NOAA Teacher at Sea
Aboard NOAA Ship Rainier
July 8 — 25, 2013
Mission: Hydrographic Survey
Geographical Area of Cruise: Shumagin Islands, Alaska
Date: July 11, 2013
Current Location: 54° 49.6 N, 159° 46.6 W
Weather data from bridge: 8.7°C, good visibility (6-8 miles), light and variable wind, overcast
Science and Technology Log:
Today, Avery and I were scientists in the field, helping the ship’s crew install tidal equipment in preparation for ocean floor survey work. This was a complex process, so we decided to walk you through it in a step-by-step question format.
What does a navigation chart show you?
The image below shows a chart of the area that we are in right now. Our first anchor point was off the north coast of Bird Island in a cove. On the chart, you can see many tiny numbers in the water areas, which represent various depths. These depths are measured in fathoms (1 fathom=6 feet). This depth information helps mariners stay in safe areas that are not too shallow. The charts also show known hazards such as sub-surface rocks and ship-wrecks. This chart clearly has a lot of white space, signifying many areas were never surveyed.
But wait, why are the depth numbers “fixed” on the charts? Doesn’t the water level change with the tides?
Yes! It sounds easy to say, “the water is 10 fathoms deep at this point”. However, water is subject to the gravitational pull of the moon and sun, resulting in various water levels or tides throughout the day. So the water will not always be “10 fathoms deep at this point.” For navigational purposes, the most hazardous water level is the lowest one, so nautical charts show the depth at the low tide water level. Depending on the location, some places have two high tides and two low tides per day (semi-diurnal) and some places have one high tide and one low tide per day (diurnal). Here in the Shumagin Islands we are on a semi-diurnal mixed tide schedule (meaning that the two highs and two lows are not the same height).
What are your experiences with high and low tides? What do you notice when you go to the beach? Leave me a comment!
How do you measure the tides each day?
There are permanent tide measuring stations all over the globe that provide information on how to “correct for” and figure out your local tide conditions. For our case, there is a tide station at Sand Point on Popof Island, which is west from our survey area. Our survey area is in two zones, one which is in the same zone as Sand Point and the other which is in a different zone. Therefore, we installed a tide gauge in the latter to verify that the tidal times and heights of this zone are accurately predicted by the Sand Point values. According to the current information, it says that in the different zone the tides should occur 6 minutes before the tides in Sand Point and to multiply the heights by 0.98.
A tide gauge is a pretty cool device that works by the laws of physics. It is installed (by divers) on the sea floor near a coast-line, in relatively deep water, so that it will always be covered with water. The tide gauge uses the water pressure above to determine the depth of the water column (density of water and gravity are the important factors in making this calculation). The tide gauge stays in place for at least 28 days (one full tidal cycle), after which there is a record of the water level throughout that time period (as we were gathering data), as well as a rough idea of the tidal cycle each month, ready for comparison to the Sand Point data.
How do you know if the tide gauge is working?
To verify that the tide gauge is working, humans (i.e.: Avery and I), take water level measurements (in an area close to the tide gauge) using a giant meter stick or “staff”. In our case, we recorded the average water level height every 6 minutes for 3 consecutive hours. This 3-hour data set can then be compared to the tide gauge data set for that same time period, and hopefully they will show similar trends.
What happens if the survey terrain changes over time? Will that affect the water depth?
The ocean floor is above a liquid mantle, so it is possible for there to be terrain changes and this would affect depth measurements. Thus, as scientists, we must make sure where our survey area is “geologically stable”. To do this, we installed “benchmarks”. If you’ve ever been to the highest point on a mountain in the United States, you might have already seen something like this: they are bronze disks that mark important places, used by NOAA as well as other agencies. We stamped our benchmarks with the year and our station data, letter A-E (by hand! with a hammer and letter stamps!), and installed them at roughly 200-foot intervals along the coastline in what we hope is bedrock. Once they were cemented in place, we determined each benchmark’s relative height in relation to the staff using a survey instrument called an optical level – this process is also called “leveling.” At the end of the survey season, the ship will come back and re-level them. If the area is geologically stable, the benchmarks should all be at the same relative heights to one another as they were when they were initially installed. More so, the scientists will also be very pleased because their depth measurements will be reliable going forward in time.
So what next?
Now that we have completed all necessary pre-survey measurements and research, we are ready to begin surveying the coastline and ocean floor. Happy Hydro!
One of my favorite parts about this particular activity was exploring the coastal wildlife along the way. A Harbor Seal spent a good portion of the day swimming near by and keeping an eye on what we were doing. Unfortunately, every time I tried to get closer for a picture he ducked under water. He was clearly very curious, though. No doubt the installation of the equipment seemed rather bizarre.
Being on the rocky outcropping where we installed the tidal gauge and the beach nearby reminded me a great deal of my childhood. From the washed up bull kelp still clinging to a barnacle (sometimes still alive) to the hermit crabs scurrying away from my hand in tide pools to the brightly colored sea anemones untucking as the tide came in, it brought back a lot of fond memories and definitely re-inspired my childhood enthusiasm for exploring nature and learning about biology by experiencing it. It also brought back that sense of heightened physical awareness as I scrambled from barnacle-covered rock to barnacle-covered rock, trying to avoid the slippery foot placements that would inevitably lead to lengthy gashes on my hands. All is well. I returned from my beach adventure in one very intact piece, slightly rosy-cheeked despite the overcast conditions.
Aside from that, as someone who loves food and eating, the Rainier has treated me very well so far. We have some wonderful stewards and cooks, who do a far better job feeding 50+ people than I do feeding one or two. Every meal includes several gourmet options, including stuffed peppers, chicken or tofu stir fry, braised beef, and countless other delicious things. And there is dessert at every meal. And a freezer full of ice cream. No wonder the crew on the Rainier seems so happy!