Hydrology Rainfall - Runoff Modeling (II) Synthetic unit hydrographs

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HydrologyRainfall - Runoff Modeling
(II)Synthetic unit hydrographs

• Prof. Ke-Sheng Cheng
• Department of Bioenvironmental Systems
  Engineering
• National Taiwan University

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Synthetic unit hydrographs

• For areas where rainfall and runoff data are not
  available, unit hydrograph can be developed based
  on physical basin characteristics.
• SCS unit hydrograph
• Clarks IUH (time-area method)
• Linear reservoir model (Nash model)

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SCS unit hydrograph
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Misuse of SCS UH an example
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Clarks IUH (time-area method)

• The concept of isochrones
• Isochrones are lines of equal travel time. Any
  point on a given isochrone takes the same time to
  reach the basin outlet. Therefore, for the
  follwoing basin isochrone map and assuming
  constant and uniform effective rainfall,
  discharge at the basin outlet can be decomposed
  into individual contributing areas and rainfalls.

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Contributing area and contributing rainfall
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• Using the basin isochrone map, the cumulative
  contributing area curve can be developed. The
  derivatives or differences of this curve
  constitute the instantaneous unit hydrograph
  IUH(t).

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• If the effect of watershed storage is to be
  considered, the unit hydrograph described above
  is routed through a hypothetical linear reservoir
  with a storage coefficient k located at the
  watershed outlet.
• For a linear reservoir with storage coefficient
  k, we have

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• Consider the continuity of the hypothetical
  reservoir during a time interval ?t.

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• Example. A watershed of 1000-acre drainage area
  has the following 15-minute time-area curve. The
  storage coefficient k of the watershed is 30
  minutes. Determine the 15-minute unit hydrograph
  UH(15,t).

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• Area between isochrones t 0 and t 0.25 hr is
  100 acres. Since we are interested in the unit
  hydrograph UH(15-min, t), the rainfall intensity
  in the time period (0.25 hr) should be 4 inch/hr.
  Therefore, the ordinate of UH(15-min, t) at time
  t 0.25 hr is
• 4 inch/hr ? (1/12 ft/inch) ? (1/3600 hr/sec) ?
  100 acres ? (43560 ft2/acre) 403.33 cfs

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Hydrological Analysis for Detention Pond Design
in Hillslope Development
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