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.

Computing Flood Discharges For Small Ungaged Watersheds (rev. 6/2008) has been updated several times since the 1991 version. The current version, dated June 2008, should be used and all previous versions discarded. The file is in PDF format. To view this file, you should use the latest version of Acrobat Reader, which is free from Adobe. An Excel spreadsheet (rev. 6/2008) incorporating the method and instructions for using the spreadsheet (rev. 6/2007) can also be downloaded.

Revision Information

• 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. 6/2008) was revised to eliminate manually-entered precipitation values.  In addition, composite curve numbers less than 30 prompt a message stating that results may not be accurate.  The precipitation values used in the spreadsheet are determined based on the climatic region of the Michigan county entered on the "Discharge" tab.  These revisions should eliminate inaccurate results.

• The Computing Flood Discharges For Small Ungaged Watersheds (rev. 6/2008) document and Calculating Runoff Curve Numbers with GIS methodology were 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 (http://directives.sc.egov.usda.gov/media/pdf/H_210_630_9.pdf).  In addition, NEH Chapter 9 has curve numbers for soil-land cover complexes not listed in Computing Flood Discharges For Small Ungaged Watersheds (rev. 6/2008).  Runoff curve numbers listed in NEH Chapter 9 are acceptable for use in hydrologic modeling reviewed by MDEQ's Hydrologic Studies Unit (HSU). 

• The revised NEH Chapter 10: Estimation of Direct Runoff from Storm Rainfall (http://directives.sc.egov.usda.gov/media/pdf/H_210_630_10.pdf), eliminates Antecedent Moisture Condition (AMC) and replaces it with Antecedent Runoff Condition (ARC). The handbook now states "No apparent relationship between antecedent precipitation and curve number exists."  HSU is evaluating this change.  Modeling using AMC will continue to be approvable by HSU.