andym on "Twenty Thirteen Sidebar Overlapping Footer"

The position of the footer is determined by the main content. The sidebar is absolutely positioned so exerts no influence over the rest of the page. Thus if the sidebar is taller than the main content it will overlap (and go beyond) the footer. Setting a minimum height for the main content greater than the height of the sidebar is one solution. You could use javascript to dynamically calculate and apply this (not for small screens which display a single column!!)
Looking at your page the difference in height is too great for a good looking solution - I suggest you re-arrange the page layout or choose a different theme!