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An unusually high oceanic tide on December 15, 1977, caused flooding of lowlying, nearshore parts of western Washington, including several areas in the Puget Sound region. At Seattle, the December 15 high tide of 14.8 feet above MLLW (mean lower low water datum; 8.55 feet above the National Geodetic Vertical Datum of 1929, or NGVD) was 0.1 foot higher than the 100-year high tide. At Neah Bay, near the western end of the Strait of Juan de Fuca, however, the high tide of 8.77 feet MLLW (4.55 feet NGVD) on that date was 3.2 feet lower than the 100-year high tide. This study has identified the observed December 15, 1977, high-tide elevations at many locations in the Puget Sound region. The observed high tide of that date was much higher than predicted in most of the Puget Sound region, primarily as the result of a very low barometric pressure. Little damage from wind waves was reported. Elevation profiles for the predicted and observed high tides on December 15, 1977, and for several other selected tide levels indicate an increase in the maximum height in the inland direction, except near Port Angeles, and show abrupt changes in tidal elevations at three constrictions Admiralty Inlet, Tacoma Narrows, and Deception Pass. INTRODUCTION The purpose of this study, done in cooperation with the Land Information and Analysis Office of the Geological Survey, was to document the elevations of the December 15, 1977, high tide at the inland marine shorelines of northwestern Washington. The December 15, 1977, high tide was caused by an unusually low barometric pressure in the Puget Sound region (the National Weather Service measured a daily average barometric pressure of 28.85 inches of mercury at the Seattle-Tacoma Airport on that date). At Seattle the tide equaled the highest on record (February 6, 1904). The documentation of these tide elevations will help to determine flood-prone shorelines and should assist in nearshore land-use planning and coastal flooding mitigation. However, the elevations determined in this study and presented herein do not reflect any increase that might result from wind waves. In some places wind-wave increases can amount to several feet and therefore should be considered when determining flood-prone or safe areas. The study area includes Puget Sound from Port Townsend to Olympia, passages through the San Juan Islands connecting the Strait of Georgia and the Strait of Juan de Fuca, and the Strait of Juan de Fuca from the Pacific Ocean at Tatoosh Island to Puget Sound at Port Townsend and Deception Pass (figs. 1-3). TIDAL-STATION ELEVATIONS Elevations were obtained from the National Ocean Survey (NOS) for several selected tide levels at tidal stations in the study area. The tide levels are those commonly used and defined in marine navigation books and in NOS tide tables and records. Elevations at tidal stations for two of the levels, mean higher high water and mean tide level, are given in table 1, along with the elevation of the December 15, 1977, high tide at locations where it could be determined. The length of the tidal elevation records used by NOS to determine the tide-level elevations varies from 80 years at Seattle to less than 30 days at many of the stations. The National Ocean Survey recorded the high-tide elevations in the Puget Sound region on December 15, 1977, at 11 s tat ions Neah Bay, Port Angeles, Port Townsend, Hansville, Olympia, Tacoma, Seattle, Bremerton, Everett, Friday Harbor, and Cherry Point (figs. 1-3). Tidal elevations for the December 15, 1977, high tide at other sites were obtained from elevation surveys of high-water marks identified by people living along the shorelines. Most of the privately observed elevations are considered reliable and were memorable because the water had entered homes or was about to enter homes. Most observers also recalled little wave action from wind. The observed high-tide elevations for December 15, 1977, are shown in figures 1-3 and table 1. The elevations used in the Puget Sound region are based on two different datums. Tidal elevations are based on mean lower low water (MLLW) datum, and land-surface elevations are based on the "Sea Level Datum of 1929 through the Pacific Northwest Supplementary Adjustment of 1947" (National Geodetic Vertical Datum or NGVD, formerly Mean Sea Level Datum or MSL). The adjustment from one datum to the other at some locations where both datums are known is given in table 1; that is, elevation in MLLW datum minus the MLLW-to-NGVD adjustment equals the elevation in relation to NGVD at that location. The December 15, 1977, high-tide elevation was 14.80 feet MLLW (8.55 ft NGVD) at Seattle and equaled the highest tide on record (February 6, 1904). On the basis of a log-Pearson III frequency curve with a skew of +0.2 (Chow, 1964) that fits the annual highest tides recorded over 71 years'(1899-1969) at Seattle (fig. 4), this tide elevation was 0.1 foot above the 100-year tide at Seattle. At Neah Bay, the elevation of the December 15, 1977, high tide was 8.77 feet MLLW (4.55 feet NGVD; fig. 4), which is much less than the highest tide on record (12.00 feet MLLW, or 7.78 feet NGVD, on November 30, 1951). A log-Pearson El frequency curve with zero skew that fits 39 years (1935-58, 1960-74) of annual highest tides at Neah Bay indicates that the 100-year tide level is 12.0 feet (MLLW).

Water-surface elevations for the high tide of December 15, 1977, in the Puget Sound region, Washington