Black Rock Processing Control Bridge. Clear out the enemies in the security room. Approach and interact with the glowing red box on the right-hand side.
Turn around and take first left follow the pipe and jump down. Clear out the enemies in the security room. Go to the premises leading to your goal and you will realize that the bridge has not been lowered.
You do that by going through the Oceanview Motel.
To your left and to your right giant stone bridges. If you continue to follow the signs you will come to a giant abyss. Turn around and take first left follow the pipe and jump down. The pathbridge leading to the Black Rock Processing is now accessible.