Runoff final
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This document discusses various methods for estimating runoff from rainfall. It begins by defining components of stream flow such as overland flow, interflow, and baseflow. It then discusses catchment characteristics and methods for classifying streams. Various factors that affect runoff are identified, including drainage area, soil type, land use, and antecedent moisture conditions. Two primary methods for estimating runoff are presented: the Rational Method and the SCS Curve Number Method. Worked examples are provided to demonstrate how to apply both methods to calculate peak runoff rates from given rainfall and catchment property data.
Runoff final
- 2. A. COMPONENTS OF STREAM FLOW B. CATCHMENT CHARACTERISTICS C. MEAN AND MEDIAN ELEVATION D. CLASSIFICATION OF STREAMS E. ISOCHRONES F. FACTORS AFFECTING RUNOFF G. ESTIMATION OF RUNOFF 2
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- 4. Overland Flow- a thin sheet of water which flows over the land surface. Interflow/Subsurface Flow/Underflow -an infiltrating water which moves laterally in the surface soil & joins the streamflow. Baseflow-groundwater contributing to the streamflow. 4
- 5. Direct Runoff- general term used to include the overland flow and interflow and snowmelt, in some case. Depression Storage- water stored in puddless, pits, and small ponds. Surface Detention/Detention Storage- value of water in transit in overlandflow which has not yet reached the stream channel. 5
- 6. Bank Storage- portion of runoff in a rising flood in a stream, which is absorbed by permeable boundaries of the stream above the normal phreatic surface. 6
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- 9. Drainage Basin- area of land drained by a river. Catchment Area- area within the drainage basin. Watershed/Drainage Divide- edge of highland surrounding a drainage basin & marks the boundary b/w two drainage basins. Source- beginning or start of a river. Confluence- the point at w/c two rivers or stream join. 9
- 10. Tributary- stream or small river w/c joins a larger stream or river. Mouth- the point where the river comes to the end usually when entering a sea. Concentration Point/Measuring Point- a single point at w/c all surface drainage from a basin comes together as outflow in the stream channel. 10
- 12. Characteristics of Drainage Net Number of Streams Length of Streams Stream Density Drainage Density 12
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- 15. Median Elevation- elevation at 50% area of the catchment & is determined from the area- elevation curve (hypsometric curve). 15 Hypsometric Curve
- 16. Example: The areas b/w different contour elevations for the River A basin are given below. Determine the mean and median elevation. 16
- 17. 17Computation of Mean Elevation of Basin
- 18. 18 Computation of Median Elevation of Basin
- 19. 19 Median Elevation for 50% of total area is read from the curve as 350 m.
- 20. Streams may be classified as: 1.Influent Streams & Effluent Streams 2.Intermittent Streams & Perennial Streams 20
- 21. Influent Streams If the GWT is below the bed of the stream feeds the groundwater resulting in the build up of water mound. 21
- 22. Effluent Streams When the GWT is above the WS elevation in the stream, the groundwater feeds the stream. 22
- 23. Intermittent Streams If the GWT lies above the bed of the stream during the wet season but drops below the bed during the dry season, the stream flows during wet season but becomes dry during dry season. 23 Perennial Streams Even in the most severe droughts, the GWT never drops below the bed of the streams & therefore they flow throughout the year. Perennial Streams Even in the most severe droughts, the GWT never drops below the bed of the streams & therefore they flow throughout the year.
- 24. Isochrones These are time contours and represent lines of equal travel time that are used to show the time taken for runoff water w/in a drainage basin to reach a lake, reservoir or outlet. 24
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- 30. 30 Curve
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- 32. 32 r=1, correlation is perfect giving a straight line plot r=0, no relationships exist b/w x & y r 1, close linear relationship
- 33. Example: Annual rainfall and runoff data for River M for 17 years (1934-1950) are given below. Determine the expected runoff for an annual rainfall 1050 mm. 33
- 34. 34 Year Rainfall (P) xRunoff ® y x^2 xy 1934.00 1088.00 274.00 1183744.00 298112.00 -211.82 -260.88 44869.21 68059.60 55261.02 1935.00 1113.00 320.00 1238769.00 356160.00 -186.82 -214.88 34903.03 46174.43 40145.08 1936.00 1512.00 543.00 2286144.00 821016.00 212.18 8.12 45018.85 65.90 1722.37 1937.00 1343.00 437.00 1803649.00 586891.00 43.18 -97.88 1864.21 9580.96 -4226.21 1938.00 1103.00 352.00 1216609.00 388256.00 -196.82 -182.88 38739.50 33445.96 35995.55 1939.00 1490.00 617.00 2220100.00 919330.00 190.18 82.12 36167.09 6743.31 15616.84 1940.00 1100.00 328.00 1210000.00 360800.00 -199.82 -206.88 39929.44 42800.31 41339.96 1941.00 1433.00 582.00 2053489.00 834006.00 133.18 47.12 17735.97 2220.07 6274.96 1942.00 1475.00 763.00 2175625.00 1125425.00 175.18 228.12 30686.80 52037.66 39960.84 1943.00 1380.00 558.00 1904400.00 770040.00 80.18 23.12 6428.27 534.43 1853.49 1944.00 1178.00 492.00 1387684.00 579576.00 -121.82 -42.88 14840.97 1838.90 5224.08 1945.00 1223.00 478.00 1495729.00 584594.00 -76.82 -56.88 5901.85 3235.60 4369.90 1946.00 1440.00 783.00 2073600.00 1127520.00 140.18 248.12 19649.44 61562.37 34780.26 1947.00 1165.00 551.00 1357225.00 641915.00 -134.82 16.12 18177.38 259.78 -2173.04 1948.00 1271.00 565.00 1615441.00 718115.00 -28.82 30.12 830.80 907.07 -868.10 1949.00 1443.00 720.00 2082249.00 1038960.00 143.18 185.12 20499.50 34268.54 26504.49 1950.00 1340.00 730.00 1795600.00 978200.00 40.18 195.12 1614.15 38070.90 7839.14 Sum 22097 9093 29100057 12128916 Mean 1299.823529 534.882353 Computation through Microsoft Excel ο࢞ൌൌൌ࢞ൌൌ࢞ ο࢞ൌൌൌ࢞ൌൌ࢞ (ο࢞࢞^2 (ο࢞࢞^2 (ο࢞࢞࢞࢞ο࢞࢞
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- 37. Rational Method 37 General Procedure Step 1: Determine the drainage area (in acres.) Step 2: Determine the runoff coefficient (C). Step 3: Determine the hydraulic length or flow path that will be used to determine the time of concentration. Step 4: Determine the types of flow (or flow regimes) that occur along the flow path. Step 5: Determine the time of concentration (Tc) for the drainage area. Step 6: Use the time of concentration to determine the intensity. Step 7: Input the drainage area, C value, and intensity into the formula to determine the peak rate of runoff
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- 39. 39 Formulas Used The rational method, used to calculate peak discharge: Q = C i A Calculating "C" in heterogeneous terrain: Estimating travel time of shallow concentrated flow: Calculating elevation change: Length of flow × Slope = Elevation change To calculate total time of concentration: Tc = Lo + Lsc + Lc
- 40. Example: 40 Given Information A project is to be built in southwest Campbell County, Virginia. The following information was determined from field measurement and/or proposed design data: Drainage Area: 80 acres 30% - Rooftops (24 acres) 10% - Streets and driveways (8 acres) 20% - Average lawns @ 5% slope on sandy soil (16 acres) 40% - Woodland (32 acres) LO = 200 ft. (4% slope or 0.04 ft./ft.); average grass lawn. LSC = 1000 ft. (4% slope or 0.04 ft./ft.); paved ditch. LC = 2000 ft. (1% slope or 0.01 ft./ft.); stream channel.
- 41. 41 1. Drainage area (A) = 80 acres (given). 2. Determine the runoff coefficient(C): Area × C Rooftops 24 × 0.9 = 21.6 Streets 8 × 0.9 = 7.2 Lawns 16 × 0.15 = 2.4 Woodland 32 × 0.10 = 3.2 Total 80 34.4 SolutionSolution
- 42. 42 Determine the hydraulic path: This has already been given. Determine flow regimes: a. Overland flow (LO) = 15 minutes (using Seelye chart). b. Shallow concentrated flow (LSC): 1. Velocity = 4 feet/second (using Diagram 1). 2. LSC = 4.2 minutes (based on the following calculations). c. Channel flow (LC): Change in elevation = 20 feet (based on the following calculations). 2000 feet × 0.01 = 20 feet LC = 13 minutes (using Kirpitch chart).
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- 45. 45 Time of Concentration = 32.2 minutes (based on the following calculations). Tc = Lo + Lsc + Lc Tc = 15 + 4.2 + 13 Tc = 32.2 Intensity = 2.3 in/hr (based on 2-year storm I-D-F curve for Pittsylvania County). Peak discharge = 79.1 cfs (based on the following calculations). Q = C i A Q = (0.43) (2.3) (80) Q = 79.1
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- 47. Soil Conservation Service(SCS) Curve Number (CN) model estimates precipitation excess as a function of cumulative precipitation, soil cover, land use, and antecedent moisture SCS developed the method for small basins (< 400 sq. mi.) to "before" and "after" hydrologic response from events. 47 SCS Curve Number Method
- 48. Where Q = runoff (in) P = rainfall (in) S = potential maximum retention after runoff begins (in) and Ia = initial abstraction (in) S)I(P )I(P Q a 2 a +− − = 48
- 49. Ia is all losses before runoff begins it includes: • water retained in surface depressions, • Water interception by vegetation • Evaporation and infiltration. Ia was found to follow: Ia = 0.2*S S)(P S)(P Q 2 *8.0 *2.0 + − = 49
- 50. S is related to the soil cover conditions of the watershed through the CN. CN has a range of 0 to 100 50 10 CN 1000 S −= con’t . . .
- 51. The ultimate total retention, S, and the initial abstraction, Ia, are assumed to be dependent on the following properties of the drainage basin: Land use Soil Type: A, B, C, D oSoil group A – Well drained sand or gravel, high infiltration rate oSoil group B – Moderately well drained soil, moderate infiltration rate, with fine to moderately coarse texture oSoil group C – Slow infiltration rate, moderate to fine texture oSoil group D – Very slow infiltration, mainly clay material, relatively impervious Hydrologic condition – good/fair/poor (rural land use only) Antecedent moisture/runoff condition (AMC) or (ARC) AMC/ARC I – Dry soil AMC/ARC II – Average soil moisture AMC/ARC III – Wet soil 51
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- 57. Example Solution Determine the CN from the Table of Runoff Curve Numbers considering the stated conditions: 57
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- 59. 59 The excess rainfall hyetograph may be determined from the rainfall hyetograph in one of two ways, depending on whether streamflow data are available or not.
- 60. 60 Example
- 61. Solution 61 1. Calculate the direct runoff hydrograph (DRH).
- 62. 62 3. Estimate the rainfall abstraction rate.
- 63. 63 4. Calculate the excess rainfall hyetograph.
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- 78. Engineering Hydrology by H.M. Raghunath Applied Hydrology by Ven Te Chow, et.al CE 374 K – Hydrology by Daene C. McKinney Rainfall-Runoff Modeling by Prof Ke-Sheng Cheng- National Taiwan University Runoff Estimation by Dr. Ali Fares-NREM 600, Evaluation of Natural Resources Management Runoff Estimation by Muhammad Khairudin bin Khalil Part 630 Hydrology Nationall Engineering Handbook,USDA-NSCS 78
- 79. That’s all about RUNOFF. 79
Editor's Notes
- #15: probability that 100 year flood occurs at least once in 100 years ° 1! P(exceedance) = 1 - (1 - 0.01)100 = 0.63