(541) 361-0194
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(541) 361-0194
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Tides 101 by Capt. Scott Howard
If you are new to boating some basic knowledge of tide tables and what tide tables mean to you will most likely prove to be the most important piece of knowledge you possess while operating your boat over the "Bars" of the Oregon Coast. Key factors to consider while checking Oregon tides are swell height, time of high & low tide, height of high tide, height of low tide and the difference in the heights of the low tide following each high tide, this difference is called either run-out or draught.
On the Umpqua River Bar I consider swell heights of 6 feet or less to usually provide non-hazardous conditions on the bar. Swells of 7 feet or greater typically create hazardous conditions during the ebb tide. Are there exceptions to this general rule? The answer is yes! Read on to find out why.
The safest times to cross a bar on any given day are during the "flood" tide. Flood tide is defined as the period of time after low tide in which the tide is incoming or rising. When the tide is at its highest point there is a period of time where the tide "stops" moving. Most Captains call this period of time "high slack". It is during this period of "high slack" that bar conditions will normally be safest for boat traffic. A conservative rule of thumb is to assume that this "slack" period will last about 30 minutes before the tide changes to an "ebb" or outgoing tide.
The most hazardous times to cross a bar on any given day usually will occur during the "ebb" tide. Ebb tide is defined as the period of time after high tide in which the tide is outgoing or falling, if you are planning to cross a bar and are new to boating try to plan your crossings both out and in over the bar to avoid the times of the day that there is an ebb tide.
Not all ebb tide bar crossings are hazardous. Each day before a trip I check my tide tables to see what the run-out is for the day’s ebb tide. For example, if the high tide is listed as 6.1 feet and the low tide following is listed as 3.7 feet the run-out would be 2.4 feet. Notice that the low of 3.7 feet is not really that low of a tide, meaning there is still a decent depth of water in the channel of the entrance to the bar. The 2.4 feet of run-out is also a reassuring bit of information. In my opinion the Umpqua River bar would most likely be safe to cross even if the swell height was 7 to 8 feet. A 10 foot swell however would make the Umpqua River Bar hazardous to cross during the ebb no matter what the run-out and height of the tide.
Looking at a different day in the tide tables we see a high tide listed as 9.6 feet and a following low listed as (-)1.5 feet. The run-out for this ebb tide is 11.1 feet. This is most definitely a hazardous tide to deal with on the Umpqua River Bar irregardless of swell height. A 5 foot swell could prove to be hazardous given these tide conditions. For the tide to drop 11.1 feet during the course of the ebb the water must move out over the bar at a high rate, this in itself makes the waters rough. On top of that notice that the actual height of the tide is listed as a minus. Remember, the shallower the water, the more likely a swell is going to “break”.
O.K. let’s try to put this all together. The Umpqua River Bar has a projected mean low water height of 26 feet of depth in the channel. In other words, at a 0.0 low tide the depth of the bar’s channel should be 26 feet. A (-) 1.5 foot low tide should make the depth of the bar’s channel 24.5 feet deep. When looking at swell height forecasts and tide tables it would be comforting to have a general idea about what a certain size swells might be expected to do on a Bar entrance. My father told me to use a factor of “3” to answer this question. Let’s assume that there is a 10 foot swell on the ocean and we want to cross the Umpqua bar. 10 times 3 equal 30. So…. A 10 foot swell can be expected to “break” in 30 feet of water. A 5 foot swell can be expected to break in 15 feet of water. Now let’s imagine that we want to cross the Umpqua at low slack water with a (-) 1.5 foot tide. The water depth is probably going to be around 24.5 feet deep. If the swell height for that day was 8 feet it is very likely that the bar could be hazardous according to the factor of “3”. An 8 foot swell can be expected to break in 24 feet of water. What about during the ebb before low slack tide? You might be wondering. Due to the fact that water is moving out over the bar before slack tide the factor of “3” doesn’t apply at all. The bar would probably be hazardous to cross due to the river water rushing out to meet the ocean swells. Using a factor of “3” will give you a rough idea of what you might encounter on flood tides and slack water conditions but ebb tides can surprise even the most experienced boaters.
I hope that this article is of use to some who read it. Whenever you plan a trip over a bar check your tide tables for your estimated times to cross both out and in over the bar. In closing I would like you to remember that every river entrance is unique and there really is not one rule of thumb that applies to all situations. Until you are familiar with the traits of your bar with different swell heights and tides exercise caution and if in doubt don’t hesitate to communicate with the local Coast Guard, they are there to help boaters keep their feet dry!
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