Dr. Phil Bedient 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    Storage Depression    Storage Direct Runoff

Equilibrium Hydrograph Uniform Rain Inflow = Outflow Equiv to summing n hydrographs in T Produces S curve

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 Horton Method F Method 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 Method Measured Hydrograph Adjusted for Net RF Adjusted for Net Rainfall Unit Hydrograph Method

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

2-Hour Unit Hydrograph Net Rainfall 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

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 And Subtract Lag by D’ and subtract both curves

3 hr. Unit Hydrograph from a 2 hr. UH (D/D’) 3 HR UH

Hydrograph Convolution Add and Lag Method 1 2 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 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

Development Effects on UH Natural Partial Fully Developed

Actual Brays Bayou UH Faster response with Development Ultimate Higher peak flow with Need to provide down- Stream capacity Ultimate Present 1959

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 Six Hours of Uniform Rainfall = 0.25 in/hr 400 1.5 Inches Total 300 200 100 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 will alter UH Peak and Timing Land Use 1 Land Use 2 Unit Hydrograph Changes

Kinematic Wave Overland Collector Channels Main Stream Manning’s Eqn

Kinematic Wave

Unit Hydrograph Methods Linear Methods Tested in numerous WS Stood the test of time Regional calibration Still most used approach Used to compute a storm hydrograph for a subbasin

Basin hydrographs- routed & combined  to produce storm hydrograph