JP 3-50 National Search and Rescue Manual Vol I - US Navy

3. Bottom Current (BC) should be considered in underwater
incidents. Bottom current is usually not strong enough to
move a sunken object, including a body. However, if current
exceeds 4 to 5 knots, as in a rain-swollen river, the sunken
target may tumble along the bottom. Bottom current
information can be obtained from the Defense Mapping Agency
Oceanographic Atlas for certain harbor, coastal, and ocean
areas. Also, Defense Mapping Agency or Naval Fleet Weather
Centers may be able to provide special analysis services for
underwater and bottom currents. For bottom currents in harbor
areas the Army corps of Engineers should be consulted.
4. Swell/Wave Currents (SWC) may, in the absence of winds, affect
rafts and other small marine craft. Because SWC speed is
slight, this drift force is usually disregarded. However, it
may be useful for determining probable direction of target
movement.
5. Surf Current (SUC) is considered only for coastal surf areas
and is more of a factor in rescue or salvage than in search
planning. Surf current will move the object perpendicular to
the line of breakers toward the shore. The object will also
be displaced in the direction of any along-shore current.
F. Total Water Current (TWC) is the vector sum of currents
affecting the search object. The best information on total water
current is usually obtained from a Datum Marker Buoy (DMB).
1. DMBs and sonobuoys are droppable floating beacons transmitting
a signal on UHF frequencies. The buoy drifts with surface
currents, but shows no leeway. Each DMB used on scene should
operate on a different frequency to preclude confusion over
DMB origin.
2. With minimal current, first-day DMB observations may be
questionable because of SRU navigational error. The average
over 2 to 3 days can reduce the effect of such error. DMBs
should be inserted or relocated as accurately as navigational
systems permit.
3. Information on currents obtained by a DMB should be used with
caution. It provides information only while in the water and
represents a total water current (sea current and wind-driven
current) valid only during the time of deployment and for the
water area through which it traveled. Even so, it is probably
a more accurate representation of current than that previously
calculated from historical and statistical data. If there is
a wide disparity between DMB and planning information, the SMC
should consider adjusting search areas and/or datum.
4. To preclude diversion from planned search patterns, SRUs
should relocate DMBs only at the beginning or end of search.