L8 unit hydrograph
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- 2. Coverage Flood pictorial views Hydrograph - Review Unit Hydrograph Unit Hydrograph. Why ? Assumptions For UH Terminology for UH Creating Unit Hydrograph Applications of Unit H
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- 26. Prediction The only hydrograph that predicts the behavior of a flood from a storm of any duration
- 27. 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
- 29. Hydrograph Graphical representation of time (hours) versus discharge (cfs or acms) at a particular point on stream or channel the watershed area which drains
- 30. Peak Discharge ?
- 31. -------------------------YES ?Then we will be able to 1. Manage the Storm water. 2. Identify the Flood Plans on downstream side. 3. To place the Hydraulic structures at safe level. 4. Efficient Urban Storm water management plan. 5. Design the Different types of Hydraulic structures. 6. Minimize the effects of Floods.
- 33. Unit Hydrograph A conceptual direct runoff hydrograph resulting from a rainfall excess of unit depth and constant intensity for a particular watershed is called a unit hydrograph
- 34. The unit calculate hydrograph method is employed to the direct runoff hydrograph at the watershed outlet given the rainfall excess produced by a storm event. This method is categorized as a lumped model in which the hydrologic characteristics of the entire watershed are combined and typified by one or more parameters, simple mathematical expressions, or graphs.
- 35. The Unit hydrograph is a useful tool in the process of predicting the impact of precipitation on stream flow. The Unit depth is 1cm in the SI unit system and 1inch in the U.S. system. It is usually abbreviate as a Uhc. The subscript “c” indicate the Duration of the rainfall excess.
- 36. For instance, the direct runoff hydrograph produced by a rainfall excess that has a duration of 3 hr and constant intensity of 1/3 in./hr is denoted by UH3 and depth of the rainfall excess is (1/3 in./hr)(3 hr) = 1 in
- 37. We can develop a unit hydrograph for a gaged watershed by analyzing the simultaneous rainfall and runoff records. Unfortunately, most small, urban/rural watersheds are ungaged. However, there are several available ungaged synthetic unit hydrograph methods to develop a unit hydrograph for watersheds e.g. Espey Ten-Minute Unit Hydrograph.
- 38. UNIT HYDROGRAPH—WHY ? Simplifying our task / work / procedures. Gives us a base line for a specified watershed. Standardize the hydrograph for different watersheds. Gives us information that how the flow of a stream will be affected over time by the addition of one unit of runoff.
- 39. The role of Unit Hydrograph theory in the flood prediction process is to provide an estimate of stream flow given and amount precipitation. Once we know how much rainfall or snowmelt has occurred, or is likely to occur, and we have an idea of how much of this will turn into runoff, we still need to know how the flow of a stream will be affected over time by that runoff. The unit hydrograph provide us with a way to estimate this, and is an integral part of many hydrological modeling systems.
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- 41. ASSUMPTIONS The primary assumption of unit hydrograph theory is that the rainfall has uniform distribution, both in space- with minimal variations across the basin-and in time; in other words, the rainfall rate did not vary much during the event. In reality, precipitation events are rarely uniform in space and time. Often, one portion of the basin experiences higher intensity precipitation than another portion.
- 42. The base duration of direct runoff hydrograph due to an effective rainfall of unit duration is constant. The ordinates of DRH are directly proportional to the total amount of DR of each hydrograph (principles of linearity, superposition, and proportionality) For a given basin, the runoff hydrograph due to a given period of rainfall reflects all the combined physical characteristics of basin (time-invariant)
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- 45. BASIN-AVERAGED RAINFALL In typical non-snow situations, we begin the hydrologic process with rainfall. In particular, we start with a basin-averaged rainfall. This simply tells us how much rain fell, or is forecast to fall, on a given basin and typically takes the form of a rainfall depth per time. In unit hydrograph theory, we assume that this rainfall has fallen uniformly across the basin
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- 47. BASIN-AVERAGED EXCESS RAINFALL From averaged rainfall, we need to know how much of the basin- averaged rainfall will become runoff. In unit hydrograph theory, runoff is often referred to as “excess precipitation” or “excess rainfall.” Rainfall runoff models will typically provide an estimate of what becomes excess rainfall. So, for example, if 25% of our 4.00 cm basin-averaged rainfall becomes excess rainfall, then we have a basin averaged excess rainfall of 1.00 cm
- 48. The unit hydrograph represents the excess prCecoipnitteantiotsn or quick – response runoff - Click to add Text - Click to add Text - Click to add Text Direct Runoff
- 49. TERMINOLOGY - UH Duration Rising Limb Recession Limb (falling limb) Peak Flow Time to Peak (rise time) Time of Concentration Recession Curve Base flow Separation line Base flow Quick Response Run off Point of inflection
- 50. UH Components / Terminology
- 51. CREATING U.HYDROGRAPH From Stream flow Data Synthetically Espey Ten-Minute Unit Hydrograph Snyder SCS Unit Hydrograph Time-Area Unit Hydrograph(Clark, 1945) Gamma Function Unit Hydrograph “Fitted” Distributions Geomorphologic My Concern
- 52. STEPS FOCRonDteEnRtsIVING THE UNIT HYDROGRAPH flow hydrograph at a given stream gauge location along with the following information: • The Basin Area • The Basin-averaged rainfall depth • The duration over which the excess precipitation - Click to add Text - Click to add Text - Click to add Text A unit hydrograph can be derived from a total stream occurred.
- 54. When deriving a unit hydrograph it is important to start with an archived hydrograph in which the quick-response runoff portion is from one single storm event. In addition, that storm should have produced its excess precipitation with nearly uniform coverage in space and time over the basin Select Appropriate Precipitation EventStep-1
- 55. Remove Base flow ContributionStep-2
- 56. The total volume of water from the quick-response runoff needs to be calculated. This is done by summing the areas under the QRR Hydrograph for each time step, in this case, hourly. Calculate Quick – Response VolumeStep-3
- 57. Determine Excess PPT Depth from Basin Step-4
- 58. Determine Excess PPT Depth from BasinStep-4 For example, assume we have a basin area of 100 square km, which is 100,000,000 sq.m and calculated volume of quick- response to be 2,000,000 cum-then the depth will be
- 59. Adjust the Quick-Response Hydrograph Step-5 The excess ppt depth probably won`t be exactly one unit as unit hydrograph requires. So, we have to adjust the QRR hydrograph to show what the response from one unit would be.
- 60. Adjust the Quick-Response HydrographStep-5 Once we multiply each point on the hydrograph by our adjustment factor of 0.5, our resulting unit hydrograph is for exactly 1 cm of excess precipitation
- 61. Determine Duration of UHStep-6 The duration of a unit hydrograph refers to a continuous time period during which one unit of excess ppt occurred. If it took 6 hours for the one unit of excess to occur, we have a 6-hr unit hydrograph. Remember, the unit hydrograph duration does not refer to the duration of the stream flow response.
- 62. The difficult part of determining the duration of a unit hydrograph is estimating which portion of the entire precipitation event actually contributes to excess ppt. Recall that the water that infiltrates & percolates into deeper storage and base flow is not part of excess ppt. We can estimate this portion of the ppt. by applying a constant loss function to the rainfall. Recall that we have already calculated the depth of the excess ppt to be 2.0 cm. Now, we need to know how long it took for that excess to occur.
- 63. So we move this loss function line such that the amount of ppt. above the line is equal to the depth of excess ppt. that we already calculated for the basin. Below that line the ppt. goes to long-term storage. Above the line is the excess ppt.
- 66. Now we have an excess precipitation bar graph of 6-hr. Notice that the amounts from hour to hour on this graph are not truly uniform. This is typical. For purposes of calculating a unit hydrograph duration, however, we assume that all excess ppt occurred uniformly in time.
- 67. Final Unit Hydrograph At the end of these steps, we have a 6-hr unit hydrograph. It show the stream flow response to 6 hrs of excess ppt that produced one unit of depth.
- 68. Application of Unit Hydrograph
- 69. The UH method is based on the assumption of a linear relationship between the rainfall excess and the DR rates. More specifically, the method assumes that The base time of the DRH resulting from a rainfall excess of a given duration is constant regardless of the amount of the rainfall excess, and The ordinates of a DRH resulting from a rainfall excess of a given duration are directly proportional to the total amount of rainfall excess
- 70. In other words, the base of the DRH resulting from a rainfall excess of, say, 1.5 in. produced over af 2-hr duration is the same as that of the 2-hr UH. Also the ordinate of this DRH are 1.5 times the ordinates of the UH2 at respective times. We can simply state that DRH = cUH2 DRH = 1.5 UH2 General Form
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- 73. Example: •Two storm each of 6-hr duration and having rainfall excess values of 3.0cm and 2.0 cm respectively occur successively. The 2-cm ER rain follows the 3-cm rain. The 6-hr UH for the catchment is the same as given in previous example. Calculate the resulting DRH.
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- 77. Unit Hydrograph of Different Durations Under condition where lack of adequate data in developement of unit hydrograph D-hour unit hydrograph is used to develop unit hydrographs of differing durations nD Two method available: 1. Method of superposition 2. The S-Curve
- 78. If a D-h unit hydrograph is available, and its desired to develop unit hydrograph of nD, its is easily accomplished by superposing n unit hydrographs with each graph separated from the previous on by D-h. Method of Superpositions
- 79. D = 2-Hr Unit Hydrograph Adjusted Net Rainfall one inch over basin Qp
- 80. Change UH Duration Consider 1 hr UH Add and Lag two UH by one hour Sum and divide by 2 Results in 2 hr UH
- 81. Example Given the ordinates of a 4-hr unit hydrograph as below derive the ordinates of a 12-hr unit hydrograph for the same catchment Time (hr) 0 4 8 12 16 20 24 28 32 36 40 44 Ordinates of 4-hr UH 0 20 80 130 150 130 90 52 27 15 5 0
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- 84. S-Curve Also known as S-hydrograph Hydrograph produced by continous effective rainfall at a constant rate for infinite period. Curve obtained by summation of an infinite series of D-h UH spaced D-h apart.
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- 86. S-Curves for UH
- 87. S-Curves • Convert 2 hr UH to 3-hr • Lag each 2-hr UH by Duration D • Add to produce S-curve S-curve
- 88. Example Solve previous example with S-curve method: Given the ordinates of a 4-hr unit hydrograph as below derive the ordinates of a 12-hr unit hydrograph for the same catchment Time (hr) 0 4 8 12 16 20 24 28 32 36 40 44 Ordinates of 4-hr UH 0 20 80 130 150 130 90 52 27 15 5 0
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- 90. Hydrograph Convolution 1 2 3 Add up the ordinates of all three to produce storm hydrograph 1 2 3 STORM HYDRO Add and Lag Method
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