At Pensacola, Florida, however the range can be less than 2 feet throughout a day. At Anchorage, Alaska, the tide range can vary up to almost 30 feet between high and low tide. These differences are caused by the interactions of the oceanic tidal motions with the continental land mass and the depths and shape of coastal bays and shelves. Large differences in the magnitude of the daily tide range occur at different locations in the US. When the tide range is at its lowest during the lunar month, the “neap tides” occur. The highest tides which occur at intervals of half a lunar month are called “spring tides.” They occur at or near the time when the moon is new or full, i.e., when the sun, moon and earth fall in-line, and the tide generation forces of the moon and sun are additive. Large differences in tide range occur at the same location throughout the month. Basic Definitions of Tides modified from /. These tides are called “diurnal” tides.įigure 3.1. Some places, like portions of the Gulf of Mexico, have only one high tide and one low tide per day. But at many other locations, there is a “mixed tide” with a clear “diurnal inequality” in the high tides as one is significantly higher than the other. These are called”semidiurnal” tides (see Figure 3.1).At many locations the two high tides that occur each day are roughly of the same elevation. In most locations in the United States, there are two high tides and two low tides every lunar day (24.8 hours). 3.1.1 Characteristics of Astronomical Tides Understanding some of the characteristics of tides is helpful in understanding some of the terminology used to define tides and tidal datums. The difference between the forecasts and actual water elevation measurements is normally a result of weather related phenomena (e.g., wind blowing from same direction over some period, i.e. Along most coasts bordered by the ocean, the astronomical tide forecasts are within 1 ft of the actual tide elevation 90% of the time. The National Oceanic and Atmospheric Administration’s (NOAA) National Ocean Survey provides on-line tidal forecasts as well as other information about tides around the nation. Tidal predictions are well understood by most coastal residents and are often included in local daily newspapers and weather forecasts. The astronomical tide is well understood and can be predicted for any time at many locations. The moon exerts a greater influence on the tides than does the sun. The usual interval between successive high tides is 12.4 hours as the arrival of the crests of these waves represent high tide. The tide is essentially a very long ocean wave with a wave period of 12.4 hours. The tide is the slow rise and fall of the ocean waters in response to the gravitational pull of the moon and the sun. Additionally, coastal water levels are often affected by meteorological conditions including storm surge in response to winds and wave sand local rainfall. The portion of the water level fluctuation controlled by the astronomical bodies, the moon and the sun, are referred to as the astronomical tide. Tidal currents can be significant as tides and storm surges enter and exit coastal bays through inlets. Storm surge, which temporarily raises the water level, can control the design water level for engineering. The definitions of tidal datums and surveying datums can be important for the design of engineering works near the coast. Geologically, sea level controls the overall location and shape of the continental shoreline. Water level is important in coastal processes and engineering in part because it controls the location of wave influence on shorelines and structures. Water level fluctuations include astronomical tides, storm surges, and long-term sea level rise or fall. From ‘Highways in the Coastal Environment: Second Edition’ by FHWA Chapter 3 – Tides, Storm Surge and Water Levels
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