CWR 4101 Hydrograph Generation Streamflow and Unit Graph Chapter 6

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CWR 4101 Hydrograph Generation Streamflow and
Unit Graph Chapter 6
Dr. Marty Wanielista 407.823.4144 wanielis_at_mail.uc
f.edu www.stormwater.ucf.edu http//classes.cecs.u
cf.edu/CWR4101/wanielista
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Topics Chapter 6 Hydrograph

• Definitions
• Separation of Streamflow hydrograph
• Construction of Unit Hydrograph from Streamflow
  Hydrograph
• Convolution for Hydrographs

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Hydrograph

• Definition a plot or listing that relates flow,
  velocity, stage or some other hydrologic
  characteristics of water to time. Most commonly
  refers to a plot of flow versus time.
• Streamflow hydrograph is a combination of the
  runoff, base flow and interflow hydrographs.

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Streamflow Hydrograph
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Runoff Hydrographs

• Effect of Watershed Characteristics on a
  hydrograph, see Figure 5.1 0n page 137
• Some Examples in Florida
• Urban, typical rational shape
• Rural, depends on slope, soils, resistance, and
  storage, to mention most important ones

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Surface Runoff Hydrograph
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• Lag time, L
• Time to peak, tp
• Time of concentration tc
• Recession time tr
• Time base tb

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Stream Flow Hydrographs
Volume of surface water
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Mathematical Description - Rising Limb
Q

• IN - OUT Accumulation
• r - Q dS/dt
• and dQ/dS K
• or dS/dt dQ/(Kdt)
• r - Q dQ/(Kdt)
• Integration
• Q(t) r(1-e-KT) (6.5)

K
S
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Mathematical Description - Recession Limb

• IN - OUT Accumulation (no r )
• - Q dS/dt
• and dQ/dS K
• or dS/dt dQ/(Kdt)
• -Q dQ/(Kdt)
• Integration from Qp to zero
• Q(t) Qp (e-KT ) for tgttp (6.9)
• (NOTE) K on recession limb not the same as the K
  on the rising limb

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Separation of Stream Flow Hydrograph

• Logarithm recession method
• Straight line method
• Fixed time method

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Given Stream Flow Data
Plot of Existing data
Peak
A
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Spread Sheet Solution
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Log CurveRecession Method
E (0.32)
C
D (0.155)
B
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• Logarithm recession method

• Plot normalized stream flow hydrograph in
  semi-log paper
• Fixed point A on the rising limb as the point the
  hydrograph starts to rise
• Locate points B and C on the recession limb
• Extent the straight lines from points B and C to
  intersect the vertical line through the peak at D
  and E, respectively
• Connect AD and AE.
• Obtain Qb/Qp from the straight lines AE and EC
  with linear interpolation of logarithm values
• Obtain Qb/Qp from the straight lines AE and EC
  with a linear interpolation of the logarithm
  values
• Obtain Qd/QpQ/Qp Qb/Qp

Now you can do problem 24 on page 254
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• Straight line method

A
B
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• Straight line method

• Plot stream flow hydrograph in regular paper
• Identify point B on the stream flow hydrograph
• Locate A at the point the stream flow starts to
  increase
• Connect point A and B
• Determine slope of line AB
• Calculate base flows at various times greater
  that the time at QA

7. Runoff flow is difference between
streamflow and base flow
Q runoff (t) Q streamflow (t) Q baseflow (t)
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• Fixed time method

A
G
F
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• Fixed time method for hydrograph separation

• Plot stream flow hydrograph
• Select a fixed point A hydrograph before it
  starts to rise
• Draw a straight line to intersect the vertical
  line passing the peak flow to fix point F
• Compute a tr with
• either at the inflection point on the recession
  limb
• or with an empirical formula see below
• Draw a vertical line with tr units away from the
  peak vertical to intersect the recession limb to
  fix point G.
• Obtain Qb from the straight lines AF and FG
• Obtain Runoff Q Qs - Qb

tr in days, DA in square miles, and n ? 0.2
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Construction of Unit Hydrograph fromStream Flow
Hydrograph due to a Uniform Rainfall Excess of
Fixed Duration

• What is it? The hydrograph from 1 inch of
  rainfall excess over the entire watershed.
• Obtain surface runoff hydrograph
• Compute the volume of surface runoff
• Divide the volume by the watershed area with
  answer in inches, which is a scale factor.
• Obtain the unit hydrograph by multiplying the
  surface runoff hydrograph with the scale factor

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Suppose the following surface runoff hydrograph
is obtained from a stream hydrograph due to a
5-hour storm over a watershed of 12 mi2, what
would be the unit hydrograph (UG)?
STREAMFLOW

• Volume of Surface Runoff is 10,881 cfs-hr
  39,171,600 CF
• So rainfall excess is 39,171,60012/(1264043560)
  1.41 in.
• Therefore the unit hydrograph is obtained by
  dividing all ordinates by 1.41 as given in the
  following slide.

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unit hydrograph of a five-hour storm
(see problem 30) note UG is one inch Rainfall
Excess
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Having constructed a five-hour unit hydrograph,
now what would be the stream hydrograph resulting
from a 10 hour storm with 0.5 in rainfall excess
for the first five hours and 1.0 in of rainfall
excess for the second five hours?
Method of Convolution Summation
2 UGs displaced by 5 hours

• Qt discharge at time t
• Rj Constant rainfall excess of the j-th
  interval of duration D
• N maximum integer such that (N-1) x D ? t
• Ut-(j-1)D ordinate of unit hydrograph at time
  t (j-1)D
• For example, Q9 R1 x U9 R2 x U9-5 0.5 x
  645.39 1.0 x 283.69 606.38 cfs.

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Composite Hydrograph
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Method of Convolution Summation Special Case

• When t i x D, the convolution summation become

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IN General
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Construction of Unit Hydrograph fromStream Flow
Hydrograph due to a Variable Rainfall

• Given the rainfall, obtain rainfall excess using
  NRCS methods, Horton, or Green-Ampt infiltration
  method
• Given stream flow data, obtain surface runoff
  hydrograph using base-flow separation methods
• Use the convolution summation method to obtain
  unit hydrograph

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(File Figure Example Problem 6-3 on page 203.xls
sheet 1)
Now you can do problem 31 on page 255
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(File Figure Example Problem 6-3 on page 203.xls
sheet 2)
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(File Figure Example Problem 6-3 on page 203.xls
sheet 2)
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CWR 4101 Hydrograph Generation Streamflow and
Unit Graph Chapter 6
Separation of Stream Flow Hydrographs Construction
of Unit Hydrograph from Stream Flow
Hydrograph Convolution of unit hydrographs