Responsive/mobile sub-navigation

Wrestling with sub-navigation for a responsive design.

Many of the examples I have come across work fine for top-level navigation but struggle to consider sub-navigation without resorting to a select drop-down approach (which I'm not a big fan of).

Accepting a convention for navigation icon at mobile layouts, be interested in whether folk think this method might work for adding ability to further interrogate a second (or third) level of navigation.

Any thoughts or feedback, or suggestions for examples where sub-navigation has been considered for narrow viewports in responsive designs?