ERT 246 Hydrology & Water Resources Engineering

1. ERT 246 Hydrology & Water Resources Engineering Siti Kamariah Md Sa’at School of Bioprocess Engineering, UniMAP

2. RUNOFF Review

3. What is runoff? • Surface Runoff? Page 3

4. Factor affecting surface runoff flow (Q) • Area of catchments: Q αvolume of rainfall • Catchment’s slope, Tc: Less water time of travel • Catchment’s orientation: where rainfall event (upstream/downstream) • Catchment’s shape: time of runoff to reach end of catchments • Soil thickness: thick soil- less runoff • Plantation: Q forest area < Q grass area • Stream density: High stream density, High Q Page 4

5. Hydrograph Page 5

6. Hydrograph • Definition: a plot or listing that relates flow, velocity, stage or some other hydrologic characteristics of water to time. • Most commonly refers to a plot of flow (m3/s) versus time. Page 6

7. Element of Hydrograph • Streamflow hydrograph is a combination of the runoff, base flow and interflow hydrographs. • Interflow = subsurface stromflow = some of water that infiltrate the soil may move laterally through upper soil zones until its enters a stream channel. • Baseflow = flow from groundwater and contributed some flow to hydrograph Page 7

8. Page 8

9. Factor Affecting Hydrograph shape Page 9

10. a) Shape of the basin • Influence the time taken for water from remote parts of the catchment to arrive at the outlet. • Thus the occurence of the peak and hence the shape of the hydrograph are affected by the basin shape. • Fan-shape catchment-give high peak and narrow hudrograph while elongated shape give broad and low-peaked hydrograph Page 10

11. b) Size of basin • Small catchment-overland flow is predominant over the channel flow. Hence the land use and rainfall intensity have important role on peak flow. • Large catchment-channel flow phase is more predominant. Page 11

12. c) Slope • Slope - control the velocity of flow in channel • Stream channel slope will effect the depletion storage. • Large stream slope- quicker depletion storage and result steeper recession limbs of hydrograph. Page 12

13. d) Drainage Density Drainage Density- a ratio of the total channel length to the total drainage area. Page 13

14. e) Land Use/Land Cover • Human factor • Vegetation and forest increase the infiltration and storage capacities of the soils, and further cause retardance to overland flow. • Vegetal reduce the peak flow (area < 150 km2) Page 14

15. f) Climatic factor/meteorological factor • Climatic factor affecting shape of hydrograph • rainfall intensity and pattern • Areal distribution of rainfall over the basin • size and duration of storm event Page 15

16. Hydrograph Element & Component Net Rainfall Rising Limb Crest Falling Limb Recession Base Flow Direct Runoff (DRO) Page 16

17. Component of hydrograph • A typical hydrograph is characterized by • Rising Limb • Crest Segment • Recession Curve Page 17

18. Rising Limb • Also known as concentration curve – represent the increase in discharge due to gradual building up of storage in channel and the catchment surface • The initial losses and high infiltration losses during the early period of storm cause the discharge to rise rather slowly in the initial periods. As the storm continues, more and more flow from distant parts reach the basin outlet. • Simultaneously the infiltration losses also decrease with time. • The basin and storm characteristics control the shape of rising limb of a hydrograph Page 18

19. Mathematical Description – Rising Limb IN - OUT = Accumulation (Storage) r - Q = dS/dt and dQ/dS = K or dS/dt = dQ/(Kdt) r - Q = dQ/(Kdt) Integration Q(t) = r(1-e-KT) Q K S Page 19

20. Crest Segment • One of the mosty important parts of a hydrograph as its contain peak flow. • The peak flow occur when the runoff from various parts of the catchment simultaneously contribute amounts to achieve the maximum amount of flow at basin outlet. • Large catchment area- peak flow occurs after the cessation (end) of rainfall, the time interval from the centre of mass of rainfall to the peak. • Being control by basin and storm characteristics. • Estimation of peak flow and occurance –very important in flood studies. Page 20

21. Recession Limb/Curve • Extent from the point of infection at the end of the crest segment to the commencement of the natural groundwater flow represent the withdrawal of water from the storage built up in the basin during the earlier phase of hydrograph. • The starting point- represent the condition of maximum storage. • Since the depletion of storage takes place after cessation of rainfall, the shape of this part independent of storm characteristics and depend entirely on the basin characteristics. Page 21

22. Mathematical Description – Recession Limb/Curve IN - OUT = Accumulation (no r ) - Q = dS/dt and dQ/dS = K or dS/dt = dQ/(Kdt) -Q = dQ/(Kdt) Integration from Qp to zero Q(t) = Qp (e-KT ) for t>tp (NOTE) K on recession limb not the same as the K on the rising limb Page 22

23. Baseflow separation method Straight Line ABC continue the recession - concave Empirical method Page 23

24. Effective Rainfall (ER) • Also known as excess rainfall • Part of rainfall becomes direct runoff at the outlet of the watershed. • ER could be define as that rainfall that is neither retained on the land surfecae nor infiltrated to the soil. • Hyetograph = effective rainfall hyetograph (ERH) Page 24

25. Hyetograph Page 25

26. Basin lag time Peak flow 3 Rising limb Overland flow Recession limb 2 mm Discharge (m3/s) Flood Hydrograph 4 Through flow 1 3 2 Base flow 0 12 24 36 48 30 72 Hours from start of rain storm Page 26

27. 3 2 Discharge (m3/s) 1 0 12 24 36 48 30 72 Hours from start of rain storm Page 27

28. Rainfall shown in mm, as a bar graph 3 2 mm Discharge (m3/s) 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm Page 28

29. Discharge in m3/s, as a line graph 3 2 mm Discharge (m3/s) 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm Page 29

30. Rising limb The rising flood water in the river 3 Rising limb 2 mm Discharge (m3/s) 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm Page 30

31. Peak flow Peak flow Maximum discharge in the river 3 Rising limb 2 mm Discharge (m3/s) 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm Page 31

32. Recession limb Peak flow Falling flood water in the river 3 Rising limb Recession limb 2 mm Discharge (m3/s) 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm Page 32

33. Basin lag time Basin lag time Peak flow Time difference between the peak of the rain storm and the peak flow of the river 3 Rising limb Recession limb 2 mm Discharge (m3/s) 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm Page 33

34. Base flow Basin lag time Peak flow Normal discharge of the river 3 Rising limb Recession limb 2 mm Discharge (m3/s) 4 1 3 2 Base flow 0 12 24 36 48 30 72 Hours from start of rain storm Page 34

35. Overland flow Basin lag time + Peak flow Through flow 3 = Rising limb Overland flow Recession limb 2 Storm Flow mm Discharge (m3/s) 4 Through flow 1 3 2 Base flow 0 12 24 36 48 30 72 Hours from start of rain storm Page 35

36. Overland flow Through flow Volume of water reaching the river from surface run off Volume of water reaching the river through the soil and underlying rock layers Page 36

37. Unit Hydrograph Page 37

38. Unit Hydrograph Theory Page 38

39. Hydrologic Analysis Change in storage over time = inflow - outflow In the case of a linear reservoir, S = kQ Transfer function for a linear system (S = kQ). Page 39

40. Proportionality and superposition • Linear system (k is constant in S = kQ) • Proportionality • If I1 Q1 then C*I2 C*Q2 • Superposition • If I1 Q1and I2 Q2, then I1 +I2 Q1+ Q2 Page 40

41. Impulse response function Impulse input: an input applied instantaneously (spike) at time t and zero everywhere else An unit impulse at t produces as unit impulse response function u(t-t) Principle of proportionality and superposition Page 41

42. Step and pulse inputs • A unit step input is an input that goes from 0 to 1 at time 0 and continues indefinitely thereafter • A unit pulse is an input of unit amount occurring in duration Dt and 0 elsewhere. Precipitation is a series of pulse inputs! Page 42

43. Unit Hydrograph Theory • Direct runoff hydrograph resulting from a unit depth of excess rainfall occurring uniformly on a watershed at a constant rate for a specified duration. • Unit pulse response function of a linear hydrologic system • Can be used to derive runoff from any excess rainfall on the watershed. Page 43

44. History • First proposed by Sherman (1932), the unit hydrograph (originally named unit-graph) of a watershed is defined as a direct runoff hydrograph (DRH) resulting from 1 cm of excess rainfall generated uniformly over the drainage area at a constant rate for an effective duration. • Sherman originally used the word “unit” to denote a unit of time. But since that time it has often been interpreted as a unit depth of excess rainfall. • Sherman classified runoff into surface runoff and groundwater runoff and defined the unit hydrograph for use only with surface runoff. Page 44

45. Assumption The unit hydrograph is a simple linear model that can be used to derive the hydrograph resulting from any amount of excess rainfall. The following basic assumptions are inherent in this model; • The excess rainfall has a constant intensitywithin the effective duration. • The excess rainfall is uniformly distributed throughout the whole drainage area. • The base time of the DRH (the duration of direct runoff) resulting from an excess rainfall of given duration is constant. • The ordinates of all DRH’s of a common base time are directly proportional to the total amount of direct runoff represented by each hydrograph. • For a given watershed, the hydrograph resulting from a given excess rainfall reflects the unchanging characteristics of the watershed. Page 45

46. Criteria for Selecting Storm Events to Derive UHs • Storms are isolated and occur individually; • Storm coverage should be uniform over the entire watershed - watershed area should not be too large, say < 5000 km2 ; • Storms should be flood-producing storms – ER is high, 10mm < ER < 50mm is suggested; • Duration of rainfall should be approx. 1/5 to 1/3 of basin lag; • The number of storm events should be at least 5. Page 46

47. Derived Unit Hydrograph • Rules of Thumb : • … the storm should be fairly uniform in nature and the excess precipitation should be equally as uniform throughout the basin. This may require the initial conditions throughout the basin to be spatially similar. • … Second, the storm should be relatively constant in time, meaning that there should be no breaks or periods of no precipitation. • … Finally, the storm should produce at least 1 cm of excess precipitation (the area under the hydrograph after correcting for baseflow). Page 47

48. Illustration of unit hydrograph Page 48

49. Derivation of UH (For simple ERH) 1.Analyze hydrograph and perform base flow separation. 2.Measure the total volume of DRH in equivalent uniform depth (EUD) 3.Find the effective rainfall such that VDRH = VERH. 4.Assume that ERHs are uniform, the UH can be derived by dividing the ordinates of DRH by VDRH 5.The duration of the UH is the duration of ERH. 6. In rainfall-runoff analysis, the times of occurrence for DRH and ERH are commonly made identical. Page 49

50. Construction of Unit Hydrograph fromStream Flow Hydrograph due to a Uniform Rainfall Excess of Fixed Duration • What is it? The hydrograph from 1 inch/1cm of rainfall excess over the entire watershed. • Obtain surface runoff hydrograph • Compute the volume of surface runoff • Divide the volume by the watershed area with answer in inches/mm, which is a scale factor. • Obtain the unit hydrograph by multiplying the surface runoff hydrograph with the scale factor Page 50