Guidelines for Determining Flood Flow Frequency--Bulletin 17C (Ver. 1.1, May 2019): U.S. Geological Survey Techniques and Methods, Book 4, Chap. B5 [open pdf - 28MB]
From the Abstract: "Accurate estimates of flood frequency and magnitude are a key component of any effective nationwide flood risk management and flood damage abatement program. In addition to accuracy, methods for estimating flood risk must be uniformly and consistently applied because management of the Nation's water and related land resources is a collaborative effort involving multiple actors, including most levels of government and the private sector. [...] Federal agencies are requested to use these Guidelines in all planning activities involving water and related land resources. State, local, and private organizations are encouraged to use these Guidelines to assure uniformity in the flood frequency estimates that all agencies concerned with flood risk should use for Federal planning decisions. This revision is adopted with the knowledge and understanding that review of these procedures will be ongoing. Updated methods will be adopted when warranted by experience and by examination and testing of new techniques."
Techniques and Methods 4-B5
U.S. Geological Survey: https://www.usgs.gov/