Joomla Standard Submenu as Megamenu

Hey

I want to make a template with 2 navigations for me from the t3-bs3-blank template.
The first navigation is the topnavigation at the header section and it is the root-menu (Topnavigation).
The second navigation should be a submenu (mainnavigation) from the root-menu as the Megamenu.
So thats my problem.
I don´t know how can I show the submenu as the Megamenu.
I hope you understand my Problem and you can give me a idea or a solution for my problem.

Thank you very much!