Hydrodynamic modeling and the realization of an international height reference system - urgent need and potential contribution

Water levels from hydrodynamic models typically lack an absolute vertical reference, and can thus not be linked directly to digital elevation models to assess coastal floodrisks. Instead, most workarounds rely on tide gauges that are connected to the land-based vertical reference to establish this link. The lack of an absolute vertical reference also makes it impossible to directly assimilate observed total water levels into models as this requires both observed and modeled water levels to refer to the same vertical datum.

The first consequence underlines the urgency of realizing an international height reference system that is easily accessible to users and which is adopted as the vertical reference in global tide gauge datasets and digital terrain models. Indeed, the lack of such an international height reference frame makes it impossible to accurately determine the impact and risks of sea level rise and changes in extreme water levels due to climate change. Let alone that an agenda for adaptation measures can be drawn up. The second consequence limits the accuracy of modeled water levels as existing workarounds do not exploit the observed long-term mean water level variations. At the same time, the assimilation of total water levels imposes strong requirements to the accuracy of an international height reference frame. Hydrodynamic models are extremely sensitive to slopes in observed water levels. Even small erroneous slopes between tide gauges introduced by errors in the vertical referencing, impose false currents that in turn result in model instabilities.

In turn, hydrodynamic models offer great and unique opportunities to assist in the realization of an international height reference system. The difference between the observation-derived mean water level (MWL) at location B and the sum of the observation-derived MWL at location A and the model-derived mean water level difference between the two locations is a proxy for the datum shift between the height datums used at locations A and B. The asset and uniqueness of the technique referred to as ‘model-based hydrodynamic leveling’ lies in the fact that it allows to transfer a height datum over large water bodies without the need to acquire new measurements. In the Dutch Versatile Hydrodynamics project, we implemented the technique and combined the data with geopotential differences from spirit leveling/gravimetry to compute a new realization of the European Vertical Reference System (EVRS).

This presentation will i) explain and demonstrate the need for an international height reference system from the perspective of hydrodynamic modelers; ii) demonstrate the potential contribution of hydrodynamic models to the realization of such a height system using the results obtained in the Versatile Hydrodynamics project, and iii) outlines ideas for future work.