Tides, Water Levels & Vertical Datums Flashcards

In areas with predominantly diurnal tides (e.g., portions of the Gulf of Mexico), the recommended chart datum is:

Answer: Mean Lower Low Water (MLLW)

MLLW remains the US chart datum even in diurnal tide areas because it adapts correctly to both semidiurnal and diurnal tidal regimes.

The 'equilibrium argument' (V0 + u) used in harmonic tidal analysis represents:

Answer: The theoretical phase of a tidal constituent at the origin of the epoch from astronomical calculations

V0 + u is the equilibrium argument giving the theoretical phase of a tidal constituent at the start of the analysis period, derived from astronomical data.

Sea level rise monitoring at NOAA tide gauges uses which statistical measure to track long-term trends?

Answer: Monthly mean sea level averaged over decadal periods

Long-term sea level trends are computed from monthly mean sea level values, which average out short-term meteorological and tidal variability.

A 'co-tidal chart' displays lines of equal:

Answer: Time of tide (phase) at which high water occurs

Co-tidal lines (co-phase lines) connect points where high tide occurs at the same time, illustrating the progression of the tidal wave across a region.

Which type of tide gauge is least susceptible to wave noise and therefore preferred for precise water level measurement in exposed locations?

Answer: Microwave radar gauge

Microwave radar gauges measure water level remotely without contact, and their signal averaging inherently rejects short-period wave noise in exposed conditions.

In tidal datum computations, what is the 'low water datum plane' for Great Lakes water level references?

Answer: Low Water Datum (LWD) set at a chart low-water plane

The Great Lakes use a Low Water Datum (LWD) set at a low-water plane specific to each lake, rather than tidal datums used on oceanic coasts.

When transferring tidal datums across large distances with differing tidal characteristics, hydrographers should use:

Answer: A network of simultaneous short-term gauges forming a datum transfer chain

A network of overlapping short-term gauges creates a datum transfer chain, preserving accuracy when tidal characteristics change significantly over distance.