Hydrologic Hydrologic AnalysisAnalysis

CIVE 6361CIVE 6361

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

hydrographhydrograph