CWR 4101 Hydrograph Generation Synthetic Hydrographs Chapter 6

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CWR 4101 Hydrograph Generation Synthetic
Hydrographs 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 Synthetic Hydrograph

• Definitions
• Types of Synthetic Hydrographs
• Rational Method
• NRCS or SCS Method
• Clark Unit Graph
• Santa Barbara

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Synthetic Hydrograph

• Definition Synthetic Hydrograph is a plot of
  flow versus time and generated based on a minimal
  use of streamflow data.
• Example A pending land use change and the
  resulting runoff hydrograph is thus unknown, but
  nevertheless must be estimated.

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Objective Determine the Surface Runoff Hydrograph
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• The Rational Method Hydrograph

where C Runoff coefficient i Intensity
(in/hr) A Watershed area (acre)
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Assumptions using the Rational Method Triangular
Hydrograph

• D gt tc
• Constant rainfall intensity
• Product of CA is linear with time, both during
  and after the rain or (on rising and recession
  limbs)
• As such method is reasonable for small
  homogeneous watersheds.
• Qp CiA at tc and
• Q (CA)t (i) for all t lt tc

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Rational Method Hydrograph

• Rising limb falling limb
• Area under hydrograph Area under hyetograph

Area 12 acres, i4in/hr Tc 30 minutes Vol of
Rain Vol of Runoff Rain Vol 4in/hr (30/60)
2 in Runoff Vol 87,100 CF or 2 in Vol rain
(CF) Vol runoff (CF) (C)(i)(A)(1.008)(D)
(tb)(Qp)/2 But D tb/2 and time in seconds Qp
1.008CiA
i4in/hr
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• The SCS (NRCS) Hydrograph - Typical

where 0.75 attenuation factor C Runoff
coefficient i Intensity (in/hr) A
Watershed area (acre)
NOTE if A is in mi2, the attenuation factor
would be 484.
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Typical SCS Hydrograph
Area 12 acres, i4in/hr Tc 30 minutes Vol of
Rain Vol of Runoff Rain Vol 4in/hr (30/60)
2 in Runoff Vol 87,100 CF or 2 in Vol rain
(CF) Vol runoff (CF) (C)(i)(A)(D)
(2.67)(tp)(Qp)/2 But D tp and time in
seconds Qp 0.75CiA
tpD 1.67tp
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• The SCS (NRCS) Hydrograph - Typical

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• The SCS (NRCS) Hydrograph - Typical

(File Table 6-7.xls sheet 1)
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Problem 4 (page 249) of 6.6.1 Hand Problems
Calculate the peak runoff from a residential area
with similar watershed soil and surface
characteristics. The area is 20 ac in size with
40 of imperviousness. Use a rainfall intensity
of 3 in./hr for 1 hr. Do the calculations by
using the rational formula and the SCS (NRCS)
typical hydrograph procedure. Compare results
and discuss assumptions. The pervious area does
not contribute to runoff.
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Problem 4 (page 249) of 6.6.1 Hand Problems
(continued)
It can be seen that the rational method yields a
greater peak, while the SCS typical method
lengthens the recession limb and yields a lower
peak flow. The volume of runoff in each case
would be the same
(File Section 6-6-1 Hand Problem No 4.xls sheet 1)
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• The SCS (NRCS) Hydrograph - General

where K 2/(1x) attenuation factor C
Runoff coefficient i Intensity (in/hr) A
Watershed area (acre)
NOTE The attenuation factor K is given in
Table 6.6 on page 213
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• The SCS (NRCS) Unit Hydrograph

• For large watersheds, time of concentration tc ??
  duration (D) of constant rainfall intensity
• Rainfall cannot last long enough that the peak
  flow, Qp, will occur at time tc
• Instead, the peak flow, Qp, will occur at time
  tp, which is a function of rainfall duration D
  and the watershed characteristics represented by
  tc

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• The SCS (NRCS) Unit Hydrograph

where 2/4.33 attenuation factor D Rainfall
duration i Intensity (in/hr) A Watershed
area (acre)
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• The SCS (NRCS) Unit Hydrograph

where R Rainfall excess and
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• The SCS (NRCS) Unit Hydrograph

Now you can do problem 19 on page 252
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• The SCS (NRCS) Unit Hydrograph

Example Problem 6.4 (page 219) For an actual
drainage basin with data shown in Table 6.8,
compute a unit hydrograph using the typical SCS
hydrograph shape (K484).
tc 55 min 0.92 hr, A 270 acre 0.42 mi2, K
484L 0.6 tc 0.55 hr, Assume D 0.5L
0.28 hr ? 0.25 hr
qp 484 x 0.42 x 1/0.68 298.94 ? 300 cfs
(File Table 6-9.xls sheet 1)
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(File Table 6-7.xls sheet 3)
(File Table 6-9.xls sheet 1)
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• Clark Unit Hydrograph

• Develop a time area (TA) curve
• Route the time area curve through a linear
  reservoir with a Clark routing parameter

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Develop a time area (TA) curve
(File Table 6-10.xls sheet 1)
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(File Table 6-10.xls sheet 1)
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Routing the time area curve
where ?t time step size (hr), R Clark
routing parameter (hr) c linear routing
coefficient IUHi the i-th increment of the
instantaneous unit hydrograph
where UHi the i-th increment of the unit
hydrograph
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Example Problem 6.5 (page 223). A 15-mi2
watershed in the western part of the United
States has a time of concentration of 7 hr. If
the Clark storage coefficient R is estimated to
be 8 hr, calculate the unit hydrograph
(File Table 6-11.xls sheet 1)
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(File Table 6-11.xls sheet 1)
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(File Table 6-11.xls sheet 2)
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• Santa Barbara Urban Hydrograph

• Compute rainfall excess for each Dt note this
  will be a function of pervious and impervious
  areas.
• Convert rainfall excess to instant hydrograph,
  I(Dt)
• SBUH is obtained by routing

where
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(File Table 6-12.xls sheet 1)
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(File Table 6-12.xls sheet 1)
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CWR 4101 Hydrograph Generation Synthetic
Hydrographs Chapter 6
Synthetic Hydrographs Methods Rational NRCS or
SCS Clark Unit Graph Santa Barbara