First of all -- where do you live? Depending on how cold a climate you live in a 2 foot deep pond may not be deep enough to hibernate your animals in the coldest months with freezing temps.
You must be below the frostline. Usually 3 feet deep is a safe bet.
As far as substrate it really does vary. I have known folks who successfully use leaves or sand and even do not put anything in the bottom and let the turtles hibernate on flat bottom.
Also, if you do consider this hibernation option be sure to use a pond heater or a bubble system to keep a hole open in the ice during the deep freeze months. This helps keep gases from rotting material from being trapped in the water and toxifying your fish and/or turtles.
Lastly, please be advised that you always do run the risk of losing your animals when you hibernate them outside.
You may know all this already but I just thought I would put the info. out there.
Steve