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. t t t r I. t , , . . i , t t t ~ i , ~ . .. ~ ~ ~ ~ t t I . , l ~ ~ ~ t f ~ ~ t t ~ , . '-, storm duration, 2-inchcs of runoff ovcr the same duration doubles the dischargc at each point in time. Supcrposition allows for the accumulation of individual runoff responses. For example, if a storm event which generates I-inch of runoff over a given duration is followcd immediately by another I-inch runoff storm event of the same duration, thc basin response will be the accumulation of the individual effects over timc. (figure II-3) A unit hydrograph is derived for a specified storm duration. Sincc storm durations vary, many different unit hydro- graphs exist for any particular watcr- shed. Techniques exist to vary thc dura- tion of a unit hydrograph such as the .S. Curve (Summation Curvc) approach. (11) These methods are useful in matching the design unit hydrograph to the duration increment of a design rainfall. Methods are also available to formulate design rainfalls using U.S. Weathcr Service data. (12,13) b. Svnthetic Unit Hvdrograoh. A synthetic unit hydrograph may be developcd in the absence of stream gage data. Thc methods used to develop synthetic unit hydrographs are generally empirical and depend upon various watershed parameters, such as watershed size, slope, land usc, and soil type. Two synthctic proccdurcs which have been widely used are the Snydcr Method and the Soil Conservation Service (SCS) Method. The Snyder Method uscs empirically defined terms and physiogra- phic characteristics of the drainage basin as input for empirical equations which charactcrize the timing and shapc of the unit hydrograph. The SCS mcthod utilizes dimensionless hydrograph par- ameters based on the analysis of a large number of watershcds to dcvclop a unit hydrograph, The only parametcrs rcquircd by the mcthod are thc peak discharge and the time to peak. A variation of the SCS syntlretic unit hydrograph is the SCS '~~,' synthetic triangular hydrograph, c. ComDutcr Models. Hydrologic computer modcls are bccoming popular for generating flood hydrographs, Somc com- putcr mod cis mcrely solvc cmpirical hand mcthods more quickly. Othcr models are thcorctical and solve thc runoff cyclc in its entirety using con tin uous simula- tion over short lime increments. Results arc obtained using mathcmalical cquations to reprcsent cach phasc of the runoff cycle such as intcrception, surface reten- tion, infiltration, and ovcrland flow. In most simulation modcls, thc drainage arca is divided into subareas with similar hydrologic charactcristics. A dcsign rainfall is synlhcsized for cach subarea, and abstractions, such as interccption and infiltration, arc rcmovcd, An over- land flow routine simulates thc movement of thc rcmaining surface watcr. Adjacent channcls receivc this overland flow from thc subarcas. The channels of thc watcr- shed arc linked together and the channcl flow is routed through them to complctc the basin's response to the design rain- fall. Computcr models arc available which simulatc a single storm event or continuous runoff ovcr a long pcriod of timc, Thc Stanford Watershed model was one of the carlicst simulation models. It is a continuous simulation model using hourly rainfall and potential evapotranspiration as input dala. The output is in the form of mcan daily flows, hourly ordinatcs of thc hydrograph, and monthly totals of the water balance. The EPA Sponsored Storm Watcr Management Model (SWMM) pcrmits the simulation of a single storm event. An assumption inherent in these models is that the return period of the computcd flood is the same as that of the input rainfall, All simulation models require calibration of modcling parame- tcrs using mcasurcd historical evcnts to increase their validity. Most simulation models rcquire a significant amount of input data and uscr expcricncc to assure reliablc results. S. Basics of Storage Routing, Mca- surcmcnt of a flood hydrograph at a strcam loca tion is analogous to recording the 17 ~~---..-/- " -