**3.2 Hydraulic considerations**

Hydraulics deals with flow in the river. For designing hydraulic structures for flood control, subsequent to hydrologic questions, there are hydraulic questions that need answering. These questions are: (1) What will be the flood stage and flood discharge in a river? (2) When will the flood stage exceed the flood threshold at a given location? (3) How long a river reach will be impacted by a given flood? (4) How much area will be flooded by such a flood? (5) How long will a flood last? (6) What will be the return period of such a stage and discharge? (7) What will be the probability or return period of a given flood stage and discharge? (8) What will be the risk of such a flood stage? These questions are answered by hydraulic modeling. Deterministic flood routing which is either empirical (relation between upstream and downstream hydrographs), or conceptual (Muskingum method), or physically-based (diffusion wave, dynamic wave) answers the first five questions. Singh [11] has given a full account of the deterministic flood routing. The last three questions are answered by stochastic hydraulics involving frequency analysis and multivariate stochastic analysis. Stochastic methods in hydraulics are similar to those in hydrology [9].
