Author(s): Markus Noack; Lydia Seitz; Stefan Haun; Silke Wieprecht

Linked Author(s): Markus Noack, Lydia Seitz, Stefan Haun, Silke Wieprecht

Keywords: Colmation; Clogging; Multiparameter approach; MultiPAC; VertiCO

Abstract: The hyporheic zone as the interface between surface water and groundwater in fluvial river systems represents an integral part of aquatic ecosystems and plays an important role in fulfilling the ecological functions of rivers including highly complex hydrodynamic and biogeochemical processes. Furthermore, it is a mandatory habitat for different life stages of salmonid fish as well as for certain species of macroinvertebrates. Both hydrodynamic and biogeochemical processes highly depend on the characteristics of the hyporheic interstitial such as hydraulic conductivity, pore space and sediment characteristics. This fragile system can be heavily disturbed by infiltration and accumulation of fine sediments into the riverbed, the so-called colmation processes. The consequence is a decreased porosity and a strong reduction of the hydraulic conductivity, which also limits the transport of oxygen-rich surface water into the hyporheic zone. Given the complexity of colmation processes no standardized quantitative measuring technique exists to determine the vertical depth and the degree of colmation. Most available measuring methods represent different mapping approaches or single parameter approaches. However, mapping methods allow only for qualitative assessments while single-parameter approaches are insufficient to encompass the complex process of colmation. Considering that colmation cannot be addressed by a single parameter, the objective of this study is the development of a multi parameter approach (MultiPAC) to quantitatively assess colmation. MultiPAC consists of four key parameters that are measured in the field to address colmation in rivers: 1.grain size distribution of the riverbed using freeze core sampling and sieving 2.vertical profiles of hydraulic conductivity by a double deep packer system 3.porosity and pore space applying a volumetric approach based on photogrammetry 4.vertical profiles of interstitial dissolved oxygen content with optodes This novel approach allows on the one hand for a quantitative description of colmation and on the other hand for the identification of clogged layers in stratified river beds. First results from different river reaches show promising results. The measured vertical profiles of the dissolved oxygen content in clogged rivers show a strong reduction with increasing sediment depth, which is in line with the measurements of hydraulic conductivity using the newly developed double deep packer system. With additional information about porosity and particle size distribution, an objective database is available to address the degree of colmation in rivers. Aiming at a holistic approach for the quantitative assessment of colmation further measurements are required to achieve distinct relations of the four parameters.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022128

Year: 2022