CH Tides, Water Levels & Vertical Datums 3 — Questions and Answers
Question 1: Which tidal constituent has the largest amplitude in most locations along the US East Coast?
- S2 (principal solar semidiurnal)
- K1 (lunisolar diurnal)
- M2 (principal lunar semidiurnal) (Correct answer)
- O1 (principal lunar diurnal)
Correct answer: M2 (principal lunar semidiurnal)
M2 is the dominant tidal constituent along the US East Coast and in most ocean areas worldwide due to the Moon's gravitational dominance.
Question 2: A water level gauge experiencing a datum shift would most likely be identified by comparing:
- Current readings to harmonic predictions only
- Staff gauge readings to the gauge benchmark network via leveling (Correct answer)
- Satellite altimetry data to the gauge record
- Tidal range to historical spring range values
Correct answer: Staff gauge readings to the gauge benchmark network via leveling
Datum shifts are detected by periodic leveling between the gauge reference and the benchmark network to verify the gauge hasn't settled or shifted.
Question 3: The Great Diurnal Range (Gt) is defined as the difference between which two tidal planes?
- MHHW and MLLW
- MHW and MLW
- Highest Astronomical Tide and Lowest Astronomical Tide
- Mean Higher High Water and Mean Lower Low Water (Correct answer)
Correct answer: Mean Higher High Water and Mean Lower Low Water
Gt (Great Diurnal Range) is defined as the difference between Mean Higher High Water (MHHW) and Mean Lower Low Water (MLLW).
Question 4: Under NOAA standards, how many months of water level data are typically required at a short-term tide station to transfer a tidal datum from a nearby control station?
- 1 month minimum (Correct answer)
- 3 months minimum
- 6 months minimum
- At least 12 months
Correct answer: 1 month minimum
NOAA's standards allow datum transfer from as little as 1 month of simultaneous water level data if the control station relationship is strong and stable.
Question 5: What is 'storm surge' in the context of water level measurement?
- The rapid rise of tide during spring conditions
- The abnormal rise of water above predicted tide level due to meteorological forcing (Correct answer)
- A calibration artifact in pressure-based tide gauges
- The tidal bore effect at river mouths during flood tide
Correct answer: The abnormal rise of water above predicted tide level due to meteorological forcing
Storm surge is the departure of water level above (or below) predicted astronomical tide caused by wind stress, atmospheric pressure, and wave setup from storms.
Question 6: When conducting a hydrographic survey in an area with no nearby tide gauge, the preferred method for obtaining water levels is:
- Using a predicted tide table for the nearest listed port
- Deploying a temporary water level recorder at or near the survey area (Correct answer)
- Estimating from satellite altimetry corrections
- Applying a fixed offset from the nearest control station
Correct answer: Deploying a temporary water level recorder at or near the survey area
Deploying a temporary tide gauge at the survey site provides the most accurate local water level data for sounding reductions.
Question 7: Which datum is used as the reference surface for nautical chart soundings published by NOAA for US coastal waters?
- Mean Sea Level (MSL)
- Mean Low Water (MLW)
- Mean Lower Low Water (MLLW) (Correct answer)
- Lowest Astronomical Tide (LAT)
Correct answer: Mean Lower Low Water (MLLW)
NOAA uses Mean Lower Low Water (MLLW) as the chart datum for US coastal nautical charts, providing conservative (safe) depth references for mariners.
CH Tides, Water Levels & Vertical Datums 3
Which tidal constituent has the largest amplitude in most locations along the US East Coast?