Processes Affecting Beaches Waves, Currents and Tides

Wave Wave – energy moving along interface of ocean and atmosphere – Energy comes from wind Wave Parts – Crest – Trough Trough Crest

Wave Measurements of a wave – Wave height – Distance between crest and trough – Wavelength – Distance between crests or troughs – Wave period – Time interval for a full wave to pass Wavelength Wave height

Wave Wave height, length and period depend on: – Wind speed – Wind duration (how long the wind blows) – Fetch (distance the wind blows over ocean) Wave Function – Water molecules move in circular motion below surface Water movement stops at ½ the wavelength – As wave interacts with shore Wavelength decreases, wave height increases, wave speed slows – Wave releases energy on shore by breaking

Waves

Currents Longshore drift Gravity currents Rip currents

Currents Longshore drift – When waves approach the shore at any angle other than 90° – Swash and backwash move particles on shore at an angle and straight back to sea – Current carries sediment down beach – Closes inlets – Creates spits

Currents Gravity Currents – Currents on or offshore based upon winds at beach – Onshore winds Local rise in sea level Current along bottom depositing sediment offshore – Offshore winds Local drop in sea level Current along bottom depositing sediment onshore

Currents Rip Currents – Created by water rushing offshore through a break in a sand bar – Swim with current

Tides Tide - Predictable rise and fall of sea level due to gravitational forces between the earth, sun and moon – Tidal Range – Difference between high tide and low tide Varies with geography – Gulf of Mexico < 1m – New England – up to 6m – Bay of Fundy 55m

Largest Tidal Range in the world Bay of Fundy 55 ft tidal range Caused by tidal resonance

Tides 3 patterns – Semidiurnal – 2 equal high tides and 2 equal low tides /day Period of 12hrs and 25 mins. At equator at all times US East Coast – Mixed – 2 unequal high tides and 2 unequal low tides /day When moon is north or south of equator Most areas above and below equator Caribbean and US Pacific Coast – Diurnal – One high tide and one low tide / day Period 24hrs 50 mins. At poles Gulf of Mexico

Tides Spring Tide – Sun and moon are in line with Earth – High tides - higher than normal 3x higher than neap tides – Low tides - lower than normal – Occurs twice a month Neap Tide Sun and moon form right angle with Earth High tides are lower than normal and low tides are higher than normal Occurs twice a month

Why are tides important? Surfing Laying on the beach Shell hunting Recreational boating Navigation Power generation

Power Production Using Tides Insert water turbine into tidal current – Energy is dependent upon tidal cycles – Turbines block ships Building ponds to collect and release high tide water through a turbine – Dams are expensive and block ships – Multiple ponds can store water to produce electricity when needed There are very few tidal systems installed – Environmental issues – Most famous one: La Rance (Rance Power Station), Brittany, France

Beach Profile