Tides & Beaches
OUTLINE
• BEACHES
– Nearshore Sediment Transport
• Tides
– What are Tides?
– Tides in the Coastal Ocean
– What Causes Tides
Nearshore Sediment Transport –
• What influences the loss of sediment on Beaches?
•
•
•
•
•
Waves
Winds
Tidal Currents
River discharge
Runoff
• Beaches divided into Segments
– Nearshore/Offshore
• Submerged portions
– backshore\foreshore
• Exposed portions
– Low-tide terrace
– Aka swash zone
» Alt. periods
» Of run up
» Waves/
» backwash
• BEACHES
– Waves supply E  affect Beach Profile
– Back/fourth movement of sediments is called?
• Cross-shore transport
• What does this result it?
– Change in the beach slope as sand is piled up or removed from beach
• b/c E of Wave run up ~ exceeds E or Backwash
– Sediments typically pile up on beaches. . . .
Run UP
Backwash
CROSS SHORT TRANSPORT
Fig. 10.22
• BEACHES
– If Wave E = High  sediments carried offshore
• Stored as Sandbar
– If Wave E = Low  sediments carried onshore
• From Sandbar  buildup of sand
• Storms etc. periodically
– disrupt the equilibrium profile
• BEACHES
– What is Longshore transport?
• Sediments moved parallel to beach face


• What type of current influences this?
– Longshore currents – strikes beach at angle
– e.g. gone out @ Manressa Beach, North of Lifegaurd tower. . .
» End up in Moss Landing. . . How’d I get here?
• Beaches
Challenge Q
What direction is the Long shore current coming from?
– Preventing beach erosion
• Groins, Jetties, etc.
• Mitigates Longshore
– Currents sediment removal
– e.g. Marin Co.
– Bolinas Bay
S
N
• Erosion Control
– Artificial Importation or beach nourishment
• Sediment budgets embody concept of a Littoral cell
– Littoral Cell
» Self contained section of coast whose beach sediments
» Controlled by identifiable local mechanisms of Supply/ loss
» E.g. So. Cal Beaches (N. Pacific) or N. Atlantic Outer Banks
Fig. 10.29
Fig. 10.29.a
Fig. 10.29.b
OUTLINE
• BEACHES
– Nearshore Sediment Transport
• Tides
– What are Tides?
– Tides in the Coastal Ocean
– What Causes Tides
–What are Tides?
What Causes Tides?
– They are just Big Waves
• Most basic understanding
– Originally proposed by Newton
• Incorporates Equilibrium Model
• As earth rotates, locations on surface
– Pass b/w beneath
» Tidal bulges
» Directly beneath
» = High Tide
» Other locations
» = lower Tide(s)
What is the Tidal Period?
Time b/w successive High and Low Tides (~12 hrs)
What is the Tidal Day?
One complete revolution of Earth beneath Tidal Bulges
What is the Tidal Range?
high tide mark (2.0 m) - low tide mark (0.5 m) = 1.5 m Tidal Range
• Tides
– Variations in Height/ Time ~ involve MOON and Sun
• Both create Tidal Bulge via Tidal Forces
– Moon = M2 tide
– Sun = S2 tide
M2
S2
For H-work 3: will use
F = 2 x G x M1 x M2x a
R3
a = radius of Earth
R = distance b/w Earth and object in meters
So What is a diurnal tide anyway? Well . . .
• Challenge Questions
– How many different types of tides actually exist/ day &
What are they?
• Diurnal
• Semi-diurnal
• Mixed Semi-diurnal
• Challenge Question
– What are the definitions of the following and give
examples of where they may occur
• Diurnal –
– only one high and one low tide/day
– (e.g. Gulf of Mexico)
• Semi-diurnal: –tidal forces  bulges both sides of earth
– Earth’s rotation causes:
– ~ 2 equivalent High Tides, 2 low tides/ day
– (e.g. N. Atlantic Coast)
• Mixed Semi-diurnal
– Unequal pattern of 2 high and low tides/
» (West. Coast N. America)
• Graphical Representation: Three Types of Tides
• Diurnal
• Semi-diurnal
• Mixed Semi-diurnal
• A typical Mixed – semi Diurnal Tide (Santa Cruz, Ca)
• Locations of the Occurring 3 Tide Types Worldwide
• Rough Explanation as to why SUCH Tidal differences exist
– Depends on MANY Factors (very COMPLEX)
• Geometry of Coastline
• Where Sun/ Moon located
• Oceans have vibratory modes (like a drum) aka Harmonic Modes, 300+ exist
– Get stimulated in changing ways from minute to minute
• Storms move large quantities of water
– e.g. Variations Tidal Sea Ports:
• Velocity of water – depends on Salinity & depth in area it passes
• Bathymetry
• Overall Harmonics play major role
• NOT as Simple as you’d think
– – here’s a simplified explanation. . .
• Equilibrium Model
NOTE:
Moon’s Tidal Force
Assists with bulging of Tides
NOTE:
Moon also Orbits around Earth
• So Here’s what you can get. . .
Confirming your knowledge
– 3 different Types of Tides observed Around the Globe
• Diurnal (@ High Latitudes & Equator)
• Semi-diurnal (Mid Latitudes)
• Mixed semi-diurnal (Mid Latitudes)
• Some known factors that influence which type of Tide is present/where are:
– Storms
– Geometry of Coastline
– Bathymetry
– Velocity of water – depends on Salinity & depth of area
– Overall Harmonics (vibrations)
– esp. declination of the Moon
• Further Insights - Challenge Questions
– What are Spring & Neap Tide?
• Spring Tide
– when the tidal Range reaches a Maximum
• Neap Tide
– when the Tidal Range reaches a Minimum
It’s Beach, Santa Cruz, CA
Fig. 11.7
Confirming Your Knowledge
Identify the date(s) of the following Figures that Represent Spring or Neap Tidal Events.
Fig. 11.7.a
Identify the date(s) of the following Figure that Represent a Spring or Neap Tidal Events.
Fig. 11.7.b
Identify the date(s) of the following Figure that Represent a Spring or Neap Tidal Events.
Fig. 11.7.c
Identify the date(s) of the following Figure that Represent a Spring or Neap Tidal Events.
Fig. 11.7.d
Identify the date(s) of the following Figure that Represent a Spring or Neap Tidal Events.
• Challenge Questions
– What actually Influences a Spring or Neap Tidal event?
• Locations of the Sun and Moon
• Influences of a Spring or Neap Tidal event (celestial)
– Spring Tide
•
•
•
•
Moon
Sun
Earth
(in strait Line)
– Neap Tide
• Moon @ 90˚angle
– vs Earth/Sun Line
illuminated
Half illuminated etc.
Quarter (Crescent)
dark
ANOTHER PERSPECTIVE
• END / Break
• Midterm
– Green Scantron Please
• Write your name on TEST and Scanton
• Lots to do with the moon and Sun
Fig. 11.6