Introduction to Surface Water Hydrology and Watersheds Lecture 1

1
Introduction to Surface Water Hydrology and
WatershedsLecture 1

• Philip B. Bedient
• Rice University
• November, 2000

2
The Hydrologic Cycle
3
Major Hydrologic Processes

• Precipitation (measured at rain gage)
• Evaporation or ET (loss to atmosphere)
• Infiltration (loss to subsurface)
• Overland Flow
• Stream Flow (measured at stage gage)
• Ground Water Flow

4
The Watershed or Basin

• Area of land that drains to a single outlet and
  is separated from other watersheds by a drainage
  divide.
• Rainfall that falls in a watershed will create
  runoff to that watershed outlet.
• All other rainfall falling outside a basin will
  not affect the runoff response.

5
Brays Bayou Watershed
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The Hydrograph

• Graph of Discharge vs Time at a Single Location
• Rising Limb, Crest Segment, Falling Limb,and
  Recession
• Base Flow is Usually Subtracted to yield DRO
• Peak Gives the Maximum Flow for the Event

7
The Hyetograph

• Graph of Rainfall Rate vs Time at a Single Gage
• Usually Plotted as a Bar Chart
• Net Rainfall is Found by Subtr. Infiltration
  Losses
• Integration of Net Rainfall in Time yields the
  Total Rainfall Vol (DRO) in inches over a
  Watershed

8
Runoff in an Urban Basin

• A portion becomes pipe flow (storm water).
• The remaining portion becomes overland flow in
  streets and yards.
• The total runoff is the sum of both components

Overland Flow
Hydrograph
Outflow
Pipe Flow (SWWM)
Pipe Flow
Time
9
Harris Gully Watershed
Brays Bayou
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Major Causes of Flooding(Excess Water that
Inundates)

• Highly Developed (urbanized) Area
• Intensity and Duration of Rainfall
• Flat Topography with Little Storage
• Poor Building Practices - Floodplains

11
Tailwater Effects
Ground surface
Bayou Water Level 3
HGL Surcharged Flow 2
HGL Surcharged Flow 1
Bayou Water Level 2
Top of Culvert
HGL Gravity Flow
Bayou Water Level 1
Flow
Bottom of Culvert
Incoming Culvert
Receiving Bayou
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Mannings Equation

• V Velocity of Flow, ft/s
• n Mannings Roughness Coefficient
• S Slope of Channel, ft/ft
• R A/P, where
• A Cross-sectional Area of Flow
• P Wetted Perimeter of Channel

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Backwater Calculation
z1 hp1 hL z2 hp2
Hydraulic Grade Line
hL
hp2
Flow
hp1
z2
z1
Datum (MSL)
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HEC-HMS Model

• Hydrologic Eng Center - Army Corps of Eng.
• Converts Input rainfall into runoff hydrographs
• Uses either historical data or design storms
• Predicts the total outflow hydrograph for a basin

Losses Removed
Losses Removed
Surface Runoff
Routing
Surface Runoff

Combination
Q
t
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HEC-HMS Theory

• Input Rainfall
• Loss Rate Function
• Unit Hydrograph
• Computes Runoff
• Flood Routing
• Combination Steps

Losses Removed
Losses Removed
Surface Runoff
Routing
Surface Runoff

Combination
Q
t
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Loss Rate MethodInitial and Uniform Loss Rate
Method

• Initial Amount Lost to Infiltration (in)
• Soil is Saturated.
• Uniform Loss of a Constant Rate (in/hr)

Example Initial Loss 0.5 in, Uniform Loss
0.05 in/hr
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Unit Hydrograph Theory

• The unit hydrograph represents the basin response
  to 1 inch of uniform net rainfall for a specified
  duration.
• Linear method originally devised in 1932.
• Works best for relatively small subareas.
• Several computational methods exist.

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Unit Hydrograph Method

• Snyders Method (1938)
• Clark TC R Method (1945)
• Nash (1958)
• SCS Method (1964)
• Espey-Winslow (1968)

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Surface Runoff TheoryClark TC R Method
Q
T
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Clark UnitHydrograph Computation
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Surface Runoff TheoryClark TC R Method
L channel length (mi) S slope of channel
(ft/mi) LCA length to centroid (mi) So
watershed slope (ft/mi)
So
channel
L1
S
outlet
LCA
L2
S
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Surface Runoff TheoryClark TC R Method

• TC Travel time of overland runoff from most
  remote point to the outlet
• R Routing Coefficient Relates Storage and
  Outflow

where L channel length (mi) S slope of
channel (ft/mi) LCA length to centroid (mi) D
0.94 for developed watersheds of Solt20
ft/mi So watershed slope (ft/mi)
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Effects of Stream Flow Routing
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Routing Theory 1Muskingum Method
A

• K Travel Time Through the Reach
• x Weighting Factor (Storage Coefficient)
• N Number of Steps in Computation

B
A
B
Outflow
Time
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Routing Theory 2Modified Puls Method
A

• Based on Mannings
• Storage vs Outflow
• Numerical Procedure
• Inflows Converted to Outflows from Reach

B
A
B
Outflow
Time
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Modified Puls (Storage)

• Storage-Indication Relationship
• Solved Numerically in Model

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Obtaining Storage-Discharge Data
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Combination StepSuperposition
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Design Rainfalls

• Design Storm from HCFCD and NWS
• Based on Statistical Analysis of Data
• 5, 10, 25, 50, 100 Year Events
• Various Durations

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Harris Gully Watershed
Brays Bayou
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Rainfall and Runoff Response
Flow Measured from USGS Gage 403 Inside Harris
Gully
Rainfall Measured from USGS Gage 400 at Harris
Gully Outlet
February 12, 1997
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Calibration Results
Average Errors Peak Flow 6 Volume
4 Peak Time 0.9 Hours
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Verification Results

• Volume Error Could Be Due to Unmeasured
  Quantities
• Calibration Successful

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Urban Basin - Low Flow
Rainfall Pattern
Inlets to Pipes
Pipe Elevations and Sizes
Bayou Level
Junction Locations
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Urban Basin - Flood
Flooding Areas
High Bayou Level
Pipe Capacity
Backflow at Outlet
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Harris Gully Models

• Determine Pipe Capacities at Six Different
  Tailwater Elevations
• DIVERT this Amount from Total Runoff Computed in
  HEC-HMS
• HEC-HMS Model Computes Total Runoff, Subtracts
  Amount Diverted to Pipes, Remaining is Overland
  Flow

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HEC-1 Diversion Operation
Total Runoff
Remaining Overland Runoff
Diversion to Pipes
capacity
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Harris Gully at Brays Bayou
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Overland Flow for Rising Brays Bayou
Rainfall 1.5 in/hr for 3 hours
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Detention Pond Locations
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Detention Pond Options

• 1) 50 Acre-ft in Hermann Park
• 2) 100 Acre-ft in Hermann Park
• 3) 100 Acre-ft in Hermann Park
• AND
• 50 Acre-ft on Rice Campus

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Pond Impact on Overland Volume
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HEC-HMS Output

• Tables
• Summary
• Detailed (Time Series)
• Hyetograph Plots
• Sub-Basin Hydrograph Plots
• Routed Hydrograph Plots
• Combined Hydrograph Plots
• Recorded Hydrographs - comparison

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Viewing Results
Hydrograph
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HEC-HMS Output

• Sub-Basin Plots
• Rainfall
• Hydrographs
• Abstractions
• Base Flow

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Conclusions for Harris Gully

• 67 Acre-ft Pond in Hermann Park AND 50 Acre-ft
  Pond at Rice University would have prevented 48
  of Overland Flow in March 1997 Storm
• Ponds are imperative for Flood Relief!