"Cause and effect" or causality is the state in which one part causes the outcome of an event that leads to the other state of the event. And both parts are partly or fully dependent on the other for their condition. In cause and effect, the cause contributes to the effect that will happen in the next phase, thus leading to the dual dependence of the two.
In the given sentence, the heavy rains cause the flood, thus leading to many people looking for "<em>alternative routes to get to their destination</em>". The heavy rain is the cause of the whole effect of the "flood", which led to the people looking for other ways to get on.
