Hydrosystems Hydrulics open channel flow iit nptel _120_ by hemlalb


									Hydraulics                                                                                      Prof. B.S. Thandaveswara

                37.1 Introduction
                The flow of water in rivers, canals, reservoirs, lakes, pools, and free- surface flow in

                storm water drains, conduits, pipes , galleries, tunnels and culverts, in which the

                velocities change with time, is defined as unsteady flow ( non - permanent, non -

                stationary , or time -variable free- surface water flow). This unsteadiness may arise

                naturally or may be caused by human action. Floods in rivers, water level variation in

                estuaries due to tidal action etc. are examples of unsteady flows occuring naturally.

                Surges created in power channels, water level variation in irrigation canals due to gate

                operation etc. are unsteady flows caused by human action. Analysis and computation of

                unsteady flow in open channels are important from the point of view of operation of

                flood control reservoirs, designing a flood forecasting system, risk assessment of

                dambreaks, designing storm water drainage systems, and assessment of surface

                irrigation systems.

                Flow in natural channel is always unsteady. When the discharge changes slowly with

                time the unsteady flow is usually approximated by steady flow. The discharge

                hydrographs in natural streams are largely comprised of using rising limb followed by

                recession limb. Those flows occurring during a prolonged drought or those occurring for

                short time intervals at the highest and lowest points of the hydrographs may be

                aproximately considered to be steady flows. In hydraulic engineering problems, it is

                important to know when to treat an unsteady flow as steady flow. For practical

                purposes, the answer is obtained by judgment rather than by definite mathematical or

                experimental criteria. (Reference: unsteady flow in open channels edited by Mahmood

                K and Yevjevich V. Vol 1, 1975).

Indian Institute of Technology Madras

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