Answer

 

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To prevent dune erosion, the beach is nourished. The beach becomes broader and attracts more tourists; this is a side-effect.
When studying coastal protection measures, to apply beach nourishments is an alternative. Sand can be used that is being dredged from a nearby access channel. The dredging costs don't have to be taken into account in the analyse as they are sunk-costs.
A government chooses to use a dune area for water treatment plants. The area is also very suitable for recreation, but there is no access road. The land is already public property, so, the total costs of the development will only be 100,000 dollar to build the plant and connect it to the existing water service pipes.

 


Touristist recreation also generates money for the society; the land has opportunity costs. For example, if the land could be auctioned (and privately exploited for recreation) for 500,000 dollar, these are the opportunity costs.