Emergency Navigation by David Burch

Author:David Burch
Language: eng
Format: epub, pdf
Publisher: McGraw-Hill Education
Published: 2008-07-14T16:00:00+00:00

COASTAL CURRENTS

Here I’m making a distinction between rotating tidal currents in coastal waters (discussed above) and a broader category of currents that can include other sources that I call coastal currents. Generally, any one point along any coast will have contributions from both; a wind-driven current, for example, could be a coastal current that is not tidal.

We can consider the region of coastal currents to be within some 20 miles of an island or coast or, alternatively, well onto the continental shelf, if it is prominent. Generally, coastal currents are the most difficult to predict. In these coastal regions, currents might be dominated by any one of the three types of currents or even be composed of a combination of all three (ocean, tidal, and wind-driven currents)—or they may be caused by still another effect. A strong onshore wind, for example, can sometimes pile up water against prominent headlands, which in turn creates significant currents when the wind dies and the water flows back to level the surface. In such circumstances, you might find strong currents without wind or tidal changes. Currents of this type are sometimes referred to as being driven by a hydraulic head.

Coastal currents can vary significantly in strength at any one location, and they can vary rapidly and irregularly from point to point along a coast. Coastal or island currents tend to be stronger closer to shore if the current has an onshore component to its direction, as many do. Sailing Directions and Coast Pilots are good sources of coastal current information.

In many places around the world, on-shelf coastal currents are primarily wind-driven currents. If the wind blows to the north, currents flow north; with wind to the south, currents flow south. This can be valuable information, since you can use the rule of thumb for strength estimates, but, more important, it helps in interpreting Sailing Directions. On the Pacific Coast shelf (Washington to California), for example, Sailing Directions often describe the near-coastal currents as southerly in the summer and northerly in the winter. The currents behave in this general way because prevailing winds of the region are from the north in the summer and from the south in the winter. A more informative description might be that these are wind-driven currents. If the winds are unseasonable, it is likely the currents will follow. The primary ocean circulation well off the shelf, however, is persistently to the south in this region throughout the year.

The sea state can often indicate a strong coastal current. A strong current flowing against the wind causes an enhanced chop and steepness to the seas, whereas a current flowing with the wind diminishes the seas just as dramatically. To recognize the effect, however, requires some experience at sea, since you must be able to conclude that the seas are not consistent with the winds. More generally, in a strong coastal current you might notice confused seas with not just steeper waves but also more frequent big ones, or frequent waves running perpendicular to the wind.

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