Equation 1: SCS Graphical peak discharge equation

Where:

q_{p}=peak discharge(cfs)

q_{u}=unit peak discharge (csm/in or cfs per sq.mile of drainage area per inch of runoff)

A_{m}=drainage area (mi_{2})

Q=runoff (in)

F_{p}=pond and swamp adjustment factor.

## Procedure

- Find out the initial abstraction for the rainfall amount, P, using the
table provided below or using the formula **S=[(1000/CN)-10]** with **I _{a}=0.2S.**

- Then depending on your region of study, use one of the exhibit for either Type I, IA, II or III rainfall distribution for the time of concentration of your drainage area. This will give you Unit Peak Discharge.

- Then use the Equation 1 to compute the peak discharge in cfs.

- Done !!

## Table for estimating I_{a} for given CN

Curve Number | Ia (in) | Curve Number | Ia (in) |
---|---|---|---|

40 | 3 | 69 | 0.899 |

41 | 2.878 | 70 | 0.857 |

42 | 2.762 | 71 | 0.817 |

43 | 2.651 | 72 | 0.778 |

44 | 2.545 | 73 | 0.74 |

45 | 2.444 | 74 | 0.703 |

46 | 2.348 | 75 | 0.667 |

47 | 2.255 | 76 | 0.632 |

48 | 2.167 | 77 | 0.597 |

49 | 2.082 | 78 | 0.564 |

50 | 2 | 79 | 0.532 |

51 | 1.922 | 80 | 0.5 |

52 | 1.846 | 81 | 0.469 |

53 | 1.774 | 82 | 0.439 |

54 | 1.704 | 83 | 0.41 |

55 | 1.636 | 84 | 0.381 |

56 | 1.571 | 85 | 0.353 |

57 | 1.509 | 86 | 0.326 |

58 | 1.448 | 87 | 0.299 |

59 | 1.39 | 88 | 0.273 |

60 | 1.333 | 89 | 0.247 |

61 | 1.279 | 90 | 0.222 |

62 | 1.226 | 91 | 0.198 |

63 | 1.175 | 92 | 0.174 |

64 | 1.125 | 93 | 0.151 |

65 | 1.077 | 94 | 0.128 |

66 | 1.03 | 95 | 0.105 |

67 | 0.985 | 96 | 0.083 |

68 | 0.941 | 97 | 0.062 |

98 | 0.041 |

## Limitations

Following limitations should be considered when using SCS graphical peak discharge method for estimating peak flows:

- Graphical method provides a determination of peak discharge only. This method cannot be used if you need a hydrograph or if the watershed subdivision is required

- The watershed must be hydrologically homogeneous, that is, describable by one CN. Land use,
soils, and cover are distributed uniformly throughout the watershed.

- The watershed may have only one main stream or, if more than one, the branches must have nearly
equal T_{c}' s.

- The method cannot perform valley or reservoir routing.

- The Fp factor can be applied only for ponds or swamps that are not in the T_{c} flow path.

- Accuracy of peak discharge estimated by this method will be reduced if I_{a} / P values are used that
are outside the range given in exhibit 4. The limiting Ia / P values are recommended for use.

- This method should be used only if the weighted CN is greater than 40.

Exhibit 4-I from chapter 4 of TR-55 for Type I rainfall Distribution

Exhibit 4-IA from chapter 4 of TR-55 for Type IA rainfall Distribution

Exhibit 4-II from chapter 4 of TR-55 for Type II rainfall Distribution

Exhibit 4-III from chapter 4 of TR-55 for Type III rainfall Distribution