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Flow routing is a procedure to determine the time and magnitude of flow (i.e. flow hydrograph) at a point on a watercourse from a known or assumed hydrograph at one or more points upstream.
If the flow is flooded, the procedure is known as flood routing and if the routing is done for the flow in the channel, the procedure is called channel routing.
For a catchment, a rainfall hyetograph is input but for a reservoir and stream channel network, an inflows hydrograph is input, and an outflow hydrograph is output from the system.
1. Flood Routing
If the flow is flooded, the procedure is known as flood routing. Flood routing ( also called storage routing or reservoir routing) is the process of computing the water levels in the reservoir and outflow rates corresponding to particular inflow at various instants of time.
The hydrograph of a flood entering a reservoir will change in shape as it emerges out of the reservoir because a certain volume of its water is stored in the reservoir temporarily and is let off as the flood subsides.
The base of the hydrograph, therefore, gets broadened, its peak gets reduced, and of course, the time of peak is delayed. The extent to which the inflow hydrograph gets modified due to the reservoir storage can be computed by a process known as flood routing (or reservoir routing).
2. Channel Routing or River Routing
Since the flood protection reservoirs are generally located many kilometers upstream of the cities which are to be saved against floods, it is sometimes necessary to route the outflow hydrograph of the reservoir up to these downstream localities.
The reservoir outflow hydrograph may then be routed through this length of the river channel, so as to obtain the final shape of the hydrograph at the affected cities. This routing, in which the stream self acts as an elongated reservoir, is known as channel routing or river
As the hydrograph travels, the peak of the outflow hydrograph is less than that of the Inflow hydrograph. This phenomenon is called attenuation f the peak due to the combined effect of storage and channel friction. Furthermore, the peak of the outflow hydrograph occurs sometimes later than the inflow hydrograph. This is called translation or lag of the peak. This is due to the travel time of the flood waves in the channel or reservoir.
3. Why route flows?
Flow routing accounts for changes in flow hydrograph as a flood wave passes
This helps in accounting for storage and studying the attenuation of flood peaks.
|Read Also: Hydrology in Civil Engineering|
4. Types of Flow Routing
There are three methods of routing:
a. Hydrological system
b. Hydraulic system
c. Hybrid system
a. Hydrological System
a. Flow is calculated as a function of time alone at a particular location.
b. Governed by the continuity equation and flow/storage relationship.
c. Hydrological method is based on the storage concept and hydrological models are conceptual models.
d. Muskingum method is pure to storage concept or hydrological method.
b. Hydraulic system
a. Flow is calculated as a function of space and time throughout the system
b. Governed by continuity and momentum equations.
c. Hydraulic model is based on the method of mass and momentum conservation. Hydraulic models are physical base models. All parameters are physically based.
d. Kinematic wave model diffusion wave, model and dynamic wave models are hydraulic model
c. Hybrid Method
a. Basically derived from storage concept but some parameters are derived from physical based.
b. Combination of hydrological and hydraulic
c. A Muskingum – Cunge method is a hybrid method.
This was for the flow routing.
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