Costiero vs. litoraneo
While both costiero and litoraneo translate to "coastal" in English, they are used in slightly different contexts. Costiero is the most common, general, and geographical term derived from "costa" (coast). Litoraneo is more specific, often formal or technical, derived from "litorale" (littoral/shoreline), and frequently refers to infrastructure or phenomena running lengthwise along the sea.
Costiero
B1This is the standard adjective for anything related to the coast. It generally refers to the geography, the landscape, the economy involving the sea, or defensive structures. If you are describing the general region or the view, costiero is usually the correct choice.
La guardia costiera ha pattugliato la zona per tutta la notte.
(The coast guard patrolled the area all night long.)
Il turismo costiero è più attivo durante i mesi estivi.
(Coastal tourism is most active during the summer months.)
Preferisco il clima costiero perché è meno rigido.
(I prefer the coastal climate because it is less severe.)
Dalla scogliera si gode di un magnifico paesaggio costiero.
(From the cliff, one can enjoy a magnificent coastal landscape.)
Le antiche torri costiere servivano per avvistare i pirati.
(The ancient coastal towers were used to spot pirates.)
Litoraneo
C1This word implies a focus on the strip of land immediately touching the water or running parallel to it. It is often used for infrastructure (roads, railways) connecting seaside towns, or in scientific contexts regarding currents and biology found strictly along the shoreline.
Abbiamo guidato lungo la strada litoranea per vedere il mare.
(We drove along the coastal road to see the sea.)
La corrente litoranea ha trasportato i detriti sulla spiaggia.
(The littoral current carried debris onto the beach.)
La ferrovia litoranea collega tutte le città balneari della regione.
(The coastal railway connects all the seaside towns in the region.)
Esiste una vegetazione specifica nella fascia litoranea sabbiosa.
(There is specific vegetation in the sandy coastal belt.)
Il traffico sulla via litoranea è bloccato.
(Traffic on the coastal road is blocked.)
Summary
Use costiero for general geographical descriptions, landscapes, authorities (like the coast guard), and economic activities. Use litoraneo when discussing transportation routes running along the shore (roads, trams) or technical aspects of the immediate shoreline strip (currents, sediments).