Section 2:Hydrology

Generating a runoff hydrograph85 Minutes

Copyright 2004 HydroCAD Software Solutions LLCAll Rights Reserved - Duplication Prohibited

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IntroductionWhat are we going to discuss?

PWe will start with a review of some fundamentalhydrologic techniques.

PA basic understanding of these calculations willimprove your ability to understand and interpretthe results from any hydrology software.

PPlease see other texts for full details, such as theHydroCAD Owner’s Manual.

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What is the Return Period?A common point of confusion!

PRoughly speaking, the return period is theaverage time between occurrences of a givenevent, such as the 25-year storm.

PTo be more precise, a 25-year storm has a 4%probability of occurring in any given year.

PTherefore, a 25-year storm can occur in twoconsecutive years!

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Intensity-Duration-Frequency Data

A basic way toquantify rainfallobservations.

Typicallycompiled foreach county inthe US.

PAn IDF curve indicates the rainfall intensity thatwill occur for a given duration and return period.

PThe rainfall intensity remains constant for theentire duration.

Rainfall Duration (minutes)0 10 20 30 40 50 60

2

4

8

6

10

12

14

16

18

20

Frequency (years)5025105

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Typical IDFCurves

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The Rational MethodA basic equation for predicting peak runoff

PQ=Peak Runoff [CFS]PC=Runoff CoefficientP i=Rainfall intensityPA=Catchment Area< The intensity is determined from the IDF curve, but

what duration do we use?

Q ' C IA

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Setting the Rainfall Durationfor the Rational Method

A simple equation gets more complicated!

PWe must use the “critical duration”, whichproduces the highest runoff.

PFor a single, homogeneous subcatchment, theduration is typically equal to the time-of-concentration, Tc.

PBut when several subcatchments are combined,the critical duration can be any value between theshortest and longest Tc.

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Other Limitations ofthe Rational Method

PThe Rational Method predicts peak runoff, butnot total runoff volume.

PThe Rational Method predicts a constant flow,rather than generating a complete hydrograph.

PRational Method is poorly suited for volume-sensitive calculations, such as detention ponds.< How do we overcome these limitations ???

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The SCS Rainfall DistributionsAllowing the rainfall intensity to vary over time

PA mass curve indicates the cumulativerainfall depth at any time during the storm.

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Features of RainfallDistributions

PAllows the intensity to vary over time.< Similar to an actual rainfall event

P Includes rainfall volume of entire (24-hour)event.< Can be used for volume-sensitive calculations.

PEach curve includes all events up to 24-hours.< Eliminates need to identify the critical duration!

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More RainfallDistribution Features

PRepresents most of the US with just a few curves.< Curve is scaled for your local 24-hour rainfall depth.

PSynthetic rainfalls provide design standards< Type II peak defined at 12 hours

PCan also create an actual rainfall distribution< Allows modeling of observed events

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The SCS Runoff EquationHow much of the precipitation appears as runoff?

PQ=Precipitation excess (runoff) [inches or mm]PP=Cumulative precipitation [inches or mm]PS=Potential maximum retention [inches]PCN=SCS Curve Number

Q'(P&.2 S )

2

P% .8S(Q=0 if P<.2S )

where S '1000

CN& 10

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Time-of-Concentration:What does it mean?Determining when the runoff occurs

PThe Tc is the time required for a particle of waterto travel from the most hydrologically remotepoint in the watershed to the point of collection.

PThere are many procedures for calculating Tc< TR-55 Sheet Flow< TR-55 Shallow Concentrated Flow< Channel Flow (based on Manning’s velocity)< Upland Method< CN Method (a.k.a. Lag Method)

– We’ll examine the individual procedures in the next section2-120

The SCS DimensionlessUnit Hydrograph

Defines the runoff hydrograph for a single “burst”

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Unit HydrographTime Axis

How long does the runoff lastfrom a single burst?

< Tp=Time to peak< D=Burst duration < Tc=Time of concentration

PEverything is based on the Tc

Tp' 5D and Tp'23

Tc ˆ D 'Tc

7.5

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UH Time Axis ExampleJust substitute the Tc !

If Tc = 30 minutesD = Tc/7.5 = 4 minutesTp = 5D = 20 minutesTb = 5Tp = 100 minutes

Time (minutes)20 40 60 80 1000

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Unit Hydrograph Flow AxisWhat is the peak runoff from a single burst?

Pqp=Peak discharge [CFS]PC=Units conversion factor (653.33)

PK=UH Shape Factor (0.75 for SCS UH)

P484=SCS UH “Peak Factor”PA=Area [sq-miles]PQ=Precipitation Excess (runoff) [inches]

qp 'C K A Q

Tp

'484 A Q

Tp

'484 A Q

2

3Tc

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UH Flow Axis ExampleDetermine the flow for a single burst,with precipitation excess Q

Time (minutes)20 40 60 80 1000

If Q= 0.2 inchand A = 0.1 sq-mileand Tc = 0.5 hours

qp = 484 A Q / 2/3Tc = 29 cfs

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A Fully Dimensioned UH !The actual runoff hydrographfrom a single burst of rainfall

Time (minutes)20 40 60 80 1000

PNow we have a completehydrograph for one burst.

PBut how do we get thehydrograph for an entire24-hour storm?

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Convolution: The Heartof TR-20

Constructing the entire runoff hydrograph

PThe unit hydrograph (when dimensioned) tells usthe runoff for a single burst of rainfall.

PTo determine the runoff for the entire storm, wemust perform a convolution of the unithydrograph with the precipitation excess.< We must break the storm into a sequence of individual

bursts, and add up the hydrographs from all the bursts.

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Convolution: Step #1Adding up the hydrographs for each burst

Time (hours)

1) Divide storminto bursts “D”

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Convolution: Step #2Adding up the hydrographs for each burst

Time (hours)

1) Divide storminto bursts “D”

2) Generate UHfor each burst

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Convolution: Step #3Adding up the hydrographs for each burst

Time (hours)

1) Divide storminto bursts “D”

2) Generate UHfor each burst

3) Add all UHTotal Hydrograph

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Complete Runoff HydrographWhy does the hydrograph look like this?

1) Zero runoff forseveral hours

2) Runoffpeaks afterrainfall peak

Time (hours)

3) Runoffcontinues afterend of storm

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Performing the ConvolutionA computation-intensive process

P If Tc = 7.5 minutes< Then burst duration D = 1 minute< And 24-hour storm contains 1440 bursts

P If UH consists of 100 coordinates< 1440 x 100 = 144,000 coordinates must be calculated

and summed to generate the runoff hydrograph !!PCalculation is not feasible by hand!

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TR-55 and theTabular Method

Doing runoff calculations by hand

PThe TR-55 Tabular method allows manualcalculation of a runoff hydrograph thatapproximates the TR-20 UH procedure.

PTR-55 tables were produced by TR-20 with:< Curve Number = 75< Runoff = 3 inches< Fixed Tc values from 0.1 hours to 2.0 hours

POther values require approximation and resultswill differ from actual TR-20 results.

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TR-55 WarningsQuoting from the TR-55 manual

PThis method (TR-55) approximates TR-20, amore detailed hydrograph procedure... UseTR-20 if the watershed is very complex or ahigher degree of accuracy is required.

PThe procedure (TR-55) should not be used toperform final design if an error in storage of25 percent cannot be tolerated... Moredetailed hydrograph development and routingwill often pay for itself through reducedconstruction costs.

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HydroCAD: An EasierTRS20 Solution

Plus lots of other calculations and capabilities

P In 1986, HydroCAD was the first PC-basedprogram to use the full UH runoff procedure.

PEliminates approximations in the tabular method.PAlso provides:< TR-55 Tc procedures< Hydraulics calculations< True hydrograph routing< Lots more! (As we’ll see throughout the day)

PBut first, let’s examine some basic concepts inmore detail... 2-270

What is a hydrograph?The flow of water over time (Flow vs. Time)

T i m eA hydrograph is ideally a smooth curve...

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But how do we represent thehydrograph curve numerically?

Use a series of pointsspaced equally in time

T i m eThe time increment is called “dt”

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What is the peak flow?HydroCAD uses an interpolated peak, just like TR-20

T i m e

PTrue peak may fallbetween points...

PSo interpolate usingthree highest points

Allows for variations in point timing2-300

What time increment should I use?The time increment (dt) specifies the point spacing

PFor best results, use dt # Tc / 2< This ensures good peak definition

P If Tc $ 0.1 hours (6 minutes), use dt = 0.05 hours< This is the default value for new projects

PA warning will occur if dt is too large< This will occur if Tc < 0.1 hours

PDt can be as small at 0.01 hours< For fastest operation, don’t go smaller than required

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What time span should I use?Long enough to cover all flow of interest!

T i m e (hours)0 3 6 9 12 15 18 21 24

Default time span5-20 hours

Use longer span ifentire volume is

required!

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Rational Method hydrographsA shorter time span andtime increment are required

0 Tc D D+Tc D+2Tc

Default dt =0.01 hours

Default time span = 0-3 hours2-330

Time for a quick stretch!

PPlease have a quick stretch at your deskPWe’ll have a full break after this section

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Rainfall Curves #1Different ways to define the rainfall distribution

Time (hours)

PThe actual rainfalldistribution is acurve

PHow do werepresent this inHydroCAD?

True RainfallMass Curve

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Rainfall Curves #2Different ways to define the rainfall distribution

Time (hours)

Fitted MassCurve

PUses a polynomialfit to each segmentfor a true curve

PUsed for standardSCS rainfalls

PFor details seeStart|Programs|HydroCAD|Rainfall Info

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Rainfall Curves #3Different ways to define the rainfall distribution

Time (hours)

Mass curvepoints

PEasy to definePLine segments may

cause runoff “steps”PSmoothing option

gives better curve

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Rainfall Curves #4Different ways to define the rainfall distribution

Time (hours)

Intensitycurve points PEasy to define

PGives smoothrunoff curve

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Sample Mass Curve FileThis is all you need to create a custom mass curve

timeunits=hoursduration=1

depthunits=inchesstormdepth=2.5

depth= 0 .020 .080 .200 .410 .625depth= .805 .915 .985 .995 1.000

1-hour duration

2.5 inch depth

Mass-curve points

For full details see HydroCAD “Sample B” file2-390

Sample Intensity Curve FileThis is all you need to create a custom curve

timeunits=hoursduration=1

depthunits=inchesstormdepth=2.5

inten= 0 .20 .60 1.2 2.1 2.15Inten= 1.8 1.1 .7 .1 .05

1-hour duration

2.5 inch depth

Intensity-curve points

For full details see HydroCAD “Sample A” file2-400

Changing the Storm DurationHydroCAD can rescale a storm to any duration

Original24-hourstorm

Time (hours)

Rescaled12-hourstorm

PStorm durationcan be changed

<But beware ofchange in peak

PDifferent rainfalldistribution may

be required!

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Tc Effects #1How does the Tc effect the runoff hydrograph?

Time (hours)0 6 12 18 24 30 36 42 48

P If Tc=0, runoff is immediate!< HydroCAD will accept Tc=0

PHydrograph has same timingas rainfall distribution< Peak runoff occurs at same time

as peak rainfall

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Tc Effects #2How does the Tc effect the runoff hydrograph?

Time (hours)0 6 12 18 24 30 36 42 48

PAs the Tc increases:< Unit Hydrograph is extended< Runoff occurs later

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Curve Number EffectsHow does the CN effect the runoff?

PLow CN values decrease the runoff volume< Runoff timing is not directly effected by CN

PHigh CN values increase the runoff volume< If CN=100 all precipitation will appear as runoff:

S '1000

CN&10 ' 0

Q'(P&.2 S )2

P% .8S

Maximum Potential Retention =

SCS Runoff Volume = 'P

NoRetention!

All precipis runoff!

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Rainfall on thesurface of a pondHow do we include thisprecipitation in the model?

P Include pond surface area in an adjacentsubcatchment.< OK for small pond areas

POr, model as separate subcat with:< Tc=0< CN=100< Produces “Runoff” equal to rainfall

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Adding HydrographsWhat happens when multiplehydrographs are combined?

7 Inflow #1

7 Inflow #2

T i m e

7 Combined FlowPPeak flows add

directly only if theyoccur at the same

time!PVolumes mustalways add-up

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Software ExercisesExploring these topics in HydroCAD

PBefore we begin the software demonstration, arethere any other questions about runoffcalculations?

P If you wish, you may perform the followingexercises yourself, or just watch the demo.

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Are You Ready?Before we begin the exercises...

P Is your computer logged-in?< The windows desktop should be visible

PCan you locate the HydroCAD shortcut?< HydroCAD icon should be on the desktop< Otherwise look in Start | Programs | HydroCAD

PDoes anyone need help?

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Subcatchment Setup #1Prepare a subcatchment toreceive our sample data

PGetting ready< Start HydroCAD< Open the “TR-20 Sample Project”

PCreate a subcatchment:< Drag a subcat from the palette

PEdit the subcatchment< Double-click the node -or-< Right-click and select Edit from the menu

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Subcatchment Setup #2Enter some sample data

PEnter a couple of sub-areas & curve numbers:< Click the “Area” tab< Enter an area and a CN value directly

– For example: Area=10 acres, CN=75< Enter a second area and lookup the CN value

– To open the lookup table, double-click the line– Select the desired CN value from the table

PA few notes about data entry tables< Right-click any table for options< Drag first column to change order of rows

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Subcatchment Setup #3Enter some sample data

PEnter some Tc values:< Click the “Tc” tab< Do a direct Tc entry

– Double-click a blank line– Select “Direct Entry”– Enter a TC value of 20 minutes– Click OK to save the TC data

PWe’ll examine the other Tc options later

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Subcatchment Setup #4View the results!

PClick “OK” to save the subcat dataPOpen a report window:< Double-click any described node -or-< Right-click and select “report”

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Subcatchment Setup #5View the results!

PExplore the report tabs:< Hydrograph

– Tabular view– Note maximum values in bold– Click “Shrink” to see more points

< Summary– Use summary to review results and input data– Click any line for help (new 7.1 feature)

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Rainfall Settings #1Global settings for the current project

PReview the calculation settings< Click the “Calculator” icon

– Note: You can leave any report windows open!

PRunoff methods< TR-20 / SCS UH< SBUH< Rational

P (We’ll return to the routing options later)

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Rainfall Settings #2Global settings for the current project

PRainfall data< Rainfall distribution & depth< Antecedent moisture condition< Rainfall duration< Back-to-back storms

PRainfall events< Define events as desired< -or- import events from another project

PFor Rational Method:< Can define Custom IDF curves

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Rainfall Settings #3Global settings for the current project

PTime Span & Time Increment< Adjustable in response to any warning messages< “Fast Hydrograph Plots” generally recommended

– Improves plot speed and appearance– Does not affect accuracy of calculations

PUnit Hydrograph selection< Changing the Peak Factor

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Rainfall Settings #4Global settings for the current project

PAdvanced Parameters< Initial Abstraction ratio< Used with new modeling techniques< (New feature added in version 7.1)

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Rainfall Settings #5Global settings for the current project

PAutomatic pipe & culvert sizing< Lots of options to explore on your own< Remember: Click HELP for more information

PClick “OK” to close the calculation settings< Repots are updated for new settings!

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*** End of Section ***

PBefore we move on, are there any other questionsabout subcatchments?

P If you performed the exercises, you may closeHydroCAD at this time.< You do NOT need to save your changes.

PWe will continue after a SHORT BREAK

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