From the Ground Up: Capturing the tides.
Projects with tidal requirements are now frequent, and the numbers continue to increase each year. The diversity of these projects is considerable, and the demands are often arduous. For example, LiDAR acquisition in coastal areas frequently requires collection within a certain time on either side of low tide so that a greater portion of the land is exposed during acquisition.
Benthic habitat mapping, or mapping of the sea floor, can also be affected significantly by water levels as it is considerably easier to see through a reduced water column at lower tide heights. Benthic mapping might include developing thematic maps for coral reefs, seagrass, shellfish beds or algal beds. Requirements for really low water levels can be extremely restrictive during data acquisition yet key to a project’s ultimate success.
Additionally, mapping of the coastline and coastal rivers at specific tide heights is important in determining the dividing line between private land ownership and state-owned lands. These projects typically require imagery capture within a very small tolerance at a specific tide height and are often the most challenging.
Before beginning a project with tidal mapping requirements, it is important to gain a clear understanding of tides, the astronomic and climatic effects on tide heights, and the sources of information for tide gauges and tide predictions.
The National Oceanic and Atmospheric Administration (NOAA) Center for Operational Oceanographic Products and Services (CO-OPS) defines a tide as “the periodic rise and fall of a body of water resulting from gravitational interactions between Sun, Moon, and Earth.” These interactions can easily be seen in the water level variations along the coast with our oceans, but they also occur to a lesser extent on our large lakes.
It is interesting to look at the differences in tide ranges at various locations throughout the country. The tide range is the difference in height between consecutive low and high waters. The mean range is the difference between mean high water and mean low water. This mean range varies considerably depending on location. Table 1 (page 42) lists the mean range for different tide gauges operated by CO-OPS.
The moon’s gravitational pull is the primary force in the creation of tides. (The sun’s gravitational force is only about 46 percent of the moon’s force.) The timing of these tides is directly related to the time of rotation of the Earth with respect to the moon. The average time of this rotation, also know as a lunar day, is 24.84 hours, or about 24 hours and 50 minutes. In simple terms, this means that the timing of high and low tides tomorrow will occur about 50 minutes later than they occur today.
The average period of time for the moon’s revolution around the Earth with respect to the sun--known as the lunar month--is 29.5 days. This is equal to the time required to move from a new moon through all the varying phases of the moon and back to the new-moon phase. The phase of the moon is extremely important in determining the range of tides. With a full or new moon, the gravitational forces of the sun and the moon are combined. This combination produces the highest and lowest waters (highest high and lowest low tides) during the lunar month. These conditions are also referred to as spring tides, although it has nothing to do with the season of the same name.
Conversely, neap tides occur at the quarter phases of the moon approximately midway between the new- and full-moon phases. At these moon phases, the gravitational forces of the sun and moon are perpendicular to one another and cancel each other out. Neap tides will typically have the least tidal range during the lunar month.
Land ownership in coastal areas is dependent on specific water levels determined over long periods of time. For most states, the legal definition is referenced to either mean high water (MHW) or mean lower low water (MLLW). MHW is determined from the arithmetic mean of all high water recordings over an 18.6 year tide epoch. Similarly, the MLLW is determined from the mean of the lower low water readings over the same period of time.
In Alaska, for example, state-owned tidelands are the land extending from the line of MHW to MLLW, state-owned submerged lands extend from MLLW to 3 nautical miles offshore, with private land ownership above MHW. This is the convention accepted by the majority of our coastal states. Virginia is one of the states, however, that define private land ownership to MLLW with state-owned lands beyond that line.
CO-OPS operates the National Water Level Observation Network (NWLON), which is extremely useful in determining both observed and predicted water levels for the nation’s coasts. The NWLON is a network of 175 long-term continuously operating water-level stations throughout the United States including its island possessions and territories and the Great Lakes. Considerable information is available for all these gauges at the CO-OPS Web site at www.tidesandcurrents.noaa.gov.
The site includes a map that allows visitors to easily navigate to areas of interest and view information about the existing tide gauges (see Figure 1, page 40). The “station information” section includes the geographic position, date of installation and mean tidal range. The “tide/water level data” section provides the ability to view both historical and predicted water levels for time periods selected by the user.
Figure 2 shows the water levels for the tide gauge at Windmill Point, Va. In this plot, the blue line is the predicted water levels, the red line the observed (actual) water levels, and the green line is the difference between the two. Note that there is a significant difference between the predicted and observed water levels at this tide gauge for this day. There are a number of other factors that affect actual tides on a day-to-day basis beyond the configuration of the sun and the moon. These include barometric pressure, wind speed and wind direction. The difference here between predicted and observed water levels illustrates the significance of these effects. Historical tide gauge data from the CO-OPS Web site reveal that the observed water levels often differ significantly from predicted levels. Therefore, it is very important to view these observed levels (as opposed to predicted levels) on a daily basis for any projects where tide coordinated data collection is a critical success factor.
NWLON stations are the foundation for reference stations for NOAA’s tide prediction products and serve as controls in determining tidal datums for all short-term water level stations. The NWLON is also a key part of the NOAA Tsunami Warning System and the NOAA Storm Surge Warning System.
If your work takes you to coastal areas, you may be involved with projects that need water level prediction and determination. Tide-coordinated data collection requires a significant knowledge base regarding the behavior of coastal water levels. Fortunately, a number of information resources are available on the Internet, and agencies like NOAA provide valuable tools that are readily available to all of us.
1. U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Ocean Service, Center for Operational Oceanographic Products and Services, “Tide and Current Glossary.”
2. The National Water Level Program (NWLP) and the National Water Level Observation Network (NWLON), NOAA CO-OPS, tidesandcurrents.noaa.gov/nwlon.html.