change enemy NavMesh Walkable at runtime

made a navmesh to span a valley that is on a layer called “bridge” . No enemies can walk across it as per their navMesh Walkable settings. When the draw bridge is lowered I want to make it so that layer is added to the enemy navmesh Walkable. How do i do this at runtime?

ok, found all sorts of info on this but did NOT understand it the way it was explained. So I just kept plugging in numbers to see what happened and managed to piece it together.
-1 Everything
0 Nothing
1 Default
2 Not Walkable
3 Default and Not Walkable
4 Jump
5 Jump and Default
6 Jump and Not Walkable
7 Jump and Not Walkable and Default
8 Terrain ( my user layer 0)
9 Terrain and default
10 Terrain and N0t Walkable
so you can see where this is going. to finally get to the new layers I wanted:
16 my Layer 1
32 my layer 2
64 my layer 3
and FINALLY what I was searching for :
97 my layer2, my layer3, and Default