How a groin changes a beach
1 min 44 s. The words are spoken and shown in the picture; the player can also show them as subtitles.
For the Earth and Space Sciences Regents. Topic: Engineering solutions to global challenges, with the past questions on it.
What Dr. Mars says (the words of the video)
How does a groin change a beach?
A beach, seen from above.
North is up. The ocean is to the south.
Waves reach the shore at an angle.
This makes a current along the shore.
This longshore current carries sand along the beach.
Engineers judge a solution by its criteria and constraints.
Criteria: what it must do. Constraints: limits, such as cost or harm.
The criterion here: keep sand on the east beach.
A constraint: do no harm to the west beach.
One solution is a groin:
a wall of rocks built out from the beach into the water.
Dr. Mars builds the groin, rock by rock.
The groin blocks the sand moving with the current.
Sand piles up on the side the current comes from.
Less sand reaches the west side, so the west beach narrows.
The east beach gains sand: the criterion is met.
But the west beach narrows: the constraint is not met.
Check yourself! Where will sand pile up: A, B, or C?
Think: where does the groin stop the moving sand?
Answer: B! Against the groin, on the side the current comes from.
Dr. Mars's groin trapped sand there, and the west beach narrowed.
Dr. Mars. Free practice for the New York Earth and Space Sciences Regents.