Natural Hazards Ch 15 Waves, beaches, coastal erosion

We build various kinds of structures in an attempt to save our beaches and beach homes, unintended effect of reducing sand migration

- jetties around a marina - seawall - rip rap - groins -breakwaters

The power of waves are in circular motion?

- largest at surface - no water movement happens below 1/2 of the wavelength

Longshore drift of beach sediments often end in submarine canyons

- move particles down the beach in a river fashion, gets stuck down the cliffs and into the deep ocean, river of sand then disappears the sandy beach is gone

Material moves along a beach, grain of sand on the beach based on the wave motion what direction would a grain of sand move?

- wherever direction the wave is approaching shore - when water goes back out it does not go back out at an angle, moves backward along the same pattern

Beach Erosion <----Sand supply is short because:

-dams trap sand -sand mining for construction use

reasons for sand loss

-rip currents -wind blows it inland to make dunes -drift into inlets that cross barrier islands forcing humans to dredge it -carried away by hurricanes

Wave Energy equation

0.125rgH^2L - r = water density - g = gravity - h = wave height - l - wavelength

beach growth occurs by:

1. due to coarser sand or gravel, much of the water soaks quickly into the ground rather than flowing back offshore over the surface; thus, less sand or gravel is eroded from the beach. 2. small waves tend to leave more of their sand onshore.

Equation of wave velocity in deep water

1.2L^1/2 L - wavelength

Replenishment Issues:

1.Expensive: Localities pay about 5% of cost and the remainder is payed by the state and federal governments. 2. Uncertain Durability: Projected up to 20 years but typically only lasts a couple 3. Sand supply: Florida is running out of sand to supply for beaches and we could start to talk from other sources such as the Bahamas 4. extra sand naturally migrates somewhere causing another issue

equation of wave velocity in shallow water

3.1D^1/2 D = water depth

waves that are 2x as high have _____ the energy

4x

Barrier islands

A near-shore, coast-parallel island of sand built up by waves and commonly capped by windblown sand dunes. Also called a barrier bar - some protection to coastline - coastal lagoon forms on landward side that is usually below sea level

Result of Groins

A zig-zag beach where the sand is carried on one side of the barrier and eroded on the next because the current pushes the sand until it hits one of the groins then a new current begins at the next barrier.

To what depth of water do waves commonly move sand?

About 10 meters

Limiting Development

Adjust controls on building via zoning. Measures E-10 to E-60 and creates a graduated system. E-10:Imminent hazard, E-30: Intermediate zone, E-60: Longer term hazards. Limits property owners rights, property damage, and relief needs.

Replinshment

Also referred to as 'Beach Nourishment'. Manually move sand to beaches and you pump from offshore or other beach zones. It involves joint projects from localities, states, and corps of engineers.

Barrier Islands

Are not permanent; develop where the landward-steepening beach profile is steeper than the general slope of the coast. Wind blows sand shoreward into dunes

Which of the following evidence does NOT indicate that a barrier island migrated landward?

Barrier island beaches are now more easily eroded than they were one hundred years ago.

Where does beach sand NOT originate?

Continental shelf as much as 50 km offshore

Littoral Cells

Develop when there are multiple outlets for sand

Natural Island Migration

Driven by changes in sea level; a rise in sea level causes waves to push the sand backwards which creates the barrier island to shift

Why are ocean waves generally larger than those on lakes?

Fetch length is much longer in the ocean.

East Coast Drift

Gentle offshore slopes: gently breaking waves; relatively slow sand loss causes large beach zones. coarser materials

Breakwaters

Hard structures parallel to the beach. Purpose is to intercept waves before shoreline

Seawalls

Hard structures parallel to the beach. They save properties and are built behind beach to protect property.

Groins

Hard structures perpendicular to the beach are used to trap the sand from moving with the longshore drift.

Jetties

Hard structures perpendicular to the beach. Purpose is to protect an inlet; not allow any boats to come from the sound side to the ocean side.

What are the three basic types of hazards? and what are they triggered by?

Hurricanes, typhoons, nor'easters; day-to-day processes

Beach Replenishment/Nourishment

Involves replacing sand on a beach, an expensive proposition that needs repetition at intervals

Result of Jetties

Jetties cause erosion on the backside and pile up of sand on either side of the barriers depending on the current

West Coast Drift

Keep offshore slopes: steeply breaking waves; relatively quick sand loss causes small beach zones

Consequence of the current

Long-shore current and waves push water towards shore and eventually the build up gets flushed out which causes a rip current.

Result of breakwaters

Longshore drift stops behind the barrier and you get a pile up of sediment

Why does sea level rise during a near-shore storm?

Low atmospheric pressure of the storm permits sea level to rise.

Lake Shorelines

Many lakes large enough for the same processes; they have a greater stability expectation and less tolerance for changes

Dune Stabilization

Protecting dune lines as a storm buffer by the use of sand fences and vegetation

Why are inlets through barrier bars or barrier islands not closed by longshore drift of sand?

Rapid flow of water back and forth through the inlet as tides rise and fall keeps it clear.

The three types of adapting:

Replenishment, dune stabilization, and limiting development

Rip Current

Rip Current

Without outlets what would happen to the beaches?

Sand gets carried along the beach for great distances

Beach erosion prevention or stopping the longshore drift of sand includes?

Seawalls; groins; jetties; breakwaters

wave refraction

Slowing and bending of progressive waves in shallow water.

What is sea cliff erosion determined by?

Strength of the rock; wave height; sea level; precipitation; resulting in amplified erosion during storms

possible solution

The cost of moving buildings may be high, but it is less than the continuing long-term cost of maintaining beach-hardening structures that tend to destroy their beaches or of continually bringing in thousands of tons of sand that get removed by following storms.

Beach Stabilization (Interfering with drift)

Trying to keep the beach in place because sand movement causes erosion which can damage beach front property

What is the ULTIMATE cause of coastal hazards?

Us; population density is increasing faster at coastal areas then any other area

Why do waves not break in the open ocean?

Water in deep-water waves does not move forward with the wave; it moves in a circular motion.

Long-shore Current

Waves approaching the beach at an angle push sand parallel to shore

Why does the beach slope under water become steeper near shore?

Waves in shallow water drag on the bottom, so they lose energy and need a steeper slope to move the sand.

equilibrium profile

Whether or not the sediment moves shoreward depends on the balance (equilibrium) between shoreward bottom drag by the waves, size of bottom grains, and downslope pull by gravity

What effects the rip current and how does it effect it?

Wind; no wind causes no rip current or riptides and lots of wind causes large rip currents

rip rap

a useless mitigation effort that requires Coarse rock piled at the shoreline in an attempt to prevent wave erosion of the shore or destruction of a nearshore structure. can do more harm than good in long run

Longshore drift

angled waves pushes a river of sand along the shore near the beach

what causes a wave to break

bottom of the circular particle motion drags on the bottom, slowing it - the top continues to move forward

waves created by storms:

can lead to waves either combining, forming rogue waves, or cancelling each other out

Sand on the beach is made of?

clear, glass like, quarts come from particular rock types not found in the ocean, quarts come from rivers so the way sand gets through the ocean is through rivers

How does sediment get to the coasts

cliff erosion, bottom of the ocean , waves and rivers

beach hardening, structures built to protect properties, backfires by

contributing to further erosion

Rivers carry sediment, it is trapped when

dams are built

Energy is associated with

density

dunes disappearing bc:

development

Wave refraction focuses

erosion

Longer wavelengths moves?

faster

wavelength and depth relationship

feeling bottom when depth = .5 wavelength. shorter wavelength means the depth at which bottoming occurs is also smaller. this causes the wave to grow tall

two factors that increase wave height

fetch and length of time the wind blows

Why does removal of logs and rocks from a beach endanger the sea cliffs behind them?

force of wave is softened by the logs and rocks acting as natural barriers

When wave hits the bottom of the ocean it deforms?

goes from circle to oval, changes shape and slows down

shore profile?

grain size strongly controls the slope of the beach

You cannot own property past the

high water mark, federal government owns from the high water mark outwards

Seawalls are built to?

keep ocean from moving backward and eroding the land, dump sediment along the base of the seawall

jetties are built to

keep out waves and keep the sand from making it too shallow. Made to protect navigation channels and require freq dredging

loss of sand occurs from

large waves of storms and high tides removing sand from the surface of the shallow portion of the beach and from the sea side of dunes the waves break against

fetch

length of water surface over which the wind blows

storm surge

local rise in sea level from low atmospheric pressure in the midst of a storm - high winds push them forward and can cause massive erosion

Groins protruding into the surf stop?

longshore drift of sand

wave energy is proportional to

mass of moving water

Dunes

mounds of wind-deposited sand -removing dunes puts buildings at risk

typical wave motion offshore

moves in circular motions, staying in place. Floating objects move up and down. Not dangerous

Breakwaters are built>

offshore and parallel to the shore, causing depostion in the protected area behind the barrier

Waves break against a seawall, stirring?

offshore sand and causing erosion

Where will erosion occur at highest

on headlands

As the wave drains back into the sea the water moves?

perpendicular to the waters edge

Shape is related to

process

Wave velocity in shallow water is

proportional to square root of water depth

Wave velocity in deep water is ?

proportional to the square root of wavelength

Homeowners placing large boulders in front of house for protection is bad bc:

protection is temporary. big waves reflect off the boulders, washing away the beach sand in front and steepening the remaining beach. Eventually, the boulders will slide into the wave-eroded trough, leaving the homes even more vulnerable

Wave energy doubles wave heigh _______ energy

quadruples

Edge of landscape submerged as sea level

rises

Barrier bars move inland with

rising sea level picks up particles and waves push it forward and deposit it further inland

how does on particle of water in a wave zone move

rolling in a circular motion, top of the circle moving in the direction of wave motion

Massive Corps Of engineers Beach replenishment project

sand and water slurry is pumped from up coast in 30 inch pipes - heavy earthmoving equipment spread the sand

sand will get trapped inside barriers?

sand builds up inside the groins, building up on upstream side makes the beach bigger and wider

beaches change with seasons

sand in summer and gravel and boulders in winter

small waves tend to leave more

sand onshore

Headlands

separated by shallower sandy bays that reach into deeper water, energy focuses against headlands

shore profile

slope of beach determined by grain size

A constant, mild onshore wind will produce waves with

smaller and shorter wavelengths.

Coarser grains need a

steeper slope to move in the surf

You should escape a ripcurrent by

swimming parallel to shore and then back to the beach

Long-shore Drift

the movement of sand along the beach. looping motion

Coast in Louisiana has muddy beaches because

the sediment is coming from the Mississippi river which carries a lot of mud

Hawaii doesnt have quart sand because

they dont have quart rocks so you get volcanic beaches, carbonate from reefs, green beaches caused by peridotite

Overtime the ocean will attempt to tear back the headlands because?

they will get eroded and positive inside and overtime the heandlands retreat and form a straight cliff

Breakers

water at the base of the wave cant move toward the beach - top of wave circles deformed moves ahead of base (rise up)

rip current

water constantly coming in at one place, going back out another place, almost creating a river - can be permanent at groins, jetties, and outcrops where water flowing against piles up and is forced to flow offshore along the structure

Shallower water, smaller waves, and coarser grains promote steeper slopes offshore

water that arrives here carrying sediment slows immensely and drops it onto the steep slow before reaching shore causing further steepening

Beach face forms by

wave action

Result of Seawalls

wave that hits erodes sand in front of wall. the beach narrows and gets steeper. water in front of barrier deepens and waves reach closer to chore b4 breaking. when the water in front of barrier gets deep enough the structure is undermined and fails

35 m high waves occur every year and develop where:

wave travel direction is opposite to a strong current direction such as off the tip of South Africa

building on a cliff is bad idea bc:

waves hitting rock face erode material and undermine the cliff. can cause a landslide

What causes waves

wind blowing across the water (contact between water and wind, air can drag the ocean with it)