Understanding Tides: Causes, Types, and Predictions, Lab Reports of Earth Sciences

Download Understanding Tides: Causes, Types, and Predictions and more Lab Reports Earth Sciences in PDF only on Docsity! TIDES Objective To understand the astronomical forces that generate changes in sea level and how these vary over the different locations on the earth. Introduction Marine coasts experience a rise and fall of the sea level due to: 1) the attractive forces of the sun and moon, and 2) the earth’s constant rotation upon its axis. Although the sun is larger than the moon, the moon is closer to the earth and thus exerts a stronger gravitational force on the oceans than does the sun. The moon thus has the greatest effect on the tides and the ocean surface bulges towards the moon. This bulge is balanced by another one on the opposite side of the earth due to centrifugal force. A tidal day is defined as the length of time that it takes for a spot on the Earth starting beneath the moon to rotate once and return to its position beneath the moon. Remember that as the Earth spins on its axis, the moon is revolving around the Earth (1 revolution in 29.5 days). So, although it takes 24 hours for the Earth to rotate once on its axis, a tidal day is 24 hours and 50 minutes long. In most places the tidal cycle occurs semi daily (semidiurnally), with a rise and fall occurring approximately every 12 hours and 25 minutes (half tidal day) as the earth spins on its axis and a given geographic position rotates under this gravitational bulge. Approximately 6 hours and 12.5 minutes after high tide, a low tide will occur. In some regions, due to the effects of the shapes of the ocean basins on the tides, only one high and one low tide are observed each day. This is called a diurnal tide. The range of the tide is the difference between high and low water levels (Figure 1). For different geographic locations, the tidal ranges measured on a given day may either be approximately equal to one another or they may be significantly different. The case of unequal tidal ranges occurring during a day is called a semidiurnal mixed tide. Every 2 weeks, when the sun, earth, and moon are aligned, a spring tide will occur where the tidal range is greater than the usual. Approximately two weeks later an extremely small tidal range called a neap tide will occur when the sun, earth, and moon form a right angle. The attractive pulls of the sun and moon are additive during spring tides and act to cancel one another during neap tide since the forces are perpendicular to one another. The flood current occurs in estuaries and bays during the rising tide. This is a landward current that is followed by a brief interval of no flow (called mid-tide or slack-tide). The ebb current occurs in estuaries and bays as the tide falls and water flows in a seaward direction. vo &9 42) (skeq uel 19 09 65 f) out, es Js 9s ss Ss es eg Lg (w) eouejsip e1oys-ssory Cor ase OOF Osz O02 3NOZ TWOLLMOT NOz TWoLL-oMn (w) *z ‘uoneney Figure 1  5 Part II: Exploring Tide Tables The National Ocean Service publishes Tide Tables that provide tide predictions for various sites in the United States. You will use these tables to make tide predictions for three National Ocean Service (NOS) Tidal Reference Stations. Go to the following website maintained by the National Oceanic and Atmospheric Administration (NOAA), National Ocean Services: http://tidesandcurrents.noaa.gov/tide_pred.html Directions for locating tide predictions: ? Scroll down and click on the desired year or look to the left and pick the desired year. ? Select the state in which the city is located ? There will be several geographical regions listed. Pick the one closest to the city. ? You will see a table with stations, latitudes, longitudes, ranges, tidal levels, and clickable links for predictions. Some of the stations are in all caps. These are the reference stations. Click on the reference station for the city in question. ? This will give you a year-long table of tidal predictions, including data, day, and times and heights of the tides. Use the information from the table to answer the following questions. 7. When and how high will the tides be on May 10th, of this year in Charleston, SC? 1st 2nd 3rd 4th Time Height --Use the back arrow to return to the Tidal Predictions main page. --Follow the instructions above to make tidal predictions for each one of the following locations: 8. For the same date, when and how high will the tides be in Pensacola, FL? 1st 2nd Time Height 6 9. For the same date, when and how high will the tides be in Seattle, WA? 1st 2nd 3rd 4th Time Height 10. For each of the locations above, what are the tidal ranges (the differences in feet between highs and lows) based on the previous predictions? Mean tidal range? Mean tidal level? A) CHARLESTON, SC Tidal ranges: difference between 1) 1st and 2nd tides ___________ 2) 2nd and 3 rd tides ___________ 3) 3rd and 4th tides ___________ Mean tidal range: __________ Mean tidal level: __________ B) PENSACOLA, FL Tidal ranges: difference between 1) 1st and 2nd tides ___________ Mean tidal range: __________ Mean tidal level: __________ C) SEATTLE, WA Tidal ranges: difference between 1) 1st and 2nd tides ___________ 2) 2nd and 3rd tides ___________ 3) 3rd and 4th tides ___________ Mean tidal range: __________ Mean tidal level: __________ 11. How would you characterize the tide at each location (diurnal, semidiurnal, semidiurnal mixed)? a) Charleston, SC ___________________ b) Pensacola, FL ___________________ c) Seattle, WA ___________________ 7 Part III: Predicting Tides from a Reference Station The three sites that you just examined are all National Ocean Service (NOS) Tidal Reference Stations. Sometimes we need tidal predictions for regions that are not reference stations. For these sites we must know the offset between local tides and tides at the nearest reference station. The Tide Tables published by the NOS provide the necessary information to correct for tide time and height at locations other than the reference stations. 12. You are in charge of planning a field trip to Huntington Beach State Park, SC scheduled for _________________. The closest NOS reference station is MYRTLE BEACH, SPRINGMAID PIER. Return to the Tidal Predictions main page. Click on SC, and choose Myrtle Beach to Pawley’s Island. A table of the tidal stations will appear. Click on the link for MYRTLE BEACH, SPRINGMAID PIER. Find your specified date and record the highs and lows, as well as the times at which they occur. MYRTLE BEACH reference station data 1st 2nd 3rd 4th Time Height Now go back, and click on the link for Oaks Creek, upper end. The correction factors are listed at the top of the page, and have been already applied. Oaks Creek, upper end 1st 2nd 3rd 4th Time Height 13. During the field trip you have three goals: a) measure the velocity of currents exiting and entering a tidal creek during ebb and flood tides, b) dig for clams on a mud flat, and c) determine the maximum landward intrusion of seawater on the beach. Using the tidal information for Oaks Creek, determine the best times of the day to plan these activities. Measure velocity of ebb tide _______________________ Measure velocity of flood tide _______________________ Dig for clams on mud flat _______________________ Determine the maximum intrusion of seawater on the beach ______________