DESIGN RAINFALL DATA

Transcription

1 Unit 4: Design Rainfall Data Page 4-1 DESIGN RAINFALL DATA For stormwater management system design purposes, most engineers use point rainfall. Design rainfall data typically is given in terms of accumulated depth or average intensity. The NRCS and South Carolina synthetic unit hydrograph methods use 24-hour rainfall depths while peak runoff methods such as the Rational method use rainfall intensity data. 24-Hour Rainfall Maps The first extended rainfall frequency study in the United States was made by Yarnell (1935) in the early 1930s and was presented in the form of a series of maps for several combinations of return periods (recurrence intervals) and durations. That work was expanded by David Hershfield and published in the U.S. Weather Bureau Technical Paper No. 40 Rainfall Frequency Atlas for the United States (NWS, 1961). Commonly called TP-40, this document provides rainfall isohyetal (rainfall depth contour) maps for durations ranging from 30 minutes to 24 hours and return periods from 1 to 100 years. The NRCS (SCS) graphical peak discharge method uses 24-hour rainfall depths. Appropriate maps are provided as Appendix B in TR-55 (SCS, 1986). Figures A-1 and A-2 (this document) show the 2- and 10-year 24-hour rainfall maps. The extended graphical peak discharge method (Meadows, 1991) also uses 24-hour rainfall data, available from the maps in TR-55 and TP-40 or from tables provided by the South Carolina Department for Health and Environmental Control (Appendix B of this document). Intensity-Duration-Frequency Curves Intensity data, used with the Rational Method and other intensity-based peak flow methods, is obtained from intensity-duration-frequency (IDF) curves that relate an average intensity during a specified time interval to the recurrence frequency (return period) for that event. IDF data generally are available from local sources, such as the State DOT or State Climatologist. IDF data are presented in tabular, graphical, or equation format. Figure 4-1 shows the Columbia, SC IDF curves (2, 5, 10 and 25-year return periods) in graphical format. One equation that approximates the IDF curve is: a I d = c (b + t c ) (4-1) where I d is the design intensity, t c is the duration which produces that intensity for a given return period, and a,b,c are best-fit parameters that vary with return period. Table 4-1 lists the parameter values for Eq. 4-1 for Columbia, South Carolina for I d in inches/hour and t c in minutes. Values for other locations in South Carolina were provided by SCDOT and are included in Appendix C (this document). Table 4-1 for Columbia, SC (Source: SCDOT) (years) a b c

2 Unit 4: Design Rainfall Data Page 4-2 Figure 4-1. IDF Curves for Columbia, SC References Meadows, M.E. (1991). Extension of Hydrograph and Pond Design Methods for Various Unit Hydrograph Peak Rate Factors, Project Report, Volume III, U. S. Geological Survey, Columbia, SC, Sept. National Weather Service (1961). Rainfall Frequency Atlas of the United States, Technical Paper No. 40, U.S. Dept of Commerce, Washington, DC. Soil Conservation Service (1986), Urban Hydrology for Small Watersheds, TR-55. South Carolina Department of Health and Environmental Control (1997). South Carolina Stormwater Management and Sediment Control Handbook, Environmental Quality Control, Bureau of Water. Yarnell, D.L. (1935). Rainfall Intensity Frequency Data, Miscellaneous Publication 204, U.S. Dept of Agriculture, Washington, DC.

3 Unit 4: Design Rainfall Data Page 4-3 APPENDIX A 24-Hour Rainfall Maps for Continental United States

4 Unit 4: Design Rainfall Data Page 4-4 Figure A-1. 2-Year 24-Hour Rainfall Map Figure A Year 24-Hour

5 Unit 4: Design Rainfall Data Page 4-5 APPENDIX B 24-Hour Rainfall Depths for South Carolina Counties Source: (SCDHEC, 1997) 24-Hour Rainfall Depth (inches) County Abbeville Aiken Allendale Anderson Bamberg Barnwell Reardon Berkeley (North) Berkeley (South) Calhoun Charleston Cherokee Chester Chesterfield Clarendon Colleton (North) Colleton (South) Darlington Dillon Dorchester (North) Dorchester (South) Edgefield Fairfield Florence Georgetown Greenville (North) Greenville (South) Greenwood Hampton Horry (North) Horry (South) Jasper Kershaw Lancaster Laurens Lee Lexington Marion (North) Marion (South) Marlboro McCormick Newberry Oconee (North) Oconee (South) Orangeburg Pickens (North) Pickens (South) Richland Saluda Spartanburg NE Spartanburg KW Spartanburg SE Spartanburg SW Use Technical Paper 40. Rainfall data in this area must be predicted carefully. Summer Union Williamsburg York

6 Unit 4: Design Rainfall Data Page 4-6 APPENDIX C for Selected South Carolina Towns Table C-1. for Anderson, SC years a b c Table C-2. for Camden, SC years a b c Table C-3. for Charleston, SC Table C-4. for Florence, SC

7 Unit 4: Design Rainfall Data Page 4-7 Table C-5. for Greenville, SC Table C-6. for Hilton Head, SC Table C-7. for Newberry, SC Table C-8. for Pickens, SC Table C-9. for Rock Hill, SC Years A b c

8 Unit 4: Design Rainfall Data Page 4-8 Table C-10. for Spartanburg, SC Table C-11. for Union, SC Years A b c Table C-12. for Walhalla, SC Table C-13. for Gaffney, SC