LS10.21 PROBABLE ERRORS OF POSITION

Section: LS10 SAR Operations

Date: 20th September 2016

Page: 2 of 4

Since most areas affected by tidal streams will be close to landmasses, wind current will usually not be a factor in determining drift. Because of this, drift occurring in in-shore waters over short periods will be more greatly affected by tidal streams than current or leeway. However, if the cumulative effect of tidal streams and coastal currents thrusts the target into areas where sea current takes effect then drift considerations will need to be revised.

River Current

River current will affect SAR incidents that occur in offshore areas near river mouths. Tidal streams affect the river current speeds near the mouths of the rivers. In large rivers this affect may be noticed several kilometres upstream from the mouth. Published current tables often give values which are combinations of tidal and river flow effects. These are among areas where reversing streams will be greater in one direction that the other.

On the other hand, river current affects both total current and sea current at its mouth. Some major rivers extend their influence quite significantly off shore. Seasonal variations in water volume and velocity should be considered.

When estimating river current in the discharge area an assumption that the current direction is a straight line from the river mouth to the discharge boundary and the river current speed decreases linearly from the river mouth to the discharge boundary should be made. The river current speed at the mouth can usually be obtained from local knowledge or by direct observation.

If any type of offshore current is present, the IC should expect that the river discharge will not fan out symmetrically, but will be displaced in the direction of the offshore current.

Long Shore Current

Long shore currents are caused by incoming swells striking the shore at an angle. Long shore current information must be obtained from direct observation or local knowledge.

Swell/Wave Current

In calm conditions, swells and waves may affect rafts and other small marine search targets. The effect is similar to leeway in that the raft is being moved through the water. However swell/wave current speed is so small, under 0.1 knots, that the drift force is usually ignored in determining general search areas. It is useful however for determining probable direction of target movement in some cases.