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Surface runoff

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Hydrology, water resources engineering

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Surface runoff

  1. 1. Runoff, Stream flow, Concept of unit hydrograph and S-curve Dr. Mohsin Siddique Assistant Professor Dept. of Civil & Env. Engg 1
  2. 2. Outcome of Lecture 2 After completing this lecture… The students should be able to: Understand surface runoff and its categories Understand the concept stream flow, velocity and discharge Understand the concept of hydrograph, unit hydrograph and its application Understand the application of rational formula and its application
  3. 3. Surface Runoff and Stream flow 3
  4. 4. Surface Runoff and stream flow 4 On hard or frozen ground, most of the precipitation is unable to seep below ground.This precipitation then flows down slopes and hills, eventually stopping in rivers, lakes, streams, and oceans. Some of this water will then evaporate and rejoin the hydrologic cycle, while other water will remain in the body of water. This process of water traveling over the ground and collecting in a body of water is called surface runoff. Stream
  5. 5. Surface Runoff and Stream flow 5 Stream flow, or channel runoff, is the flow of water in streams, rivers, and other channels, and is a major element of the water cycle. Importance: Stream flow is one of the most important topics in engineering hydrology because it directly relate to water supply, flood control, reservoir design, navigation, irrigation, drainage, water quality, and others. Point of interest 1. Discharge 1.1Velocity 1.2 Cross-sectional area 2. Stage (water depth),
  6. 6. Stream Flow Measurements (Discharge) 6 Serves as the basis for many water resources engineering designs Measurement of Discharge; Measurement of flow area Measurement of flow velocity
  7. 7. Devices for Flow Velocity Measurement Floats: Suitable for straight channel, V = L/T PitotTubes: Suitable only for clean water Current Meters Cups Propellers V = a + b×N whereV = flow velocity; a = starting velocity to overcome mechanical friction; b = equipment calibration constant; N = revolutions/sec.
  8. 8. Floats 8 Surface velocity of flow= = L/T
  9. 9. Current Meters
  10. 10. Mean Flow Velocity Estimation Velocity Profile Deph < 0.6m 0.6d VV = ; 0.6 water depth from the water surface 2mDepth0.6m ≤≤ 2 0.8d V 0.2d V V + = 2mDepth ≥ 4 0.8d V 0.6d 2V 0.2d V V ++ =
  11. 11. Current Meters 11
  12. 12. Measurement of Stream Flow Discharge 1. Mid-Section Method i A i V i i QQ i A i V i Q i bd i A ∑=∑= = = ∑= = += + += + i i QQ i A i V i Q )V i V( 2 1 i V ) 1i d i (d 2 b i A 1i 2. Mean-Section Method
  13. 13. Example 13 Estimate the discharge in stream as shown. Current meter measurements are also given in table Distance from left edge depth Current meter m m No. of Rev Time (s) 3 1.4 12 50 6 3.3 38 52 23 55 9 5 40 58 30 54 12 9 48 60 34 58 15 5.4 34 52 30 50 18 3.8 35 52 30 54 21 0 18 50 0 3 6 9 12 15 18 21 Velocity equation by current meter is given by V=2.3N+0.05 Where N is revolution per second.
  14. 14. Example: 14 Distance from left edge depth Area Current meter N V Vavg. Q m m sq.m No. of Rev time RPS m/s m/s cu.m/s 3 1.4 2.1 12 50 0.24 0.602 0.602 1.2642 6 3.3 7.05 38 52 0.73 1.73 1.37 9.6723 55 0.42 1.01 9 5 12.45 40 58 0.69 1.64 1.48 18.4530 54 0.56 1.33 12 9 21 48 60 0.80 1.89 1.64 34.5334 58 0.59 1.40 15 5.4 21.6 34 52 0.65 1.55 1.49 32.2330 50 0.60 1.43 18 3.8 13.8 35 52 0.67 1.60 1.46 20.1930 54 0.56 1.33 21 0 5.7 18 50 0.36 0.88 0.88 5.00 Total. Q= 121.331
  15. 15. Example 15 Estimate the discharge in stream as shown. Current meter measurements are also given in table 0 4 8 12 16 20 24 28 Velocity equation by current meter is given by V=2.3N+0.05 Where N is revolution per second.
  16. 16. Example: 16 Distance from left edge depth Area Current meter N V Vavg. Q m m sq.m No. of Rev time RPS m/s m/s cu.m/s 4 1.68 18 50 8 33 57 52 34.5 55 12 6 60 58 45 54 16 10.8 72 60 51 58 20 6.48 51 52 45 50 24 4.56 52.5 52 45 54 28 0 18 50 Total. Q=
  17. 17. Measurement of Stream Flow Discharge (3) Area-Slope Method 2/13/21 SAR n Q = A= cross-sectional area R=hydraulic radius S= channel slope n= Manning’s roughness coefficient
  18. 18. Measurement of Stream Flow Discharge (4) Chemical dilution method (5) Hydraulic structures
  19. 19. Hydraulic Structures for Discharge Measurement
  20. 20. Measurement of Water Stage 20 Measurement of Water Stage Water stage: the elevation above some arbitrary datum of water surface at a station.
  21. 21. Measurement of water stage 21 Types of Gages Measuring River Stage: Staff gage – vertical or inclined Suspended – weight gage Recording gage Crest – stage gage ( used to indicate high water mark)
  22. 22. Measurement of Water Stage 22
  23. 23. Stage-Discharge Relation Rating curve is a graph of discharge versus stage for a given point on a stream, usually at gaging stations, where the stream discharge is measured across the stream channel with a flow meter Q HH Q t t Stage Hydrograph Stage-Discharge Curve or Rating Curve Discharge Hydrograph
  24. 24. Stage-Discharge Relation 24 During the event of large flood, it is impossible or impractical to measure discharge directly. More often, the flood stage goes beyond the range of the data range used to define the rating curve.Therefore, extrapolation of the ration curve is needed when water level is recorded below the lowest or above the highest level. Q H Stage-Discharge Curve or Rating Curve Q H Extension of Rating Curve Methods for extension (1). Logarithmic method (2). Chezy’s method
  25. 25. 25 Part II
  26. 26. Hydrograph 26 A hydrograph is a graph showing the rate of flow (discharge) versus time past a specific point in a river, or other channel or conduit carrying flow. The rate of flow is typically expressed in cubic meters or cubic feet per second (cms or cfs).
  27. 27. Hydrograph 27 Time to Peak, tp:Time from the beginning of the rising limb to the occurrence of the peak discharge. Time of Concentration, tc:Time required for water to travel from the most hydraulically remote point in the basin to the basin outlet LagTime, tl:Time between the center of mass of the effective rainfall hyetograph and the center of mass of the direct runoff hydrograph Time Base, tb: Duration of the direct runoff hydrograph.
  28. 28. Essential Components of Hydrograph 28 Essential components of a hydrograph are: (i) the rising limb, (ii) the crest segment, and (iii) the recession limb. (iv) Direct run off (v) Baseflow D.R.O baseflow Falling limb crest Rising limb Q (m3/s) Concentration curve Recession curve Inflection Point t D.R.O=Direct run-off
  29. 29. Separation of Base flow 29 N (days)=0.83A0.2 A= area of catchment
  30. 30. Separation of Base flow 30 Hydrograph-Baseflow=Direct runoff Hydrograph Direct runoff
  31. 31. Unit Hydrograph 31 The hydrograph that results from 1- inch (or 1 cm) of excess precipitation (or runoff) spread uniformly in space and time over a watershed for a given duration. The key points : 1-inch (or 1cm) of EXCESS precipitation Spread uniformly over space - evenly over the watershed Uniformly in time - the excess rate is constant over the time interval There is a given duration
  32. 32. Unit Hydrographs of Different Duration 32 In practice the unit hydrographs of different duration are needed. e.g., 1h- unit hydrograph, 2h-unit hydrograph, etc Two methods are available to generate hydrograph of different durations 1. Method of Superposition 2. the S-Curve
  33. 33. 1. Method of Superposition T-H unit duration is available and it is needed to make UH of nT-H, where n is and integer Superposing n UH with each graph separated from the previous one by T-H
  34. 34. 2- The S-Curve 0.00 10000.00 20000.00 30000.00 40000.00 50000.00 60000.00 0 6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 102 108 114 120 Time (hrs.) Flow(cfs) The S-curve method involves continually lagging a T-h unit hydrograph by its duration and adding the ordinates.
  35. 35. Solution of Example 6.9 by S-curve method
  36. 36. Problem 38 Time from start Ordinate of 4h- UH (cu. m/s) (Hours) cu.m/s Col.1 Col. 2 0 0 2 8 4 20 6 43 8 80 10 110 12 130 14 146 16 150 18 142 20 130 22 112 24 90 26 70 28 52 30 38 32 27 34 20 36 15 38 10 40 5 42 2 44 0 Ordinate of 4h-Unit Hydrograph are given, Determine (1). Ordinate of S-Curve (2). Ordinates of 2-h Unit Hydrograph
  37. 37. 39 Time from start ordinate of 4h-UH (cu.m/s) S Curve addition S Curve ordinates S-Curve lagged by 2 h D.R.O Hydro. of (2/4) cm of precipitation Ordinate of 2-H UH Col.1 Col. 2 Col.3 Col.4=Col.2+Col.3 Col.5 Col.6=Col.4-Col.5 Col.7=Col.6/(2/4) 0 0 0 0 0 2 8 8 0 8 16 4 20 0 20 8 12 24 6 43 8 51 20 31 62 8 80 20 100 51 49 98 10 110 51 161 100 61 122 12 130 100 230 161 69 138 14 146 161 307 230 77 154 16 150 230 380 307 73 146 18 142 307 449 380 69 138 20 130 380 510 449 61 122 22 112 449 561 510 51 102 24 90 510 600 561 39 78 26 70 561 631 600 31 62 28 52 600 652 631 21 42 30 38 631 669 652 17 34 32 27 652 679 669 10 20 34 20 669 689 679 10 20 36 15 679 694 689 5 10 38 10 689 699 694 5 10 40 5 694 699 699 0 0 42 2 699 701 699 2 (0) 4 (0) 44 0 699 699 701 -2 (0) -4 (0)
  38. 38. Thank you Questions…. 40

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