Here is the original, tweaked very slightly:
Shake the Rope Bridge
At-Will Terrain (Level 4)
You grab the ropes and swing the bridge.
Minor Action; Melee touch
Check: Athletics DC Moderate by Level
Target: Each creature on the rope bridge.
Attack: + Level + 3 vs. Reflex
Hit: The target is knocked prone. A target already prone is slide off the bridge.
And then a simple added effect for when the ropes are cut:
Cut the Rope Bridge
Single Use Terrain
The poorly-made rope bridge is easily destroyed.
Immediate Reaction or Minor Action; Melee touch
Trigger: A creature moves onto the rope bridge.
Effect: The rope bridge is cut, sending it and any creature on it swinging to the opposite side of the chasm. Any creature on the bridge is knocked prone and must make a saving throw or fall into the pit.