Given the diversity of its forms, we can no longer talk about a single type of flow, but of many. First, we need to distinguish two main types of flow: “fast” flows, as opposed to groundwater flows which qualify as “slow” flows. Groundwater flow is basically the infiltrated portion of the rainfall, which is slowly transported down into the water table to end up at the outlet. The flows that reach the outlets quickly to constitute the flood can be divided into surface flow and subsurface flow.