Oceanography Research Paper

Chapter 10: Beaches, Shorelines, and the Coastal Ocean

• Beach terminology • Sand transport on the beach • Erosional and depositional shores • Sea-level changes • Types of coastal waters

Coastal Terminology Shore – the zone that lies between low tide and the highest elevation on land affected by storm waves. Can range from few meters to 100’s of meters.

Coast – extends inland from the shore as far as ocean features can be found. Can range from <1km to tens of km.

Coastline – the boundary between the shore and coast. The landward limit of the effect of the highest storm waves on the shore.

Beach Terminology Backshore – area above high tide and is covered in water only during storms.

Foreshore – portion of shore that is exposed at low tide and submerged at high tide.

Shoreline – the water’s edge; line marking the intersection of the water surface with dry land. This line migrates back and forth with the tide.

Beach Terminology Nearshore – extends seaward from the low tide shoreline to the low tide breaker line. It is never exposed to atmosphere but it is affected by waves that touch the bottom.

Offshore – area beyond low tide breakers which is deep enough that waves rarely affect bottom.

Beach – a deposit of the shore area. Sediment deposit seaward of the coastline through the surf zone. Consists of wave worked sediment and is the active area of the coast.

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Beach Terminology Wave-cut bench – flat, wave eroded surface that sediment moves along.

Berm – dry gently sloping, slightly elevated margin of the beach that can be found at the foot of coastal cliffs or sand dunes. This is where you hang out when you go to the beach!

Beachface – offshore of berm, it is a wet, sloping surface that extends from the berm to the shoreline. It is more fully exposed during low tide. Sand is more hard-packed so good for runners!

Beach Terminology Longshore bars – offshore of beachface. They are sand bars that are parallel to the coast. They may become exposed during certain times of the year when low tide is extremely low.

Longshore trough – low area of beach that separates beach face from longshore bars.

Beach Composition Beaches are composed of whatever material is locally available. • Nearby coastal cliffs or mountains → coarse

texture. • Rivers → fine texture. • Fine clay particles → mudflats along shore. • High biological component with little rock/sand

input → beaches mostly composed of shell fragments.

Beach Composition Regardless of composition: Ø Sediment on beach does not stay in one

place, but instead the waves that crash along the shore are constantly moving it.

Ø Beaches can be thought of as material in transit along the shoreline.

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Movement of Sand on Beach

Transport of sand on the beach can happen in two directions:

1. Perpendicular to the shoreline. • Toward shore • Away from shore

2. Parallel to the shoreline. • Up coast • Down coast

Movement Perpendicular to Shoreline Swash – a thin layer of water that washes up over exposed beach as waves break at the shore.

Backwash – the flow of water down the beachface toward the ocean from a previously broken wave.

Some swash soaks into beach but most drains away as backwash.

When next wave breaks on beach, it sends its swash on tip of previous wave’s backwash.

swash

backwash

Swash and backwash transport sand up and down beachface perpendicular to the shore.

Whether swash or backwash dominates will determine if sand is deposited on or eroded from beach.

Light versus Heavy Wave Activity

Light wave activity – characterized by less energetic waves.

Ø Much swash soaks into beach, so backwash is reduced.

Ø Swash dominates sand transport so net movement sand up the beachface toward berm.

Ø Creates well-developed berm.

Light versus Heavy Wave Activity Heavy wave activity – characterized by high energy waves.

Ø The beach is saturates by previous waves, so little swash gets soaked into beach

Ø Backwash dominates sand transport so net transport of sand down beachface.

Ø Erodes the berm.

Ø Where does sand go? Sand accumulates just beyond where the waves break creating longshore sand bars.

Ø Light and heavy wave activity alternate seasonally at most beaches.

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Summertime and Wintertime Beaches

Summertime beach – light wave activity with wide sandy berm and overall steep beachface with no longshore bars. (depositional beach)

Wintertime beach – heavy wave activity produces narrow rocky berm and overall flattened beachface that builds prominent longshore bars. (erosional beach)

Movement Parallel to Shoreline • Within surf zone, waves refract (bend), causing the

swash to move up the beach at a slight angle.

• Then gravity pulls the backwash down the beachface perpendicular to the shore.

• Therfore water moves in a zigzag pattern along the shore.

Longshore current – the zigzagging movement of water along the shore. Ø Can have speeds up to 4km/hr and speeds increase

as: 1. Beach slopes increase 2. Angle on incoming waves increases 3. Wave height increases 4. Wave frequency increases

Movement Parallel to Shoreline

Longshore drift – or longshore transport is the movement of sediment in a zigzagging pattern caused by the longshore current.

Both longshore currents and longshore drift only occur in the surf zone.

Longshore Currents Longshore currents can change directions, but typically they flow southward along both Atlantic and Pacific coasts of US.

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Erosional and Depositional Shores Although all shores exhibit some erosion and some deposition, most can be identified as primarily one or the other.

Erosional shores – shores with well- developed cliffs in areas where tectonic uplift of the coast occurs. Ø Ex: Pacific Coast of US

Depositional shores – shores that have sand deposits and offshore barrier islands in areas where the shore is gradually subsiding. Ø Ex: US Southeastern Atlantic and Gulf

Coast

Features on Erosional Shores Headlands – a steep-faced irregularity of the coast that extends out into the ocean. Ø Wave refraction concentrates the

wave energy on the headlands causing the headlands to erode more quickly.

Wave-cut cliffs – cliffs formed as waves crash at the base of headlands, undermining the upper portions which eventually collapse.

Sea caves – waves eroding the base of cliffs.

Features on Erosional Shores Sea arches – as waves continue to erode headlands, sea caves can erode to the other side of the cliff forming sea arches.

Sea stacks – as erosion continues, sea arches become unstable and crumble to produce sea stacks.

Features on Erosional Shores Sea arches – as waves continue to erode headlands, sea caves can erode to the other side of the cliff forming sea arches.

Sea stacks – as erosion continues, sea arches become unstable and crumble to produce sea stacks.

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Features on Erosional Shores Marine terrace – gently sloping area above sea level that is an uplifted or elevated wave cut bench.

Terraces are especially seen in areas with episodic uplift due to earthquakes (like CA). In some places you can observe multiple stacked terraces in a progressive series.

Marine Terraces Marine terrace – gently sloping area above sea level that is an uplifted or elevated wave cut bench.

Terraces are especially seen in areas with episodic uplift due to earthquakes (like CA). In some places you can observe multiple stacked terraces in a progressive series.

Features of Depositional Shores

Spit – a linear ridge of sediment that extends in the direction of longshore drift from land into the deeper water near the mouth of the bay.

Bay barrier – or bay mouth bar – a buildup of sand usually <1m above sea level that cuts off the bay from the open ocean.

Tombolo – a sand ridge that connects an island or sea stack to the mainland or two adjacent islands. Formed in the wave-energy shadow behind islands.

Features of Depositional Shores

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Features of Depositional Shores

Barrier island – extremely long offshore deposits of sand that are parallel to the coast. Ø Protects the shoreline against rising sea level and high energy storms. Ø Origin is complex but appear to be related to sea level rise associated with melting glaciers from

last ice age ~18 kyr ago. Ø ~300 barrier islands along Atlantic and Gulf coasts. Ø Length <100km. Ø Separated from land by lagoon.

Barrier Islands

Features of Barrier Islands

From ocean landward: 1. Ocean beach 2. Dunes 3. Barrier flat 4. High salt marsh/Low salt marsh 5. Lagoon

Features of Barrier Islands

1. Ocean Beach

In summer → gentle waves carry sand to the ocean beach, so it widens and becomes steeper.

In winter → higher energy waves carry sand offshore and produce a narrow, gently sloping bench.

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Features of Barrier Islands

2. Dunes

Winds blow sand producing coastal dunes, which are stabilized by dune grasses.

Features of Barrier Islands

3. Barrier flat

Barrier flat forms behind dunes from sands that pass through dunes. Grasses colonize flats which can turn into woodlands and forests if flood infrequently.

Features of Barrier Islands

4. Salt Marsh (high and low)

Salt marshes typically lie inland of barrier flat and are divided into low marsh and high marsh. Ø Low marsh – mean sea level to high neap tide line. Most biologically productive part of marsh. Ø High marsh – high neap tide line to highest spring tide line.

New marsh land forms as overwash carries sediment into the lagoon.

Barrier Island Migration

A gradual rise in sea level can cause barrier islands to migrate landward.

This has been observed on the North Atlantic Coast of US.

Island slowly rolls over itself as it migrates towards coast, leaving a trail of peat deposits (organic remains from salt marshes).

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Deltas

Deltas – formed when rivers carrying large sediment loads outlet into the ocean.

The river is carrying more sediment than the longshore current can distribute so the river deposits sediment at its mouth.

Delta formation begins when a river has filled its mouth with sediment.

Deltas The delta grows through the formation of distributary channels – branching channels that deposit sediment as they radiate out over the delta in finger like extensions.

Over time these channels fill with sediment and flood the entire delta.

New channels form and the process repeats, growing the delta larger.

Beach Compartments Beach compartment – a series of rivers, beaches, and submarine canyons involved in the movement of sediment to the coast, along the coast, and down a submarine canyon.

Beach compartments consists of: 1. A series of rivers that supply

sand to a beach. 2. The beach itself. 3. Offshore submarine canyons

where sand is drained away from the beach.

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Beach Compartments Beach compartments drain away sand from the beaches into the ocean where they are lost from the beach forever.

To the south of beach compartments, beaches are typically thin and rocky with little sand.

The process begins all over again at the upcoast end of the next beach compartment where rivers add sediment to the beach.

Farther downcoast beaches widen and has abundance of sand until it is diverted offshore to submarine canyon.

Beach Compartments Beach starvation – Caused by an interruption of the sediment supply. Occurs when sediment loads are reduced in rivers delivering sediment to beach compartments (dams, flood control, etc).

The sediment is still being delivered to submarine canyons causing beaches to narrow.

Sea Level Change

In addition, to a shoreline being described as erosional or depositional, shoreline can also be classified based on their position to relative sea level.

Sea level changes through time due to: 1. Level of land changes (uplift or subsidence) 2. Level of sea changes (rising or lowering water levels) 3. Combination of both

Emerging Shorelines – shorelines that are rising above sea-level.

Submerging Shoreline – shoreline that are sinking below sea level.

Features of Emerging Shorelines Marine terraces – flat platforms backed by cliffs, which form when a wave-cut beach is exposed above sea level.

Stranded beach deposits – ancient beach deposit found above present sea level.

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Features of Submerging Shorelines Drowned beaches – an ancient beach now beneath the coastal ocean because of rising sea level or subsidence of the coast.

Submerged dune topography – ancient coastal dune deposits found submerged beneath the present shoreline because of a rise in sea level or submergence of the coast.

Drowned river valley – the lower part of a river valley that has been submerged by rising sea level.

Changes is Sea Level

Two ways to change sea level: 1. Raising and lowering of land. 2. Worldwide changes in the amount of water in ocean.

Sea Level Change Due to Movement of Earth’s Crust

This change is sea level is called a “relative change in sea level” because it is the land that is changing not the sea.

Tectonic movement – changes in land level sue to uplift and subsidence of the Earth’s crust usually happening at plate boundaries.

Isostatic Adjustment – The Earth’s crust sinks under the accumulation of heavy loads of ice, vast piles of sediment, or outpourings of lava, and it rises when these loads are removed.

Ex: ice caps/ice sheets/glaciers during last ice age were heavy loads on crust. There are still areas today that are rebounding from the last ice age 18kyr ago.

Worldwide Eustatic Changes in Sea Level Eustatic sea level change – changes in sea level that are experienced worldwide due to changes in sea water volume or ocean basin capacity.

Examples include: • Formation and

destruction of large lakes • Sea floor spreading rates

and the size of the ocean basins.

• Ice Ages when water is tied up in glaciers lowering sea level, but also thermal contraction of water.

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Estuaries Estuary – partially enclosed coastal body of water in which fresh water runoff from a river dilutes input of salty ocean water. Ex: river mouth, bays, inlet, gulfs, sounds.

There are 4 major types based on their geological origin:

1. Coastal plain estuary

2. Fjord

3. Bar-built estuary

4. Tectonic estuary

Estuaries Coastal Plain Estuary – forms as sea level rises and floods existing river valley. Also called drowned river valleys.

EX: Chesapeake Bay, VA/MD

Estuaries Fjord – Forms as se level rises and floods a glaciated valley. They are u-shaped valleys with steep walls.

EX: Norway, Alaska, Canada

Estuaries Bar-build estuary – shallow estuary that is separated from the open ocean by sand bars that are deposited parallel to the coast by wave action. Lagoons that separate barrier islands from the mainland.

EX: Laguna Madre, TX

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Estuaries Tectonic estuary – Forms when faulting or folding of rocks creates restricted down- dropped area into which sea has flooded.

EX: San Francisco Bay, CA

Water Mixing in Estuaries

Less dense freshwater from rivers mixes with dense salty sea water in estuaries.

Estuaries are classified bases on how water mixes within them:

1. Vertically mixed estuary

2. Slightly stratified estuary

3. Highly stratified estuary

4. Salt-wedge estuary

Water Mixing in Estuaries

Vertically mixed estuary – very shallow estuaries such as lagoons in which fresh water and sea water are totally mixed from top to bottom so that the salinity at the surface and he bottom is the same in most places.

Water Mixing in Estuaries Slightly stratified estuary – an estuary of moderate depth in which sea water invades beneath the freshwater runoff. The two water masses mix so that the bottom water is slightly saltier than the surface water in most places.

Estuarine circulation pattern – a flow pattern in an estuary characterized by a net surface flow of low salinity water toward the ocean and an opposite net subsurface flow of seawater toward the head of estuary.

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Water Mixing in Estuaries

Highly stratified estuary – a relatively deep estuary in which a significant volume of sea water enters as a subsurface flow. A large volume of fresh water runoff produces a widespread low surface salinity condition that produces a well-developed halocline throughout most of estuary. Pronounced circulation pattern.

Water Mixing in Estuaries

Salt wedge estuary – a very deep river mouth with a very large volume of freshwater beneath which a wedge of saltwater from the ocean invades.

Ex: Mississippi River

Questions???