The existence of a beach underneath Spaghetti Junction (or the Gravelly Hill Interchange as it is known to people who are something in highways) is regarded as an urban myth, but as you can see from the picture it does indeed exist, just to the left of the promenade. Bathers have the choice of three canals and two rivers, and free changing rooms are provided. Those not wishing to take a dip can take advantage of the fact that it never rains on the beach, and if that wasn't enough as can be heard the transport links are excellent.
http://www.birminghamitsnotshit.co.uk/articles/a-trip-to-the-beach