Biostabilisation of fluvial sediments: an improved device to address an old problem.

Moritz Thom, Holger Schmidt, Silke Wieprecht, Sabine U. Gerbersdorf

Monday 29 june 2015

14:20 - 14:35h at Oceania (level 0)

Themes: (T) Sediment management and morphodynamics, (ST) River morphodynamics

Parallel session: 2B. Sediment - River

In present studies the stability of sediments is predominantly described by means of gravitational forces of single particles or aggregates and physico-chemical interactions (like van der Waals, etc.). In contrast, adhesive forces caused by the presence of biofilm are less considered. This is despite the fact that they can influence the stability of sediments significantly (up to ten times as compared to abiotic sediments in recent experiments). Neglecting these stabilization mechanisms (called hereafter biostabilization) in river morphology can thus introduce large errors. However, these impacts are difficult to incorporate due to the absence of easy-to-apply methods that are capable of handling the high heterogeneities of biological growth on sediment surfaces. In order to close this gap a method (MagPI: Magnetic Particle Induction) has been developed that determines the adhesiveness of a biofilm surface on the meso-scale (measuring surface: A = 0.25 cm²). Briefly, an electromagnet in combination with an adjustable power supply is used to attract ferromagnetic particles which have been spread on the measuring surface before. Then, the corresponding magnetic flux density (mTesla) for different thresholds is related to adhesiveness. Even though this method is already well-established and has been successfully applied in different studies, its full potential could not yet be utilized due to a lack of standardization. In this paper a further development of the MagPI is presented, implementing a standardized measurement procedure by the synchronization of the power supply and a camera system. The attraction of particles is automatically evaluated with the help of image analysis such that additional parameters and mechanical properties of the biofilm are obtained to address the phenomenon of biostabilization in spatially and temporally high resolution. In order to demonstrate the applicability of this method, representative results of long-term experiments (under defined environmental boundary conditions, at different seasons) are analyzed. A regression analysis, between results from erosional studies in a straight flume (SETEG) and the adhesion measurements, demonstrates a high applicability of the improved method with R² = 0.84. The presented method marks a necessary contribution to the investigation of biostabilization in fluvial sediments and is a new way to address an old problem.