Sebastian Inlet Tides and Currents — Sebastian, Florida

Sebastian Inlet is the narrow, jetty-lined opening in the barrier island that lets the Atlantic Ocean tide into the Indian River Lagoon east of Sebastian. The Sebastian Inlet District describes the inlet as manmade, not a naturally occurring feature of Florida's east coast, and it was opened at its present location in June 1923 after repeated earlier cuts filled with sand. Twice a day the ocean tide pushes water west through the cut, and twice a day the ebb carries lagoon water back out past the north and south jetties.

The striking feature of the Sebastian Inlet tide is how quickly it fades once it enters the lagoon. NOAA tide records show a mean range of roughly two feet at the A1A bridge over the inlet, while the same records show a range of only a few inches at the Sebastian waterfront on Indian River Drive. Harbor Branch Oceanographic Institute research, now held by Florida Atlantic University, documents the same pattern in the currents: tidal flow along the Intracoastal Waterway weakens steadily with distance north of Sebastian Inlet.

NOAA's National Ocean Service keeps published bench mark sheets for three historical tide stations around Sebastian. At station 8722004, Sebastian Inlet, the tide gage and staff were located on the north fender of the State Highway A1A bridge over Sebastian Inlet. Its datums come from one year of observations, August 1969 to July 1970, adjusted to the 1983 to 2001 National Tidal Datum Epoch using Miami Beach (City Pier) as the control station. NOAA lists mean high water there at 0.683 meters and mean low water at 0.041 meters above mean lower low water, so the average rise and fall at the Sebastian Inlet bridge is about 0.64 meters, or a little over two feet.

Inside the lagoon the numbers shrink sharply. At station 8722029, Sebastian, Indian River, the gage was on the Sebastian Yacht Club dock, reached from Cleveland Street and Indian River Drive. Based on two years of records (August 1969 to July 1970 and September 1972 to August 1973), NOAA lists mean high water at 0.114 meters and mean low water at 0.021 meters, a mean range under 0.1 meters, or roughly three to four inches. The Micco station 8721994, on the fuel pier at Sebastian River Marina north of the U.S. 1 bridge over Sebastian Creek, recorded only one month in April 1972 and shows a similarly small spread, with mean high water at 0.104 meters and mean low water at 0.022 meters.

The extremes in the NOAA record tell the same story. The highest observed water level at the Sebastian Inlet gage was 1.174 meters on October 29, 1969, while the highest observed level at the Sebastian waterfront gage was 0.701 meters on October 19, 1973. The gap between the two Sebastian stations shows how much of the ocean tide is lost in the narrow inlet channel and the shallow lagoon west of it.

The strongest currents are in the inlet throat between the jetties, where the whole tidal exchange for this part of the lagoon squeezes through a narrow channel. The Sebastian Inlet District calls Sebastian Inlet a local knowledge, fair-weather inlet used largely by smaller recreational fishing boats, and the only charted inlet between Cape Canaveral and Fort Pierce. The District lists the north jetty at 1,000 feet and the south jetty at 575 feet, and describes a 42-acre dredged basin within the inlet, the sand trap, where sand carried by the currents settles until it is dredged.

West of the bridge the flow spreads out and slows. In a 1996 study, a Harbor Branch oceanographer recorded currents at three sites along the Atlantic Intracoastal Waterway in the lagoon near Sebastian Inlet. The semi-diurnal M2 tide dominated, with current amplitudes decreasing from 11.0 centimeters per second near the inlet to 7.6 centimeters per second 13 kilometers north. The share of current variance explained by the tide fell from 49 percent to 33 percent over that distance, and the M2 tidal wave moved through the lagoon at about 5.5 kilometers per hour.

The same Sebastian Inlet study found a steady drift underneath the tidal back and forth: net along-channel flow was to the south, toward the inlet, at 0.5 to 2.1 centimeters per second during the 42-day study, and lower-frequency motions at all three sites were inversely related to wind stress. Wind, in other words, moves the lagoon water off Sebastian and Micco as much as the tide does.

Tidal exchange through Sebastian Inlet is the main way ocean water reaches the central Indian River Lagoon. Public radio station WFIT reported in 2019 that the lagoon has five inlets along its 156 miles, and that it is almost 90 miles from Sebastian Inlet north to Ponce de Leon Inlet in New Smyrna Beach. As a result, water in the southern lagoon is refreshed almost daily, while in parts of the northern end it can be over a year before lagoon water is exchanged with ocean water.

Harbor Branch modeling quantified that imbalance. A 1992 Harbor Branch paper calculated that 38.7 million cubic meters of water enter and leave the Indian River Lagoon on an average semidiurnal tidal cycle, and that the northern part of the lagoon, with 65 percent of the surface area, contributes only 9 percent of the intertidal volume. A 2003 study in Ocean and Polar Research considered tidal flushing in the central lagoon because of its proximity to Sebastian Inlet: it estimated 50 percent flushing times of 6 days in the dry season in the northern central zone and 2 days in the southern central zone, against 17 days in the northern lagoon without a tidal effect.

Modern measurements at Sebastian Inlet come from a partnership between the Sebastian Inlet District and Florida Tech. According to the District, Florida Tech installed wave and weather gauges at the inlet in 1993, and in 2007 the District entered into a long-term coastal monitoring agreement with the university to track sea level changes and natural sand transport. The State of the Inlet Reports page describes the network as a meteorological station on the north jetty and two nearshore acoustic Doppler current profilers about 0.5 kilometers north of the inlet in about 8 meters of water, collecting currents, waves, water temperature and water level.

Those Sebastian Inlet instruments feed public products. Florida Tech's Coastal Processes Research Group posts real-time readings from the north jetty weather station and an offshore current profiler, and the District's navigation page links a three-day real-time inlet conditions forecast that covers wave height, currents and water levels. Each year the District publishes a State of the Inlet report built on the current gauge records and semi-annual hydrographic surveys.

On shore, the Sebastian Inlet District notes that beach warning flags are stationed at both the north and south jetty entrances, with flag colors indicating hazardous conditions and dangerous marine life. Inside the park on the lagoon side, the manager of Sebastian Inlet State Park has described the tidal cove as a popular destination for families, a sheltered contrast to the fast water in the main Sebastian Inlet channel.