MultiPAC as a tool to monitor the sustainability of riverbed restoration measures

Many water bodies are, as a result of anthropogenic influences, such as river straightening, river bank fixation, or damming in accordance with the EU Water Framework Directive (2000/60/EC) not in a good ecological state anymore. With the aim to return to a good ecological status for surface waters, restoration measures are implemented in many rivers. However, the success and sustainability of such measures are often site-dependent and require hence an objective assessment.

In this study, the Multi-Parameter Approach to assess Clogging (MultiPAC) was used to assess the suitability and sustainability of different riverbed restoration strategies. MultiPAC is based on several measured physico-chemical parameters, which enable a detailed investigation of in-situ conditions of gravel-bed rivers. The approach includes measurements of the sediment composition for identifying surface and subsurface grain size distributions and fine sediment fractions. In addition, measurements of the porosity are obtained by using Structure-from-Motion and the Water Replacement Method of freeze-core samples. Finally, measurements of the interstitial oxygen concentration and so-called slurping rates, which are converted into hydraulic conductivity, were performed with a double-packer system called VertiCo.

The residual river stretch between Jettenbach and Töging at the Inn River in Germany provided a means to evaluate riverbed restoration measures, implemented in February and March 2020. Investigations were performed for several gravel bars, where sediment was replenished and a mechanical break-up of the bed armour layer was conducted. The MultiPAC investigations were performed before measure implementation, shortly afterwards (March 2020) and in November 2020 to investigate the impact of a flood event with a 10-year return period, which occurred in August 2020, and thus may have influenced the sustainability of the restoration measures.

From the measurements, it can be seen that sediment replenishment and the mechanical break-up of the armour layer significantly improved the ecological functioning of the riverbed. However, it became evident that the increase in the quality of the riverbed was only temporary. Hence, these measures will need to be repeated regularly with the aim of maintaining ecologically-valuable riverbed habitat conditions. The results of this study also showed that MultiPAC provides detailed insights into the riverbed sediments, their composition, and the permeability of the riverbed.