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Diving Deeper: Episode 24 (June 16, 2010) —
How do we measure tides?

(INTRO)
HOST: Welcome to Diving Deeper where we interview National Ocean Service scientists on the ocean topics and information that are important to you! I’m your host Kate Nielsen.

Today’s question is….How do we measure tides?

Tides are basically very long-period waves that move through the oceans in response to the forces exerted by the moon and the sun. Tides begin in the oceans and then move towards the coast where they appear as the regular rise and fall of the sea surface.

To help us dive a little deeper into this question, we will talk with Tom Landon on measuring tides – both how we do this and then what the data is actually used for. Tom is an oceanographer with NOAA’s Center for Operational Oceanographic Products and Services. Hi Tom, welcome to our show.

TOM LANDON: Hi Kate, thanks for having me.

(IMPORTANCE OF MEASURING TIDES)
HOST: Tom, we interviewed your colleague Steve Gill back in April 2009 about tides. Can you remind us what the difference is between tides and currents?

TOM LANDON: Well, it’s actually quite simple Kate. Tides are the alternating rise and fall of the water level. It’s a vertical movement in oceans, bays, rivers, estuaries, any of the water bodies along the coast. And those vertical water level movements are caused by the gravitational forces of the sun and the moon. Tidal currents on the other hand are the horizontal movement of the water. The speed and direction of the currents in those bodies of water.

HOST: What kind of data do scientists, yourself and your colleagues, what do you typically collect for tides? What is tidal data?

TOM LANDON: Tidal data is basically the time and the height of the water level relative to a datum, which is a reference plane, a height that you use as your reference plane. And for example, what we call mean lower low water which is the average of all of the lower low waters over a 19-year period, that’s used as the reference plane for nautical charting so all the depths are referred to mean lower low water. And then all the bridge clearances and things like that, overhead cables, they’re all referred to the average of all the high waters.

HOST: So why is it important to measure tides? Who really uses this tidal data and what do they use it for?

TOM LANDON: Well Kate, there’s a wide variety of uses of tidal data. NOAA uses it specifically for hydrographic and shoreline mapping support. We produce all of the nautical charts and they have to have a shoreline reference and they also have to have a reference for the depths.

Ship captains and pilots use both real-time data and averages for real-time navigation support. They want to ensure safe navigation in and out of harbors and they also want to get the most efficient commerce. So they use tide data quite a bit in their travels and one of the things they also refer to in terms of safety is a thing we call air gap, which is the distance between the bottom of a bridge and the water level.

The Army Corps of Engineers uses tide data extensively for all their dredging projects because they have to have a reference datum on how deep to dredge the channel. Coastal engineers use it for any kind of construction project along the coast – bridges, condominiums, bulkheads, anything like that. Ecosystem managers now use tide data extensively for both long-term planning and for special projects such as marsh restoration projects. A good example right now is the oil spill in the Gulf. The people who run the Hazmat group, the hazardous materials response team, they use tide data extensively to map where oil spills are going to travel and what the tide measurements might do in terms of the effect of the oil spill along the coast.

Emergency managers during hurricanes use tide data extensively to both plan for the safety of people and protection of property. Scientists and engineers both use tide data, long-term tide data, to monitor changes in sea level, sea level changes over time and they’ve been watching that very closely over the last ten years probably and it will become a more and more popular topic as time goes on. And then lastly even fishermen use tide data, they want to know when the best times to go fishing are.

HOST: You mentioned air gap in your last response. What exactly is air gap and why is this important?

TOM LANDON: Air gap is that distance between the bottom of a bridge and the water level. And as tide changes, that distance also changes. And if you could picture a tide curve with a high and a low tide, an air gap measurement would actually be the reverse of that. So when the tide was high the air gap would be the least and when the tide was low the air gap would be the greatest.

Now, a really dramatic example of that is a recent article in the news about the USS New York, which is a billion-dollar Navy ship that was built on the Mississippi River upstream of New Orleans and when it came time to go to sea trials, they had to get it down the Mississippi River under at least two different bridges and out to sea. Well, the water level was such that the air gap, the clearance that they had to work with was very, very tight, and using our information with recently installed air gap sensors, we gave them the proper water level measurements and the air gap height so that they safely navigated under the Huey P. Long bridge, they cleared it by about two and a half feet. So less than a yard stick between the top of the ship and the bottom of the bridge. So that’s how critical that air gap measurement is.

HOST: Thank you, great example there. Tom, what types of tools are used to measure tides? Do we use things like buoys?

TOM LANDON: The primary method of measuring tides is a tide station, Kate. Nowadays, they actually use buoys that have GPS receivers attached to them and as the buoy rises up and down with the changes in water level, the actual measurement, the height measurement, can be tracked through the GPS satellite. So that’s pretty cool.

But, when you get right down to the very simplest tool, it would be equivalent to putting a yard stick in the water and measuring how far up and down the yard stick the water rises and falls and you can measure that. And if you were to sit there hour after hour after hour for days and days, you could actually get measurements and draw a tide curve. That’s how we started, way back when Thomas Jefferson started the Coast Survey, it was a simple thing as a tide staff.

Then as time went on, we migrated to recording-type gauges that had a float that rose and fell inside a protective pipe and they used that rise and fall of the float to record the actual heights on a gauge. Well, that continued to develop and where we are now is that we use things like acoustic sensors, microwave sensors, we still use float gauges, but it’s more modern technology now and we transmit all that data through the GOES satellite. It can collect data every six minutes now rather than once every hour or once every day when looking at the tide staff. It’s very, very real-time now.

(MEASURING TIDES - TIDE STATIONS)
HOST: That’s great, so there’s a lot of different sensors that collect different tidal information for your needs. Are they housed together in some way?

TOM LANDON: Yes, basically a tide station is a collection of instruments within some kind of protective enclosure. It could be a little house, it could just be an enclosure like you would see at an intersection, a traffic light enclosure. It’s slightly bigger than that, but that’s the look of the more modern tide stations and the tide station also would include permanent reference marks on the land in the vicinity of the tide station to which the tidal heights could be referred to through standard surveying techniques.

But anyway, within these enclosures we have a data collection platform which is basically the electronics box which gathers all the measurements and then processes them and sends them out through the GOES transmitter through the satellite and back down to the ground station. So there’s the data collection platform and then you have the various water level sensors, it could be an acoustic sensor, it could be a sensor that measures the difference in water pressure above a certain point as the water rises and falls the pressure changes.

There’s also a more modern microwave sensor and a microwave sensor basically sends microwaves from the sensor down to the water and measures that distance using microwave technology. The acoustic sensor basically sends a sound pulse down a tube, bounces it off the water, and measures the height from the sensor to the water level from the time that it takes the acoustic sound to go down and back again. The last thing that a typical tide station would include would be a very little GPS antenna that basically gets the timing signal from the GPS satellites and it keeps exact time, never varies by more than a couple microseconds. It’s a very accurate timing system and keeps all the measurements exactly on time continuously.

HOST: How is a tide station set up out in the water? How do you get all this equipment out there and set up one of these stations?

TOM LANDON: It can either be a very simple process or it can be quite involved. We do have short-term tide stations that are used for hydrographic projects or Army Corps of Engineers projects. And they may be installed and only operate for three to six months. The permanent stations that we install are rather involved and like I said the components would be all the sensors that I previously mentioned and the enclosure and if you were to go to any coastal area, no two tide stations are going to look the same. Some of them might be similar, but each and every one is kind of like your fingerprint, it’s distinct. The one in Baltimore is an old wood frame house that looks old, it’s been there a long time, the one at say Clearwater Beach is one of these more modern, white aluminum boxes and it has a NOAA symbol on the outside so you know that it’s related to NOAA.

One of the things I forgot to mention at the beginning, in addition to measuring tides, we’re also responsible for measuring all of the water levels in the Great Lakes. And in the Great Lakes a typical water level station looks like a small brick building. They build it out of brick and block and it’s meant to last 50 to 100 years. And some of the more recently installed houses actually have some artwork on the outside. We have a connection up in the Great Lakes where this lady paints the door and the front of the house in a coastal scene with the NOAA symbol on it so it’s very unique and that just pops right out at you when you see it.

HOST: What kind of equipment is part of a station?

TOM LANDON: A lot of the more recent, say in the last five years, tide stations that we’ve installed include a standard set of meteorological sensors and that would include a set of anemometers, wind sensors, we always use two now so in case the first one breaks, the second one hopefully would be still recording the data and the wind measurements are really critical during hurricanes of course. We usually have an air temperature sensor, and then we also have a barometric pressure sensor.

HOST: You just talked a little bit about hurricanes and some of the strong winds that can come with these. How are tide stations able to hold up or withstand, what is their track record during that time?

TOM LANDON: Sometimes the storms get the better of us. We have lost tide stations in the past on some of the stronger hurricanes. Ivan, back in 2004, destroyed a couple different stations down on the Gulf coast. And of course Katrina, Katrina got a number of them. So they are vulnerable, depending on where they’re located and there’s been a very planned effort in the last six to seven years probably to build stronger and better tide stations, better able to withstand the strength of at least a category three to a category five hurricane.

Now there are some places where the tide station is located on a very strong pier and really that’s about all you can do, you don’t need anything better than that. The tide stations are located on a variety of different types of piers and when they’re vulnerable to hurricanes, we have made an attempt to what we call ‘harden’ them. And hardening would involve putting them at a level, a height level, which would pretty much keep them out of harm’s way for any major hurricane and most recently we’ve designed these upgrades to withstand a category 4 hurricane storm surge levels as well as winds and waves. And this was all done through a very extensive engineering review process and the granddaddy of all this work was what we call the NOAA Sentinels.

And the NOAA Sentinels are a single pile, single pipe, elevated platform, well the four that we installed were in very remote places. Tthey’re typically 25 to 30 feet above the water, and the pipe is three feet in diameter when it goes from the water level up to the top platform, and then below ground its four feet in diameter. So it’s this huge pipe, and the pipe is driven anywhere from 60 to 120 feet into the bottom. So these were designed to be the ultimate answer to the hurricanes and I think it’s going to take one heck of a hurricane to damage these things. And there’s four of them down in Mississippi and Louisiana.

HOST: Great, so strategically placed then in areas that are known to be prone to those kind of coastal storms.

TOM LANDON: They were basically designed and constructed in a direct response to the damage that Katrina did. We wanted to be able to record water level and meteorological information throughout the storm and not lose the tide station. So that was our answer.

(NOS'S ROLE IN MEASURING TIDES)
HOST: Tom, what is the role of the National Ocean Service in measuring tides?

TOM LANDON: We’re authorized by Congress as the nation’s experts in tidal datums and measuring water levels. Our information is used in any kind of litigation so we’re the legal experts. We go to court with tidal information when requested. Any kind of engineering firm, Army Corps of Engineers, any of the basic users of the data also rely on the fact that we’re the nation’s experts, they all come to us for information. We’re also authorized through the Hydrographic Services Improvement Act to install and operate real-time navigation systems in major ports. So, Congressional authorization as well as the Hydrographic Services Improvement Act basically give us our orders to do this type of work.

HOST: Where does your office collect tidal data? How far off of the coast?

TOM LANDON: With the exception of say a GPS buoy and a rare offshore drilling platform, we do not go any further than the coastline. We do all our measurements from piers and docks near the coastline and the Sentinels are all within probably 60 meters of a shoreline. So they’re just offshore too. We do really no work out in the open ocean. And the areas that we cover are every place in the United States from Maine to Texas. We measure water levels in the Caribbean, also all along the West Coast, all of Alaska, and the Pacific Islands.

HOST: How many stations are in all of those areas?

TOM LANDON: We have a total of 210 permanent stations right now and then there’s probably another 100 short-term water level stations that come and go. And those are all for special projects and the real-time navigation support and things like that.

HOST: How long do stations typically last for? Is there some kind of standard maintenance that’s required as well as upgrading to new technology over the years?

TOM LANDON: Yes, there is a standard maintenance schedule and that’s annually. We try to visit each and every station at least annually to do all the basic preventative maintenance, repairs, and the surveying from the sensors to the reference markers on land, called bench marks. We also do what we call emergency maintenance, when something breaks or fails, then we send a crew out, and do emergency maintenance to restore it to operation.

Now, a standard tide station might last 30 years before you have to replace the actual enclosure. The equipment basically is just refurbished as necessary and a single data collection platform might last 20 years before you have to replace it. The equipment is pretty reliable. The sensors occasionally fail and you just replace the sensor, so it’s just swapping parts as you need to.

HOST: You’ve talked a lot about all of this data that’s out there, the many different kinds of people that use it. How can the public access this data?

TOM LANDON: Well, all our data is available on the internet. The address is www.tidesandcurrents.noaa.gov. That will take you to our Web site, from which you can get any number of different types of information regarding tides and all the different projects that we are involved with.

One of the more useful and popular places on the Web site is a page called Tides Online, its Web address is tidesonline.nos.noaa.gov. And that is an application that you can pull up any given tide station, you can search by it by state, you can search by it by region – East Coast, Gulf Coast – and it pulls up a list of all of the different tide stations and when you select a particular tide station it will show you a graphic of the actual water level, both predicted and observed.

HOST: Thanks Tom, do you have any final closing words for our listeners today?

TOM LANDON: Yes, I do Kate. NOAA does a lot of great things for the people of the United States. Our products and services are used extensively and I’ve been privileged to work here a long time and it’s always been fun, it’s always been exciting and challenging and I just encourage people to find out more about what we do and how it affects their lives.

HOST: Thanks Tom for joining us on Diving Deeper and talking more about how and why we measure tides. To learn more, please visit tidesandcurrents.noaa.gov.   

(OUTRO)
That’s all for today’s show. Please join us for our next episode in July.  

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