mocacinno is right, BCH and BTC are two different coins. If you sent BCH to a BTC address and you have that wallet's private key, you will be able to get the BCH back by yourself. However in your case, only Coinbase could help you. In their
Bitcoin Cash FAQ updated last Dec 22, 2017, it was mentioned:
At this time, Coinbase has already credited accounts who received any BCH at their existing BTC addresses. Going forward, we cannot guarantee that funds will be properly credited when one asset is sent to the address of a different asset. Please make sure that you only send funds to the specific deposit address displayed in your Coinbase account for the asset you are sending. You can view a list of these addresses, or create a new one, by visiting this page.
BUT, I tried to send a message the chat bot in Coinbase and asked "What happens if I mix up BTC and BCH addresses?":
*If you accidentally send Bitcoin Cash (BCH) to your Bitcoin (BTC) Coinbase wallet, we may not be able to assist you in recovering your Bitcoin Cash(BCH).
*If you accidentally send Bitcoin (BTC) to a Bitcoin Cash (BCH) wallet outside of Coinbase, they may be recoverable. That being said, we aren't able to offer recovery support in that instance.
*Always be a bit cautious when sending BTC and BCH, and double-check that the address you're sending them to is compatible with the respective network.
Still, confirm it by sending a support ticket to them.