Lesson 14: Tides and Currents

Need To Know

Tide is the vertical movement of water. When water momentarily stops rising (flood) or falling (ebb), (such as a baseball “hangs” before it begins to fall when thrown upwards), this moment is called the stand.

Wet rock = ebb. Dry rock = flood.

Sounding Datum a designated reference point which tide heights are compared to.

High Tide is the highest level of water caused by the ascending or flood tide during a tidal cycle. This height is expressed in relation to the sounding datum.

Low Tide is the lowest level of water of water caused by the descending or ebb tide during a tidal cycle. This height is expressed in relation to the sounding datum.

Range of Tide is the vertical difference between the high and low water levels during a tidal cycle.

Under stand the differences between Mean Higher High Water (MHHW), Mean High Water (MHW), Mean Low Water (MLW), and Mean Lower Low Water (MLLW) and how this impact chartered depth soundings and vertical clearance under bridges (MHW).

Semidiurnal tide is probably the most common of the world’s tide pattern where there are two high and two low tides occurring during the tidal day (24 hours 50 minutes in duration). Diurnal tide is where there is only one high and one low tide occurring during the tidal day. Mixed Tide is a semidiurnal pattern characterized by a greater variation in range in one of the pairs of high-low tides occurring during the tidal day. This type of tide is commonly seen along the US Pacific coast.

Current is the horizontal movement of water. Current is comprised of both direction (set) and velocity (drift). Slack or slack water occurs when there is no horizontal movement of water or current.

Most open ocean tidal currents rotate in direction through a tidal period of 12 hours 25 minutes.
There are no slack waters in a rotary current. Instead, there is a minimum and maximum
current, separated by three hours, thus corresponding to low and high tides.

Tide and current tables make use of a primary or Daily Predictions reporting station in which daily predictions are made. From this station, time, depth, and current speed corrections are made for other Subordinate or secondary coastal locations within the same geographic area.
Always correct for Day Light Savings Time (DST) when required. Not all charts observe this correction and accordingly must be made by the navigator.

Tidal Datums and Tidal Epochs –Sea level averages are taken over several years to obtain a tidal datum - a vertical reference based on some phase of the tide - to slow the process if only temporarily. This is a workable idea because, in addition to sinking crusts and melting ice, tidal variations also have their effect on sea level. One such effect is the 18.6-year cycle of the lunar nodes – a cycle accompanied by variations in tidal range. Another force for change is the annual variation in solar declination that modulates solar heating and density of ocean waters. To account for both, a 19-year period of water level averaging – the National Tidal Datum Epoch (NTDE) – has been established by NOAA/NOS in the United States. NTDEs have included the years 1924-1942, 1941-1959, 1960-1978, and most recently, 1983-2001. NTDEs thus are being updated roughly every twenty years.

Here are the basic definitions for tidal datum commonly used in the U.S. and its territories:

Mean Sea Level (MSL) – Arithmetic mean of hourly water levels observed during current NTDE.
Mean Higher High Water (MHHW) – Mean of higher high water heights during current NTDE.
Mean High Water (MHW) – Mean of all high water heights observed during current NTDE.
Mean Low Water (MLW) – Mean of all low water heights observed during current NTDE.
Mean Lower Low Water (MLLW) – Mean of lower low water heights during current NTDE.
Mean Tide Level (MTL) – A datum located midway between MHW and MLW; i.e., MTL = ½ (MHW+MLW).