Hi This is all about subject/verb 'agreement', or concord (notional concord to be precise). Notional concord occurs when the verb agrees with the singular or plural meaning of the subject, regardless of any grammatical marker. For example, in two miles is a long way, the verb is singular because 'two miles' is viewed as a single entity.
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BillJ
So, the answer to your question is: do we consider the phrase '1000km of the coastline' to be a single entity, or more than one entity?