Understanding tides.

Abstract

This book presents an elementary explanation of tides and tidal datums. It is written to explain the natural phenomenon of tides in terms and concepts readily understandable by students as well as those in all walks of life merely wishing to be enriched by additional knowledge of their environment. Also, scientists in fields other than physical oceanography and astronomy may wish to use the book as a primer to the fundamentals of tidal theory. Although not at all essential to a complete understanding of tides, a mathematical development of the tide-generating forces is provided. Chapter 10 Tidal Datums should be of particular interest to coastal zone managers, coastal engineers, geologists specializing in beach processes, attorneys concerned with boundary litigation, and legislators (at all government levels) representing marine activities and jurisdictions adjacent to the sea. Finally, it is hoped that new employees in the tides and tidal datum areas of the National Oceanic and Atmospheric Administration will find this book a very helpful introduction to their careers. The term tide is often restricted to th e vertical rise and fall of the water usually occurring twice in a little more than a day. This rise and fall are best observed on a breakwater or on the piles of a pier. When the water is viewed near the time of its lowest poi nt, called low tide or low water, the extent of marine growth and discoloration indicates the general range of the vertical excursion. The tidal range varies, from place to place over time, from almost nothing to many feet. With tides on a sloping beach, the water moves up the beach landward and down the beach seaward. Similarly, but on a grander scale, it moves inland and then seaward across the sloping mud flats and marshes of estuaries as the tide alternately rises and falls. The horizontal component of this phenomenon is called the tidal current. It is best seen at an inlet connecting the ocean with a barrier sound. The tidal current floods and then ebbs with slack waters in between. At any particular location, the high and low tides; together with the floods, ebbs, and slacks of the tidal current; have a particular sequence of occurrence with near constant time intervals between each other. The tide (in its restricted sense) and tidal current are both integral parts of one major phenomenon that will be called hereafter, the tide. Tides 2 should be thought of as being in the form of waves. These waves are thousands of miles in length. Their crests are the high tides, their troughs, the low tides, and the horizontal component of the water particles that make up the wave, the tidal currents. To complicate the matter, these waves combine to reinforce or interfere with each other in varying amounts, partially contributing to the wide differences in tidal characteristics as actually observed. The tide is fundamentally caused by gravitational interactions between the sun, moon, and earth. These interactions of the gravitational forces are the same as those causing the moon and earth to remain in their respective orbits. It is often said of science that the ability to predict a natural event is indicative of understanding. Since tides are one of the most accurately predictable natural phenomena, it follows from the axiom that the tide is truly understood. Nothing could be further from the truth. The sciences of astronomy and geophysics provide very accurate quantitative determinations of the tide-generating forces on the earth. The science of physical oceanography provides a detailed understanding of wave dynamics and the response of the ocean to the tide-generating forces. The Center for Operational Oceanographic Products and Services (CO-OPS) of the National Ocean Service (NOS), National Oceanic and Atmospheric Administration (NOAA) provides very accurate measurements of the tide over time at numerous locations throughout the United States and its territories. But, between the fundamental tide producing forces and the observed tide at a particular place and time, there is a vast area of numerous unknowns that will be referred to as terrestrial factors. Some of the factors that the unknowns are associated ...