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<br />5. CLARK METHOD. <br /> <br />As discussed previously, there are many possible unil hydrographs for the same <br />basin, since the shape of the hydrographs vary with different unil durations and other <br />factors. To define a unique unit hydrograph for a basin that can be adjusted to account <br />for some of the factors, C. O. Clark developed a technique which uses the concept of the <br />instantaneous unil hydrograph (IUH). This is conceptually the hydrograph that would resuit <br />from 1 unit of excess occurring over the basin in a specified areal pattern and zero time. <br />The IUH can then be used to compute a unit hydrograph for any unit duration equal to or <br />greater than the time interval used in the computations. <br /> <br />The Clark method used two parameters and a time-area relation to define the <br />instantaneous unit hydrograph. The first parameter, time of concentration (t 0) is the travel <br />time of a water particle from the most upstream point in the basin to the outflow location. <br />An estimate of this lag time is the time from the end of effective rainfall plus snowmeit over <br />the basin to the inflection point on the recession limb of the surface runoff hydrograph. <br />The time of concentration is used in developing the time-area relation. <br /> <br />The second parameter is the attenuation constant, R, which has the dimension of <br />time. This parameter is used to account for the effect that storage in the river channel has <br />on the hydrograph. This parameter can be estimated by dividing the flow at the point of <br />inflection of the surface runoff hydrograph by the rate of change of discharge (slope) at the <br />same time. Another technique for estimating R is to compute the volume remaining under <br />the recession limb of the surface runoff hydrograph following the point of inflection and <br />divide by the flow at the point of inflection. In either case, R should be an average value <br />determined by using several hydrographs. <br /> <br />The other necessary item to compute an IUH is the time-area relation. When t 0 <br />has been determined, the basin is djvidEld into incremental runoff-producing areas that <br />have equal incremental travel times to the outflow location. The distance from the most <br />upstream point in the basin is measured along the principal watercourse to the outflow <br />location. Dividing this distance by t , givEls an estimate of the rate of travel. Isochrones <br />representing equal travel time to the outflow location are laid out using the rate of travel <br />to establish the location of the lines. The areas between the isochrones are then <br />measured and tabulated in upstream sequence versus the corresponding incremental <br />travel time for each incremental area. <br /> <br />The increment of time used to subdivide the basin need only be small enough to <br />adequately define the areal distribution of runoff while the time period selected as the <br />computation interval must be equal to or less than the unil duration of excess. Since the <br />former is frequently larger than the latter, a plot percent of time of concentration versus <br />accumulative area is useful in determining time-area relationships. Such a curve facilitates <br />rapid development of unil hydrographs for various computation intervals and unit durations <br />of excess. This is especially helpful when making flood predictions and for basins where <br />t 0 is not firmly established at the outset, since unit hydrographs may be easily modified <br />to reflect subsequent changes in t ,. Also. il is possible to refine the curve by considering <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />7-41 <br /> <br />ffiIJFT <br />