No, in everyday written or spoken English, you should try not to use two relative clauses in the same sentence. (This is sometimes used as a poetic device, though.) As a native speaker of English, I would tend to say: There was a sudden heavy rain, causing a quick flash flood, which hid many potholes (you would use the past tense of the verb 'to hide', which is 'hid') OR There was a sudden heavy rain, which caused a quick flash flood, hiding many potholes.

No, you shouldn't. A sentence with two relative clauses is clumsy and ambiguous. You should find a way of rephrasing it to avoid the double relative. For example: There was a sudden heavy downpour which caused a flash flood, hiding many pot holes.

The "relative clause is the non-essential part of this sentence. When you remove it you lose some details, but the meaning of the sentence remains the same. There was a sudden heavy rain (which quickly caused a flash flood) that hide many potholes. Hope this helps