Computing Flood Discharges for Small Ungaged Watersheds

Concern for potential flooding is a critical factor in the safe design of water-related projects. The magnitudes of floods are described by flood discharge, flood elevation, and flood volume. This report will detail a procedure that can be used to estimate both the discharge and the volume of a flood given a design rainfall and a physical description of the watershed. 

NOAA National Weather Service Atlas 14, Volume 8 was finalized in 2013.  Atlas 14 updates design rainfall depth values by annual probability and storm duration based on an additional 30 +/- years of collected rainfall data; replacing “Rainfall Frequency Atlas of the Midwest”, NOAA Midwestern Climate Center, 1992 (informally known as “Bulletin 71”).

Atlas 14 also provides updated information on storm distribution; as a result, the SCS Type II rainfall distribution is now replaced with a location-specific rainfall distribution (based on the county where the watershed is located).  At the same time, the method has been revised to reflect the effects of storm size on storage (S), based on the ratio of Initial Abstraction to Precipitation depth (Ia / P).  Together, these changes reflect major revisions to the SCS Method.  (The Michigan office of the NRCS may decide to incorporate EGLE’s state-specific dimensionless unit hydrograph as well.)

EGLE will be incorporating the same changes in the state-specific SCS Method [as documented in “Computing Flood Discharges for Small Ungaged Watersheds”  (Sorrell, 2010)].  The Michigan SCS Method was developed with Bulletin 71 rainfall data to reproduce the peak flows that would be expected if the watershed was gaged.  If Bulletin 71 rainfall values are replaced with Atlas 14 rainfall values, the method will estimate unrealistically high peak discharges. 

We strongly recommend that the method (as implemented in the Excel spreadsheet available for download below), be used only with Bulletin 71 rainfall data until such time as we have completed incorporating all the changes included in Atlas 14.

Computing Flood Discharges For Small Ungaged Watersheds (rev. 2010)  has been updated several times since the 1991 version. The current version should be used and all previous versions discarded.  An Excel spreadsheet (rev. 3/2012) incorporating the method and instructions for using the spreadsheet (rev. 2007)  can also be downloaded.  If you need help enabling the macros, please refer to EGLE Peak Discharge Spreadsheet - Enabling Macros in Excel 2010 document.

Revision Information, contact Susi Greiner, GreinerS@Michigan.gov, 517-927-3838

• 7/2015:  Information related to Atlas 14 rainfall and its impact on the Peak Discharge Spreadsheet added
• 10/2012: Peak Discharge Spreadsheet - Enabling Macros in Excel 2010 document added
• 3/2012: The spreadsheet revision unlocks the flow type cells on the time of concentration sheet.
• 1/2012: The spreadsheet revision affects only watershed basin 55, the Portage in the Upper Peninsula.
• 6/2010:  Computing Flood Discharges For Small Ungaged Watersheds (rev. 2010)  revised as follows. Clarifies that the Appendix B hydrologic soil groups are not current and provides reference for current soils data.  Clarifies maximum length for sheet flow and use of ponding adjustment at design point.  Presents ordinates of Michigan unit hydrograph for use in WinTR-55.  Changes unit hydrograph peak designation from Qup as qp' to match SCS designation.
• 8/2008, Michigan Unit Hydrograph: The Michigan Unit Hydrograph equation is not applicable when Tc is less than one hour because the equation becomes increasingly non-linear as Tc decreases below one hour.  This has been clarified in both the Excel spreadsheet (rev. 2012)  and Computing Flood Discharges For Small Ungaged Watersheds (rev. 2010)  document. 
• 6/2008 Precipitation: The runoff curve number methodology was developed to estimate peak flood flows and flood volumes, and was not intended to be used as a basis for continuous simulation or computing low flows. For this reason, the Excel spreadsheet (rev. 2012)  was revised to eliminate manually-entered precipitation values.  The precipitation values used in the spreadsheet are determined based on the climatic region of the Michigan county entered on the "Discharge" tab. 
• 6/2008 Curve Numbers: The Computing Flood Discharges for Small Ungaged Watersheds (rev. 2010) document was revised to increase runoff curve numbers that were less than 30 up to 30.  This is consistent with the revised NRCS National Engineering Handbook (NEH), Chapter 9: Hydrologic Soil-Cover Complexes.  The Excel spreadsheet (rev. 2012) was modified so that the use of a composite curve number less than 30 prompts a message stating that results may not be accurate.