A groyne or jetty blocks part of the sediment moving along the coast. Sand builds up on the updrift side and the downdrift side is starved, so one beach gains and the next loses.
This lesson is part of coastal processes and management. It uses the direction rule from tracing sediment movement.
What is the method?
- Find the drift direction from the wave arrows.
- Label the side sediment comes from as updrift, and the other as downdrift.
- State the effect on each side: build-up updrift, starvation downdrift.
- Quote the change in beach width on each side.
The order matters. If you label the sides before finding the drift, a turned diagram will catch you out.
Worked example: an original groyne
An original coast runs north to south, with the sea to the east. The waves drive the drift from north to south. A groyne is built across the beach. Beach widths before and after are measured on each side.
| Side | Position | Before | After | Change |
|---|---|---|---|---|
| North | Updrift | 20 m | 35 m | +15 m |
| South | Downdrift | 25 m | 12 m | −13 m |
Model answer: “Drift moves sediment from north to south. The groyne traps sand on the north side, so that beach widens by 15 m, from 20 m to 35 m. Less sand reaches the south side, so its beach narrows by 13 m, from 25 m to 12 m.”
The mistake: saying the groyne stops erosion everywhere
The common slip is to write “the groyne protects the coast”. The data show a winner and a loser, so the claim is half true at best.
| Sentence | Problem |
|---|---|
| “The groyne stops the beach eroding” | Ignores the narrowing south beach |
| “The south beach is updrift” | Sides swapped because drift was not found first |
| “Sand builds up in the north” | Correct, but gives no figure or cause |
A strong answer names the side, the direction of change, the cause and a number.
Check yourself
An original jetty is built at a river mouth. Drift runs from south to north.
The south beach widens from 10 m to 24 m. The north beach narrows from 30 m to 18 m. State the updrift side, the two changes and the reason.
Answer
The south side is updrift, because the drift comes from the south.
The south beach widened by 24 − 10 = 14 m. The north beach narrowed by 30 − 18 = 12 m.
The jetty traps sand on the south side, so less sand reaches the north side.
What to study next
Next, weigh a structure against other choices in comparing coastal responses against a stated objective and limitation. Or test all four skills in the coastal practice set.
If you want a teacher to go through your structure answers on your own diagrams, see online one-to-one Geography tuition.