1. Narnarayan Shashtri Institute of Technology SUBJECT :– HYDROLOGY AND WATER RESOURCES ENGINEERING TOPIC :-HYDROLOGIC ANALYSIS GROUP MEMBER Patel Darshak -130340106019 Dudani Bhavesh- 130340106025 Patel Harsh – 130340106030 Kakadiya Akash – 130340106036 SUBMITTED TO :- Nimisha Modi

2. TOPIC COVER Introduction Design flood Flood estimation Flood frequency analysis Theoretical probability distribution methods Flood routing Channel routing

3. Introduction A flood may defined as an overflow coming from some streams to river or from some other body of water. Any flow which is relatively high which overtops the natural or artificial banks in any reach of a river may be called a flood. A flood is an unusually high stage of a river due to runoff from rainfall/or melting snow.

5. Design flood The maximum flood that any structure can safely pass is called the design flood. A design flood is the flood design adopted for the design of a structure after careful consideration of economic and hydrologic factors. While designing ay important hydraulic structure, provision must be made for the flood that is likely to occur during the life of that particular structure. The design flood is related to the project feature, for example small structures such as culverts and storm drainages can be designed for less severs floods as the consequences of a higher-than-design flood may not be very serious. other hand, storage structures such as dams demand greater attention to the magnitude of floods used in the design. The failure of these structures causes large loss of life and great property damage on the downstream of the structure. Therefore, while designing structures, we have to think of a flood value against which these structures can be designed as safe.

6. As the magnitude of the design flood increases, the capital cost of the structure also increases but the probability of annual damages will decrease. On the other hand, a very low value chosen for the design, if exceeded during the life time of the structure, will result in the failure of the structure causing much more damage. The most economical design flood can be found after studying the various alternatives. It is apparent that the type, importance of the structure and economic development of the surrounding area dictate the design criteria for choosing the flood magnitude.

7. In the design flood estimates reference is usually made three classes : In the design flood estimates reference is usually made three classes : i.Ordinary floods : These are the floods which are sure to be equalled in magnitude once or more times in the estimated Life of the project. ii.Standard Project Flood (SPF) : This is the estimate of the flood likely to occur from the most severe combination of the meteorological and hydrological factors which are reasonably applicable to the region. Extremely rare combinations of factors are excluded. iii.Maximum Probable Flood (MPF) :The extreme flood that is the physically possible in a region as a result of severe most combinations, including rare combinations of meteorological hydrological factors.

8. Flood Estimation In general the methods used in the estimation of the flood flow can be grouped as under 1. Physical indication of past floods 2. Empirical formulate and curves 3. Concentration time method 4. Rational method 5. Overland flow hydrograph and unit hydrograph 6. Flood frequency studies

10. Physical indication of past floods :- The maximum flood discharge may be approximately by enquiring from the old person in the village situated on the banks of the river about the flood marks that the higher flood in their memory in the past about 35 years may have left on the river banks. By noting the high water marks along the banks of the flow section as well as the water surface slope may be computed and using the manning’s formula, with suitably assumed value of the flood discharge may be determined. This procedure should be repeated at several villages or water marks, to get consistent result.

11. Empirical formula and Envelope Curves Several empirical formulae have been developed for estimating the maximum or peak value of the flood discharge. In these formulae the maximum flood discharge Q of a river is expressed as a function of the catchment area A. Most of these formulae may be written in a general form as Q= CA n where, C = coefficient and n= index Both C and n depend upon various factors, such as size, shape, and location of catchment, intensity and duration of rainfall and distribution pattern of the storm over the catchment area.

12. 1. Dicken’s formula Q=CA^(3/4) Where Q = maximum flood discharge in cumec A = area of catchment in square kilometer C = coefficient The maximum value of C is 35. This formula is applicable for moderate size catchments of river of North and Central India. RegionValue of C North India11.4 Central India13.9 to 19.5 Western Ghats22.2 to 25

13. 2.Ryve’s Fromula Q = CA^2/3 Where Q = discharge in cumec A = catchment area in sq. km. C = coefficient RegionValue of C 1. Area within 24 km from the coast6.75 2. Area between 24 km to 161 km from the coast 8.45 3. Limited areas near hills10.00

14. 3.Inglis formula Q = 124 A / √A + 10.4 Q = 124 A½ where Q = discharge in cumec A = area of catchment in sq.km Inglish formula is dervied by using the data of rivers of Maharashtra, where it is commonly used.

15. 4.Ali Nawah Jang Bahadur formula Q = CA^(0.993 - 1/ 14 logA) where Q = discharge in cumec A = area of catchment in sq. km. C = coefficient The coefficient C varies from 48 to 60. The maximum value of C is 85. This formula is derived on the basis of the data of rivers of former Hyderabad state. However, the formula is applicable with lower values of C for the catchments of rivers of South India and with higher values of C for the catchment of rivers of North India.

16. 5.Myer's formula Q = 175,/A where Q = discharge in cumec A = area of catchment in sq. km.

17. Envelope Curves Areas having similar topographical features and climatic conditions are grouped together. All available data regarding discharges and flood formulae are compiled along with their respective catchment areas. The maximum flood discharges are then plotted against the areas of the drainage. basins and a curve is drawn to cover or envelope the highest plotted points, which is known as.envelope curve. If the ploting is done on a log - log paper then the envelope curve is usually a straight line. By using the envelope curve the maximum flood discharge may be estimated if the area of the drainage basin is known. Envelope. curves are generally used for comparison only and the design floods got by other methods, should be higher Ih nn those obtained from envelope curves. For Indian rivers enveloping curves from observed data of floods have been developed by Kanwar Sain and Kanwar which are shown in Fig.

18. Concentration Time Method The concentration time is nothing but the time required for the surface runoff from the remotest part of the catchment area to reach the basin outlet. The concentration time method of estimating the peak discharge consists of two steps : a) Determination of the concentration time, etc. There are a number of empirical equations available for the estimation of the time of concentration, Two of these described below. US Practice : For small drainage basins, the time of concentration is approximately equal to the lag time of the peak flow. Thus,

19. tc = tp = CtL ( LL ca / √s) n where tc = time of concentration in hours L = basin length measured along the main water course in km. Lca = distance along the main water course from the gauging station to a point opposite the watershed centroid in km. S = basin slope P CtL and n = basin constants. b) Selection of the period of maximum rainfall for the concentration time duration:- Q = 2.78 (i- Φ ) A where Q = maximum discharge in cumecs i = the maximum intensity of rainfall in cm/hr Φ = infiltration index in cm/hr,A = area of catchment in km2

20. Rational Method In this method it is assumed that the maximum flood flow is produced by a certain rainfall intensity which lasts for a time equal to or greater than the period of concentration time. When a storm continues beyond concentration time every part of the catchment would be contributing to the runoff at outlet and therefore it represents condition of peak runoff. The runoff rate corresponding to this condition is given by Q = 2.78 CI c A where Q = discharge in cumecs C = coefficient which depends upon the characteristics of the catchment Ie = the critical intensity of rainfall (cm/hr) corresponding -to the time of concentration (tc) of the catchment for a given recurrence interval obtained from the intensity duration frequency curves A = area of catchment in km2

21. Overland flow Hydrograph and unit Hydrograph When a unit hydrograph is available for the catchment under consideration, it can be applied to the design storm to yield design flood hydrograph from which peak flood value can be obtained. Whenever possible it is advisable to use the unit hydrograph method to obtain the peak flood. It gives not only the flood peak but also the complete flood hydrograph which is essentially required in fixing spillway capacity after incorporating the effect of storage of the reservoir on flood peak through flood routing. When the area of the basin is very large it is better to frequency analysis methods described in the next method.

22. FLOOD FREQUECNCY ANALYSIS It is the best method out of all the earlier methods discussed so for provided sufficient data of flood is available. In this method on the basis of the past records of the maximum flood discharge predictions for the maximum flood discharges which may occur in future are made by deterring the flood frequency and using the probability concept. Floods are extremely complex natural event. It is very difficult to model flood analytically as it is the outcome of may component parameters. Therefore, estimation of peak flood is complex problem leading to many approaches. Frequency analysis make use of the along with their probabilities or return periods.

24. Determination of discharges for different frequency floods by using statistical or probability methods : In these methods, the predictions for the future floods are made on the basis of the available records of the past floods. These probability methods are however, unable to give precise results, where lesser number of past records are available. For the success of any probability method, sufficient past records of flood must, therefore, be made available. Chance of flood :- If a flood of a given magnitude occurs with an average frequency of 50 years, then there exists 1/50 X 100 = 2 percent chance for this flood to occur, and such a flood is generally called 2 percent chance flood. This is flood which can occur in 50 years in any year. If a flood of a given magnitude occurs with an average frequency of 100 years, then there exist 1/100 x 100 = 1 percent chance for this flood to occur, and such a flood is generally called 1per cent change flood.

25. frequency or probability of flood (p)

26. Return period or recurrence interval (T) It is defined as the number of years in which a flood can be expected once or a flood of given magnitude will be equaled or exceeded only once. It is usually denoted by T and it given by T = 1/p where p is probability of flood Probability of Occurrence (p) : The probability of an event (rainfall or flood) being equaled or exceeded in any one year is the probability of its occurrence. This probability of occurrence or exceedance is generally represented by p, and would be equal to T ∴ p = 1/T where T is the recurrence interval Probability of non-occurrence (q) : If the probability of occurrence of an event such as rainfall or is p, then the probability of its non-occurrence in given by q, which would be equal to 1-p. ∴ q = 1-p.

27. Probability of non occurrence in n successive years : The probability of an event not occurring at all in n successive would be equal to = q n which is equal to (1-p) n Probability of occurrence at least once in n successive years : The probability of an event occurring at least once in n successive years (R) would be equal to 1-qn or [1 - (1- p )n ] This probability is called risk R = [1- (1 - p) n].

28. Flood Routing The flood routing is a process of determining the reservoir storage volume, out flow rates and the rise of water level in the reservoir. corresponding to Any inflow during any peak flood discharge. Flood routing may be defined as a technique of determining the flood discharge and arresting the same in a reservoir for some period so that sudden flood discharge may not create any damage in the down stream areas. Flood routing is technique used for the flood control. When a flood passes over a spillway, the outflow rate is different from the inflow rate because of the effect of the storage of the reservoir, the flood hydrograph is thus modified after passing over the spillway. Its peak is reduced. However, the base of the out hydrograph is longer than the base of the inflow hydrograph. It is achieved by detaining the flood water for some longer period closing the spillways and then the excess flood water is released gradually by opening the spillways.

30. Procedure of flood routing The relationship governing the computations for flood rou t.i ny simple and it states that over any interval of time, the volume of inflow must be equal to the volume of outflow plus the chance in storage during this period. This basic relationship of the flood routing is based on the continuity of flow. The relationship may be expressed as follows : Inflow volume = Outflow volume ± change in storage This may be expressed mathematically as I = O ± δ s Where, i= average inflow during a given time period O = average outflow during the same period and δ s = the change in storage during the same period

31. Flood routing may be divided into two basic types : (i) The reservoir routing and (ii) The channel routing Reservoir routing :-In normal cases, the rain water from the catchment area enters the reservoir and the water level reaches up to full reservoir level (F.R.L.), the excess water flows through the spillways. But due to the excessive rainfall during any period the flood discharge is highly increased and the reservoir level exceeds the full reservoir level ( normal pool level) and reaches up to high flood level (H.F.L.). This volume of water is absorbed temporarily for some period in the reservoir and then allowed to flow to the down stream through the spillways.

33. The reservoirs can be either controlled or uncontrolled. The controlled reservoir have spillways with gates to release water at the time of need. The uncontrolled reservoirs are those spillways are not controlled by gate operation. The flood routing involves i. The fixation of maximum reservoir level up to which the structure is completely safe. ii. Reservoir routing requires the relationship among the reservoir elevation, storage and discharge to be known. iii. Implementation of outflow patter from the reservoir so that it may not create any damage in the down stream area.

34. Methods of Reservoir Routing The most common and simple methods that are frequently used by field engineers are : 1. Trial and error method 2. I.S.D. curve method 3. Goodrich method 4. Modified Pul's method Modified Pul's method (semi-Graphical Method): This method is also known as the storage indication method. In this method the basic continuity equation is given below (I 1 + I 2 /2) Δ t – (O 1 +O 2 /2) Δ t = S 2 -S 1 A CURVE is plotted between the elevation as ordinate and (2S/ Δ t+O) as abscissa

35. CHANNEL ROUTING In a river during floods, the flow is non-uniform and unsteady. This type of flow is very difficult to solve. The hydraulic characteristics vary from stage to stage and also from· channel to channel. There may be a lateral inflow or outflow also. Thus, in reservoir routing the Storage was a unique function of the outflow discharge, S = f(O). However, in channel routing the storage is a function of both outflow and inflow discharges. All these conditions make the analysis of the problem increasingly difficult. However, neglecting all the above conditions and assuming changes occurring gradually with time, the problem can be simplified and solved. It is also known as the stream flow routing.

37. In channel routing, the flood hydrograph at various sections of the reach is predicted by considering a channel reach and an input hydrograph at the upstream end. The specific length of a stream channel between the upstream section where the hydrograph is known and the down stream section where the hydrograph is to be determined is called a channel reach. The hydrograph at the upstream end of the reach is the inflow hydrograph and the hydrograph at the downstream and of the reach is the outflow hydrograph.

38. Prism Storage The prism storage is formed by a volume of constant cross-section along the length of the river. It is the volume that would exist if uniform flow occurred t the downstream depth, i.e the volume formed by an imaginary plane parallel to the channel bottom drawn at the outflow section water surface.

39. Wedge storage The wedge storage is the volume between the top of the prism and the water surface. It is the wedge like volume formed between the actual water surface profile and the top surface of the prism storage. At a fixed depth at a downstream section of a river reach the prism storage is constant while the wedge changes from a positive value at an advancing flood to a negative value during recending flood.

40. The prism storage SP is similar to a reservoir and can be expressed as a function of the outflow discharge, S p = f(O). The wedge storage can be accounted for by expressing it as S w = f(I). The total storage in the channel reach can then be expressed S = K [x I m + (1- x ) o m ] Where K and x are coefficients and m = a constant. It has been found that the value of m varies from 0.6 for rectangular channels to a value of about 1.0 for natural channels.

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