Tidal Research Page

Your Name: ______

PART B

Research Question: Why are tidal currents at Race Rocks so strong?
Comparing tidal velocity in the Juan de Fuca Strait.
Tidal current varies within the Juan de Fuca Strait due to geography and geology. If you were to remove all of the water in the Juan de Fuca strait, it would resemble a long, narrow and deep valley with rocky peaks on the northwest side. At the southern tip of Vancouver Island, these underwater mountains reach up over the water and peak as Race Rocks.
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Why are the tidal currents so fast at Race Rocks? There are three main factors. First, Race Rocks is situated in the Juan de Fuca Strait, whose mouth is approximately 22 to 28 km wide. From the northwest entrance, the strait narrows as it swings south east and reaches its narrowest point (18 km wide) between Victoria, Canada and Port Angeles in the United States. Similar to a narrow point in a running river, the water picks up speed as it is forced through a smaller area. Second, Race Rocks is located on the southern end of Vancouver Island. Like a car turning a corner, the water passing by picks up speed as it is forced around the tip.

This picture, obtained from a program on the following website illustrates how as the water passes Race Rocks, it turns a corner and picks up speed. The larger, thicker arrows represent increased tidal velocity. The (+) shows where Race Rocks is located.

Third, geology of the area is also a major factor in the high speeds of tidal currents at Race Rocks.

In 1999, the following picture of the underwater landscape surrounding Race Rocks was generated by the Institute of Ocean Sciences while conducting a bathymetric survey. /

The dark areas of the survey are the islands of Race Rocks, with the deep blue part being an area of 100m or more. This shows the steep incline between the bottom of the ocean and the islands. As the water moves through the strait and encounters Race Rocks, it is forced up and over the rocky structure. This causes it to increase in speed as the same amount of water, which was previously in a large space, now must fit through a smaller space. This causes the water to swirl and eddy, like water running over stones in a river.

Now, let’s see what the tidal currents are actually like!

Go to Race Rocks and look out over the water. Note the time of day and date. Can you see the rock in the foreground or not? Describe the water... is it smooth, rippled or really wavy? Does it look like it is moving fast or slow? Is it moving from the left to the right or right to left?

Let’s learn about what we are looking at. Go to the CanadianTide Tables. Follow these steps to see what the tides at Race Rocks are like for the next few days.

1.Go to section 2 and select “Becher Bay”, if not already selected. This is the area closest to Race Rocks.

2.Go to section 3 and select “Pacific Standard Time”.

3.Go to section 4 and select “3 day period”.

4.Press “Query”.

Up will come the tides for Race Rocks area. On the left hand side of the chart is the height of the water and on the bottom is the time of day, over the next three days.

Your teacher will print out a copy of the chart. Referring to the chart, answer the following questions:

A)What is the range of the tide at Race Rocks (from highest to lowest points?)

B)When is the tide flooding (coming in from the Pacific Ocean) and when is it ebbing (going back out to sea)? Mark a minus sign beside the plotted line where the tide is ebbing and a plus sign where it is flooding.

C)What do you think it means when there is a flat line on the chart?

D)Now go to the NOAA’s tide current prediction table for Race Rocks. This table predicts the speed of tidal currents that flow past Race Rocks as the water moves in and out of Juan de Fuca Strait. Note again that a minus sign (-) means that the tide is ebbing (moving outward).

E)The table gives times of day when the velocity of the current changes. Write these current velocities beside their times on the chart. What kind of relationship is there between the increase in velocity of the current and the angle of the slopes plotted on the tidal chart?

F)Now looking at the chart, predict what kind of water flow you will see at Race Rocks. (HINT: Choose a time of day when the slope is at its steepest, if you can, as it will be easier to see the results).

G)Write your prediction in the chart below, with your supporting reasons:

At _____ (time) on the ___ day of ______, 200_. The water will be ___ flooding or ___ ebbing and the velocity will be _____.

My reasons for this are

H)Once again, go to Race Rocks (preset 4) and see which way the tide is going. (Note that it is hard to determine water flow direction around the slack tide.) Was your prediction correct?

I)Check your prediction against the following chart.

When you have completed this page, go to Tidal Research Page – Part C