TROY

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GEM 4409HydrologyFall 2005

• TROY
• UNIVERSITY

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Hydrographs

• Graphs flow vs. time
• Shape depends on rainfall basin characteristics
• Provides time distribution of runoff peak
  flow
• Important where storage is significant

• Can estimate time that roads/bridges are flooded

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Hydrographs

• USGS Synthetic Hydrographs
• Rural urban streams in Alabama
• Drainage areas lt 500 square miles
• Based on a statewide dimensionless hydrograph
• Applied to individual sites by using lag time
  peak flow

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Fall Line
Birmingham
Fall Line
Montgomery
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Seven Major Soil Areas in Alabama
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Hydrographs

• Unit Hydrographs
• Produced by a unit depth of runoff uniformly
  distributed over a basin for a specific excess
  rainfall duration
• Example
• The 5-minute unit hydrograph for a basin is a
  graph of the direct runoff resulting from 1 of
  excess rain occurring at a constant rate over
  5-minutes

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Hydrographs

• Unit Hydrographs
• A basin will have many different unit hydrographs
• For each duration of runoff
• 5-minute, 10-minute,24 hour, etc.

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Hydrographs

• Unit Hydrographs
• Excess rainfall
• The rainfall that contributes to the runoff
• i.e. (Total Rainfall Hydrologic Losses)

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Hydrographs

• Unit Hydrographs
• Excess rainfall volume runoff volume
• Runoff volume area under hydrograph
• Rainfall volume depth x area

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Hydrographs

• Unit Hydrographs
• Applicable basin size
• No well defined upper limit
• Midsize basins (1 mi2 100 mi2)
• Has been used with basins up to 3,000 mi2
• Accuracy decreases with area

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Hydrographs

• Unit Hydrographs
• Lag Time (LT)
• Central concept to the development of the unit
  hydrograph
• A measure of the time elapsed between the
  occurrence of unit rainfall and unit runoff
• i.e. basin response time
• Involves flow path, land use, slope, etc.

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Hydrographs

• Defining
• Lag Time (LT)
• T1
• T2
• T3
• T4
• T5
• T6
• T7

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Hydrographs

• Defining
• Lag Time (LT)
• Most Common
• T2 Time from
• centroid of
• excess rainfall
• to peak runoff

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Hydrographs

• Lag Time (LT)
• Short lags ? High peak, narrow base
• Long lags ? Low peak, wide base

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Hydrographs

• Unit Hydrographs
• Calculated
• Directly By measured data
• Indirectly By using a synthetic formula

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Hydrographs

• Unit Hydrographs
• Direct Method
• Gaged basins only
• Rainfall-runoff data
• Selected storms should have uniform rainfall
  intensity (in time and space)

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Hydrographs

• Unit Hydrographs
• Indirect Method
• SCS unit hydrograph

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Hydrographs

• Unit Hydrographs
• SCS Unit Hydrograph
• Based on the relationship between the lag time of
  a basin and the time of concentration
• SCS defines lag time as time from center of
  excess rainfall to peak (T2)

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SCS Unit Hydrograph
tr duration of runoff tbt time base of
DRH tl lag time for the basin
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SCS Unit Hydrograph
tr duration of runoff tbt time base of
DRH tl lag time for the basin
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SCS Unit Hydrograph
tr duration of runoff tbt time base of
DRH tl lag time for the basin
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SCS Unit Hydrograph
mi2, hours, cfs
tr duration of runoff tbt time base of
DRH tl lag time for the basin
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SCS Unit Hydrograph

• Shape based on a ratio of 8/3 leading to the
  constant, 484
• This constant is called a peak factor (PF)

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SCS Unit Hydrograph

• The ordinates of the SCS UH can also be
  approximated using the following equation

(For PF484, x 3.79 and for PF 300, x 1.5)
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SCS Unit Hydrograph

• The ordinates of the SCS UH can also be
  approximated by combining and solving for flow
  rate

(For PF484, x 3.79 and for PF 300, x 1.5)
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SCS Unit Hydrograph
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SCS Unit Hydrograph
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SCS Unit Hydrograph

• Calculate tr, tp, qp
• tc 37.5 / 60 0.627 hours
• tr 0.133tc 0.133(0.627) 0.0834 hrs 5 min
• tp 5 tr 5 (5) 25 min 0.417 hrs
• qp 484 (A)/tp 484(0.117)/0.417 135.8 cfs

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SCS Unit Hydrograph
2. Determine the coordinates of the dimensionless
unit hydrograph
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SCS Unit Hydrograph
Q (cfs)
Time (min)
0
25
125
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SCS Unit Hydrograph

• 3.Check to see that the depth of runoff is 1
• Sum the UH flows ? 905.9 cfs