Section 2: Hydrology. Introduction. What is the Return Period? Intensity-Duration-Frequency Data. Generating a runoff hydrograph 85 Minutes
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1 Section 2: Hydrology Generating a runoff hydrograph 85 Minutes Press Space, PageDown, or Click to advance. Press PageUp to reverse. Esc to exit. Right-Click for other options. Copyright 2004 HydroCAD Software Solutions LLC All Rights Reserved - Duplication Prohibited Introduction What are we going to discuss? PWe will start with a review of some fundamental hydrologic techniques. PA basic understanding of these calculations will improve your ability to understand and interpret the results from any hydrology software. PPlease see other texts for full details, such as the HydroCAD Owner s Manual What is the Return Period? A common point of confusion! P Roughly speaking, the return period is the average time between occurrences of a given event, such as the 25-year storm. P To be more precise, a 25-year storm has a 4% probability of occurring in any given year. P Therefore, a 25-year storm can occur in two consecutive years! Intensity-Duration-Frequency Data A basic way to quantify rainfall observations. Typically compiled for each county in the US Frequency (years) Rainfall Duration (minutes) P An IDF curve indicates the rainfall intensity that will occur for a given duration and return period. P The rainfall intensity remains constant for the entire duration Typical IDF Curves 2-040
2 The Rational Method A basic equation for predicting peak runoff Q ' CIA PQ=Peak Runoff [CFS] PC=Runoff Coefficient Pi=Rainfall intensity PA=Catchment Area < The intensity is determined from the IDF curve, but what duration do we use? Setting the Rainfall Duration for the Rational Method A simple equation gets more complicated! P We must use the critical duration, which produces the highest runoff. P For a single, homogeneous subcatchment, the duration is typically equal to the time-ofconcentration, Tc. P But when several subcatchments are combined, the critical duration can be any value between the shortest and longest Tc Other Limitations of the Rational Method The SCS Rainfall Distributions Allowing the rainfall intensity to vary over time PThe Rational Method predicts peak runoff, but not total runoff volume. PThe Rational Method predicts a constant flow, rather than generating a complete hydrograph. PRational Method is poorly suited for volumesensitive calculations, such as detention ponds. < How do we overcome these limitations??? P A mass curve indicates the cumulative rainfall depth at any time during the storm
3 Features of Rainfall Distributions More Rainfall Distribution Features P Allows 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. P Each curve includes all events up to 24-hours. < Eliminates need to identify the critical duration! P Represents most of the US with just a few curves. < Curve is scaled for your local 24-hour rainfall depth. P Synthetic rainfalls provide design standards < Type II peak defined at 12 hours P Can also create an actual rainfall distribution < Allows modeling of observed events The SCS Runoff Equation How much of the precipitation appears as runoff? 2 (P&.2 S) Q ' (Q=0 if P<.2S ) P %.8S where S ' 1000 & 10 CN PQ=Precipitation excess (runoff) [inches or mm] PP=Cumulative precipitation [inches or mm] PS=Potential maximum retention [inches] PCN=SCS Curve Number Time-of-Concentration: What does it mean? Determining when the runoff occurs P The Tc is the time required for a particle of water to travel from the most hydrologically remote point in the watershed to the point of collection. P There 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 section 2-120
4 The SCS Dimensionless Unit Hydrograph Defines the runoff hydrograph for a single burst Unit Hydrograph Time Axis How long does the runoff last from a single burst? T p ' 5D and T p ' 2 3 T c ˆ D ' T c 7.5 < Tp=Time to peak < D=Burst duration < Tc=Time of concentration P Everything is based on the Tc UH Time Axis Example Just substitute the Tc! 0 If Tc = 30 minutes D = Tc/7.5 = 4 minutes Tp = 5D = 20 minutes Tb = 5Tp = 100 minutes Time (minutes) Unit Hydrograph Flow Axis What is the peak runoff from a single burst? q p ' C K A Q ' 484 A Q ' 484 A Q T p T 2 p T 3 c Pq p =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] 2-160
5 UH Flow Axis Example A Fully Dimensioned UH! Determine the flow for a single burst, with precipitation excess Q If Q= 0.2 inch and A = 0.1 sq-mile and Tc = 0.5 hours q p = 484 A Q / 2/3Tc = 29 cfs The actual runoff hydrograph from a single burst of rainfall PNow we have a complete hydrograph for one burst. PBut how do we get the hydrograph for an entire 24-hour storm? Time (minutes) Time (minutes) Convolution: The Heart of TR-20 Constructing the entire runoff hydrograph P The unit hydrograph (when dimensioned) tells us the runoff for a single burst of rainfall. P To determine the runoff for the entire storm, we must perform a convolution of the unit hydrograph with the precipitation excess. < We must break the storm into a sequence of individual bursts, and add up the hydrographs from all the bursts. Convolution: Step #1 Adding up the hydrographs for each burst 1) Divide storm into bursts D
6 Convolution: Step #2 Adding up the hydrographs for each burst Convolution: Step #3 Adding up the hydrographs for each burst Total Hydrograph 3) Add all UH 1) Divide storm into bursts D 2) Generate UH for each burst 1) Divide storm into bursts D 2) Generate UH for each burst Complete Runoff Hydrograph Why does the hydrograph look like this? Performing the Convolution A computation-intensive process 2) Runoff peaks after rainfall peak 1) Zero runoff for several hours 3) Runoff continues after end of storm 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!! P Calculation is not feasible by hand!
7 TR-55 and the Tabular Method TR-55 Warnings Doing runoff calculations by hand Quoting from the TR-55 manual P The TR-55 Tabular method allows manual calculation of a runoff hydrograph that approximates the TR-20 UH procedure. P TR-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 P Other values require approximation and results will differ from actual TR-20 results P This method (TR-55) approximates TR-20, a more detailed hydrograph procedure... Use TR-20 if the watershed is very complex or a higher degree of accuracy is required. P The procedure (TR-55) should not be used to perform final design if an error in storage of 25 percent cannot be tolerated... More detailed hydrograph development and routing will often pay for itself through reduced construction costs HydroCAD: An Easier TRS20 Solution Plus lots of other calculations and capabilities P In 1986, HydroCAD was the first PC-based program to use the full UH runoff procedure. P Eliminates approximations in the tabular method. P Also provides: < TR-55 Tc procedures < Hydraulics calculations < True hydrograph routing < Lots more! (As we ll see throughout the day) P But first, let s examine some basic concepts in more detail What is a hydrograph? The flow of water over time (Flow vs. Time) T i m e A hydrograph is ideally a smooth curve
8 But how do we represent the hydrograph curve numerically? Use a series of points spaced equally in time What is the peak flow? HydroCAD uses an interpolated peak, just like TR-20 P True peak may fall between points... P So interpolate using three highest points T i m e The time increment is called dt T i m e Allows for variations in point timing What time increment should I use? The time increment (dt) specifies the point spacing P For 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 P A warning will occur if dt is too large < This will occur if Tc < 0.1 hours P Dt can be as small at 0.01 hours < For fastest operation, don t go smaller than required What time span should I use? Long enough to cover all flow of interest! Default time span 5-20 hours Use longer span if entire volume is required! T i m e (hours) 2-320
9 Rational Method hydrographs Time for a quick stretch! A shorter time span and time increment are required Default dt = 0.01 hours P Please have a quick stretch at your desk P We ll have a full break after this section 0 Tc D D+Tc D+2Tc Default time span = 0-3 hours Rainfall Curves #1 Different ways to define the rainfall distribution Rainfall Curves #2 Different ways to define the rainfall distribution True Rainfall Mass Curve P The actual rainfall distribution is a curve P How do we represent this in HydroCAD? Fitted Mass Curve P Uses a polynomial fit to each segment for a true curve P Used for standard SCS rainfalls P For details see Start Programs HydroCAD Rainfall Info
10 Rainfall Curves #3 Different ways to define the rainfall distribution Rainfall Curves #4 Different ways to define the rainfall distribution Mass curve points P Easy to define P Line segments may cause runoff steps Intensity curve points P Easy to define P Gives smooth runoff curve P Smoothing option gives better curve Sample Mass Curve File Sample Intensity Curve File This is all you need to create a custom mass curve This is all you need to create a custom curve timeunits=hours duration=1 1-hour duration timeunits=hours duration=1 1-hour duration depthunits=inches stormdepth= inch depth depthunits=inches stormdepth= inch depth depth= depth= Mass-curve points For full details see HydroCAD Sample B file inten= Inten= Intensity-curve points For full details see HydroCAD Sample A file 2-400
11 Changing the Storm Duration HydroCAD can rescale a storm to any duration Tc Effects #1 How does the Tc effect the runoff hydrograph? Rescaled 12-hour storm Original 24-hour storm P Storm duration can be changed < But beware of change in peak P Different rainfall distribution may be required! P If Tc=0, runoff is immediate! < HydroCAD will accept Tc=0 P Hydrograph has same timing as rainfall distribution < Peak runoff occurs at same time as peak rainfall Tc Effects #2 How does the Tc effect the runoff hydrograph? PAs the Tc increases: < Unit Hydrograph is extended < Runoff occurs later Curve Number Effects How 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: Maximum Potential Retention = S ' 1000 & 10 ' 0 CN No Retention! SCS Runoff Volume = Q' (P&.2 S)2 P P %.8S ' All precip is runoff! 2-440
12 Rainfall on the surface of a pond How do we include this precipitation in the model? P Include pond surface area in an adjacent subcatchment. < OK for small pond areas P Or, model as separate subcat with: < Tc=0 < CN=100 < Produces Runoff equal to rainfall Adding Hydrographs What happens when multiple hydrographs are combined? 7 Combined Flow 7 Inflow #2 7 Inflow #1 P Peak flows add directly only if they occur at the same time! P Volumes must always add-up T i m e Software Exercises Exploring these topics in HydroCAD P Before we begin the software demonstration, are there any other questions about runoff calculations? P If you wish, you may perform the following exercises yourself, or just watch the demo. Are You Ready? Before we begin the exercises... P Is your computer logged-in? < The windows desktop should be visible P Can you locate the HydroCAD shortcut? < HydroCAD icon should be on the desktop < Otherwise look in Start Programs HydroCAD P Does anyone need help?
13 Subcatchment Setup #1 Prepare a subcatchment to receive our sample data P Getting ready < Start HydroCAD < Open the TR-20 Sample Project P Create a subcatchment: < Drag a subcat from the palette P Edit the subcatchment < Double-click the node -or- < Right-click and select Edit from the menu Subcatchment Setup #2 Enter some sample data P Enter 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 P A few notes about data entry tables < Right-click any table for options < Drag first column to change order of rows Subcatchment Setup #3 Enter 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 Subcatchment Setup #4 View the results! P Click OK to save the subcat data P Open a report window: < Double-click any described node -or- < Right-click and select report
14 Subcatchment Setup #5 View the results! P Explore 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) Rainfall Settings #1 Global settings for the current project P Review the calculation settings < Click the Calculator icon Note: You can leave any report windows open! P Runoff methods < TR-20 / SCS UH < SBUH < Rational P (We ll return to the routing options later) Rainfall Settings #2 Global settings for the current project P Rainfall data < Rainfall distribution & depth < Antecedent moisture condition < Rainfall duration < Back-to-back storms P Rainfall events < Define events as desired < -or- import events from another project P For Rational Method: < Can define Custom IDF curves Rainfall Settings #3 Global settings for the current project P Time 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 P Unit Hydrograph selection < Changing the Peak Factor
15 Rainfall Settings #4 Global settings for the current project Rainfall Settings #5 Global settings for the current project P Advanced Parameters < Initial Abstraction ratio < Used with new modeling techniques < (New feature added in version 7.1) P Automatic pipe & culvert sizing < Lots of options to explore on your own < Remember: Click HELP for more information P Click OK to close the calculation settings < Repots are updated for new settings! *** End of Section *** P Before we move on, are there any other questions about subcatchments? P If you performed the exercises, you may close HydroCAD at this time. < You do NOT need to save your changes. P We will continue after a SHORT BREAK 2-590
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