unit hydrograph by anwar
TRANSCRIPT
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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FLOOD PICTORIAL VIEWS
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PredictionThe only hydrograph that predicts the behavior of a flood from a storm of any duration
WhyConstruct & Analyse
Hydrographs ? To find out discharge patterns ofa particular drainage basinHelp predict flooding events,therefore influence implementation of flood prevention measures
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Hydrograph Theory
Hydrograph
Graphical representation of time (hours) versus discharge (cfs or cms) at a particular point on a stream or channel which drains the watershed area
Peak Discharge
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-------------------------YES?Then we will be able to1. 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.
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Micro Hydro Projects
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
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The unit hydrograph method is employed to calculate 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.
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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.
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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
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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 synthetic unit hydrograph methods available to develop a unit hydrograph for ungaged watersheds e.g. Espey Ten-Minute Unit Hydrograph.
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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.
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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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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.
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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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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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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
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Direct Runoff
The unit hydrograph represents the excess precipitation or quick – response runoff
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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
UH Components / Terminology
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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
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A unit hydrograph can be derived from a total stream 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 occurred.
STEPS FOR DERIVING THE UNIT HYDROGRAPH
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Text in hereWhen 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
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Remove Base flow ContributionStep-2
Text in hereThe 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
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Determine Excess PPT Depth from BasinStep-4
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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
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Adjust the Quick-Response HydrographStep-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.
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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
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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.
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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.
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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.
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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.
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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.
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Application of Unit Hydrograph
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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
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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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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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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 superposition2. The S-Curve
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
D = 2-Hr Unit Hydrograph
Adjusted Net Rainfall one inch over basin
Qp
Change UH Duration
Consider 1 hr UHAdd and Lag two UH by one hourSum and divide by 2Results in 2 hr UH
ExampleGiven 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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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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S-Curves for UH
S-Curves
• Convert 2 hr UH to 3-hr• Lag each 2-hr UH by Duration D• Add to produce S-curve
S-curve
ExampleSolve 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 catchmentTime (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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Hydrograph Convolution
1
2
3
31 2
Add up the ordinates of all three to produce storm hydrograph
STORMHYDRO
Add and Lag Method
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