CWR4101: Hydrology

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CWR4101 Hydrology
Mr. Jeff Earhart Hydrographs http/classes.cecs
.ucf.edu/cwr4101/shagen
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Hydrograph

• Definition a plot that shows stage, flow,
  velocity or some other characteristics of water
  with respect to time.

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Hydrograph for a Draining Tank

• Flow from a tank without losses

Area Ao
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Hydrograph for a Draining Tank

• Hydrograph of the water that has drained from the
  tank

Q
0
0
t
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Hydrograph for a Draining Tank

• The volume of water that has drained from tank

• Apply Conservation of Energy, or, in this case

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Hydrograph for a Draining Tank

• This reduces to

• or

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Hydrograph for a Draining Tank

• From continuity

Substitute this expression for V2 into previous
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Hydrograph for a Draining Tank

• Now consider the flow from the viewpoint of the
  top of tank

where the surface area of the tank
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Hydrographs

• Watershed Characteristics from a hydrograph, see
  figure 5.1

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Stream flow Hydrographs
Volume of surface water
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t
Intensity(in/hr)
D duration tp time to peak flow L lag
time tc time of concentration tr recession
time tb base time tptr
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Isolate Surface Water Runoff

• Estimation of losses
• SCS Curve Number method
• Green Ampt Model
• Hortons Equation

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Estimation of losses Ø index

• Constant rate of abstractions will yield excess
  rainfall with a total depth equal to the depth of
  direct runoff, rd, as averaged over the entire
  watershed.

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Estimation of losses Ø index

• where
• rd average depth of direct runoff over the
  entire watershed
• Rm observed rainfall in time interval m
• constant rate of abstraction
• Dt time interval
• M number of times that actually contribute to
  direct runoff.

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Estimation of losses Ø index

• Ø and M are adjusted until rd the depth of
  excess rainfall (see Applied Hydrology, by Chow).

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Synthetic Hydrographs

• How can we improve how well a hydrograph
  represents our watersheds response?
• Determine losses as accurately as possible
• Record many storms, making sure they are
  independent, and average
• Make sure storms produce a significant runoff

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Rational Method for Synthetic Hydrograph
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Rational Method

• where
• C Runoff coefficient
• i Intensity (in/hr)
• A Watershed area

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Rational Method Assumptions

• Constant intensity ? time of concentration
• Qp _at_ tc
• Constant runoff coefficient during storm
• No change in watershed area during storm
• As a result, this method is appropriate for tclt
  20 minutes.

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Rational Method

• The runoff coefficient, C, is commonly defined as
  the ratio of peak rate of direct runoff to
  average intensity.
• Also defined as the ratio of runoff to rainfall
  over a given time period.

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SCS Unit Hydrograph
where K peak attenuation factor (see Table
6.6) A watershed area R 1 inch of rainfall
excess (Unit Hydrograph) tp time to peak
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SCS Unit Hydrograph, typical
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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 instantaneous
  hydrograph, I(Dt)

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Santa Barbara Urban Hydrograph

• 3.

with