Hydrologic Analysis

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Hydrologic Analysis

• Dr. Phil Bedient
• Rice University

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Mansfield Dam - Lake Travis
Unit Hydrographs S Curve Method Synthetic
UH Multi-period storms UH Applications Kinematic
Wave
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Hydrograph computed at outlet of each subarea
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Hydrographs routed to the outlet of the
watershed
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Hydrographs routed thru one reach of the watershed
I Q dS/dt
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Distribution of Rainfall

• Infiltration
• Detention    Storage
• Depression    Storage
• Direct Runoff

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Hydrograph Relations
Net Rainfall Rising Limb Crest Falling
Limb Recession Base Flow Direct Runoff DRO
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Base Flow Methods

• Straight Line
• ABC continue the recession - concave
• Empirical method

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Infiltration Loss Curves

1. Horton Method
2. F Method
3. Initial and Constant Loss

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Typical Rainfall and Hydrograph Response
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Time-Area Method

• Watershed Isochrone
• Hyetographs
• Time Area Histograms
• Add and Lag Method
• Resulting Hydrograph

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Time Area Hydrograph

• P1 A1
• P2A2 P1A2
• P3A1 P2A2    P1A3

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Unit 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
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Measured Hydrograph Adjusted for Net RF
2 inches
Adjusted for Net Rainfall Of 1 Inch
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Base Flow and Infiltration

• Subtract BF
• Subtract Infiltration
• Determine Net Rain
• Integrate hydrograph
• Adjust hydrograph ordinates to match net rainfall
  Vol
• Determine UH

Net rainfall
Infiltration
Peak Flow
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D 2-Hr Unit Hydrograph
Adjusted Net Rainfall one inch over basin
Qp
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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

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Equilibrium Hydrograph
Uniform Rain Inflow Outflow Equiv to summing n
  hydrographs in T Produces S curve
One Hydrograph
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S-Curves for UH
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S-Curves

• Convert 2 hr Unit   hydrograph to 3-hr
• Lag each 2-hr UH   by Duration D
• Add to produce S   curve

S-curve
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Lag S Curves in time, subtract them, and
multiply by D/D
Lag by D and subtract both curves
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3 hr. Unit Hydrograph from a 2 hr. UH (D/D)
3 HR UH
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Hydrograph Convolution
Add and Lag Method
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1
2
1
2
STORM HYDRO
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Add up the ordinates of all three to produce
storm hydrograph
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NET RF
Multi-Period Storm

• 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
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Development Effects on UH
Natural
Partial
Fully Developed
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Actual Brays Bayou UH
Faster response with Development Higher peak
flow with Development Need to provide
down- Stream capacity
Ultimate
Present
1959
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Synthetic UH
Developed 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
Ultimate
Present
1959
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Synthetic 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
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Synthetic UH Methods

• Snyders Method (1938)
• Clark Method (1945)
• Nash (1958)
• SCS (1964, 1975)
• Espey-Winslow (1968)
• Kinematic Wave (1970s)

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Snyders Method
5 to 7 points
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SCS Methods
Triangular UH
Dimensionless UH
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SCS Triangular UH Example 1 inch of RF over 1.6
hr duration
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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
400
1.5 Inches Total Rain
300
200
100
2 4 6 8 10 12 14 16 18 20 22
24 26 30
Time in Hours
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SCS RAINFALL RUNOFF CURVES
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Unit Hydrograph Applications
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Land Use will alter UH Peak and Timing
Land Use 1
Land Use 2
Unit Hydrograph Changes
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Kinematic Wave
Overland Collector Channels Main
Stream Mannings Eqn
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Kinematic Wave
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Basin hydrographs- routed combined  to
produce storm hydrograph