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Author Previous Topic: Fraser River freshet and new boaters. Topic Next Topic: Dear God give me the patience to wait.  

Old_Salt

RO# 20541

Posted - Jun 04 2012 :  18:09:32  Show Profile  Reply with Quote
Tides and Currents and Lagoons

DO NOT enter a lagoon at High Water outside the Lagoon.

By its very nature, the entrance to a lagoon restricts the flow of water into the lagoon.

The water in the typical lagoon entrance is shallower than the water outside the lagoon and shallower than the body of water forming the anchorage(s) within the lagoon. The typical lagoon entrance is narrower than the body of water inside the lagoon.

At Low Water (Low Tide) outside the lagoon, the water level will be BELOW the water level inside the lagoon, because the shallower and narrower entrance/exit to/from the lagoon has restricted the flow of water out of the lagoon.

As the tide rises outside the lagoon entrance, the water level outside the lagoon rises to meet the diminishing level of water inside the lagoon. The point at which the two levels are equal is the moment of Low Water Slack Current, when the current as a whole has minimized. Because of the shape of the entrance and the variation in the depth of water in the channel, due to an irregular bottom, there may be some continuing small currents within the entrance, but overall the current will be benign.

As the tide continues to rise outside the lagoon entrance, the water level outside the lagoon rises to exceed the now increasing level of water inside the lagoon. The lagoon entrance is restricting the flow of water into the lagoon because of its shallow and narrow nature.

At the point of High Water outside the lagoon, the tide has been rising for about six hours. The filling of the lagoon continues at a high rate, but the level of water in the lagoon has not reached its maximum point, however, because of the entrance restrictions, and the relative height of water inside will be less than the height outside.

At some time after High Water outside the lagoon has been reached, the decreasing rate of flow into the lagoon will bring the water level in the lagoon up to the level of water outside the lagoon. At the point where the water level outside matches the water level inside, high water slack current has been achieved and the height of water within the lagoon has been maximized for that tidal cycle.

This is the best time to enter or depart the lagoon, when the low water depths and contours are known to the boater, because the flow rate is very low for a short period of time centered around that moment when the tide level outside the lagoon equals the tide level inside the lagoon. This is the moment of “high water, slack current”. Go!

If the entrance channel to the lagoon is deep enough to accommodate a vessel at Low Water Slack Current, this can be a good time to survey the entrance channel and to compare what you see with any charts of the channel and lagoon in your possession. With sufficient depth in the channel, the time of Low Water Slack Current may allow passage into or out of the lagoon.


Pacific Yachting, June 2012, contains an article by Deane Hislop, “Gunkholing the Broughtons”, where he offers Broughton Lagoon Nook, in Greenway Sound, as an anchoring spot in the Broughtons. A sidebar “Tips for Tidal Lagoons” is included.

The statements, in the sidebar, “High slack is usually the best time to either enter or leave a lagoon”, and “Another alternative is to enter on low water…” are made. Many boaters will read these statement and understand them correctly, and as they were intended.

I feel strongly that some other boaters need to see a refinement of “high slack” to read “high water, slack current (in the lagoon entrance channel)” in order to better understand that the writer was not referring to the moment of high water, or high tide, outside the lagoon, but was referring to the condition of slack current in the lagoon entrance, after the high water tide level outside the lagoon had been reached, and had then started dropping until it reached the highest level that the lagoon would reach in that cycle.

Entering the lagoon at “low water … while the water is still flowing out” could place the boater against a considerable head of water still contained within the lagoon. “Low water, just prior to slack current (in the lagoon entrance channel)” might allow the boater to see the rocks nearer the bottom of the entrance, but at a more modest outflow current.

This is not intended to be “nit-picking” but is intended to help avoid the nasty situation where the flow into the lagoon may still be strong, at the moment of high water “high tide” being reached outside the lagoon, and high water, slack current not yet having occurred within the lagoon entrance, and the situation of “low water” interpreted as “low tide outside the lagoon” having the boater facing a still quickly moving outward flow from the lagoon, as opposed to a quieter and shallower stream nearer to “low water, slack current (in the entrance channel)”.

I have heard the terms “high slack” used to indicate “high water” or “high tide”, and “low water” to mean “low tide” too often to let this pass.


Sechelt Inlet and Seymour Inlet are two of the biggest lagoons on the B.C. coast.

Their “entrance channels”, Sechelt Rapids (the Skookumchuck) and Nakwakto Rapids, are the two fastest salt water rapids in North America, and the timing in entering and exiting these “entrance channels” to the lagoons can be critical .


Excerpts from the article in the
Vancouver Province, June 3, 2012

Regarding the tragic death of two RCMSAR volunteers in Sechelt Rapids:
“Two volunteers from the Royal Canadian Marine Search and Rescue died Sunday morning while on a training mission in the treacherous Skookumchuck Narrows on the Sunshine Coast.
The two victims, both female, were trapped under their rigid-hull inflatable when it flipped in the infamous tidal rapids at around 11:30 a.m., Royal Canadian Navy Capt. Annie Djiotsa told The Province.”
“The RCMSAR inflatable, based out of Sechelt, entered the world-famous tidal rapids at 11:30 a.m., at slack tide.”


At 11:30 am, June 3 the “Tide” at Point Atkinson is given as about 3 ft. , with a low of 1.0 ft at 10:20 am PST, and the following high of 15.1 ft. at 5:43 pm PST.

The tide at Point Atkinson can be converted (approximately) to the tide level at Egmont, just outside the Sechelt Rapids , but very much in the “flow of things”, including the ebb current out of Sechelt Rapids. The timing of High Water and Low Water are within minutes of their occurrence at Point Atkinson.

However, at Sechelt Rapids, the “Current” is given at about 13.6 knots maximum ebb at 8:40 am PST, and 13.7 knots maximum flood at 4:31 pm PST, with the time of the turn from ebb to flood given as 1:11 pm PST. At 10:30 am PST (11:30 am PDT) the cycle from maximum ebb to the turn to flood would have been 1hr 50 min of the way through the 4 hr 31 min cycle, and the current still ebbing at about 7-9 knots.


Help me out here, folks.

OS

Homeport: Porpoise Bay, BC

Jgsons2

RO# 32496

Posted - Jun 04 2012 :  23:49:11  Show Profile  Reply with Quote
Good Post Old Salt. A lot of very good information. It's why I use current charts and tide charts for my trips in to tight places.
JGSONS2


May your keel stay wet and your seat stay dry

Homeport: Leavenworth,WA Go to Top of Page

Jgsons2

RO# 32496

Posted - Jun 04 2012 :  23:53:14  Show Profile  Reply with Quote
Only 48 more days till I head north to Port Hardy and beyond

May your keel stay wet and your seat stay dry

Homeport: Leavenworth,WA Go to Top of Page

BillV

RO# 17370

Posted - Jun 05 2012 :  10:22:44  Show Profile  Reply with Quote
Read it 3 times and still learn something every read. Good detail OS. In the past I had never really considered entering at low water/low tide primarily because of it's shallow nature. I did learn (almost the hard way too) about leaving a logoon at 30 minutes past low tide and against the incoming current when leaving Sandy Point, my home base marina. Shallow and narrow entrance causes very strong currents that I never considered very much at the time. I was in a hurry and decided that I had enough power to go for it. Long story short, practically stood the boat on its rear and then onto the rocks. Had applied too much power too fast and in a too narrow a location. Fortunately the boat missed everything and we got out safely but only BTGOG (By the Grace of God) and not my skills. I now wait untill almost high slack. Good to see you back OS. Have missed our chats.

1978 Glasply 28' mid-cabin, repowered, remodeled, rewired, rebuilt, replaced, repaired, oh well you get the picture! It's my full time job now that I'm retired.

Homeport: Lynden, WA Go to Top of Page

Old_Salt

RO# 20541

Posted - Jun 05 2012 :  17:47:09  Show Profile  Reply with Quote
Bill:

The original post was inspired by the Pacific Yachting article, when I saw the author use
the term “high slack” as in “High slack is usually the best time to either enter or leave a lagoon” to mean “the time after the high water tide level outside the lagoon entrance channel, at which the current in the channel becomes slack, when changing from inflow to the lagoon to outflow from the lagoon, as the tide level outside the lagoon drops from high tide toward the following low tide outside the lagoon”. I wondered if some readers might interpret “high slack” as “high tide” or “high water” and get themselves in trouble. When he then followed that by using the term “low water” as in “Another alternative is to enter on low water…while the water is still flowing out …” I wondered if some readers might interpret this “low water” as the “low water tide level for the cycle” and head into the lagoon at a time when the outflow current was bound to be fast for the present cycle, because the head would still be fairly large.

It seemed to me that “high water slack current” and “low water, just prior to slack current” would have described his desired conditions more accurately, but I suppose editors do edit from time to time.

When I read about the Sechelt Rapids tragedy the next morning, and read the reporter’s words “… at slack tide” being attributed to an RCN officer, I couldn’t believe that the person in charge of this training procedure would refer to “slack tide” as a safe time to enter these rapids, known to be the fastest salt water rapids in North America, if they understood how the laws of nature, and hydrodynamics apply to lagoons. I have boated these rapids since 2001, and I have gone through at both “high water slack current” and “low water slack current” in both directions, safely, many times. I have never thought about going through at “slack tide” at a near maximum tidal swing, because I have viewed that from shore and it really impressed me that survival in 14-16 knot tidal rapids might be “iffy”.

If you haven’t had the opportunity to view Sechelt Rapids from shore or boat at maximum ebb conditions, I’d love for you to see it. Sechelt Rapids and Nakwakto Rapids are “special”.

OS



Homeport: Porpoise Bay, BC Go to Top of Page

BillV

RO# 17370

Posted - Jun 09 2012 :  09:10:05  Show Profile  Reply with Quote
I look forward to seeing Sechelt Rapids and Nakwakto Rapids but will excercise due caution and will undoubtedly travel thru them at “high water slack current” for my peace of mind. Thanks for the information and I only wish I had half the knowledge you have OS about these areas.

1978 Glasply 28' mid-cabin, repowered, remodeled, rewired, rebuilt, replaced, repaired, oh well you get the picture! It's my full time job now that I'm retired.

Homeport: Lynden, WA Go to Top of Page

Old_Salt

RO# 20541

Posted - Jun 09 2012 :  16:57:53  Show Profile  Reply with Quote
The Coast Reporter, “Voice of the Sunshine Coast”, carried the story of the Sechelt Rapids mishap in their weekly Friday edition, on June 8. The following quotes are from the news story.

“The tragedy struck during a time when the currents leaving leaving the Skookumchuck were near their strongest.”

“ … the Sechelt Rapids may have been moving as fast as 13.5 knots (25 km/h) when the Sunshine Coast RCMP received the 11:30 am Mayday call.”

“ … the strongest outward currents would have occurred that day close to 10:20 a.m.”

It really doesn’t seem to me that these two nice ladies, entering the Rapids at 11:30 a.m., were going to be entering a body of water “at slack tide”, as claimed by the Navy spokesperson. The Rapids don’t ebb at all during the moment of slack current, and at high tide at Egmont, they are flooding at a significant speed, and at low tide they are ebbing at a significant speed.

If you are boating on the B.C. coast, in the beautiful and complex waterways, get a copy of CHS chart 3000, “Juan de Fuca Strait to Dixon Entrance”, and the Sailing Directions for the entire coast, and the Canadian Tide & Current Tables Volumes 5,6,&7 and spend a little time with them. It will cost you about the same as dinner and drinks for two ashore, and will serve you well with respect to understanding the complexities of the waterways on this beautiful boating coast.

OS



Homeport: Porpoise Bay, BC Go to Top of Page

Jgsons2

RO# 32496

Posted - Jun 09 2012 :  21:56:00  Show Profile  Reply with Quote
I would like to repeat what BillV said about seeing Nakwakto Rapids and thank you for all the good information you put out. Like BillV I wish I had your knowledge about B.C. waters. BillV and my self are learning and I am sure we will pass on what ever we learn to other people when we can. You can not always help the person who help you but you can pass it on. That's way it should be.
One month and 14 days and I will be up there at Port Hardy and beyond.


May your keel stay wet and your seat stay dry

Homeport: Leavenworth,WA Go to Top of Page
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