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Hydrologic Hydrologic AnalysisAnalysis
Dr. Phil BedientDr. Phil Bedient
Rice UniversityRice University
Mansfield Dam - Lake Mansfield Dam - Lake
TravisTravis Unit HydrographsS Curve Method Synthetic UHMulti-period stormsUH Applications Kinematic Wave
Hydrograph computed at Hydrograph computed at outlet of each subareaoutlet of each subarea
Hydrographs routed to Hydrographs routed to the outlet of the watershedthe outlet of the watershed
Hydrographs routed thru one Hydrographs routed thru one reach of the watershedreach of the watershed
I – Q = dS/dt
Distribution of RainfallDistribution of Rainfall
• Infiltration
• Detention
Storage
• Depression
Storage
• Direct Runoff
Net Rainfall
Rising Limb
Crest
Falling Limb
Recession
Base Flow
Direct Runoff DRO
Hydrograph RelationsHydrograph Relations
Base Flow MethodsBase Flow Methods
• Straight Line
• ABC continue the
recession - concave
• Empirical method
1. Horton Method
2. Method
3. Initial and
Constant Loss
Infiltration Loss CurvesInfiltration Loss Curves
Typical Rainfall and Typical Rainfall and Hydrograph ResponseHydrograph Response
Time-Area Time-Area MethodMethod
• Watershed Isochrone
• Hyetographs
• Time Area Histograms
• Add and Lag Method
• Resulting Hydrograph
Time Area HydrographTime Area Hydrograph
• P1 * A1
• P2*A2 + P1*A2
• P3*A1 + P2*A2 +
P1*A3
Unit HydrographUnit Hydrograph
Linear Methods
Tested in numerous WS
Stood the test of time
Regional calibration
Still most used approach
Basin response from 1 inch of uniform rainfall of given duration, D
Measured Hydrograph
Adjusted for Net RF
Adjusted forNet RainfallOf 1 Inch
2 inches
Base Flow and InfiltrationBase Flow and Infiltration• Subtract BF
• Subtract Infiltration
• Determine Net Rain
• Integrate hydrograph
• Adjust hydrograph
ordinates to match
net rainfall Vol
• Determine UH
Infiltration
Net rainfall
Peak Flow
D = 2-Hr Unit HydrographD = 2-Hr Unit Hydrograph
Adjusted Net Rainfall one inch over basin
Qp
Change UH DurationChange UH Duration
Consider 1 hr UH
Add and Lag two UH
by one hour
Sum and divide by 2
Results in 2 hr UH
Equilibrium HydrographEquilibrium HydrographUniform Rain
Inflow = Outflow
Equiv to summing n
hydrographs in T
Produces S curveOne Hydrograph
S-Curves for UHS-Curves for UH
S-CurvesS-Curves
• Convert 2 hr Unit
hydrograph to 3-hr• Lag each 2-hr UH
by Duration D• Add to produce S
curve
S-curve
Lag by D’ and subtract both curves
Lag S Curves in time, subtract them, and multiply by D/D’
3 hr. Unit Hydrograph 3 hr. Unit Hydrograph from a 2 hr. UH (D/D’)from a 2 hr. UH (D/D’)
3 HR UH
Hydrograph Hydrograph ConvolutionConvolution
1
2
3
31 2
Add up the ordinates of all three to produce storm hydrograph
STORMHYDRO
Add and Lag Method
Multi-Period Multi-Period StormStorm
• Pg - F = Net Rainfall
• RF units = UH
duration (i.e. 1 hour)
• Adjust UH by Net RF
• Add and Lag UH
• Produce Storm
Hydrograph
UH
STORM
NET RF
Development Effects on UHDevelopment Effects on UH
Natural Partial Fully Developed
Actual Brays Bayou UHActual Brays Bayou UHFaster response with
Development
Higher peak flow with
Development
Need to provide down-
Stream capacity1959
Present
Ultimate
Synthetic UHSynthetic UHDeveloped for basin
that were ungaged
Based on data from
similar gaged basins
Most methods are very
similar in nature
Revolutionized ability to
predict hydro response
1959
Present
Ultimate
Synthetic UH MethodsSynthetic UH Methods
Methods to characterize ungaged basins
Use data and relationships developed from gages
Variety of approaches but most based on Tp and Qp
England and U.S. led the efforts to standardize UH
Synthetic UH MethodsSynthetic UH Methods
• Snyder’s Method (1938)
• Clark Method (1945)
• Nash (1958)
• SCS (1964, 1975)
• Espey-Winslow (1968)
• Kinematic Wave (1970s)
Snyder’s MethodSnyder’s Method
5 to 7 points
SCS Methods
Triangular UH
Dimensionless UH
SCS Triangular UH Example
1 inch of RF over 1.6 hr duration
2-hr Duration UH Example
Qp at 400 cfs at 10 hr with 24 hr Tb
Six Hours of Uniform Rainfall = 0.25 in/hr
1.5 Inches Total Rain
2 4 6 8 10 12 14 16 18 20 22 24 26 30
100
200
300
Time in Hours
400
SCS RAINFALL RUNOFF CURVES
Unit Hydrograph Applications
Land Use 1
Land Use 2
Unit Hydrograph Changes
Land Use will alter UH Peak and Timing
Kinematic Wave
Overland
Collector Channels
Main Stream
Manning’s Eqn
Kinematic Wave
Basin hydrographs- routed & Basin hydrographs- routed & combined to produce storm combined to produce storm hydrograph hydrograph