On Long Island\'s ocean shoreline, you have probably heard that beach sand moves

Question

On Long Island's ocean shoreline, you have probably heard that beach sand moves alongshore from east (Montauk) to west (Coney Island), but on any particular day you might see waves pushing sand to the east instead. At East Hampton waves on some days move 265,000 cubic yards of sand to the east every year and on other days, waves move 387,000 cubic yards of sand annually. What is the magnitude of the net average longshore transport? In which direction is it going?

There’s a rule of thumb that tells you how much the shoreline will erode if you’re losing sand or how much wider the beach will become if you add sand.

The rule is “one cubic yard per foot per foot”. This means that it takes one cubic yard of sand to change the shoreline position by one foot for every foot of shoreline length. To use this you need to know any two of the following three numbers:

1.the number of cubic yards added or lost

2.length of the shoreline in feet

3.change in the shoreline position (i.e. the number of feet the shoreline recedes or the number of feet the beach widens).

Example:

If we have a beach 6000 feet long and add 1,000,000 cubic yards of sand, how much wider does the beach get?

“One cubic yard per foot per foot” =             1,000,000 cubic yards/6000 feet/WHAT

We want to put in some value for “WHAT” so that this number comes out to be one.

1,000,000/6,000/WHAT = 1 or WHAT = 1,000,000/6,000 = 166 feet

We can check this by putting in the 166 and recalculating. It does equal one, so it works.

1,000,000/6,000/166 = 1

Got it?

Likewise, if we have a beach 6,000 feet long and let’s say 60,000 cubic yards of sand cave in at one end and 100,000 go out at the other. That is,

                                                  <-----------6000 ft --------à

BEACH

________________________________________________________________________<===100,000 yd 3                     OCEAN          <=== 60,000 yd 3

Then, this beach is losing 40,000 yd 3

40,000 yd3/6000 feet/WHAT = 1 cubic yard per foot per foot

WHAT = 6.6 feet = amount the shoreline will go back

Explanation / Answer

Here 265,000 cubic yards of sand came in eastwards and 387,000 go out to westwards

Net the beach is losing 122,000 yd ^3

let x be net average longshore transport

122,000/l/x =1 as per rule one cubic yard per foot per foot where l is length of beach.

x=122000/l where l is length of beach as length of beach here not given.

and the direction will be towards west.

thank you

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