Destroying ditches when there's a bridge spanning them

[Plain Ian]

Not sure I have ever seen that situation and I wonder if the only way to "attack" the AT ditch is to destroy the bridge first??

Just a guess.......

The Scheldt campaign has bunches of them on Walcheren Island.

Any chance of a picture of this? I'm having problems understanding the situation. 

We have a ditch with a bridge across it but the ditch is still preventing movement? 

So we need to destroy the bridge to fill in the ditch? Or really we need to destroy the bridge first so that the game engine sees the ditch and allows us to destroy this.