Chapter-7 Flood routing notes pdf (1).pptx

#engineering flood routing

1. CHAPTER -THREE 3. FLOOD ROUTING The technique of determining the characteristics of a flood hydrograph at the outlet of a reservoir or at a downstream section of a river channel by utilizing the data of flood flow at one or more upstream sections. Two broad categories of routing can be recognized. These are: 1. Reservoir or storage or Lumped routing or Hydrologic and 2. Channel or Distributed routing or Hydraulic

2. RESERVIOUR ROUTING The effect of a flood wave entering a reservoir is studied. This form of reservoir routing is essential in: 1.the design of the capacity of spillways and other reservoir outlet structures 2.the location and sizing of the capacity of reservoirs to meet specific requirements. A variety of routing methods are available and they can be broadly classified into two categories namely: i . Hydrologic routing and ii. Hydraulic routing

3. Cont… Hydrologic routing methods employ essentially the equation of continuity . Hydraulic routing methods, on the other hand, utilize both the continuity equation and the equation of motion (derived from Newton’s second law of motion) for unsteady flow. The limitations of data unavailability notwithstanding, the basic differential equations used in the hydraulic routing, known as the St. Venant equations afford a better description of unsteady flow, than hydrologic methods . A flood hydrograph is modified in two ways as the storm water flows downstream

4. Cont… Firstly , the time of the peak rate of flow occurs later at downstream points. This is known as translation. Secondly, the magnitude of the peak rate of flow is diminished at downstream points, the shape of the hydrograph flattens out, and the volume at the floodwater takes longer to pass a lower section. This modification of the hydrograph is called attenuation.

5. Hydrologic storage routing There are essentially two types of storage routing. These are: reservoir hydrologic-routing and channel hydrologic-routing . In both these methods, the characteristics of an outflow (O), either at a given channel section or at the outlet of a reservoir are estimated by utilizing the information on inflow (I) at a given upstream section or sections . A flood wave I(t), enters a reservoir provided with an outlet flow control such as a spillway. Due to the passage of the flood wave through the reservoir, the water level in the reservoir changes with time.

6. Hydrologic reservoir routing Thus , the discharge and storage also change with time. But the outflow is a function of the reservoir elevation only, i.e. O=O(H). The relationship between reservoir storage and outflow can be expressed in the following general form: k = storage coefficient Reservoir storage routing involves finding the variation of S, H, and O with time, i.e. finding S=S(t), O=O(t) and H=H(t) given I=I(t).

7. Hydrologic reservoir routing cont.. For the reservoir storage routing, the following data have to be known: A relationship between the reservoir volume and the water level above the crest of the spillway A relationship between the reservoir water-surface elevation and the outflow and hence a relationship between the reservoir storage and the outflow discharge. The temporal distribution of the inflow hydrograph I=I(t) The initial values of S, I and Q at time t=0

8. Hydrologic reservoir routing cont.. There are a variety of methods available for the routing of floods through a reservoir. All of them use the finite difference form of the lumped continuity equation (mass balance) Eq. The initial reservoir water elevation is often assumed to be horizontal. Thus, the storage routing is also known as Level Pool Routing. Many methods have been developed for Level Pool routing. Two of the most commonly used level pool routing methods are: The Coefficient method The storage indication ( Puls ) method

9. The coefficient (Linear reservoir) routing method Assumes that the reservoir or channel storage is directly proportional to the outflow from the reservoir. The storage concept is well established in flow-routing theory and practice. Figure 1 shows two consecutive time levels, 1 and 2 separated between them by an interval ∆t, and two spatial locations or sections depicting inflow (section 1) and outflow (section 2), with the reservoir located between them . I1= inflow (section 1) at time level 1; I2 = inflow (section 1) at time level 2; O1 = outflow (section 2) at time level 1; 02 = outflow (section 2) at time level 2; S1 = storage at time level 1; S2 = storage at time level 2; and ∆t = time interval.

10. The coefficient routing method Cont.. C0 + C1 + C2= 1. Thus, the routing coefficients are interpreted as weighting coefficients. These routing coefficients are a function of ∆t/K, the ratio of time interval to storage constant. Example 1: A linear reservoir has a storage constant K = 2 h, and it is initially at equilibrium with inflow and outflow equal to 100 m'/s. Route the following inflow hydrograph through the reservoir.

11. cont... The reservoir exerts a diffusive action on the flow, with the net result that peak flow is attenuated and time base is increased. In the linear reservoir case, the amount of attenuation is a function of ∆t/K. The smaller this ratio, the greater the amount of attenuation exerted by the reservoir. Conversely, large values of ∆ t/K cause less attenuation. Values of ∆t/K greater than 2 can lead to negative attenuation. This amounts to amplification; therefore, values of ∆t/K greater than 2 are not used in reservoir routing.

12. Hydrologic channel routing In reservoir routing presented in the previous sections, the storage was a unique function of the outflow discharge. However, in channel routing , the storage is a function of both the inflow and the outflow discharges. Hence, a different routing approach is required. It is important to note that the flow in a river during a flood passage belongs to the category of gradually varied unsteady flow. The water surface is not only unparallel to the channel bottom but also varies with time. Thus, considering a channel reach having a flood flow, the total volume in storage can be considered under two categories as: 1. Prism storage 2. Wedge storage

13. Cont Prism storage is the volume that would exist if uniform flow occurred at the downstream depth, i.e. the volume formed by an imaginary plane parallel to the channel bottom drawn at the outflow section of the flow. Wedge storage is a wedge-like volume formed between the actual water surface profile and the surface of the prism storage.

14. The Muskingum channel routing method

15. Example

16. Solution