Signage in English often uses short, clipped phrases instead of complete sentences. If you were to rewrite the message from this sign as a full, grammatically standard sentence, it would be something like:
Dumping is prohibited. This drain flows to the bay.
Phrases such as ‘No Dumping. Flows to Bay’ would not normally be used in spoken or written English. However, they occur quite frequently on signs, where brevity is essential and the meaning is usually easy to understand. Other common examples include:
Beware of Dog. [full sentence: ‘Beware of the dog.’]
Keep Off Grass. [‘Keep off the grass.’]
Insert Coin. [‘Insert a coin.’]