On 04/04/2021 17:31, JohnP wrote:
Out walking and idly looking at the various manhole covers and hydrant
covers.

I saw one that had this cast on "Sewer Wash Out".

Purpose obvious - but why and were are this situated? It was fire hrdrant
sized.

I can see two possible purposes - a point to put water in to wash out
the sewer or a hydrant (by another name) used purely as a source of
water for washing out the sewer. I assume the latter?