GEOS chapter 20

Waves

form as wind energy is transferred to ocean’s surface by friction.

In the open ocean, wave height depends on: wind speed, wind duration, and *Fetch (*the distance of open water over which the wind blows*)*

finer material goes back to water, coarser material goes to the beach

Waves of oscillation

waves in which water moves in a circular path

A floating object on the ocean’s surface, such as a bottle, moves in a circular pattern as the waves pass beneath it.

object does not move forward or horizontally

Wave of translation

water (and any object in it) moves forward horizontally in the direction of wave movement

only occurs along coastlines.

Friction with the seafloor slows the base more than the top and, as a result, the wave crest grows higher

The wave crest collapses into a *breaker,* in *surf zone*.

As breakers collapse onto a beach, they create a *swash* and *backwash.*

When the **wave steepness** (H/L) exceeds a ratio of 1:7, it becomes unstable and breaks.

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Wind generated waves

travel downwind

are ***non-dispersive*** (they advance on a straight front), i.e., they conserve most of their energy

***wave height*** depends on ***wind speed***, on ***duration*** of wind, and on ***fetch***; the distance over which wind can act on the waves

for long fetches (waves reach equilibrium for the given wind speed),

Spilling 

(short steep waves, gentle beach, breaking far from shore)

Plunging

(curling waves, typical of surfing; long waves)

Surging

(low flat waves, steep beach, crest does not break)

collapsing

(between plunging and surging)

Wave Refraction

* Results in bending of ***wave crest***
* Focuses energy where crests become ***concave*** (where water

shallows first)
* Disperses energy where crests become ***convex***
* Result is wearing away of typical ***headland*** (sediment source) and expansion of ***beach*** (sediment storage) configuration
* wave closer to shore moves slower

Longshore currents

* Waves move up the beach ***normal to wave front***, ***swash***
* They move down the beach ***normal to the beach contours*** (move down the

“dip” of the beach), ***back wash***
* When the two are not parallel, produces ***edge waves***
*  ***a net lateral transfer of water results***


* Sediment is entrained and transported (as ***bedload*** and in ***suspension***) with this ***longshore current***

Longshore current

a current that flows parallel to beach in direction of wave movement creating a *longshore drift*

Longshore drift

the process by which sediment on a beach is moved down length of beach in direction of wave movement

Only the top most meter or so of sand actively moves.

Deeper layers move only during strong storms, if at all.

Rip current:

a short-lived jet of backwash formed as water from breakers flows back into sea

Most likely to form when wave heights are at a maximum.

Range in width from few meters to hundreds of meters.

Can carry people away from shore.

Must swim sideways /parallel to the shore to escape a rip current.

Longshore currents

Most transport occurs in the ***surf zone***

Can produce transport in grains ranging from *sand* up to *cobbles*

Key to development of ***rip currents***

Type of Coastlines

Some coastlines are flat and sandy, while others are composed of steep and rocky cliffs.

Emergent coast

a coast in which either sea level is dropping, or the land is rising

Dominatedby*erosional landforms,* like rocky coasts.

Occur on *active continental margins* (where two plates meet).

Example: the west coasts of North and South America

Submergent coast

occurs where either sea level is rising, or the land surface is subsiding

Dominatedby*depositional landforms*, like beaches.

Occur on *passive continental margins* (far from plate boundaries).

Example: the southeastern United States and Gulf Coast.

Eustasy

a change in global sea level as a result of change in amount of water in oceans (ex: melting of ice sheets)

Other important factors that control the morphology of a coast are:

Amount of wave energy reaching it

Presence of biogenic structures (e.g., coral reefs)

Tidal range

Amount and type of sediment available

Sea level can change without raising or lowering of lithosphere.

Quartz beaches

are mostcommon because quartz is resistant to weathering and erosion

Coral reef beaches

are particularly white

Beach Zones I

Beaches have an *offshore zone*, *nearshore zone*, *foreshore zone* (or intertidal zone), and *backshore zone*.

offshore zone

waves encounter sea floor and slow down

nearshore zone

includes surf zone and extends out beyond the breakers where the water depth increases

foreshore zone

continues up to the high tide line

backshore zone

above high tide line

a berm of sand marks beginning of back shore environment

sand dunes anchored by vegetation are found beyond the berm

Summer profiles

net transport of sediment towards beach by ***small storms,*** steep ***beach face*** with no ***bars***

small to medium sized waves

don’t have a lot of strength to carry back sediment

Winter profile

***larger storms*** erode the ***beach face***, and redeposit that sediment ***offshore*** as a ***bar,*** beach retreats, and attains a less steep ***beach face***

most sediment moves back to sea

Beach Deposition and Erosion

* Sediment, usually sand, is continually entering beach from “upstream” end, and is continually leaving beach on “downstream” end.
* Only topmost meter or so of sand actively moves.
* Through time, most beaches are maintained by an

equilibrium between erosion and deposition of sand.
* If we consider the beach as a geomorphic system, thechange in storage of sediment is equal to the difference between the input (deposition) and the output (erosion)

Baymouth bar

a continuous sandspit formed by longshore drift that creates a **lagoon** of brackish water behind it

Tombolo

a landform consisting of an island and sandspit

Ripples often form on beaches

**symmetric in cross section** \n **form in response to bi-directional flow (swash-backwash)**

Cusps

crescent shaped features that develop on a beach ***foreshore*** Comprised of a ***bay*** of finer sediment and coarser ***horns***

***Rhythmic*** form, repeating with nearly ***constant spacing*** (develop in beach sediments ranging from ***sand*** to ***boulders***)

spacing typically between 3 and 30 m

Likely formative process is ***edgewave swash/backwash***

swash is stronger than backwash

Sand waves (bars)

large-scale rhythmic features, transverse to wave front

may be primarily submerged in ***surf zone***

***length*** (100-3000 m)

produced as a consequence of onshore- offshore sediment movement and longshore transport

may be solitary or multiple

Tombolo

Produced by ***longshore drift***

Related to ***refraction*** of ***wave crests***

Creates a sediment deposit behind the island, eventually connecting it to the mainland

Coastal sand dunes

dunes formed where prevailing winds blow sand inland, vegetation traps sand, accumulates into dune formations

Sandspit

an elongated dry bar of sand that extends from beach out into water, usually parallel with shore

dunes can be source of sediment

Spits

* larger than ***sand waves***
* associated with ***high sediment supply*** rates, ***longshore***

***transport*** in the ***breaker zone***
* may be gravel to medium sand
* **tend to straighten shorelines by filling in irregularities**

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Bay Mouth Bars

* ***Bay mouth bar*** is a spit that closes off the entrance to a bay.

Barrier island

an offshore sandbar that runs parallel to coast

* ***sand bars (“banks”)***, typically 3-30 km offshore, parallel to ***coast***
* separated from ***coast*** by ***lagoons***, ***salt marshes***, ***mudflats***, broken by ***tidal***

***inlets***
* ***length***: 10-100 km, ***width***: 100’s-1000’s m, ***height***: \~ 5 m
* subject to ***wash over*** during ***storm events***, producing ***wash over fans***
* topped by ***aeolian dunes***, produced by sand driven shoreward by ***onshore winds***

barrier island formation

* still incompletely understood
* Thought to be associated with post-glacial sea level rise: (10-20 Ka B.P.,sea level was 120 m lower, coastline was 60-150 km further out to sea)
* Islands have migrated shoreward as sea level rose, reaching present configuration \~ 5 Ka B.P.
* Currently continue to migrate shoreward, but much more slowly (since sea level rise has slowed)

Groin

a linear structure of concrete or stone that extends from beach into water to slow erosion of sand

Jetty

an artificial wall placed at mouth of harbor to prevent sand from closing harbor entrance

Seawall

an artificial hard structure designed to protect backshore environments from wave erosion during large storms

Rocky Coasts

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* Rocky coasts often have deeper water leading up to them than beaches
* Wave energy is not expended on seafloor
*  Instead, waves expend full force of their energy against rocks
* Makesthempowerfuland relentless force of erosion.

Sea stack

a steep or vertical tower of rock separated from but near a rocky coastal cliff face

Wave-cut bench

a flat coastal platform resulting from wave erosion and exposed at low tide

**Processes:**

**1. Chemical/physical weathering Solution, slat crystals and others**

**2. Abrasion** \n **Physical action of the sediment moving over the rock**

**3. Hydraulic action**

Wave-cut notch

a notch formed at base of coastal cliff as a result of weathering and erosion

Three groups of processes eroding coastlines

chemical and physical ***weathering***

***mass wasting, fluvial erosion***

***wave action*** (mechanical)

Typical features are ***sea cliffs, wave-cut platforms, wave-cut notches,*** and ***sea stacks***

Arch and seas stack

arches formed by differential erosion.

when arch collapses a sea stack will form. sea stacks are steep vertical towers

sea cave

form where waves erode weaker areas of bedrock along joints