Whenever signals the habitual aspect, i.e., an action repeated either still in the present or no longer. In your case, the barrage is apparently no longer in use, so the first clause is cast in the past habitual.
The is + something, regardless of the rest of the verb phrase, must be in some form of the present tense, but since all this repeated action took place in the past, that choice can be readily eliminated.
For the clause after whenever, the vast majority of writers will use the simple past:
The barrage would be used whenever an exceptional tide was forecast.
A Google NGram querying for whenever he would go and whenever he went yielded only a few hits for extending the use of the past habitual after whenever, even when the whenever-clause opens the sentence.
You can read more about the various ways English signals the habitual aspect here.