HEC-HMS Users Manual Subbasin Elements Current: References: Subbasin Elements PDF Download PDF Download page References: Subbasin Elements. Current page All pages References: Subbasin Elements Flood Control District of Maricopa County (Maricopa County). 2011. Drainage Design Manual. Maricopa County, AZ.Natural Resources Conservation Service (NRCS). 2007. National Engineering Handbook: Chapter 16 Hydrographs. Washington, DC.Welle, P., D. Woodward, and H. Moody. 1980. "A Dimensionless Unit Hydrograph for the Delmarva Peninsula." American Society of Agricultural Engineers Paper 80-2013, St. Joseph, Michigan. ×