B E A C H E S . M A R I N A S . D E S I G N . C O N S T R U C T I O N .

OECS Regional Engineering Workshop September 29 – October 3, 2014

Coastal Erosion and Sea Defense:

Introduction to Coastal Dynamics

David A Y Smith

OBJECTIVE • To provide a basic understanding of the primary coastal processes: What

are the wave and current processes that dominate the nearshore zone and how do they interact with the shoreline?

TOPICS • Understanding Waves

• Waves in motion – Diffraction

– Refraction

– Shoaling/Breaking

– Reflection

• Tides and Tidal Currents – How are tides generated

– Tidal currents

• Sediment Transport – Alongshore transport

– Cross-shore sediment transport

TYPES OF WAVES

WAVES IN MOTION – WIND WAVE PROPERTIES

• Waves generated by winds transmit energy through the water column.

• In deep water, an orbital current flow is created below the crest of waves which reverses direction under the trough

• In shallow water, the orbital motion becomes a “flat ellipse” which eventually becomes a to-and-fro current

• Waves under the influence of winds are called wind waves while those that outlast this influence are called swells

WAVES IN MOTION – WAVE PROPERTIES

WAVES IN MOTION - REFRACTION • REFRACTION

– Bending of wave crests as one part of the wave crest reaches shallow water first, slowing down while the other part “catches up”.

• The distance between wave

crests (wave length) decreases but the time between the wave crests (wave period) does not change.

• As waves approach shallow water: C = L

T remains constant decreases

decreases

WAVES IN MOTION - REFRACTION

• Wave orthogonal – imaginary line drawn perpendicular to the crest of the wave. This essentially shows the direction of progression of the waves.

• Waves react to the nearshore contours and bend until they become almost perpendicular to the contours

• This causes focusing on some sections of the shoreline such as headlands and dispersion in other areas such as bays

WAVES IN MOTION – SHOALING AND BREAKING – Increase or decrease in wave height due to the change in depths as the wave

approaches shallow water – As the waves bunch closer together in shallow water, they increase in height – Waves increase in height until they become so steep they break – As the waves break, the Kinetic energy (from the forward motion of the waves) is

converted to Potential Energy (increase in height of water) during breaking

Wave crest

MSL

Breaking

Seabed

WAVES IN MOTION - DIFFRACTION • Diffraction occurs when waves pass

through a gap or against the edge of a physical structure

WAVES IN MOTION - REFLECTION • Vertical surfaces absorb, transmit and

reflect wave energy. • Reflection occurs when the wave energy

that impacts on to a physical structure is not totally absorbed by the structure or transmitted through the structure.

• Reefs and rock breakwaters absorb and transmit some of the energy to their lee side (in the incident direction of the waves) but they also reflect a portion of this energy.

• Vertical walls reflect most of the energy in the opposite direction of the incident waves.

• Reflection can reduce or increase total wave energy depending on where in the wave cycle the impact occurs

THE RIVER OF SAND…WAVES TO SEDIMENT TRANSPORT

– Waves are generated in deep water by winds. – As waves approach the shoreline they undergo

several transformation processes such as refraction, shoaling and breaking.

– As waves break, the dissipated energy causes an increase in water level

– Different water levels along a shoreline cause an alongshore current.

– The breaking process also disturbs the sediments on the seafloor, bringing them into suspension.

– The alongshore currents move the sediments along the shoreline.

SEDIMENT TRANSPORT

• ALONGSHORE MOVEMENT – When waves break, their Kinetic Energy is transferred to Potential

Energy in the form of an increased water level. The variations in water level along the shoreline cause water flow (currents) from areas of high to low water level. The turbulence from the breaking waves brings sediments on the seabed into suspension. The wave-generated currents transport these sediments along the shoreline.

• CROSS-SHORE MOVEMENT – Waves breaking on a shoreline over a period of time shape the cross-

shore profile into a stable equilibrium profile. Higher waves result in steeper beach slope formations. Therefore, the profile of beaches tend to vary with the seasons.

– Larger than normal waves (eg. Hurricanes) can have a profound impact on a beach profile, moving sand from the shoreline to the offshore area and eroding beach dunes.

SEDIMENT TRANSPORT -ALONGSHORE TRANSPORT

• ALONGSHORE – ZONE OF TRANSPORT

Point of breaking

shoreline

Wave crests

S.T.

Surf Zone

Depth contours

SEDIMENT TRANSPORT -CROSS-SHORE

Area of erosion

Area of accretion Steeper beach profile after storm or during seasons of high waves

Dune Storm or Winter Swell Waves

Summer waves

Cross-shore sand movement

• CROSS-SHORE - SEASONAL BEACH PROFILE CHANGES

SEDIMENT TRANSPORT -SOURCES AND SINKS

• SEDIMENT SOURCES – Rivers – Coral Reefs – Seagrass/algae – Erosion of cliffs

• SEDIMENT SINKS – Canyons – Shelves – Beaches – Structures

SEDIMENT TRANSPORT – IMPACTS OF STRUCTURES

• IMPACT OF NATURAL AND ARTIFICIAL STRUCTURES IN THE SEA ON SEDIMENT MOVEMENT AND BEACH FORMATION. – Reefs – Natural offshore formations that create calm areas in their lee

thereby reducing the potential for cross-shore movement of sand (and alongshore movement to a lesser extent), often resulting in build up of sand on the beach.

– Breakwaters – artificial shore-parallel reef structures that create build up of sand depending on their level of submergence and distance from the shoreline.

– Groynes – Shore-perpendicular structures that interrupt the alongshore movement of sand. Groynes tend to create build up on the upstream end and erosion on the downstream end of the dominant sediment transport direction.

GROYNE IMPACT

Breakwaters

BREAKWATER IMPACT

TIDES

• Three principal forces are involved in the production of tides: (1) gravitational attraction between the moon and the earth; (2) gravitational attraction between the sun and the earth; and (3) the force of the earth's gravity, which pulls every particle of the earth toward the earth's center. The moon is mainly responsible for the tides (its effect is about 2.2 times as great as the sun's).

TIDAL CYCLES • Tidal currents and wave-driven

currents interact to produce resulting nearshore current flows

• In aggressive wave environments wave-driven currents usually dominate in the nearshore

• Tidal currents and oceanic currents always dominate in the offshore

OCEANIC CURRENTS

• Tidal currents and oceanic currents always dominate in the offshore