Sindoh D310 SERIES User Manual
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Sindoh D310 SERIES User Manual

Sindoh D310 SERIES User Manual

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CHAPTER 11

Determination of peak runoff rate

By:
J.A.M. de Brouwer
Department of Earth Resources Surveys,
International Institute for Aerospace Survey and Earth Sciences (ITC),
P.O. Box 6, 7500 AA Enschede, The Netherlands.
Tel: +31 53 4874283, Fax: +31 53 4874336, e-mail: BROUWER@ITC.NL
Summary
Determine the peak runoff rate of a small watershed using the Modified Rational
Formula method, and the SCS Dimensionless Unit Hydrograph for an excess
rainfall of 1 cm.
Getting started
The data for this case study are stored on the ILWIS 2.1 CD-ROM in the directory
d:\appguide\chap11. If you have already installed the data on your hard-disk, you
should start up ILWIS and change to the subdirectory where the data files for this
chapter are stored, c:\ilwis21\data\appguide\chap11. If you did not install the
data for this case study yet, please run the ILWIS installation program (see ILWIS
Installation Guide).
F
Now you are ready to start the exercises of this case study.
ILWIS Applications Guide
•
Double-click the ILWIS program icon in the ILWIS program group.
•
Change the working drive and the working directory until you are in
the directory c:\ilwis21\data\appguide\chap11.
131

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  • Page 1 CHAPTER 11 Determination of peak runoff rate J.A.M. de Brouwer Department of Earth Resources Surveys, International Institute for Aerospace Survey and Earth Sciences (ITC), P.O. Box 6, 7500 AA Enschede, The Netherlands. Tel: +31 53 4874283, Fax: +31 53 4874336, e-mail: BROUWER@ITC.NL Summary Determine the peak runoff rate of a small watershed using the Modified Rational Formula method, and the SCS Dimensionless Unit Hydrograph for an excess...
  • Page 2 Determination of peak runoff rate 11.1 Determination of peak runoff rate using the Modified Rational Formula One important empirical formula for determining the peak rate of runoff is the Rational Formula. This approach was introduced more than 100 years ago. Since then it has become the most widely used method for designing drainage facilities for small urban and rural watersheds.
  • Page 3 Determination of peak runoff rate Time of concentration [min]. Length of main river [m]. Distance weighted channel slope [m/m]. Available data The materials used in this exercise are: − Contour and drainage map of the catchment. − Land use map. −...
  • Page 4 Determination of peak runoff rate of the slopes to be encountered. • Study the land use map. • Calculate the time of concentration using the formula of Kirpich/Ramser. Determine the area weighted runoff coefficient The rational formula uses C, the runoff coefficient. This coefficient is related to the different land covers and hydrologic soil groups.
  • Page 5 Determination of peak runoff rate 11.2 Determination of peak runoff rate using the SCS Dimensionless Unit Hydrograph One important empirical formula for determining the quantity of runoff is the SCS Dimensionless Unit Hydrograph (U.S. Soil Conservation Service, 1964). It belongs to the group of isochronal methods for determining peak runoff rates.
  • Page 6 Determination of peak runoff rate   1000 ∗   2.587 L   [11.7] ∗ 1900 H where, Lag time [hr]. Hydraulic watershed length [m]. CN = Hydrologic area-weighted curve number. Average watershed land slope [%]. The hydraulic watershed length L can be approximated for small watersheds (<2000 ha) by the formula 11.8.
  • Page 7 Determination of peak runoff rate Table 11.3: Runoff curve number for hydrologic soil-cover complexes for antecedent rainfall condition II and I =2S (Cont.) Hydrologic soil group Land use or crop Treatment or Hydrologic practice condition Terraced Good Close-seeded legumes Straight row Poor or rotation meadow Straight row...
  • Page 8 Determination of peak runoff rate • Calculate the Hydraulic watershed length. • Determine the average watershed land slope. • Calculate the lag time t Determine the unit duration of excess rainfall The application of the SCS dimensionless unit hydrograph is only valid for storms with a duration less than 0.133 x time of concentration.

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