FIeld measurements - do they provide the ground truth?

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Ivo Wenneker, Niels Jacobsen

Chair(s): dr. Makropoulos

Tuesday 30 june 2015

16:30 - 16:45h at Central America (level 0)

Themes: (T) Special session, (ST) Coasts at threat in Europe

Parallel session: 7H. Special session: Coasts at threat in Europe

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Field measurements at sea of water levels and waves yield data for practical and research purposes. For example, they feed operational numerical models for ship navigation and storm surge prediction, and can be used for model validation. In such and other applications, field measurements are assumed to provide the ‘ground truth’. One expects that ‘redundant measurements’, i.e. measurements of some quantity at the same location by more than one instrument, would lead to (almost) identical data. We studied several years of ‘redundant data’ of: • water levels, measured at sea poles located offshore, near the coast, and sheltered in an estuary (instruments: Digital Level Meter (DLM), Radar and Step Gauge); • waves, measured at sea poles located in the surf zone (instruments: Step Gauge, Pressure Sensor, S4, Cap Wire). By comparing data of one instrument with that of another, we concluded that: • for ‘mild’ conditions, data differences are often minor; but • for ‘extreme’ conditions (e.g., storms, breaking waves, large tidal currents), data differences between instruments go up till 20 cm (water level), 50 cm (wave height) and 0.5 s (wave period). This implies that one should be cautious to classify field data obtained in ‘extreme conditions’ as ‘ground truth’. This is unfortunate, also because in such conditions numerical models are often less reliable. We will present various causes that can lead to these differences.