Page 58 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

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CHAPTER 2 The Ocean Environment
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There is a vertical density current through the Straits of Gibraltar. The evaporation of water over the Mediterranean drives the salinity of the water in that sea slightly higher than that of the Atlantic Ocean. The relatively denser high-salinity waters in the Mediterranean flow out of the bottom of the Straits while the relatively lower (less dense) salinity waters from the Atlantic flow in on the surface. This is known as a “density current.” Trying to conduct an ROV operation there will probably result in a very bad day.
Currents flow from areas of higher elevation to lower elevation. By figuring the elevation change of water over the area, while computing the water distribution in the area, one can find the volume of water that flows in currents past a given point (volume flow) in the stream, river, or body of water. However, the wise operator will find it much easier to just look it up in the local current/tide tables. There are people who are paid to make these computations on a daily basis, which is great as an intellectual exercise but is not recommended to “recreate the wheel.”
2.3.1.1 Currents
The primary generating forces for ocean currents are wind and differences in water density caused by variations in heat and salinity. These factors are further affected by the depth of the water, underwater topography, shape of the basin in which the current is running, extent and location of land, and the earth’s rotational deflection. The effect of the tides on currents is addressed in the next section.
Each body of water has its peculiar general horizontal circulation and flow patterns based upon a number of factors. Given water flowing in a stream or river, water accelerates at choke points and slows in wider basins per the equations of Bernoulli. Due to the momentum of the water at a river bend, the higher volume of water (and probably the channel) will be on the outside of the turn. Vertical flow patterns are even more predictable with upwelling and downwelling patterns generally attached to the continental margins.
Just as there are landslides on land, so are there mudslides under the ocean. Mud and sediment detach from a subsea ledge and flow downhill in the oceans, bringing along with it a friction water flow known as a turbidity current. Locked in the turbidity current are suspended sediments. This increase in turbidity can degrade camera optics if operating in an area of turbidity currents—take account of this during project planning.
Currents remain generally constant over the course of days or weeks, affected mostly by the changes in temperature and salinity profiles caused by the changing seasons.
Of particular interest to ROV operators is the wind-driven currents culled into the so-called Ekman spiral (Figure 2.14). The model was developed by physicist V. Walfrid Ekman from data collected by arctic exploration legend Fridtjof Nansen during the voyage of the Fram. From this model, wind drives idealized homogeneous surface currents in a motion 45 from the wind line to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Due to the friction of the surface water’s movement, the subsurface water moves in an ever-decreasing velocity (and ever-increasing vector) until the momentum imparted by the surface lamina is lost (termed the “depth of frictional influence”). Although the depth of frictional influence is variable depending upon the latitude and wind velocity, the Ekman frictional transfer generally ceases at approximately 100 m depth. The net water transfer is at a right angle to the wind.