Revisiting the observational strategy: Toward more robust inference of compound water level extreme processes in estuaries

Most studies on estuarine compound flooding patterns and processes either utilize paired station data of streamflow and sea level (or simulated data), readily available at national, continental and global scales, or are confined to one estuary system and specific events, where detailed water level observations exist. These approaches do not provide a full understanding of the processes underlying the compound events, as reflected in observed spatial patterns and temporal variation.

 

We call for revised observational strategies for integrating estuary-scale process and the manifestation of compound water level extremes in the estuary with larger-scale patterns and trends. Proving methodological insights from a review of existing compound flooding literature and our multi-scale analysis in Europe, we establish recommendations for such a setup.

 

As a baseline, our analysis uses regional-scale patterns and long-term trends in compound flooding potential in three contrasting countries in Europe (Norway, Finland and Spain, differing in terms of tidal amplitude, seasonality and relative sea level trends), quantified with state-of-the-art methods. We then document the availability of direct estuarine water level data for validating the inferred flooding potential. Finally, three different field setups in northern Norway, central Finland and northern Spain are used to test how compounding streamflow and sea level conditions inferred from national stations are visible in different parts of the estuary. Unlike previous studies, we also examine the manifestation of low water conditions to maximize the usefulness of the acquired data for extreme-event studies. Moreover, we test how robust the state-of-the-art infrastructure is in extreme conditions, including meso-tidal variation, river ice, snowmelt-induced flooding and drying up of estuaries.

 

This multi-scale study allows us to present recommendations for robust observational strategies that allow inference of processes governing compound water level extremes at multiple scales, and explaining the observed patterns and trends. Such strategies have potential in improving our understanding of current and future compound hazards. Accommodating low-water conditions and hydrometeorological extreme conditions facilitates continental and global comparative studies.