including Root Area Ratio (RAR) and Root Length Density (RLD), were quantified from extracted soil plugs. Root reinforcement was also indirectly estimated using biomechanical models, incorporating measured root tensile strength and root distribution parameters as inputs. A total of 12 excavations, each 0.25 m2, were conducted for comprehensive root trait analysis at both locations. Direct measurements revealed substantial root reinforcement (max 36 kPa; avg 6–19 kPa). RAR showed effective stabilization values between 0.03 and 0.65 %, peaking at 0.65 % in the area close to canal. Root systems were dominated by fine roots (<0.5 mm diameter), comprising >80 % of total root length and creating dense reinforcing networks. Corkscrew measurements yielded conservative values. Modeled estimates significantly exceeded these field measurements, which is consistent with conventional shear testing. The extensive root surface area (>3.9 m2 m−2) further demonstrates the species' soil-binding capacity, with higher values occurring in hydrologically favorable zones. While the ecological implications of using this widespread species must be contextually considered, its pronounced mechanical reinforcement makes it a highly effective biotechnical tool, particularly in managed environments like canals.