From: narad@Eng.Sun.COM (Chuck Narad - Diver/Adventurer/Engineer)
Newsgroups: rec.scuba
Subject: Re: Down Currents
Date: 8 Jan 93 19:22:46 GMT
Reply-To: narad@Eng.Sun.COM
Organization: SMCC Architecture Group
NNTP-Posting-Host: nonsequitur

hi dan-

I have experienced down currents twice, once in the Solomon Islands and
once in Grand Cayman.  The Grand Cayman one was most recent, so I
can give you an accurate description:

	We were diving at Tarpon Alley on the North wall of
Grand Cayman.  This dive spot is on a wall which is about 1/2 mile north
of the barrier reef that forms the bay where you can find Stingray City;
it is touted as the largest inland bay on any carribean island. 

	Wind that morning was mild, from the North.  There was a
north-to-south current at the surface, which disappeared at a depth
of around 10 feet.  We descended to the sandy bottom at around 50
feet, and there was a mild south-to-north current at the bottom.  As
we got closer to the wall (swimming north) the current increased but
was still light enough to hold still by swimming upcurrent lightly.

	Just before the wall there was a coral reef that rose 
about 20 feet.  We went through the reef in a sand channel, and got
pulled very rapidly out to the wall (felt like a rip current).  We came
out on the wall at about 70 feet, and were  immediately pulled down to
about 100 feet.  Both of us were kicking very hard upwards, and also
were adding air to our BC's, but were still being pulled downward!

	I finally guessed that the down current was right along the
wall, and kicked away from it; the down current lessened, and I was
able to ascend (still kicking some).  My buddy was still hugging the
wall, and managed to ascend a while beheind me through brute force.  We
made it to the bow line at around 50 feet.  We both rested there for a
few minutes to catch our breath, and then ascended hand-over-hand up
the bow line, doing a 5 minute safety stop at 15 feet, then swam back
to the dive deck.  Total dive time was about 17 minutes.

	During the hand-over-hand ascent, there was a downward and
south-to-north current most of the way up; at 15 feet swimming back to the
deck there was almost no current, and on the surface it was still with the wind.

	Oh, this was the second dive of the day, around 11:00; the first
dive at around 8:30 had no wind and also complete calm at depth (and lots of
tarpon :-)

	For background, we are both experienced divers and have good
buoyancy control.  I have over 180 dives.

	Our conclusions at the time were that the wind was forcing a 
surface current into the bay, and the outflow and down-current at depth
were caused by the bay spilling water to match it:

		-----> wind direction

	>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
                                                                                                     v
                                                                                                        v     water flow
                                                                                                        v
                                          <<<<<<<<<<<<<<<<<<<<<<<<<<<
                                      v
                                   v
                                 v
                     %          v                                 "%" = flailing divers
                                 v
                             %  v


regards,
chas/