Learn French: Quel est le protocole en cas d'inondation ? - What is the protocol in case of a flood?

The prepositional phrase en cas de is typically followed directly by a noun without an article when referring to a general event or hazard. You will see this often on safety signs, such as en cas d'incendie (in case of fire) or en cas d'urgence (in case of emergency).

Can I use au cas où in this sentence instead of en cas de?

Not in this exact sentence, because they take different grammatical structures. En cas de must be followed by a noun or noun phrase (like inondation). Au cas où (just in case) must be followed by a full clause with a subject and a conjugated verb, usually in the conditional mood: au cas où il y aurait une inondation (in case there were a flood).

Why do we use Quel est here instead of Qu'est-ce que?

When asking "What is [noun]?" to find out specific information about a situation, you must use Quel est (or Quelle est, matching the gender and number of the noun). If you used Qu'est-ce que c'est qu'un protocole ?, you would be asking for the dictionary definition of the word "protocol", rather than what the specific protocol is for this event.

Is en cas de only used in formal, official contexts?