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<br />By definition, a unit hydrograph contClins 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 hydrograph will be lengthened and the <br />peak will be lowered. A separate unit hYClrograph is theoretically necessary for each <br />possible duration of rainfall and snowmeit. Actually, a few unit hydrographs for short <br />durations will serve most requirements, and a unit hydrograph for a short duration of <br />rainfall and snowmeit 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 hydrographs <br />and the effect is directly related to basin size. On large basins, changes in storm intensity <br />must last lor several hours to cause distinguishable effects on the hydrograph. On the <br />other hand, clearly defined peaks in the hYdrographs may be caused by short bursts of <br />rainfall lasting only a few minutes in very small basins. For large basins, valley storage <br />tends to eliminate the effects of short-time intensities and only major changes in the time- <br />intensity pattern can usually be lessened by selecting the computation interval to be used <br />in develOPing a unit hydrograph short eno~gh that the changes are not great from one <br />computation interval to the next. <br /> <br />Up to this point, it has been assumed that the physical characteristics of a basin <br />have remained relatively constant. But it is known that changes in the physical <br />characteristics can and do occur from natutal and manmade causes. A typical example <br />is the drastic changes that can occur in tha shape of hydrographs and unit hydrographs <br />developeCl over a period of time for basins that are being urbanized. In this case, because <br />of the reduction of natural valley storage, the unil hydrographs will tend to have higher <br />peaks anCl shorter times of concentration. Under natural conditions, change!l in physical <br />characteristics can occur due to seasonal aM long-term changes in vegetation or to other <br />causes, SUch as fires. It should be noted that these variations also affect loss rates. For <br />example, the increase in impervious area Ci;lused by urbanization will not only reduce the <br />natural valley storage of a basin, but will also lower the average loss rates, since some <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 unil hydrographs. <br /> <br />Since the ordinates of a unit hydrograph represent the distribution by time and <br />magnitudE) of 1 unit of runoff from a drainage basin, the unit hydrograph may be applied <br />to rainfall and snowmeit excess of any magnitude to determine the resuiting hydrograph, <br />provided the duration of excess coincides with the unit duration of the unit hydrograph. <br />For a storm with only one period of excess, the ordinates of the hydrograph are computed <br />as follows: <br /> <br />0;= Ui' E <br /> <br />(IV-1) <br /> <br />. <br /> <br />where: <br /> <br />Q. = the hydrograph ordinClte for any period, i <br />, <br /> <br />Colo,.ado Flood <br />Hydrology Manual. <br /> <br />7-36 <br /> <br />a=w=r <br />