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<br />By definition, a unit Ilydrograph contains 1 unit of runoff from rainfall and snowmelt <br />occurring in a specified unit time. If the amount of runoff remains constant (1 unit) and <br />the duration in increased. the time base of the unit rlydrograph will be lengthened and the <br />peak will be lowered. A separate unit hydrograph IS theoretically necessary for eactl <br />possible duration of rainfall and snowmelt. Actually, a few unit hydrographs fOI' short <br />durations will serve most requirements, and a unit hydrograpll for a short duration of <br />rainfall and snowmelt Can be used to develop unit hydrographs for storms of longer <br />duration. <br /> <br />Time-intensity patterns of rainfall Can have a significant effect on unit hydroqraplls <br />and the effect is directly relaled to basin size. On lal'qe basins, cilanges in storm intensity <br />must last for several hours to Cause distinguishable effects on the hydrograph. On the <br />other hand, clearly defined peaks in the hydrograplls Illay be caused by short bursts of <br />rainfall lasting only a few minutes in very slllall basins. For large basins, valley storage <br />tends to eliminate the effects of short-time intensities and only major changes in the tlme- <br />intensity pattern can usually be lessened by selecting tile computation interval to be used <br />in developing a unit hydrograph short enough t1lat the changes are not great from one <br />computation interval to the next. <br /> <br />Up to this point, it has been assumecl that the physical characteristics of a basin <br />have remained relatively constant. But it is known that cllanges in the physical <br />characteristics can and do occur from natural and manmacie causes. A typical exarnple <br />is the drastic changes that Can occur in the shape ot Ilyclrographs and unit hydroqraphs <br />developed over a period of time for basins that are beinq urbanized. In this case. because <br />of the reduction of natural valley storage, the unit Ilydrographs Will tend to have hiqher <br />peaks and shorter times of concentration. Unejer natural conditions, changes in pllysical <br />characteristics Can occur due to seasonal and long-term changes in vegetation or to other <br />causes. such as fires. It should be noted that these variations also affect loss rates. F'or <br />example, the increase in impervious area caused by urbanization will not only reduce Hie <br />natural valley storage of a basin, but will also lower tile average loss rates. since sorne <br />rainfall will contribute directly to streamflow with no loss. <br /> <br />There is no prescribed set of rules for solving this problem, but the engineer <br />should be aWare that many factors cause variations in unit hydrographs. <br /> <br />Since the ordinates of a unit hycirograph represent the distribution by time and <br />magnitude of 1 unit of runoff from a drainage basin, the unit hyclrograph may be applied <br />to rainfall and snowmelt excess of any magnitude to deternine the resulting hydroqraph. <br />provided the duration of excess coincides witll the unit cluration of the unit hydrograph. <br />For a storm with only one period of excess, the ordinates of the Ilydrograph are computed <br />as follows: <br /> <br />Q, = U E <br /> <br />(IV.I) <br /> <br />where: <br /> <br />0, = the hydrograph ordinate tor any period, i <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />7-36 <br /> <br />CFW..T <br />