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e . .. e . e MANUALS-CORPS OF ENGINEERS U. S. ARMY EM 1110--2--1406 31 Aug 59 ENGINEERING AND DESIGN FLOOD-HYDROGRAPH ANALYSES AND COMPUTATIONS INTRODUCTION 1. PURPOSE AND SCOPE. This manual describes and illustrates certain methods of deriving fundamental hydrologic factors by analyzing observed hydrographs of stream flow and related meteoro- logical events and suggests methods of utilizing these deduced factors in computing hypothetical hydro- graphs of runoff for conditions differing, in specified respects, from those prevailing during the observed floods. This manual is also issued for guidance in hydrologic studies for Corps of Engineers water resource development projects. 2. REFERENCES. a. Reference is made in the text to EM 111O-2~1408, Routing of Floods Through River Channels, This manual was last published under a now superseded numbering system as Part CXIV, Chapter 8, dated September 1953. b. Appendix II consists of a selected bibliography. References in the text to items in the bibliog, raphy are indicated by numbers in an exponrntial position. Other references are also given in the text. 3. RESCISSION. a, Engineering Manual for Civil Works Construction, Part CXIV, Chapter 5, March 1948; Hydrologic and Hydraulic Analyses, Flood,Hydrograph Analy,es and Computations (EM 1110--2-; 1405). b. Civil Works Engineer Bulletin No. 49-22, Unit Hydrograph Compilations, 7 December 1949. 4. TYPES OF ANALYSES COVERED. a. Analysis of a flood hydrograph as discussed herein includes- (I) Estimates of the intensity, depth and areal distribution of precipitation causing the runoff (and rate of snowmelt, where this is significant); (2) Computation of difference between precipitation and direct runoff, expressed herein as initial loss and infiltration index; (3) Determination of the combined effect of drainage area and channel characteristics on the regimen of runoff, expressed herein in terms of "unit hydrographs." b. The computation of hypothetical hydrographs, based on data derived from natural hydrograph analyses and supplementary studies, includes: (I) Determination of desigp,storm rainfall values, the areal distribution and sequence of occur- rence of unit increments that would produce critical rates of runoff, or rates corresponding to specified lesser magnitudes aceording to the purpose of the estimate. In certain cases, rainfall values correspond- ing to certain average frequencies of occurrence may be adopted as design storms in order to serve as a basis for estimating economic benefits; (2) Selection of infiltration indices and initial loss values to correspond with the given drainage area and assumptions regarding season of the year, antecedent rainfall and other considerations affecting volume of runoff; (3) Selection of unit hydrographs to represent the probable order of runoff from adopted design storm rainfall-excess; (4) Computation of hypothetical hydrographs by combining factors referred to above. c. The previous issue of this manual (Part CXIV, Chapter 5) contained a section entitled "Flood Flow Contributions from Melting Snow." This material has been eliminated from this manual and the subject will be treated in an expanded manual entitled "Runoff from Snowmelt" (EM 1110--2-1406). 5. BASIC CONSIDERATIONS. a. The characteristics of natural drainage basins and meteorological influenees affecting runoff are too complex and variable to be evaluated accurately by mechanical com- 1