From: The Last Book Read
Bridging Engineers can destroy bridges, repair bridges, and create pontoon bridges. If in a hex beside a bridge, a bridging engineer has the option to destroy it. If in a hex beside a destroyed bridge, a bridging engineer has the option to repair it. If in a hex beside a major/minor river, a bridging engineer has the option to create a pontoon bridge. They can only do one of these actions in a hex. For example, you can't create a pontoon in a game hex that already has a bridge hexside. The right click button will only be present if the hex is valid (i.e. only get the choice to repair bridge if there's a destroyed bridge present). Note: Combat engineers cannot perform any bridging tasks.
When applicable, the hexside on the map will display an icon that indicates which of these engineer actions applies (destroy bridge, repair bridge, create pontoon, clear mines).
The probabilities to repair / destroy / create bridges:
• Repair/Destroy a Major bridge = 20% / 40% respectively.
• Repair/Destroy a Minor bridge = 40% / 80% respectively.
• Create a pontoon bridge = Major River: 30% / Minor River: 60%
Pontoon Bridges. When ordered (a button on the right click pop up unit menu), a pontoon engineer unit that is adjacent to a river creates a pontoon that crosses every adjacent river hexside that does not contain enemy units. The pontoon remains in place until the pontoon engineer unit moves. A pontoon bridge is removed if unit moves/enemy moves adjacent.
Designer’s Note: The pontoons and support boats are equipment. They travel with the pontoon engineer unit. The pontoon engineer unit will remain with the bridge to maintain it; add/subtract pontoons as the river rises and falls due to rain, etc. They assist units in crossing safely and repair any damage caused by the crossing units.