NOAA Teacher at Sea
Avery Marvin
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, Rosalind 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).
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.: Rosalind 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 water depth measurements. Thus, as scientists, we must make sure the location of 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 ocean 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!
Personal log
It’s a pretty cool feeling to know that you stepped foot on an island that hasn’t seen human visitors in 20+ years. It was also refreshing to get off the big boat and head to shore for some science fieldwork. I learned all about tide gauge and benchmark installation. I had several small but important tasks:
- stamp each bronze benchmark with year and appropriate code using hammer and metal letter stamps
- mix up cement batter and add to drilled rock hole and under benchmark disc to secure it in place for years to come (much harder than it looks because the cement was like “oobleck” and not very cooperative)
- measure distance between each benchmark using extra long tape measure
- take water level data using staff (big meter stick) in water every 6 minutes
In between tasks, I perused the tide pools for various critters. I saw a few new anemones and got a great shot of one with my new underwater camera. I absolutely love tide pooling and could spend most of the day doing it. I also enjoyed observing the puffins flying in and out of their cliff-side home. They tended to leave the cliff in packs probably to do some offshore fishing for herring and capelin. Upon return, presumably with a belly full of fish, some puffins would fly in large circles near their dwelling a few times before finally landing. This bewildered me. I thought, what a waste of energy! So I researched this and found out the following: Puffins are much better swimmers than flyers and have poor maneuverability while in the air. They sometimes are involved in mid-air collisions or crash landings into rocky slopes. Thus, they “size up” their landing a few times by circling near it before finally flying directly into their vertical burrow entrance.
Their body is mostly adapted for swimming, with short rigid wings helping them to “fly” underwater, to 30+ ft. depths! They have durable bones that endure pressure changes while diving and their body tissues store oxygen. They use anaerobic respiration for long dives. To waterproof their wings, puffins rub their bill on their oil gland several times and then smear this oil all over their feathers. How cool!
We are seeing a lot of Tufted Puffins out here in the Shumigans because it is breeding season (June-August), the time when they return from lonely open waters to rocky islands to mate and raise young. Puffins are monogamous, usually having one partner for many years. Interestingly, a female puffin only lays one egg, which is incubated for around 45 days! Both parents share incubation and feeding duties. Right on! The chick then stays in the nest for around 45 days until ready to fly. I love puffins! They are not only adorable but very well-adapted creatures.
Fun/sad factoid: Alaskan and Canadian natives made reversible parkas out of puffin skin. When it was rainy out, they wore the feathers on the outside and in cold dry weather, they wore the feathers on the inside. It took 45 puffins to make one parka!
