Walker Menu Output Help

I am trying to output divs around every ul submenu and I am at a stand still. How can I get the walker to end a submenu with a closing div and then restart with another div?

<li class="top-level-item">
<a class="top-level-link>Title</a>
<div class="grid-container">
  <div class="grid columns">
   <ul class="submenu">
    <li class="menu-item">item</li>
    <li class="menu-item">item</li>
    <li class="menu-item">item</li>
    <li class="menu-item">item</li>
   </ul>
  </div>
  <div class="grid columns">
   <ul class="submenu">
    <li class="menu-item">item</li>
    <li class="menu-item">item</li>
    <li class="menu-item">item</li>
    <li class="menu-item">item</li>
   </ul>
  </div>
</div>
</li>