Coas Geol wk 4 – ch 6 – Beaches, dunes, barriers
Beaches and dunes found both on mainland and barrier islands, but with diffs
On the beach, “each wave shifts sediment”…the beach changes to protect the
landward side, by dissipating incoming wave energy
B.I.s found mostly on trailing edge cont. margins, with some leading edge too.
Formed by combo of wave action and Ls drift.
We’ll focus upon barrier beaches, spits (like Sandy Hook), and barrier islands
I.
Beaches
Important terminology in Fig p.155

“Nearshore zone”: from low tide mark seaward, across the surf zone. Bottom
topography typically a series of bars and troughs..often see a pair of bars
paralleling the beach (we see 2 and sometimes 3 in the Great Lakes)

“Beach”: from low tide mark landward to beginning of permanent veg or next
major geomorphic feature in the landward direction, (naturally occurring dune,
seawall, etc)
beaches subdivided into a foreshore and backshore….
 “foreshore” is intertidal zone (low tide to high tide) extending landward to the
berm, contains ridge and runnel system (bar and trough on ephemeral scale)
 “backshore” is beach that is landward of high tide, rarely hit with wave energy
except during storms (when much erosion CAN occur…)
HT
backshore
(rarely submerged)
LT
Foreshore
(intertidal)
Nearshore
(always submerged)
Wk 4 p.2
Variety of factors control the profile of beach and nearshore:
 sediment supply
 wave climate
 general cont shelf slope geometry
reflective beach characs:
- steep, big waves an come in with lots of energy
- lack of sand bars
- “erosive conditions” predominate
dissipative beach characs:
- shallow slope, wave energy gradually lost
- sandbars present
- nearshore bars, rip channels, depositional conditions
II. Beach Materials
“nearly anything” can form a beach
Mud example – huge amts of mud from Amazon yield mud beaches in Surinam,
on North coast of S. America – “consistency of yogurt” – yeccchhh
low wave climate allows mud to stay in this area, rather than be carried away
offshore
gravel example – with high wave energy and the right material, coarse grained
gravel beaches develop – often see in conjunction with glacial deposits and
eroded bedrock in high northern latitudes (fig p. 156)
certain areas with barrier islands in mid to low latitudes are composed of gravelsize shell material, because this is the primary sediment source in these
carbonate regimes
Sand example – sand typically quartz, but certain areas with specific mineralogy
have particular composition – e.g., Hawaii with black/green volcanic sand, or
carbonate sands of Bahamas, Australia
Wk 4, p.3
Sed size and relation to wave activity – “at any one place, the sediment particles
are the same size” or well-sorted
So how do the particle sizes get categorized??
Smaller
Larger
1/16 mm
diam
Silt, clay
2 mm
diam
sand
gravel
III.Beach Processes
Waves, tides, currents - tides serve to “spread” the wave attack over span from
high to low tide
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



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Breaking waves – each wave “suspends an amt of sediment proportional to
size of the wave”
4 types of currents produced in the surf zone:
combined flow
longshore
rip
swash zone
1. “combined flow current” - moves water landward up the shoreface
2. longshore we’ve covered
3. sandbars serving as “modest, temporary traps” for landward-moving sand get
cut by rip currents that are transporting water back offshore
4. swash zone current – cross-shore transport, both uprush and
backwash…sediment carried up the beach may not be same as that carried
down the beach.
Wk 4, p.4
Permeability a partic important factor – in coarse-grained beach, water carries
sed up the shore, water can travel down through the beach….this helps sed get
“piled up” on the beach:
Water uprush
Return to ocean
Contrast this with finer-grained, less permeable beach – more swash back down
the shoreface carries sed back down , tending to flatten out the profile
Note how the coarser-grained material has higher angle of repose too…this in
conjunction with above diagram can explain the steep vs shallow beach angles
IV.Beach Cycles
On one hand, very diff to predict beach change, because change can occur
immediately, suddenly.
But cyclic changes do tend to occur in response to weather cycles
Wk 4., p.5
As we saw in movie Beach: River of sand, Davis generally categorizes beaches
2 ways:
1. fair-weather, low-energy, accretional beaches
2. foul-weather, high-energy, erosive beaches
1. fair weather exhibits low wave height, 8 to 12 sec period
result is an accretional beach with deposition or stability
features include;
- a wide “backbeach”
- narrow, steep foreshore; they are reflective
- nearshore sandbars well-formed w/ high relief
 this is typical of Gulf of Mexico and Atlantic US coast
2. storms are short in duration, but are the “dominant physical process” along
the vast majority of coasts
- dominant wave becomes steep
- sediment entrained, currents carry it away offshore and alongshore
- high swash zone energy, much uprush and backwash, more so than in fair
weather
- removal of sed produces a “storm profile”
- backbeach gone, all sand in the foreshore
- nearshore sandbars moved down and further offshore
- high density heavy mineral lag deposit left on the beach, left there when
lighter quartz is pulled away by storm waves (placer gold deposit analogy)
 this is the “storm beach” profile
over time, there is a recovery to “fair weather” geometry:
- landward sand transport as sandbars migrate back toward land
- landward migration of intertidal ridge, and change to asymmetrical profile,
steep side facing land
- this migrating ridge “rebuilds the beach”, makes the beach reflective again
note that in winter, storms can be frequent, keep the beach in a “winter profile”
which is a storm profile
Wk 4, p.5
V. Coastal Dunes
Formed by processes, and in the shapes and patterns, similar to those in
deserts.
2 things you need:
1. sand supply
2. wind to move it
usually have plenty of wind – the sediment is the limiting factor
where you have both wind and sediment, can build dunes over 100 meters high
A. Dune formation and distribution
2 poss wind mechanisms – diurnal sea breeze or a prevailing wind direction –
important to have transport inland from the beach
non-wet portion of beach is subject to wind transport, backbeach in particular
signs of wind transport can include sand shadows and gravel lag –
 sand shadows show scour on upwind side of obstacle, lack of scour in
the lee….
 Gravel lag is left when wind carries fine sand away
Enough gravel lag to protect surface from further erosion in known as “desert
armor”
Vegetation plays important role in dune development…sand starts collecting
around plants, espec on backbeach, piles begin to increase in size
Remember that dunes are very fragile, easily destroyed/eroded
Just landward of the beach is the first dune ridge, aka “foredune ridge”.
- Presence of several dune ridges indicates a prograding coastline,
moving toward the sea (recall North end of Sandy Hook)


Landward dune movement – requires lack of stabilization, and a big sediment
supply – example is southern Oregon, where migrating dunes have actually
buried forests (!)
Another example – Padre Island in S.Texas, wide island and mainland with
dunes as well
Week 4, p.7
B. Dune dynamics
2 major factors –
1. mobility of sand – dunes are made, dunes are destoyed
Can take “many years” to restore dune losses from a single storm
2. wind is other factor
winds both form and move the dune
Process of migration –
“blowover” – essentially ripple migration, grain by grain
wind pushes sand grain upslope
gravity fall downslope
lee face at angle of repose
(=30o for sand)
VI.Barrier Islands
One of several types of coastal barriers – others include:
-spits
-bars / beaches
-reefs
but we’ll focus upon BI, because they contain the environments found in other
barriers
some stats:
 Found worldwide
 Few 100 m long to 100 km long
 Some just above high tide, some 30 m high
 Constitute about 15% of world’s coastlines
A. BI Origins
What you need:
1. sediment
2. transport agents
3. accumulation site
Week 4, p.8
BI usually on stable, trailing-edge coasts (Fig p.167)
Why??
-have sediment
-have moderate energy (enough to move sand, but not so much as to pull it all
away offshore)
-gentle slope of cont shelf (not too steep)
see these factors on the Atlantic coast and Gulf of Mexico
DON’T usu see BI s on leading edge coasts – waves too energetic, shelf too
steep…so you see barrier spits that come off headlands, but not B. Islands
B. Timing / theories of origin –
All BI s currently on earth are quite young – only 7000 yrs old, some as young as
3000.
Theories of formation proposed in 19th century (some controversy…)
1. waves cause sediment to accumulate, eventually reach above
sea level, stabilizes as a Barrier Island
2 low spot between beach ridges and dune ridge becomes drowned, beach
ridge becomes substrate for a BI
3 spit first forms trailing off a headland, then is breached, separated from
mainland – this has been seen to happen in many places, but Davis says
“doesn’t work for GOM or the US Atlantic coasts”
Davis discusses air photos that allow us to document BI development…. “all
these…have developed essentially according to the scheme of upward-shoaling
through wave action”… “and none has developed by the drowining of a ridge
system”
This supports one of the oldest theories of BI formation (#1), which has been
subject of much criticism and debate….
Starting at 7000 yrs ago, rapid sea level rise started to slow down, and this may
be when some stability allowed for initial development of BI s….after this, they
may have been migrating landward and/or growing bigger….
Wk 4 p. 9
C. Barrier Island Components
Beaches
Dunes
Marsh
Tidal flat
Washover fans
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(Fig p. 170)
Washover fan gets special mention because it is the sedimentary substrate
upon which all the back island environments develop
Note too that this fan development, over geologic time, provides the mechanism
whereby BI s can migrate landward (like Pilkey suggested in his movie)
Storm washover
Original BI
Subsequent fans
Migration landward through time

Lagoons – special, quiet-water environment that is situated between BI and
the mainland
Lagoons receive sediments through both washover from storms and blowover
from wind; rate of sed accumulation tends to be slow, sediments are often finegrained, very muddy (look at back-side of Sandy Hook).
Environment tends to be hypersaline (v. high salinity), and few species tolerate
this well. Certain clams, worms, birds, and certain fish do well.
You can find evaporites, carbonates
Don’t confuse lagoons with estuarine environments – estuaries have more
freshwater influx and tidal flux
Week 4. P.10
VII.Tidal Inlets
Inlets are dynamic links – breaches in the BI s between open marine
environment and lagoons behind the BI s
Origins are primarily due to storms, and humans
3 main components: (Fig p. 172)
 Inlet channel
 Flood tidal delta (deposited during flood tide)
 Ebb tidal delta (deposited during ebb tide)
A. Deltas
The deltas are similar to typical river deltas – sediment is carried by a current
through inlet channel, velocity and carrying capacity drops when current hits
open water in either open ocean or lagoon – when carrying capacity drops,
sediment falls out of suspension, is deposited in a delta.
Tide-dominated inlets appear pretty stable…don’t migrate much….
Wave-dominated inlets migrate….
We see Inlets tending to migrate along the Jersey shore. They’re pushed in
direction that longsh drift is moving.
Flood and ebb deltas do have some signif diffs, because of wave action on the
open marine side affecting the ebb tidal delta.
Flood deltas get a lot of lobes over time (Fig p. 173) , generally fan-shaped.
Shapes don’t get as heavily modified as ebb deltas
Ebb deltas have many diff shapes – (fig p. 174)
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Tide-dominated ebb delta goes mostly perpendicular to the trend of the BI
Wave-dominated and mixed-energy types show the delta smeared
alongshore, or smoothed out
Key element, Davis re-iterates, is the tidal prism – large tidal prisms result in tidedominated deltas (the tidal prism is overwhelming the wave energy component)
Week 4, p.11
B. Storms and Inlet Dynamics