Tides, Water Levels & Vertical Datums Flashcards

The relationship between NAVD 88 (land survey datum) and MLLW (chart datum) at a given coastal location:

Answer: Varies spatially and must be determined by leveling from benchmarks

The separation between NAVD 88 and MLLW varies by location and must be determined through leveling connections to tidal benchmarks at each site.

A 'bench mark' in the context of a tide station refers to:

Answer: A fixed, stable physical mark with a known elevation relative to the tidal datum

Tide station bench marks are stable physical monuments with published elevations referenced to the station's tidal datum, used to recover or verify the gauge datum.

What hydrographic phenomenon explains why tides in the Bay of Fundy can exceed 16 meters in range?

Answer: Near-resonance between the tidal forcing period and the bay's natural oscillation period

The Bay of Fundy's natural resonant period (~13 hours) is close to the semidiurnal tidal period (~12.4 hours), causing near-resonant amplification of the tidal range.

In harmonic tidal prediction, the 'epoch' (κ) of a tidal constituent is:

Answer: The phase lag of the constituent relative to the equilibrium tide at the prime meridian

The epoch κ (kappa) is the local phase lag of an observed tidal constituent relative to the corresponding equilibrium constituent at Greenwich.

Which statement correctly describes 'mean tide level' (MTL)?

Answer: MTL is the arithmetic mean of mean high water and mean low water

Mean tide level (MTL) is computed as the simple average of mean high water (MHW) and mean low water (MLW), which may differ from the average of all water level observations.

Subsidence of a tide gauge benchmark would cause the recorded sea level trend at that station to appear:

Answer: Higher than the true sea level rise rate (apparent faster rise)

Land subsidence causes the gauge to sink relative to the sea surface, making sea level appear to rise faster than it actually is in absolute terms.

For hydrographic surveys in estuaries with strong riverine flow, the primary complication for tidal datum determination is:

Answer: River discharge causing non-tidal water level variations that must be separated from tidal signals

River discharge creates non-tidal water level changes (setup/drawdown) that must be identified and removed before tidal analysis to avoid contaminating datum computations.