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<br />5. CLARK METHOD. <br /> <br />As discussed previously. there are many possible unit tlydrograpils for thl) same <br />basin, since the shape of the tlydroqraphs vary with different unit durations and otller <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 tecilnique which uses tile ccncept of the <br />instantaneous unit hydro graph (IUH). This is conceptually tile hydrograph that would result <br />from 1 unit of excess occurring over the basin in a specified areal pat1ern and zero time. <br />The IUH Can then be used to compute a unit hydrograptl for any unit duration equal to or <br />greater than the time interval used in the computations. <br /> <br />Tile Clark method used two parameters and a time-area relation to define the <br />instantaneous unit hydrograph. The first parameter, time of concentration ('I J is the travel <br />time of a water particle from tile most upstream point in tile basin to the outflow location. <br />An estimate of this lag time is tile time from tile end of effective rainfall plus snowmelt 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 timf,-area relation. <br /> <br />The second parameter is the at1enuation constant, R, which has tile dimension of <br />time. This parameter is used to account for the effect that storage in the river cllannel has <br />on the hydrograpil. This parameter can be estimated by dividing the flow at tile 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 tile volume remaining under <br />tile recession limb of the surface runoff ilydrograph following the point of inflection and <br />divide by the flow at tile point of inflection. In either case, R Sllould be an average value <br />determined by using severalilydrographs. <br /> <br />Tile other n<ocessary item to compute an I UH is the time-area relation. When t c <br />ilas been determined, the basin is divided into incremental runoff-producing areas that <br />ilave equal incremental travel times to tile outflow location. The distance frorn the most <br />upstream point in tile basin is measured along the principal watercourse to tile outflow <br />location. Dividing this distance by t , ~Jives an estimate of the rate of trav<el. Isochrones <br />representing equal travel time to the outflow location are laid out using tile rale of travel <br />to establisil the location of the lines. Tile areaS between the isochron8s are then <br />measured and tabulated in upstream sequence versus the corresponding incremental <br />travel time for eactl incremental area. <br /> <br />The increment of time used to subdivide tl'e basin need only be small enough to <br />adequately define the areal distribution of runoff wilile tile time period sellleted as the <br />computation interva must be equal to or less than the unit duration of ewess. Since the <br />former is frequently larger than the latter, a plot percent of time of concentration versus <br />accumulative area i~, useful in determining time-area relationships. Such a curve facilitates <br />rapid development cf unit 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 , is not firmly estaJlished at the outset, since unit hydro!Jraphs may be easily modified <br />to reflect subsequent changes in t c' Also. it is possible to refine tile curve by considering <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />7-41 <br /> <br />fRLlFT <br />