Predicting Current

[DennisH]

Hi folks,

I did a search on this topic, but didn't find what I'm looking for -- if I missed it, please direct me.

I went for a dive at a local sight this weekend. I looked at the tide tables, and we were diving at the lowest tide for the day. The exchange between low tide and high tide 6 hours later was relatively low -- about 9 feet (three meters). Given low tide and a slow, low exchange, I was expecting calm conditions with minor current. Instead, current was very strong, and strangely multi-directional. Looking for a simile, I'd have to describe it as a "toilet bowl current". Regardless, it was unexpected. We ended up doing a drift dive across a usually sheltered bay...in a large circle. Like I said, it was a strange day.

Can anyone educate me on how to predict currents? Thanks for your help!

 

[ScubaSarus]

If you can get a copy of the Eldridge Guide if on the Northeast coast or something similar for your area, you may get the current charts for the sites in question.

Tides and currents do go hand in hand but one can not always judge a current based on low or high tides due to the topography of an area and water flowing in and out of narrow inlets and around land masses as such. You can have current differences with in a quarter mile and rips a stone throw away from dead calm water.

So what you really need is:

A current chart for the area (ask at the fishing shops).

or

Lots of logs noting the current felt and at what stage of tide it was felt at so you can interpolate and make your own current chart.

 

[TSandM]

Wow, I wouldn't think a nine foot exchange was a small exchange . . . I'd expect pretty significant current off slack on that day.

I don't know what's available for Vancouver. Here in Puget Sound, we have a great book, Northwest Shore Dives, which has correction factors for slack for a variety of shore diving sites. You might ask around the dive shops, to see if there is something similar available for your area. We also have a good online site for checking tides, but it looks as though you did that.

Sites really vary in their exposure to current -- Some are always high current off slack, some are bad on ebb tides or worse on floods. Some, weirdly enough, are worse on small exchanges! Local knowledge is really invaluable. But the other thing is that, even if you know a site and its behavior with various tides, you can get surprised. We have a current-insensitive site here called Redondo, where there is rarely much current at all, but I've been caught in a whirlwind there where the current was not only strong but made absolutely no sense, going one way and then another. The bottom line is that, if you dive in areas of big water movement, you always have to be prepared to cope with conditions that aren't what you expected, or to call a dive if the conditions get too risky.

 

[sschlesi]

Hi folks,

I did a search on this topic, but didn't find what I'm looking for -- if I missed it, please direct me.

I went for a dive at a local sight this weekend. I looked at the tide tables, and we were diving at the lowest tide for the day. The exchange between low tide and high tide 6 hours later was relatively low -- about 9 feet (three meters). Given low tide and a slow, low exchange, I was expecting calm conditions with minor current. Instead, current was very strong, and strangely multi-directional. Looking for a simile, I'd have to describe it as a "toilet bowl current". Regardless, it was unexpected. We ended up doing a drift dive across a usually sheltered bay...in a large circle. Like I said, it was a strange day.

Can anyone educate me on how to predict currents? Thanks for your help!

Local knowledge is really the only source for reliable information on current relevant to scuba diving. There are current tables/charts available for many current affected areas including around Vancouver/Vancouver Island, but those predict the main flow of water in the open channels, and specific high current passes. There is also a free program called TideView which covers most of the Strait of Georgia and Juan de Fuca Strait. While the tables can give you a general idea of whether a current will be strong or weak, the topography near shore can totally change the behavior of a current. While the main channel could be running several knots, a specific area near shore may be dead still, a short swim around the corner could put you into the main current or even a current moving in the opposite direction. Wind can also play a big role in how a current behaves.

The best thing you can do is talk to local divers/shops for info on specific sites. I'm pretty sure some questions in the Western Canada forum will get several responses for Vancouver local sites.

If you were diving around Vancouver this weekend, I'd double check your source of tide data. The main exchanges were very large this weekend, at ~14-15', while the small exchanges were overnight and ~3-4' each. I use Tide/Current Predictor as a reference for tide data around Vancouver.

Nine feet is not unusual for our area, and very diveable on most local sites. Anything more than 8' and I am cautious about the current, I try to dive near slack and specifically plan for and pay attention to the current situation during the dive. Above 12' I generally avoid The Cut, Telegrah/Lookout and Copper, though I have dove from The Cut into Whytecliff bay, cautiously on bigger exchanges. To dive local big current sites like Skookumchuck or Tzoonie, I'd look closely at the Skookumchuck current tables, and I'd call Suncoast dive shop or Porpoise Bay charters in Sechelt for their opinion, even if I wasn't going out in their boat.

 

[Peter Guy]

I think the best source of information about local currents is from your local sailors -- sailors or kayakers, not power boaters. Go to a local marine supply store that really caters to sailors and they may have a Tidal Current Chart that will give "normal" current vectors and speeds based on one of the standard current points.

 

[DennisH]

Thanks everyone. Sschlesi, I was diving at Whytecliff, which is usually a very tame dive, as you know. I actually did a drift dive across the bay, then back across the bay in a big circle. I've never experienced a toilet bowl flush there before...

For tide tables, I was using the online ones from Canadian Hydrographic Services, which are the government of Canada tables. I was using Point Atkinson for the reference.

Again, thanks folks. I will look into current charts too.

 

[flw]

Hi folks,

I did a search on this topic, but didn't find what I'm looking for -- if I missed it, please direct me.

I went for a dive at a local sight this weekend. I looked at the tide tables, and we were diving at the lowest tide for the day. The exchange between low tide and high tide 6 hours later was relatively low -- about 9 feet (three meters). Given low tide and a slow, low exchange, I was expecting calm conditions with minor current. Instead, current was very strong, and strangely multi-directional. Looking for a simile, I'd have to describe it as a "toilet bowl current". Regardless, it was unexpected. We ended up doing a drift dive across a usually sheltered bay...in a large circle. Like I said, it was a strange day.

Can anyone educate me on how to predict currents? Thanks for your help!

I've no knowledge of Vancouver area, however I do live in an area with big tidal currents and ranges.
First stop would be to check the local charts, plus whatever the canadian equivalent of the north sea pilot book which will give you local information
However - the best bet is to be a pain in the neck and simply ask any local divers - or better still if you get chance offer to help out the local creel boat men ( don't know what you call them - lobster fishermen??) - they will have intimate knowledge or the tides in a given area, and hauling creels is hard work, but fun for an afternoon - and at the end you'll know a whole heap more. it's worth buying them a pint in the local bar as well
3m might be on neaps, but it's still a lot of sea moving around - in some areas ( like here in places) there may in fact be bigger currents on neaps than springs which does seem a little counter-intuitive

Fi

 

[sschlesi]

Thanks everyone. Sschlesi, I was diving at Whytecliff, which is usually a very tame dive, as you know. I actually did a drift dive across the bay, then back across the bay in a big circle. I've never experienced a toilet bowl flush there before...

For tide tables, I was using the online ones from Canadian Hydrographic Services, which are the government of Canada tables. I was using Point Atkinson for the reference.

Again, thanks folks. I will look into current charts too.

Are you sure you checked the tide tables correctly? I just checked this DFO site http://www.waterlevels.gc.ca/cgi-bi...zone=10&language=english&region=1&stnnum=7795 which also shows 14'+ exchanges over the weekend. Which aligns with how the tides looked in Indian Arm yesterday. The low was much lower than average.

Whytecliff bay is usually tame, though if you get out around the day marker, or past the islet much there can be currents there. One time I also experienced an offshore current in the middle of the bay flowing so rapidly that for a short time it was all I could to to stay in one place.

Sometimes I don't even look at the tide tables if I plan to dive in Whytecliff Bay, or at Porteau, but current can sneak up on you at any time around here.

 

[dkktsunami]

A nine foot tide is a large movement of water. Full moon and new moon will cause the tide to be more extreme (stronger and higher/lower). If you are in a tidal river the outgoing tide will be more forceful than the incoming, at least it is that way in S.E. Georgia. A nine foot tide is a big tide.

 

[Slonda828]

I have to agree with DKK. A nine foot tide is ripping, that means all that water is going to move in (or out) in a relatively short period of time. An average diver can only swim 2 knots in full kit, which makes him useless in high current without the aide of a line or tender. The May River, for example, is totally not dive-able when the tides are above 8 feet. You need to consult your tide charts for when there is going to be a predicted lower tide. 6 feet or less is usually dive-able around slack tide. I would be shocked if you found anyone at all who considered doing anything other than tended diving in conditions like that, and even if they did I would bet it was with a river creeper or other specialty equipment. I personally would not do it, and I am a glutton for punishment. Two U.S. Army Special Forces guys died in the Savannah River in 1999 diving during those kinds of conditions.