Shorelines and Shoreline Processes

Shorelines

        The areas between low tide and the highest land affected by storm waves. The shape and position of shorelines are constantly being modified by the processes of erosion and deposition by waves and currents.

        Waves consist of the following parts or characteristics:

1. Wave crest - highest part of a wave. Swells are waves that have broad, rounded crests; seas, sharp-crested, irregular waves that develop during storms.
2. Wave trough - lowest part of a wave.
3. Wavelength - horizontal distance between adjacent wave crests or troughs.
4. Wave height - vertical distance between the wave crest and wave trough.
5. Wave period - time necessary for two successive wave crests or troughs to pass a given point.
6. Wave speed (C) - can be calculated by dividing wavelength (L) by wave period (T), i.e., c = L/T

        Waves can be formed by several processes:

• Wind-generated waves (most common) - waves are formed by shear stress between water and air when the wind speed is higher than about 3 km/hr. Factors determining the size of waves:

1. wind velocity - higher wind velocity, larger waves.
2. wind duration - longer the wind blows, larger the waves.
3. fetch - distance the wind blows over a continuous water surface. Larger water body permits larger waves to be generated.
• Displacement of water - can be caused by landslides.
• Displacement of seafloor - can be caused by faulting and volcanic eruptions.

• Circular orbits - waveform moves forward, but the water molecules move in a circular path in deep water.
• Depth of motion - diameter of the circular orbit decreases with depth and is about zero at a depth = 1/2 the wavelength (wave base).
• Breakers - waves that collapse forward (break) as they approach shore. When the water depth is less than wave base, friction between the ocean bottom and the wave occurs. Bottom friction causes:

1. decrease in wavelength.
2. decrease in wave speed.
3. increase in wave height.

Breaking waves create surf (zone of active wave erosion).

        Wave refraction occurs where wave fronts approach shore at an angle (obliquely), but are bent to become more parallel to the shoreline by frictional drag on the bottom. The part of the wave in shallow water slows down because of bottom friction, while the part in deep water keeps moving at regular speed. Refraction causes:

• Headland erosion - Wave energy is concentrated on headlands (points of land extending out offshore).
• Deposition in bays - Wave energy is weaker in bays.

        These are currents that occur in the area from the shoreline to beyond the surf zone. There are two major kinds of currents recognized:

• Longshore Currents - currents moving parallel to shore in the same general direction as the approaching waves. They are produced by the movement of oblique waves in the surf zone, and can transport large amounts of sediment by longshore drift.
• Rip Currents - narrow surface currents that flow out to sea through the breaker zone represent circulating cells that are fed by longshore currents. Water moves seaward from the nearshore zone. These currents develop in areas with lower wave heights (deeper water depths).

        The shoreline can be divided into three major areas:

• Coast - Land inland from the base of the sea cliff (cliff produced by the undercutting of bedrock at sea level by wave erosion).
• Beach (Shore) - area between low tide level and dunes, sea cliff, or permanent vegetation. It can include a wave-cut platform (a bedrock terrace left behind by the receding cliff) and is separated into:

 
1. backshore - area of beach from the sea cliff/sand dunes to the berms (sand terraces built up by storm waves). Usually dry.
2. foreshore - beach seaward of the beach face (slope below the berms that is exposed to wave swash) that includes the tidal flats (area of shore lying between high and low tide levels).

• Offshore - area, continuously underwater, can include a wave-built platform (relatively flat accumulation of unconsolidated sediments eroded from surf zone and deposited farther offshore by waves).

        Sediment comes from wave erosion and from rivers opening into the ocean (most important source), and is transported by longshore currents. The following depositional features can be developed:

• Beach - long, narrow strips of unconsolidated sediment (usually sand) that are deposited by longshore drift. Pocket beaches (small, discontinuous beaches existing in protected areas) are common in areas with high wave energy. Sediment is transported along beaches by beach drift (zigzag motion of sediment particles in the swash and backwash of waves). The amount of sand in the beach system remains fairly constant, but the profile of a beach changes with the season:

• Summer beaches - usually sand-covered and characterized by:
1. wide berms
2. gently-sloping beach faces
3. smooth offshore profiles
• Winter beaches - beach profile that is modified by storm wave erosion and possesses the general characteristics:
1. coarser-grained sediment
2. steeper beach faces
3. berms are small or missing; sand is eroded from beach.
4. offshore sand bars parallel beach; sand is transported and deposited in offshore bars.

• Spits - elongated ridges of sand that project from the land into the mouth of an adjacent bay, and form as a result of longshore drift.
• Baymouth bars - sand bars that form as a result of longshore drift and completely cross a bay, sealing it off from the open ocean..
• Tombolo - a ridge of sand that connects and island to the mainland, formed as the result of wave refraction around an island.
• Tidal inlet - a break in a spit or baymouth bar, caused by storm erosion, through which tidal currents rush.
• Barrier islands - low offshore ridges of sediments that parallel the coast, and are separated from the mainland by lagoons. The barriers islands, which commonly contain wind-blown sand dunes, migrate landward with rising sea level and with the longshore current direction. They may originate 3 different ways:

1. as spits that were severed from the mainland by wave erosion or by a rise in sea level.
2. as former beach dune ridges, drowned by rising sea level.
3. as mounds of sand piled up by storm waves at high tide in the surf area.

            Wave action may lead to shorelines which lack beaches and have prominent sea cliffs. The net effect is to straighten the shoreline through:

• Wave erosion, where storm waves erode by:

1. Corrosion - solution (minor)
2. Hydraulic action - force of breaking water compresses air into cracks. The trapped air expands rapidly after water subsides, prying rock apart.
3. Abrasion - particles carried by water act as grinding tools in the surf zone.

• Sediment transport, where nearshore areas lose sediment by:

1. longshore drift - redistributes sediment along shoreline.
2. wind erosion - transports and deposits sand inland as dunes.
3. offshore transport - deposits fine-grained sediment in deeper water.
4. submarine canyons - can funnel large amounts of sediment to deeper water.

1. Sea cliffs - form by storm wave erosion which undercuts higher land, making it susceptible to mass wasting. Sea cliffs can erode very slowly or very rapidly, depending on the rock type and wave energy.
2. Wave-cut platform - bevelled surface produced by the retreat of a sea cliff. Slopes gently in a seaward direction.
3. Headlands - seaward projections of shore eroded by wave refraction.
4. Sea caves - caves formed by accelerated wave erosion along a zone of weakness in bedrock.
5. Sea arch - formed when two sea caves on opposite sides of a headland unite.
6. Sea stack - Isolated pinnacle of rock formed when a sea arch collapses.

        Sea level has changed during the course of Earth history as the result of several factors:

• Earthquakes - can lead to sudden elevation or down-drop of shorelines locally along faults.
• Isostasy - rebounding of crust after melting of glacial ice has been documented in various areas. Subsidence of land can occur from loading with sediments or ice.
• Glaciation - sea level was 130 meters lower than present during the maximum extent of Pleistocene glaciation. If the remaining ice caps should melt, sea level would rise about 70 meters. Sea level is currently rising about 1-2 mm/yr as glaciers continue to retreat.
• Human activity - withdrawal of fluids (water or petroleum) have caused subsidence in some coastal areas.

        Coastlines can be classified in several ways:

• By gain and loss of sediment

• Erosional coasts characterized by:
• steep, irregular coastline.
• lack of well-developed beaches.
• evident erosional features.
• Depositional coasts characterized by:
• abundance of detrital sediment.
• presence of depositional landforms.
• By relationships to sea level

• Submergent (drowned) coasts - very irregular coasts, dominated by erosional processes, develop when a coastal area subsides or sea level rises. They are characterized by:
1. Estuaries - drowned river mouths.
2. Fiords - drowned glacial troughs.
• Emergent coasts - Form where land has risen relative to sea level. They can be straight or irregular, depending on the erosional processes that dominated prior to uplift. These coast are characterized by:
• Elevated marine terraces
• Sea cliffs
• Wave-cut platforms

        Tides are the rhythmic rise and fall of sea level at a specific location. Tidal ranges are most extreme in narrow inlets and bays. Tides are produced primarily by the gravitational attraction exerted upon the Earth by the moon, which causes oceans to deform toward and away from the moon. Two high tides and two low tides occur each day at most locations because of the Earth's rotation. Tidal motions are described in terms of:

1. High tide
2. Low tide
3. Spring tides - alignment of sun and moon (occurs every 2 weeks) produces very high and very low tides.
4. Neap tides - Moon and sun are at right angles to one another (occurs every 2 weeks), so that gravitational pulls are partially offset, resulting in less contrast between high and low tides.

        These currents are the horizontal flow of water accompanying the rise and fall of the tide. They help keep waterways clear. These currents are much stronger in restricted areas (like harbors) than in the open ocean, and can be described as:

1. Flood tides - where a shoreward flow of water occurs with high tide.
2. Ebb tides - where an oceanward flow of water occurs with low tide.

        Beach drift and longshore currents can create problems for populated shoreline areas. Artificial structures can be built which interfere with the natural deposition and erosion and often create worse problems than existed originally:

• Jetties - parallel barriers built at entrances to harbor or rivers to prevent deposition in the channel. They can cause deposition on the up-current side and erosion on the down-current side.
• Groins - short walls built at right angles to the shore to trap moving sand and maintain or widen beaches can cause erosion down-current.
• Breakwaters - structures built parallel to shore to protect it from the force of large breaking waves. Deposition occurs in the quiet water area, whereas erosion can strip the shoreline down-current.
• Dams, etc. - other structures built on rivers can reduce the amount of sediment load reaching the ocean, resulting in increased beach erosion.