Transcription of APPENDIX F – RATIONAL METHOD - Oregon
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APPENDIX F – RATIONAL METHOD - Oregon
Hydrology 7-F-1 APPENDIX F RATIONAL METHOD Introduction One of the most commonly used procedures for calculating peak flows from small drainages less than 200 acres is the RATIONAL METHOD . This METHOD is most accurate for runoff estimates from small drainages with large amounts of impervious area. Examples are housing developments, industrial areas, parking lots, etc. The RATIONAL METHOD is included in the HYDROLOGIC module of the FHWA's Watershed Modeling System computer software package and a RATIONAL METHOD calculator is available within the FHWA Hydraulics Toolbox program. Input data such as runoff coefficients, rainfall information, etc., should be based on the procedures and information in the ODOT Hydraulics Manual when using this program. RATIONAL Equation The RATIONAL METHOD (or RATIONAL Formula) is: Q = Cf C i A (Equation 1) Where: Q = Peak flow in cubic feet per second (cfs) Cf = Runoff coefficient adjustment factor to account for reduction of infiltration and other losses during high intensity storms C = Runoff coefficient to reflect the ratio of rainfall to surface runoff i = Rainfall intensity in inches per hour (in/hr) A = Drainage area in acres (ac) L
The kinematic wave equation can be used to estimate the time of concentration associated with overland sheet flow. The equation is shown below, and it is only applicable for travel distances equal to or less than 300 feet. Where: T osf = Travel time for …
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