Texas Bureau of Economic Geology Padre Island National Seashore: A Guide to the Geology, Natural Environments, and History of a Texas Barrier Island

TIDES AND THEIR EFFECTS ON LAGUNA MADRE

Compared with tidal ranges of the Atlantic and Pacific Coasts of the United States, the Texas Gulf Coast has a uniformly small tidal range. Astronomical tides — those produced by the gravitational pull of the moon and sun — average about 1.4 feet along the Gulf shoreline in the vicinity of Padre Island; the astronomical tidal range in Laguna Madre is even smaller, with a mean of less than one-half foot (U.S. Department of Commerce, 1978). These kinds of tides are chiefly diurnal or consist of only one high and one low tide a day, although they may be semidiurnal (two high and two low tides a day) or mixed during certain times of the month.

More important than astronomical tides in the Padre Island area are wind tides. Wind tides are produced when strong and steady winds elevate the water surface, flooding areas of low elevation (wind-tidal flats). Strong onshore winds force Gulf waters against the shoreline and produce a rise in sea level. Where channels or inlets are present, elevated Gulf waters flow through them and raise the water levels in the lagoons and bays. Offshore winds have the opposite effect.

Tidal levels that occur during the year are caused by both astronomical and meteorological tidal conditions. Maximum water levels typically occur in October, April, and May when strong onshore winds are present (Behrens and others, 1977). Minimum tidal levels are recorded in January and February when strong offshore winds occur. Behrens and others (1977) report that June and July are also months characterized by low tidal levels.

Wind tides can have a relatively sudden and dramatic effect on the shallow waters of Laguna Madre. Wind tides can produce a rise or fail in water levels by as much as 1 or 2 feet when water is pushed onto low-lying marginal areas. E.G. Simmons, in a 1957 study of Laguna Madre north of the Land-Cut Area (see pl. I), observed on one occasion that water levels near North Bird Island (North Section, pl. I) dropped 18 inches in 1 hour as a result of strong (50 mph) winds from the west. Such rapid changes in shallow Laguna Madre can leave boats stranded because of insufficient water depths. Occasionally, air boats that have maneuvered into shallow water in some parts of Laguna Madre have been stuck for hours because of a fall in water level as a result of strong, steady winds.

Fisk (1959) reported that before the Intracoastal Waterway was dredged through that section of flats now known as the Land-Cut Area, winds occasionally would drive waters completely across the flats, thereby joining the waters of north and south Laguna Madre. North winds are especially effective, causing water to advance over the flats at rates as high as 7.6 miles per day to cover the entire Land-Cut Area within a period of 36 hours (Fisk, 1959).