Transcript Lecture 3 – CH 2 - Rice University

Hydrologic
Analysis
Dr. Phil Bedient
Rice University
Mansfield Dam - Lake Travis
Unit Hydrographs
S Curve Method
Synthetic UH
Multi-period storms
UH Applications
Kinematic Wave
Hydrograph computed at outlet of
each subarea
Hydrographs
routed to the
outlet of the watershed
Hydrographs routed thru one reach of
the watershed
I – Q = dS/dt
Distribution of Rainfall
• Infiltration
•
Detention
e
•
Depression
ge
Storag
Stora
• Direct Runoff
Hydrograph Relations
Net Rainfall
Rising Limb
Crest
Falling Limb
Recession
Base Flow
Direct Runoff DRO
Base Flow Methods
•
Straight Line
•
ABC continue the
recession - concave
•
Empirical method
Infiltration Loss Curves
1. Horton Method
2. F Method
3. Initial and
Constant Loss
Typical Rainfall and
Hydrograph Response
Time-Area
Method
• Watershed Isochrone
• Hyetographs
• Time Area Histograms
• Add and Lag Method
• Resulting Hydrograph
Time Area Hydrograph
• P1 * A1
• P2*A2 + P1*A2
• P3*A1 + P2*A2
+
P1*A3
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
Measured Hydrograph
Adjusted for Net RF
2 inches
Adjusted for
Net Rainfall
Of 1 Inch
Base Flow and Infiltration
Net rainfall
•
Subtract BF
Infiltration
•
Subtract Infiltration
•
Determine Net Rain
•
Integrate hydrograph
•
Adjust hydrograph
Peak Flow
ordinates to match
net rainfall Vol
•
Determine UH
D = 2-Hr Unit Hydrograph
Adjusted Net Rainfall
one inch over basin
Qp
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
Equilibrium Hydrograph
Uniform Rain
Inflow = Outflow
Equiv to summing
n
One Hydrograph
hydrographs
in T
Produces S curve
S-Curves for UH
S-Curves
S-curve
• Convert 2 hr
Unit hydrograph to
3-hr
• Lag each 2-hr
UH by Duration D
• Add to produce
S curve
Lag S Curves in time, subtract them,
and multiply by D/D’
Lag by D’ and subtract both curves
3 hr. Unit Hydrograph
from a 2 hr. UH (D/D’)
3 HR UH
Hydrograph Convolution
1
2
Add and Lag Method
3
1
2
STORM
HYDRO
3
Add up the ordinates of all three to produce storm hydrograph
Multi-Period
Storm
• Pg - F = Net Rainfall
NET RF
UH
• RF units =
UH
duration (i.e. 1
hour)
• Adjust UH by Net RF
• Add and Lag UH
• Produce Storm
Hydrograph
STORM
Development Effects on UH
Natural
Partial
Fully Developed
Actual Brays Bayou UH
Faster response with
Ultimate
Development
Higher peak flow with
Present
1959
Development
Need to provide downStream capacity
Synthetic UH
Developed for basin
Ultimate
that were ungaged
Based on data from
Present
1959
similar gaged basins
Most methods are very
similar in nature
Revolutionized ability to
predict hydro response
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
Synthetic UH Methods
• Snyder’s Method (1938)
• Clark Method (1945)
• Nash (1958)
• SCS (1964, 1975)
• Espey-Winslow (1968)
• Kinematic Wave (1970s)
Snyder’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
400
300
Six Hours of Uniform Rainfall = 0.25 in/hr
1.5 Inches Total Rain
200
100
2 4 6 8 10 12 14 16 18 20 22 24 26 30
Time in Hours
SCS RAINFALL RUNOFF CURVES
Unit Hydrograph Applications
Land Use 1
Land Use will alter
UH Peak and
Timing
Land Use 2
Unit Hydrograph Changes
Kinematic Wave
Overland
Collector
Channels
Main Stream
Manning’s Eqn
Kinematic Wave
Basin hydrographs- routed &
combined to produce storm
hydrograph