Some points here, Hannibal whanted to throw a trap. Romans were very suspycious about ambushes, so they'll refuse battle with forests nearby. In the demo we have forest, so not historicall. In real Haniball send Magos cav and infantry to bushes side the river. Because of the terrain those bushes acted like almost forest.

Romans were forced, after a camp atack, to cross the frozen Trebia. They'll formed the other side while those guys frozen also.