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The infiltration and accumulation of fine sediments in gravel riverbeds (clogging, colmation) is a natural process, especially in rivers with heterogeneous particle size distributions. However, natural rivers are characterized by regularly occurring flood events that lead to bed alterations and hence, flushing of infiltrated fine sediments. In the case of high non-natural fine sediment inputs and/or regulated low flows, this fragile balance between clogging and de-clogging is disturbed and may finally lead to heavily clogged riverbeds with well-known ecological consequences, especially for macroinvertebrates and gravel-spawning fish.
This study presents the application of a novel approach called MultiPAC (Multi-Parameter Approach to assess Colmation) that assesses the efficiency of an artificial flood event on de-clogging of the riverbed of a near-natural bypass channel.
In contrast to existing methods for determining colmation, which typically use qualitative approaches (e.g., mapping) or single-parameters (e.g., fine sediment contents), MultiPAC is designed to measure four in-situ key parameters, notably the particle size distribution, the porosity, the hydraulic conductivity, and the dissolved oxygen content. In particular, the combined measurements of hydraulic conductivity and dissolved oxygen along vertical profiles of the riverbed (VertiCO – Vertical profiles of hydraulic Conductivity and dissolved Oxygen) with a spatial resolution of 3.0 cm enable insights into gravel riverbeds and provide an exact vertical localization of clogged layers.
The sediment characteristics of the near natural bypass channel show a distinct difference before and after the artificial flood. The vertical profiles of the measured hydraulic conductivities show increasing values up to a sediment depth of approx. 10 - 15 cm, which proves the efficiency of the artificial flood regarding de-clogging. In addition, the particle size analyses of most freezecore samples show a reduction in fine sediment fractions along with increasing porosity, which confirms the effectiveness of the flood operation. However, the vertical profile measurements show a reduction in dissolved oxygen concentrations after the artificial flood, which cannot be explained by the changed sediment characteristics or differences in the water temperatures in the hyporheic zone. Most likely, an apparent and widely spread Algae layer on the riverbed significantly influenced the oxygen measurements and a before-and-after comparison is not feasible because the Algae layer was removed during the artificial flood.
The conclusion of this study is twofold: On the one hand, it could be proven that the artificial flood was sufficient to trigger de-clogging effects. On the other hand, the application of MultiPAC showed its potential for evaluating clogging/de-clogging processes in gravel riverbeds. Especially the detection of the impact depth of de-clogging events represent highly valuable information for designing artificial floods.
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