I'm surprised no one has answered you. I jumped on here to find advice myself. I've had my pond for 10 years so I have a little experience but I'm by no means an expert. I will try to help you as best I can.
First off, that is a really cool pond! It will be worth any effort you put into it! Be sure to post before and after pics when its finished!
When I need to empty my pond I use a pump. Since my pond is functioning, I have a fountain so I just put a hose on the fountain and pump it out. If you don't have some sort of pump you could use a shop vac if you have one. I've done that too. You just have to keep emptying it. You could go to a local big box store and get a cheap sump pump. If you have any other questions pm me. I'll try to help. Good Luck!!
