logging in or signing up Explaining Tides DeathScythe_22 Download Post to : URL : Related Presentations : Share Add to Flag Embed Email Send to Blogs and Networks Add to Channel Uploaded from authorPOINT lite Insert YouTube videos in PowerPont slides with aS Desktop Copy embed code: (To copy code, click on the text box) Embed: URL: Thumbnail: WordPress Embed Customize Embed The presentation is successfully added In Your Favorites. Views: 181 Category: Education License: All Rights Reserved Like it (0) Dislike it (0) Added: October 22, 2010 This Presentation is Public Favorites: 0 Presentation Description No description available. Comments Posting comment... Premium member Presentation Transcript Tides : Tides Jay Charland Oregon Coastal Management Program October 8, 2007 Why do we have tides? : Page 2 Why do we have tides? The common explanation is that the gravitational forces of the Moon and Sun pull water toward themselves, causing bulges of water on the near and far sides of the Earth. More precisely, the Earth and Moon, and the Earth and Sun rotate around common centers of mass. This wobble is the actual source of tidal energy. There are 37 separate periods which affect the tides. It takes 18.61 years to cover them all. Both the atmosphere and the Earth’s crust also have tides, as does the Sun. M = 1 M = 1/6 Center of Mass About 29,000 mi above the Earth’s surface. Earth Moon Magnitude of tidal range : Page 3 Magnitude of tidal range Tides are waves which rotate around amphidromic points. There are 12 open ocean amphidromic points. The farther one is from an amphidromic point, the greater the tidal range. Interaction with land masses will also influence tidal range. Daily Cycles:Diurnal, Semi-Diurnal, Mixed : Page 4 Daily Cycles:Diurnal, Semi-Diurnal, Mixed Diurnal tide - Having a period of one tidal day. The tide is said to be diurnal when only one high water and one low water occur during a tidal day (tidal day = 24 hours and 50 minutes). Semidiurnal tide - Having a period of approximately one-half of a tidal day. The predominant type of tide throughout the world is semidiurnal, with two high waters and two low waters each tidal day. Mixed Semi-Diurnal - Type of tide characterized by a conspicuous diurnal inequality in the higher high and lower high waters and/or higher low and lower low waters. The interaction between waves may give rise to the various diurnal tidal cycles.A location may be dominated by a single diurnal wave : Page 5 The interaction between waves may give rise to the various diurnal tidal cycles.A location may be dominated by a single diurnal wave 1 tidal day Interaction between wavesOr a location may be dominated by a single semidiurnal wave : Page 6 Interaction between wavesOr a location may be dominated by a single semidiurnal wave 1 tidal day Interaction between wavesIf a location is significantly influenced by a diurnal and semi-diurnal wave, a mixed semi-diurnal system results. : Page 7 Interaction between wavesIf a location is significantly influenced by a diurnal and semi-diurnal wave, a mixed semi-diurnal system results. 1 tidal day Interaction between wavesIf a location is significantly influenced by a diurnal and semi-diurnal wave, a mixed semi-diurnal system results. : Page 8 Interaction between wavesIf a location is significantly influenced by a diurnal and semi-diurnal wave, a mixed semi-diurnal system results. 1 tidal day Slide 9: Page 9 Green is semi-diurnal Yellow is diurnal Red is mixed Monthly Cycles: Spring and Neap Tides : Page 10 Monthly Cycles: Spring and Neap Tides Spring tides The greatest range of tidal eleva-tion during the month. Spring tides occur around full and new moons. Neap tides The smal-lest range of tidal ele-vation. Neap tides occur around first and last quarters of the lunar cycle. Spring Tide Neap Tide Common Tidal Elevations : Page 11 Common Tidal Elevations MHW - Mean High Water. The average height of all high tides. MHHW - Mean Higher High Water. The average height of the higher of the two daily high tides (mixed systems only). MLW - Mean Low Water. The average height of all low tides. MLLW - Mean Lower Low Water. The average height of the lower of the two daily low tides (mixed systems only). MSL - Mean Sea Level. The average height of the sea measured over 18.61 years. Sea level is measured over a period of 18.61 years with reference to a geodetic datum. Changes may be due to uplift or subsidence of the land, and also to sea level rise. MSL is the datum on USGS topographic maps and aeronautical charts. MLW or MLLW used on nautical charts. Tidal Elevations Defined : Page 12 Tidal Elevations Defined MHW - Mean High Water. The average height of all high tides. MHHW - Mean Higher High Water. The average height of the higher of the two daily high tides (mixed systems only). MLW - Mean Low Water. The average height of all low tides. MLLW - Mean Lower Low Water. The average height of the lower of the two daily low tides (mixed systems only). MSL - Mean Sea Level. The average height of the sea measured over 18.61 years. Tidal Elevations Defined : Page 13 Tidal Elevations Defined MHW - Mean High Water. The average height of all high tides. MHHW - Mean Higher High Water. The average height of the higher of the two daily high tides (mixed systems only). MLW - Mean Low Water. The average height of all low tides. MLLW - Mean Lower Low Water. The average height of the lower of the two daily low tides (mixed systems only). MSL - Mean Sea Level. The average height of the sea measured over 18.61 years. Head of Tide : Page 14 Head of Tide Farthest point upriver where tidal variations can be detected. Often at the fall line, or where exposed gravel bars begin as you move upriver. Legal definition of “head of tide” usually corresponds to some visible landmark, like a bridge. Sometimes marked. May be boundary of Coast Guard authority. Often boundary between resource agencies. May change due to landslide or new dam. Slide 15: Page 15 Elevations in Depoe Bay, Oregon : Page 16 Elevations in Depoe Bay, Oregon MHHW 11.64 MHW 10.93 MSL 7.81 MLW 4.77 MLLW 3.40 NAVD 4.03* * The datum for these elevations is not NAVD. It is arbitrary and applies only to elevations at this tidal station. Tide Stations in Oregon : Page 17 Tide Stations in Oregon Geodetic Vertical Datums : Page 18 Geodetic Vertical Datums National Geodetic Vertical Datum of 1929 (NGVD 29) NGVD 29 is no longer supported by NGS. NGVD 1947 was a slight adjustment to the 1929 model. North American Vertical Datum of 1988 (NAVD 88) Supersedes NGVD 29 and 1947. Based on many thousands more observations of local gravitational vectors. Also uses a better system for defining its datum. Vertical Datums are tied to Geoids. A Geoid is a model of a theoretical sea level. Thus an elevation measurement based on NGVD 1929 or NAVD 88 represents an elevation relative to a theoretical sea level. Datums and Elevations : Page 19 Datums and Elevations To convert between NGVD 1929 and NAVD 88: http://www.ngs.noaa.gov/cgi-bin/VERTCON/vert_con.prl You will enter a longitude and latitude. To find data on tidal elevations at tide stations: http://egisws01.nos.noaa.gov/website/co-ops/stations/viewer.htm?ActiveLayer=0&Layers=100000000 MSL elevation to MLLW : Page 20 MSL elevation to MLLW Find a tide station near the site. Find the local elevation of MSL. Find the local elevation of MLLW. ElevMLLW = ElevMSL + (MSL-MLLW) Note: MSL and MLLW will be based on a local datum, not the NGVD or NAVD. You only need the difference in elevations, so the local datum does not matter. MSL elevation to MLLWExample : Page 21 MSL elevation to MLLWExample A site near Astoria is 4 feet above MSL. MSL in Astoria is 6.74;MLLW is 2.23. The site is 4 feet + (6.74-2.23) feet, or 8.51 feet above MLLW. MSL elevation to MHHWExercise : Page 22 Find the elevation of a site near Astoria with respect to MHHW if the site is 3 feet above MSL. MSL elevation to MHHWExercise ElevMHHW = ElevMSL + (MSL – MHHW) ElevMHHW = 3 + (6.74 – 10.84) ElevMHHW = 3 + (-4.10) ElevMHHW = -1.10 feet Tidal Elevations on Site : Page 23 Tidal Elevations on Site Tidal amplitudes decrease as you move upriver. May increase as you move up a fjord, sound, or narrow bay. Mean water level may also rise with the bottom. Some methods Vegetation breaks (MD, MS) Wrack line (deposited debris) Marine growth; decayed pilings; eroded rocks; jellyfish Stains on rocks; silt or clay on the ground Land survey from a benchmark (NJ) Survey grade GPS receiver Local knowledge Corps of Engineers: No tricks or special methods. World Famous TidesBay of Fundy, Nova Scotia : Page 24 World Famous TidesBay of Fundy, Nova Scotia Obscure Tidal Elevations : Page 25 Obscure Tidal Elevations MHWS - Mean High Water Spring HW - High Water MHWN - Mean High Water Neap ML - Mean Level MLWN - Mean Low Water Neap MLWS - Mean Low Water Spring LAT - Low Astronomical Tide (I’ve never heard of any of these. I got them off of a sailing website, but also found them in a coastal engineering manual.) Parallax and Declination : Page 26 Parallax and Declination Parallax Effects (Moon and Sun). The distance between the Earth and Moon will vary throughout the month by about 31,000 miles. Once each month, when the moon is closest to the earth (perigee), the tide-generating forces will be higher than usual, thus producing above-average ranges in the tides. Approximately two weeks later, when the moon (at apogee) is farthest from the earth, the lunar tide-raising force will be smaller, and the tidal ranges will be less than average. Similarly, in the earth-sun system, when the earth is closest to the sun (perihelion), about January 2 of each year, the tidal ranges will be enhanced, and when the earth is farthest from the sun (aphelion), around July 2, the tidal ranges will be reduced. Lunar Declination Effects: The Diurnal Inequality. The plane of the moon's orbit is inclined only about 5 degrees to the plane of the earth's orbit (the ecliptic). The ecliptic is inclined 23.5 degrees to the earth's equator, north and south of which the sun moves once each half year to produce the seasons. In similar fashion, the moon, in making a revolution around the earth once each month, passes from a position of maximum angular distance north of the equator to a position of maximum angular distance south of the equator during each half month. (Angular distance perpendicularly north and south of the celestial equator is termed declination.) Twice each month the moon crosses the equator. Geoid : Page 27 Geoid An imaginary elliptical surface around the earth with constant gravitational potential. A marble placed anywhere on the Geoid would not roll. The surface of the sea, measured over time and without waves, would be a Geoid. The surface of a still body of water like a bath tub is a geoid. Any one particular Geoid model is probably not at sea level. The Geoid reflects surface and sub-surface geology. : Page 28 The Geoid reflects surface and sub-surface geology. You do not have the permission to view this presentation. In order to view it, please contact the author of the presentation.
Explaining Tides DeathScythe_22 Download Post to : URL : Related Presentations : Share Add to Flag Embed Email Send to Blogs and Networks Add to Channel Uploaded from authorPOINT lite Insert YouTube videos in PowerPont slides with aS Desktop Copy embed code: (To copy code, click on the text box) Embed: URL: Thumbnail: WordPress Embed Customize Embed The presentation is successfully added In Your Favorites. Views: 181 Category: Education License: All Rights Reserved Like it (0) Dislike it (0) Added: October 22, 2010 This Presentation is Public Favorites: 0 Presentation Description No description available. Comments Posting comment... Premium member Presentation Transcript Tides : Tides Jay Charland Oregon Coastal Management Program October 8, 2007 Why do we have tides? : Page 2 Why do we have tides? The common explanation is that the gravitational forces of the Moon and Sun pull water toward themselves, causing bulges of water on the near and far sides of the Earth. More precisely, the Earth and Moon, and the Earth and Sun rotate around common centers of mass. This wobble is the actual source of tidal energy. There are 37 separate periods which affect the tides. It takes 18.61 years to cover them all. Both the atmosphere and the Earth’s crust also have tides, as does the Sun. M = 1 M = 1/6 Center of Mass About 29,000 mi above the Earth’s surface. Earth Moon Magnitude of tidal range : Page 3 Magnitude of tidal range Tides are waves which rotate around amphidromic points. There are 12 open ocean amphidromic points. The farther one is from an amphidromic point, the greater the tidal range. Interaction with land masses will also influence tidal range. Daily Cycles:Diurnal, Semi-Diurnal, Mixed : Page 4 Daily Cycles:Diurnal, Semi-Diurnal, Mixed Diurnal tide - Having a period of one tidal day. The tide is said to be diurnal when only one high water and one low water occur during a tidal day (tidal day = 24 hours and 50 minutes). Semidiurnal tide - Having a period of approximately one-half of a tidal day. The predominant type of tide throughout the world is semidiurnal, with two high waters and two low waters each tidal day. Mixed Semi-Diurnal - Type of tide characterized by a conspicuous diurnal inequality in the higher high and lower high waters and/or higher low and lower low waters. The interaction between waves may give rise to the various diurnal tidal cycles.A location may be dominated by a single diurnal wave : Page 5 The interaction between waves may give rise to the various diurnal tidal cycles.A location may be dominated by a single diurnal wave 1 tidal day Interaction between wavesOr a location may be dominated by a single semidiurnal wave : Page 6 Interaction between wavesOr a location may be dominated by a single semidiurnal wave 1 tidal day Interaction between wavesIf a location is significantly influenced by a diurnal and semi-diurnal wave, a mixed semi-diurnal system results. : Page 7 Interaction between wavesIf a location is significantly influenced by a diurnal and semi-diurnal wave, a mixed semi-diurnal system results. 1 tidal day Interaction between wavesIf a location is significantly influenced by a diurnal and semi-diurnal wave, a mixed semi-diurnal system results. : Page 8 Interaction between wavesIf a location is significantly influenced by a diurnal and semi-diurnal wave, a mixed semi-diurnal system results. 1 tidal day Slide 9: Page 9 Green is semi-diurnal Yellow is diurnal Red is mixed Monthly Cycles: Spring and Neap Tides : Page 10 Monthly Cycles: Spring and Neap Tides Spring tides The greatest range of tidal eleva-tion during the month. Spring tides occur around full and new moons. Neap tides The smal-lest range of tidal ele-vation. Neap tides occur around first and last quarters of the lunar cycle. Spring Tide Neap Tide Common Tidal Elevations : Page 11 Common Tidal Elevations MHW - Mean High Water. The average height of all high tides. MHHW - Mean Higher High Water. The average height of the higher of the two daily high tides (mixed systems only). MLW - Mean Low Water. The average height of all low tides. MLLW - Mean Lower Low Water. The average height of the lower of the two daily low tides (mixed systems only). MSL - Mean Sea Level. The average height of the sea measured over 18.61 years. Sea level is measured over a period of 18.61 years with reference to a geodetic datum. Changes may be due to uplift or subsidence of the land, and also to sea level rise. MSL is the datum on USGS topographic maps and aeronautical charts. MLW or MLLW used on nautical charts. Tidal Elevations Defined : Page 12 Tidal Elevations Defined MHW - Mean High Water. The average height of all high tides. MHHW - Mean Higher High Water. The average height of the higher of the two daily high tides (mixed systems only). MLW - Mean Low Water. The average height of all low tides. MLLW - Mean Lower Low Water. The average height of the lower of the two daily low tides (mixed systems only). MSL - Mean Sea Level. The average height of the sea measured over 18.61 years. Tidal Elevations Defined : Page 13 Tidal Elevations Defined MHW - Mean High Water. The average height of all high tides. MHHW - Mean Higher High Water. The average height of the higher of the two daily high tides (mixed systems only). MLW - Mean Low Water. The average height of all low tides. MLLW - Mean Lower Low Water. The average height of the lower of the two daily low tides (mixed systems only). MSL - Mean Sea Level. The average height of the sea measured over 18.61 years. Head of Tide : Page 14 Head of Tide Farthest point upriver where tidal variations can be detected. Often at the fall line, or where exposed gravel bars begin as you move upriver. Legal definition of “head of tide” usually corresponds to some visible landmark, like a bridge. Sometimes marked. May be boundary of Coast Guard authority. Often boundary between resource agencies. May change due to landslide or new dam. Slide 15: Page 15 Elevations in Depoe Bay, Oregon : Page 16 Elevations in Depoe Bay, Oregon MHHW 11.64 MHW 10.93 MSL 7.81 MLW 4.77 MLLW 3.40 NAVD 4.03* * The datum for these elevations is not NAVD. It is arbitrary and applies only to elevations at this tidal station. Tide Stations in Oregon : Page 17 Tide Stations in Oregon Geodetic Vertical Datums : Page 18 Geodetic Vertical Datums National Geodetic Vertical Datum of 1929 (NGVD 29) NGVD 29 is no longer supported by NGS. NGVD 1947 was a slight adjustment to the 1929 model. North American Vertical Datum of 1988 (NAVD 88) Supersedes NGVD 29 and 1947. Based on many thousands more observations of local gravitational vectors. Also uses a better system for defining its datum. Vertical Datums are tied to Geoids. A Geoid is a model of a theoretical sea level. Thus an elevation measurement based on NGVD 1929 or NAVD 88 represents an elevation relative to a theoretical sea level. Datums and Elevations : Page 19 Datums and Elevations To convert between NGVD 1929 and NAVD 88: http://www.ngs.noaa.gov/cgi-bin/VERTCON/vert_con.prl You will enter a longitude and latitude. To find data on tidal elevations at tide stations: http://egisws01.nos.noaa.gov/website/co-ops/stations/viewer.htm?ActiveLayer=0&Layers=100000000 MSL elevation to MLLW : Page 20 MSL elevation to MLLW Find a tide station near the site. Find the local elevation of MSL. Find the local elevation of MLLW. ElevMLLW = ElevMSL + (MSL-MLLW) Note: MSL and MLLW will be based on a local datum, not the NGVD or NAVD. You only need the difference in elevations, so the local datum does not matter. MSL elevation to MLLWExample : Page 21 MSL elevation to MLLWExample A site near Astoria is 4 feet above MSL. MSL in Astoria is 6.74;MLLW is 2.23. The site is 4 feet + (6.74-2.23) feet, or 8.51 feet above MLLW. MSL elevation to MHHWExercise : Page 22 Find the elevation of a site near Astoria with respect to MHHW if the site is 3 feet above MSL. MSL elevation to MHHWExercise ElevMHHW = ElevMSL + (MSL – MHHW) ElevMHHW = 3 + (6.74 – 10.84) ElevMHHW = 3 + (-4.10) ElevMHHW = -1.10 feet Tidal Elevations on Site : Page 23 Tidal Elevations on Site Tidal amplitudes decrease as you move upriver. May increase as you move up a fjord, sound, or narrow bay. Mean water level may also rise with the bottom. Some methods Vegetation breaks (MD, MS) Wrack line (deposited debris) Marine growth; decayed pilings; eroded rocks; jellyfish Stains on rocks; silt or clay on the ground Land survey from a benchmark (NJ) Survey grade GPS receiver Local knowledge Corps of Engineers: No tricks or special methods. World Famous TidesBay of Fundy, Nova Scotia : Page 24 World Famous TidesBay of Fundy, Nova Scotia Obscure Tidal Elevations : Page 25 Obscure Tidal Elevations MHWS - Mean High Water Spring HW - High Water MHWN - Mean High Water Neap ML - Mean Level MLWN - Mean Low Water Neap MLWS - Mean Low Water Spring LAT - Low Astronomical Tide (I’ve never heard of any of these. I got them off of a sailing website, but also found them in a coastal engineering manual.) Parallax and Declination : Page 26 Parallax and Declination Parallax Effects (Moon and Sun). The distance between the Earth and Moon will vary throughout the month by about 31,000 miles. Once each month, when the moon is closest to the earth (perigee), the tide-generating forces will be higher than usual, thus producing above-average ranges in the tides. Approximately two weeks later, when the moon (at apogee) is farthest from the earth, the lunar tide-raising force will be smaller, and the tidal ranges will be less than average. Similarly, in the earth-sun system, when the earth is closest to the sun (perihelion), about January 2 of each year, the tidal ranges will be enhanced, and when the earth is farthest from the sun (aphelion), around July 2, the tidal ranges will be reduced. Lunar Declination Effects: The Diurnal Inequality. The plane of the moon's orbit is inclined only about 5 degrees to the plane of the earth's orbit (the ecliptic). The ecliptic is inclined 23.5 degrees to the earth's equator, north and south of which the sun moves once each half year to produce the seasons. In similar fashion, the moon, in making a revolution around the earth once each month, passes from a position of maximum angular distance north of the equator to a position of maximum angular distance south of the equator during each half month. (Angular distance perpendicularly north and south of the celestial equator is termed declination.) Twice each month the moon crosses the equator. Geoid : Page 27 Geoid An imaginary elliptical surface around the earth with constant gravitational potential. A marble placed anywhere on the Geoid would not roll. The surface of the sea, measured over time and without waves, would be a Geoid. The surface of a still body of water like a bath tub is a geoid. Any one particular Geoid model is probably not at sea level. The Geoid reflects surface and sub-surface geology. : Page 28 The Geoid reflects surface and sub-surface geology.