7.6 Tidal Ranges

The tidal-range data set was obtained from tide tables published by NOAA's National Ocean Service (NOS) for 410 stations located on the West Coast (NOS 1992). These station data were entered into the ARC/INFO GIS as point data and are available in the supplemental data group. The supplemental data group contains the name, identification number, longitude/latitude, mean tidal range, maximum tidal range, and mean tide level for each tide-range station. The data for each station were overlaid onto the 0.25° grid cells used in this data set, and the variables calculated based on the stations that fell within each grid cell (values expressed in meters) as follows:

  1. The number of tide stations that fell within each 0.25° grid cell was calculated. The stations within each cell were then used to derive the mean tide level and the mean and maximum tidal ranges for each 0.25° grid cell.
  2. The mean tidal range for each grid cell is the average of the diurnal tidal ranges of all stations within a given cell.
  3. The maximum tide range for each grid cell is the largest value found within the diurnal tide ranges of all stations within a given cell.
  4. The mean tide level for each grid cell is the average of the mean tide levels of all stations within a given cell.

The gridded data obtained from this process were then overlaid onto the U.S. West Coastline coverage using the ARC/INFO IDENTITY command to transfer the calculated data into the line segment version of these data.

The mean tidal range at a given tide station in this data set is defined as the difference in height between mean high water and mean low water in 1992. Tide heights vary annually, but their differences are relatively constant in relation to one another. The maximum tide range variable contains the "diurnal tide range." The diurnal tide range is defined as the difference in height between mean higher high water and mean lower low water (NOS 1992). The mean tide level variable is defined as a plane midway between mean low water and mean high water in 1992. This value is reckoned from chart datums. The chart datums used in the tide tables for the mean tide level variable are from the West Coast Low Water Datum.

The magnitude of the tidal-range variables defined above has been linked to both inundation and erosion hazards. Although a large tidal range dissipates wave energy, it also delineates a broad zone of low-lying intertidal wetlands susceptible to inundation. Furthermore, the velocity of tidal currents in estuaries depends on the tide range, as well as the asymmetry of the tidal cycle and channel morphology. Therefore, when holding these other factors constant, high-tide ranges are associated with stronger tidal currents capable of eroding and transporting sediment offshore.