Meteo 22 lecture 26

Lecture Notes - Meteo 022 - Fall 2001

Lecture 26 - 27-Nov

Human control of beaches

Why control a beach?

Littoral drift

Natural

Barrier islands edging along - Chatham, MA
Barrier islands retreating inland - High Island, TX

Human modified - show figure 72 page 233 W+B

One human beach construction blocks littoral drift
So beach vanishes "downstream" i.e. DOWNDRIFT
So more human effort is needed to put the beach back

Combined (a typical example) - show figure 71 page 222 W+B

Natural

Barrier island edges along
Closing a natural pass (gap, inlet, etc)
Which was in use as a harbor entrance.

Human

So people put up jetties to stop the drift
Creating sand convergence "updrift"
and sand divergence "downdrift"
So the beach "downdrift" of the pass vanishes

Resulting problems

Passes in barrier islands move
Recreational beaches vanish
Coastal buildings undercut
Beach front property moves inland
Beach front roads undercut
More on this in the last lecture

How to control a beach?

Rules of the road

Sand transported by wave induced turbulence - show beach erosion hatteras mapquest map

So it starts moving where the turbulence begins
Continues drifting with the waves as long as there IS turbulence
And settles out where the turbulence ends

Taking action just moves the problem

If you stop erosion in one place
It becomes worse in another
Who do you save?
How do you explain it to those you destroy?
More on this in the last lecture

Net littoral drift

Sand moving into a strip of beach from one side
minus sand moving out at the other side
The goal is to make net littoral drift zero everywhere
Nature won't let us
But we can pick our poison

Jetties - show figure 72 page 233 W+B

Stop drift
So there is more net drift "updrift"
and less net drift "downdrift"
If you need jetties

to keep a pass open
or to protect a port
You need to install a sand-transfer plant

Sand-transfer plants

Humans moving sand from one side of a jetty (or set of jetties) to the other
Dredge up sand updrift of the jetty system
Pump it downdrift in a pipeline OR barge it downdrift
Dump it downdrift of the jetty system
Expect to move 100,000 or more cubic meters of sand per year
Pipelines allow continuous dredging

but take a lot of building
But give continuous movement
So downdrift beaches don't come and go

Barging allows only intermittent dredging

so you get intermittent movement of sand
and downdrift beaches come and go
NOT very popular with the beachgoers and property owners

Beach nourishment systems

Mine sand somewhere

Dunes
or bothersome sandbars
or wherever

Truck or barge the sand in
Dump it on the beach
Bulldoze it flat
Examples

Lake Erie
San Diego