Estimates of peak flows and runoff volumes (hydrographs) are necessary for the design of a wide variety of hydraulic structures, including culverts, detention basins, and bridge openings. Such estimates are made by modeling.
In general there are two different classes of models, or ways to estimate peak flows and hydrographs. The first is based on historical records used in statistical models. In other words we use historical records of stream flows to predict what might occur in the future. The second class of models are referred to as deterministic models where physical properties about the watershed and stream are used to numerically simulate peak flows.