Module 2 ch-1 heytograph and hydrology analysis

heytograph and hydrology analysis

1. MODULE-2 HYETOGRAPH AND HYDROGRAPH ANALYSIS PREPARED BY:- Prof. ANKIT N PATEL

2. HYETOGRAPH A hyetograph is a graphical representation of the relationship between the rainfall intensity and time. It is the plot of the rainfall intensity drawn on the ordinate axis against time on the abscissa axis. The hyetograph is a bar diagram. The area under the hyetograph gives the total rainfall occurred in that period. This chart is very useful in representing the characteristics of storm, and is particularly important in developing the design storm to predict extreme floods. Bits Edu Campus Prof. Ankit Patel 2

3. Mass Curve of Rainfall: The total accumulated precipitation is plotted against time Bits Edu Campus Prof. Ankit Patel 3

4. Hyetograph A hyetograph is a graphical representation of the relationship between the rainfall intensity and time Bits Edu Campus Prof. Ankit Patel 4

5. Rainfall Hyetograph Bits Edu Campus Prof. Ankit Patel 5

6. RUNOFF Runoff is that portion of rainfall that is not evaporated. The runoff is defined as the portion of the rainfall that makes its way towards river or ocean as surface or subsurface flow. The discharge flowing in a river is the runoff from the basin drained by that river. Runoff is usually expressed as volume per unit time, the common unit being m3/s or cumec. Bits Edu Campus Prof. Ankit Patel 6

7. Classification of Runoff Surface runoff : Water flows over the land and is first to reach the streams and rivers which ultimately discharge the water to the sea. Inter flow or subsurface flow: A portion of rainfall infiltrates into surface soil and depending upon the geology of basins, runs as subsurface runoff and reaches the streams and rivers. Ground water flow or base flow: It is that portion of rainfall which after infiltration, percolates down and joins the ground water reservoir which is ultimately connected to the ocean. Bits Edu Campus Prof. Ankit Patel 7

8. PROCESS OF RUNOFF When rainfall a part of it is intercepted by vegetation some of it is stored as depressions on the ground surface and is known as depression storage which later infiltrate or evaporates some of the rain is absorbed by soil. If the rain continues further, the water starts infiltrating into the surface soil and if rate of rainfall exceeds the infiltration rate then this excess water start collecting on the surface as surface detention and this water flows overland and joins the stream, river oceans. The water which percolates without joining the water table and flows below the ground surface and then joins the stream or river as a sub surface flow and is considered as a part of surface runoff. The water that percolates to the ground water table and later after long time joins the river or stream is known as ground water flow or base flow Bits Edu Campus Prof. Ankit Patel 8

9. PROCESS OF RUNOFF Bits Edu Campus Prof. Ankit Patel 9

10. FACTORS AFFECTING RUNOFF Climatic Factors Type of precipitation Rain intensity Duration of rainfall Distribution of rainfall Direction of storm movement Soil moisture deficiency Other climatic conditions 2. The physiographic Factors Size of basin Shape of the basin Topography of the basin Soil characteristics of basin Cultivation and vegetative cover in basin area Bits Edu Campus Prof. Ankit Patel 10

11. FACTORS AFFECTING RUNOFF Bits Edu Campus Prof. Ankit Patel 11

12. FACTORS AFFECTING RUNOFF Bits Edu Campus Prof. Ankit Patel 12

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15. Characteristics of drainage basin 1. Divide Lines: The boundary line, along a topographic ridge separating two adjacent drainage basins is called drainage divide. 2. Concentration Point: The single point or location at which all surface drainage form a basins comes together or concentrates as outflow from the basin in the stream channel is called concentration point 3. Time of concentration: the time of a concentration of a drainage basin is the time required by the water to reach the outlet from the most remote points of the drainage area. Bits Edu Campus Prof. Ankit Patel 15

16. HYDROGRAPH A hydrograph is a graphical plot of discharge of a natural stream or river versus time. It shows variations of discharge with time, at a particular point of a stream. It also shows the time distribution of total runoff at the point of measurement. Discharge is plotted on Y-axis and the corresponding time is plotted on X-axis. Flood analysis and derivation of unit hydrograph a single peaked hydrograph is required. Hydrograph analysis is the most widely used method of analyzing surface runoff. Bits Edu Campus Prof. Ankit Patel 16

17. Hydrograph Record of River Discharge over a period of time River Discharge = cross sectional area rivers mean (average) velocity X (at a particular point in its course) Storm Hydrographs Show the change in discharge caused by a period of rainfall

18. Why Construct & Analyse Hydrographs ? To find out discharge patterns of a particular drainage basin Help predict flooding events, therefore influence implementation of flood prevention measures

19. Construction Of Storm (flood) Hydrographs

20. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Base flow Through flow Overland flow Rising limb Recession limb Basin lag time mm 4 3 2 Peak flow Flood Hydrograph

21. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s)

22. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) mm 4 3 2 Rainfall shown in mm, as a bar graph

23. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) mm 4 3 2 Discharge in m 3 /s, as a line graph

24. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Rising limb mm 4 3 2 Rising limb The rising flood water in the river

25. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Rising limb mm 4 3 2 Peak flow Peak flow Maximum discharge in the river

26. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Rising limb Recession limb mm 4 3 2 Peak flow Recession limb Falling flood water in the river

27. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Rising limb Recession limb Basin lag time mm 4 3 2 Peak flow Basin lag time Time difference between the peak of the rain storm and the peak flow of the river

28. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Base flow Rising limb Recession limb Basin lag time mm 4 3 2 Peak flow Base flow Normal discharge of the river

29. 0 12 24 36 48 30 72 Hours from start of rain storm 3 2 1 Discharge (m 3 /s) Base flow Through flow Overland flow Rising limb Recession limb Basin lag time mm 4 3 2 Peak flow Overland flow Through flow + = Storm Flow

30. Volume of water reaching the river from surface run off Overland flow Through flow Volume of water reaching the river through the soil and underlying rock layers

31. COMPONENTS OF A HYDROGRAPH Rising limb:  The rising limb of hydro graph, also known as concentration curve, reflects a prolonged increase in discharge from a catchment area, typically in response to a rainfall event Recession (or falling) limb:  The recession limb extends from the peak flow rate onward. The end of storm flow (direct runoff) and the return to groundwater-derived flow (base flow) is often taken as the point of inflection of the recession limb. The recession limb represents the withdrawal of water from the storage built up in the basin during the earlier phases of the hydrograph . Bits Edu Campus Prof. Ankit Patel 31

32. Peak discharge:  the highest point on the hydro graph when the rate of discharge is greatest Lag time:  the time interval from the center of mass of rainfall excess to the peak of the resulting hydrograph Time to peak:  time interval from the start of the resulting hydro graph Discharge:  the rate of flow (volume per unit time) passing a specific location in a river or other channel Bits Edu Campus Prof. Ankit Patel 32

33. SEPERATION OF BASE FLOW The total runoff comprises direct runoff and base flow Direct runoff: it is that water which reaches the river shortly after it falls as rain. It is a overland flow Base Flow: It is the initial flow of the river before the rainfall comes. It is sustained or dry weather flow of the river resulting from the outflow of perennial or almost permanent ground water contribution that reaches the river. Bits Edu Campus Prof. Ankit Patel 33

34. Unit Hydrograph Unit Hydrograph is defined as the hydrograph of surface runoff of a catchment area resulting from unit depth of rainfall excess or net rainfall occurring uniformly over the basin and at uniform rate for a specified duration. Bits Edu Campus Prof. Ankit Patel 34

35. UNIT HYDROGRAPH The effective rainfall also called rainfall excess is that part of total rainfall that enters the stream directly. The specified or unit duration is that period within which the effective rainfall is considered to be uniformly distributed. A unit hydrograph is designed according to the unit duration Since the area under a hydrograph represents the total runoff, it is evident from the definition of unit hydrograph that the area under the unit hydrograph represents a direct runoff of 1 cm Bits Edu Campus Prof. Ankit Patel 35

36. Text in here Text in here ASSUMPTIONS The effective rainfall is uniformly distributed within its duration or specified period of time The effective rainfall is uniformly distributed over the entire area of drainage basin. The base or time duration of the hydrograph of direct runoff due to an effective rainfall of unit duration is constant. The ordinates of all the direct runoff hydrographs of a common base period are directly proportional to the total amount of direct runoff represented by each hydrograph. Bits Edu Campus Prof. Ankit Patel 36

37. Text in here Text in here For a given drainage basin the hydrograph of direct runoff corresponding to a given period of rainfall reflects all the physical characteristics of the basin. Bits Edu Campus Prof. Ankit Patel 37

38. LIMITATIONS OF UH This theory is not applicable to large areas because uniformly distributed effective rainfall cannot be expected in large area. The UH method cannot be applied when an appreciable portion of the storm precipitation falls as snow Practically no two storms have the same nature in space and time period. So it is not possible to construct UH for each pattern. Bits Edu Campus Prof. Ankit Patel 38

39. 39 Derived Unit Hydrograph Bits Edu Campus Prof. Ankit Patel

40. APPLICATION OF UH Use of UH for deriving a flood hydrograph resulting from rainfall of unit duration Use of UH for deriving a flood hydrograph resulting from a series of rainfalls each of same unit duration Use of UH for deriving a UH of other duration. Bits Edu Campus Prof. Ankit Patel 40