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<br />PART IV-UNIT HYDROGRAPHS <br /> <br />1. BACKGROUND THEORY. <br /> <br />A unit hydrograph is the direc': runerlt tllat Noulel be observed at the outlet of tile <br />watershed as a re"ult ot 1 unit of precipitation excess occurring within tile unit time <br />interval. The unit of excess is normally taken as equal to 1 inch. Since the physical <br />features of the basin (shape, size. slopes, soils. etc.) do not vary from storrn to storm. <br />hydrographs from ~;torms ot like duration and pattem are assumed to Ilave the same <br />shape, but with ordnates of flow in proportion to the runoff volumes. Thus, if 2 units of <br />excess occurred in 1 unit time interval. H'len it would be expected that ltm resulting <br />hydrograph would have the same shape as the hydrographs from 1 unit of excess in Hle <br />same time, except that all of the ordinates would be twice as large. As another example. <br />if 1 unit of excess occurred in each of 2 consecutive unit time intervals. the resulting <br />hydrograph would s;mply be the sum of the two uni'l hydrographs, but with the second unit <br />hydrograph beginning 1 time unit later. <br /> <br />While it is known that some assumpl ions of linearity involved in tile unit Ilydrograph <br />technique do not accurately apply, extensive experience inclicates that the limitations of the <br />technique are not a major drawback. considering the quality of rainfall and snowmelt data <br />that are usually a;ailable, provided that the procedures involved are applied with <br />appropriate knowledge and judgment. <br /> <br />It would be erroneous to assume Hlat one unit hydrograph would suffice for any <br />basin. Although HIe physical characteristics of natural basins may remain relatively <br />constant, variations in storm characteristics will have a si(jnificant effect on lhe shape of <br />resulting hydrograplls. The primary vadations in unit hydrol~raphs are due to variations (a) <br />in areal distribution of rainfall and sncwmelt. (b) rainfall and snowmelt duration, and (c) <br />time intensity pattem". <br /> <br />In general, a hydrograph resulting from precipitation concentrated in the lower part <br />of a basin will have a rapid rise, a sharp peak. and a rapid recession, wtlile precipitation <br />concentrated in the upper part of the same basin will have a slower rise ane! recession and <br />a lower, broader peak. It Can be seen that unit hydrographs developed from these <br />different areal distritlutions of runoff would have distinctly different shapes. On occasions. <br />unit hydrographs have been developed for upstream, uniform. or downstream <br />concentrations of rLnoff. However, this is not wholly sati"factory due to the subjectivity <br />involved, and a better solution is to limit the application of the unit hydrograprl technique <br />to basins small enough so that the differences in spatial distribution ot rainfall and <br />snowmelt do not si9nificantly affect runoff. This limitation does not ordinarily apply to <br />rainfall variations caused by topograpllic effects, since thE' effects can be considered as <br />relatively fixed characteristics of the basin. It is variations from the normal areal pattern <br />of precipitation that cause the differences in unit hydrographs. <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />l-:35 <br /> <br />CPIFT <br />