Unlocking Insights in Historic Tidal Records with Analysis Methods Tailored to High-Low Tidal Data

Water-level measurements, sometimes spanning centuries, offer a valuable historical perspective. Although contemporary tidal data is collected digitally at high frequencies, historical records often merely consist of basic high and low water levels. Recognizing the value of these low-resolution tidal records, recent 'data rescue efforts' focus on digitizing and preserving them. Current tidal analysis methods, optimized for high-frequency data, fall short in exploiting the potential of high- and low-water observations.Here, we introduce a specialized tidal analysis methodology tailored for high- and low-water observations. Leveraging equilibrium tide information and the unique characteristics of these observations, such as a derivative constraint, we enhance the analysis of historical records. Additionally, we explore interpolation methods for high- and low-water observations, aiming to address the possibilities and limitations associated with these data.Our approach has the potential to offer valuable insights into century-scale water level changes, and to unravel the contributions by tides, river discharge, mean sea level, storm surges and interactions among those governing factors to water level variation. A key ambition we have is to reveal the hydrodynamic consequences of human interventions, which are difficult to distinguish from each other. We hope the new technique will encourage to continue digitization of historic high-low-tidal observations, and allow to demonstrate the role of intertidal areas in modulating water level extremes.