1. Tides and Currents

2. Tides 101 Vertical movement of water Predictable Standing wave circling an amphidromic point 3 Kinds of Tides – Semi-diurnal Two high and two low tides per day (Atlantic Coast) – Diurnal One high and one low tide per day (Northern Gulf of Mexico) – Mixed semi-diurnal Two high and two low tides per day (Seattle) Different amplitudes (MLLW) Primarily influenced by the position of the Moon and Sun relative Earth

3. NOAA, 2008

4. Equilibrium theory of tides Earth Moon Gravity Centrifugal SUN water

5. Tidal Forces Sun and moon are aligned (new & full) – Syzygy – Spring Tides, higher high tides and lower low tides Moon Earth Sun Sun’s gravity + Moon’s gravity = tidal amplitude

6. Compared to… Moon Earth Sun Sun and moon are at 90 degrees (first and third quarter) Quadrature Neap Tides, less extreme heights BUT WE DON’T HAVE SPRING AND NEAP TIDES!!!!!

7. Tropic and Equatorial Tides Occur in Diurnal Systems Declination of the moon Elliptical orbit of the moon… EarthMoon Equatorial Tropic Moon

8. Tidal Questions Moon to circle Earth – 29.5 days (Lunar Month) Moon declination above equator max to max: – 29.5 days (Tropical Month) How long between two successive tropic tides? – Two successive spring tides? – Successive tropic and equatorial tides? How long will it take to complete one complete diurnal tidal cycle? (High  Low  High) – Why?

9. Currents Horizontal flow of water – Impacted by rainfall, wind, water density (temperature/salinity), tides VERY IMPORTANT WHEN DIVING!!!! – Do NOT underestimate the power of moving water!!!!!!

10. Density Currents NOAA 2011

11. Tidal Currents When is the best time to dive, with respect to tidal currents? Slack Water (GOOD FOR DIVING) – Two-three per day (high and low tides) in Mobile Bay and occur when the tides switches directions (from ebb to flood and vice versa) Maximum Current (BAD FOR DIVING) – Two per day and occur at the midpoint between high and low tides. Ask Dardeau about the flow directions in the bay… they really don’t make sense… What else can impact the flow rate of tidal currents?

12. Seiche Similar to storm surge Consistent wind direction causes water to pile up and can negate tides… What happens when wind direction changes? – When it changes during a low tide?

13. Online Resources Tides for Mobile Bay – http://tidesandcurrents.noaa.gov/tide_predictions.html?gi d=226 http://tidesandcurrents.noaa.gov/tide_predictions.html?gi d=226 Currents for Mobile Bay – http://tides.mobilegeographics.com/locations/3842.html http://tides.mobilegeographics.com/locations/3842.html NOAA tidal currents – http://tidesandcurrents.noaa.gov/currents14/cpred2.html #FL http://tidesandcurrents.noaa.gov/currents14/cpred2.html #FL Note the units on the y-axis between the first two…

14. Diving Relevance How strong is a 2 knot current? – When should you call the dive? – What can you do to mitigate current? VIDEO