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Judging inlets

Jan 1, 2003

Every inlet has unique characteristics that determine the wave conditions at the entrance. Passage into a foreign inlet or through an inlet during foul weather can be made comfortably when the conditions that affect an inlet are understood. This can also make a smoother offshore approach to the inlet.

A coastal current may affect the initial approach to an inlet. Coastal currents usually run parallel to the shore and may be due to an ocean current. Strong, fast moving ocean currents often have a countercurrent flowing next to them which may be producing the coastal current. These currents can also produce circular eddies that spin off the side of the current. General current information can be obtained in Sailing Directions and the Coast Pilot published by NOAA.

Besides affecting the approach to an inlet, coastal currents can form bars on the upstream side of a jetty as well as at the entrance. These currents tend to erode the beach on the downstream side of a jetty. Inlets with marked channels are usually dredged; thus bars at the entrance are not a problem. However, inlets that are not marked or that have an extremely strong tidal current may not be dredged. Navigation through these inlets may be difficult.

Offshore depth curves should be evaluated at the entrance of an inlet before entering. Incoming waves will normally align with bottom depth curves as they approach shallow coastal water. When a wave approaches the coast at an angle, it will slow successively as the end of the wave reaches bottom contours that usually run parallel to the coast.

Waves begin to slow when the depth of water is one half of the wavelength. The bottom of the wave slows when it encounters the decreasing bottom slope. The wave height increases as the wave crest continues at the same speed. The wave period shortens as following waves catch up. This can produce breakers when the wave height becomes unstable. A countercurrent is also present in the trough of the wave as the water recedes back into the ocean. The strength of the countercurrent depends on wave height, bottom slope and tidal current. The slope of the bottom at the entrance of an inlet will effect the height of waves and breakers. A shallow slope will produce small waves and breakers. If the bottom slope is steep, waves will slow down faster and create higher waves and breakers. The slope can change dramatically in some inlets with the change between high and low tide. When planning your approach to an inlet, check the tide tables along with the charted depth curves at the entrance.

The type of coastline at the entrance of an inlet plays a significant role in the surf conditions present. As waves approach jetties or prominent coastal points, they tend to converge at the point. Waves converging at a point cause higher waves and possible breakers. When waves meet a basin or a depression in the coastline they tend to diverge and decrease in height.A steep coastline can create high dangerous waves as the sea rebounds against the wall. If waves hit the bluff coast at an angle along with the rebounding effect, the inlet may become a mass of confused, dangerous breakers.An aspect often overlooked when charting an approach to an inlet is the effect of bottom obstructions on waves at the inlet entrance. Obstructions include bars produced by jetties and coastal currents as well as boulders on the ocean floor. The coastal shelf will also act as an obstruction if located close to the inlet. When a wave hits an obstruction, a large amount of the wave volume will deflect upward. The location and shape of an obstruction will determine the direction of the wave deflection. After hitting an obstruction a portion of the wave continues in its original direction. In shallow inlet entrances the deflection may cause breakers. When inlets have multiple bars or obstructions, this effect is compounded as the wave strikes each obstruction. These inlets are extremely dangerous since the navigator cannot find a clearly defined channel through the breakers in which to maneuver. Tidal current has another important effect on incoming waves. A flooding current with incoming waves will produce smaller waves than an ebb current with incoming waves. An ebb tidal current with incoming waves will produce higher waves with shorter periods as the two opposing forces meet at an inlet entrance. Where bars are present, wave height will increase even more.

In a narrow entrance to a wide bay the tidal current may run extremely high. With an ebb tide and onshore swells the waves at the entrance may heighten tremendously. The strength of the tidal current, wave height, and countercurrent in the wave troughs can make navigating dangerous under these conditions. A flooding tidal current with a shallow continental slope will produce smaller waves than a steep slope. The change between high and low tide should be considered before an approach. The best approach in a steep, sloping inlet would be approximately 20 minutes before the reversal of tide when the wave height will be at the minimum.The most difficult part of an inlet entrance to navigate is where the incoming waves and tidal current meet. The area in the direction of the incoming swells and in the center of the strongest tidal current will produce the highest waves. You can avoid this area by entering or departing from the main flow of current as soon as possible once clear of the inlet entrance. Also take into consideration the direction of a coastal current if it exists at the entrance and maneuver on the upstream side of the inlet.Coastal features, bottom contours, obstructions, wind, waves, and current all have important effects on the surf conditions present in an inlet. By studying tide tables, the bottom topography and coastal features of a harbor you can plan a more comfortable inlet navigation.