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<br />TABLE 20\ Summary 01 Se.nsttivlt'j Analysis Results Ear'y-Peaked Tempera' Rainfall Distribution: Assumed Input
<br />
<br /> ,
<br />Meteorological
<br />factor Values assumed for meteorological factors in sensitivity analysis
<br />(Input)
<br />L, DC DC DSC DSC USC USC DC DC DSC DSC USC USC DC DC DSC DSC use usc
<br />4> 181 270 181 270 181 270 181 270 181 270 181 ,270 181 270 181 270 181 270
<br />P, 2,5 2.5 2,5 2,5 2.5 2.5 1.0 11.0 1.0 1.0 1.0, 1.0 4,0 4,0 4,0 4,0 4,0 4,0
<br />S, so 175 100 300 100 300 50 - - - - - so' - - - - -
<br />PMSV 23 20 22 17 22 17 22 - - - - - 23 - - - - -
<br />
<br />, TABLE 2b Results
<br />
<br />Assumed
<br />land
<br />cover PMF
<br />(input) (oulput) , '" '
<br />HFD Qp 154 134 138 1I0 144 ,119 160 139 144 115 149 123 147 128 132 105 137 113
<br /> I, 4,9 4,9 4,9 4,1 5,0 6.4 - - - - - - - - - - - -
<br />DHFD Qp 143 124 131 106 133 107 ISO 130 137 III 139 III 136 118 124 101 126 101
<br /> " 4,8 4,2 4,0 3,9 4,9 , 5,0 - - - - - - - - - - - -
<br />UMHFD Q, 141 122 127 llJ() 131 106 147 127 132 105 137 III 133 lIS 120 95 124 IIJ()
<br /> " 4,9 - - - - - - - - - - - - - - - - -
<br />UHFD Qp 134 Il5 119 9I 124' 99 140 '120 124 95 130 104 127 109 ' Il2 86 118 94
<br /> I, 4,9 4,9 4,9 4,1 5,0 6.4 - - - - - - - - - - - -
<br />HUD Qp 124 107 III 88 115 93 130 113 Il7 93 121 98 Il7 101 105 83 108 88
<br /> " 4,8 - - - - - - - - - - - - - - - - -
<br />
<br />Note: L.. = location of storm center: DC = drainage centered; DSe = downstream subbasins centered; use = upstream "subbasins centered (see
<br />Table 1 for,coordinates); 4> = orientation of storm (degrees); P, = isohyetal pattern (elliptic31 axes ratio: to 1.0); S~ = storm.area size (sq mi),
<br />HMR52 optimized; PMSV = 6-hr average. PMS volume (in.); LD = land-cover distribution:" f:IFD = homogeneous fully developed; DHFD =
<br />d9wnstream half fully developed; UMHFD = upper middle half fully developed; UHFD = upstream half fully developed; HUD = homogeneous
<br />fully undeveloped; Qp = PMF discharge (cfs x 1.0(0); tp = PMF time to' peak (br); to convert sq mi to km2. multiply by 2.59; to convert in. to mm.
<br />multiply by 25.4; to convert cu ft/see (ds) to mJ./s, multiply by 0~02832.
<br />
<br />cedent storms modeled indirectly by the <l>-indexvalue. The"
<br />meteorological factors that were varied in the sensitivity
<br />analysis include: (I) The storm center location, Le: (2) the'
<br />storm orientation, 4>; (3) the elliptical axes ratio of the
<br />isohyetal pattern, P, and; (4) Ihe temporal rainfall distri-
<br />bution, RT' The storm-area size, S" is 'selected by HMR52
<br />hased on the selected values for Le, and PI with Ihe objec-
<br />live of maximizing the total PMS rainfall volume, PMSV
<br />(see Tables 2-4), Tables 2-4 present the individual values
<br />of PMF discharge; Qp, and its time peak, Ip resulting from
<br />the variouscombinations of Lc, <t>. P" and Rr (as illustrated
<br />by Tables 2, I, and 3), Due 10 the current lack of knowledge
<br />required 10 establish relationships both between the factors
<br />and the PMF and among the 'factors, Ihe results of the
<br />sensitivity analysis (given in Tables 2-4) were used to es-
<br />timate these relationships, It is froni these defined rela-
<br />tionships and interactions between the factors "that a set of
<br />guidelines has been proposed to overcome Ihe problem in-
<br />Iroduced by the NWS Irial-and-error procedure,
<br />
<br />Interactions between Factors
<br />
<br />Based on plols (not shown) of the results from Tables 2-
<br />4, there appears to be sigr:tlficant interaction between the
<br />temporal rainfall distribution and: (I) The storm-center lo-
<br />cation; and (2) the landcover distribution, Knowledge ofthese
<br />interactions would allow the designer to reduce the number
<br />of combinations of factors ut;cdeu. UnJers[anding which in-
<br />teractions are not significant can significantly reduce the num-
<br />'ber of HMR521HEC-I computer tuns required to estimale
<br />the PMF,
<br />
<br />Volume of Rainfall
<br />
<br />One of the objectives in determining the PMS is 10 selecl
<br />the stonn-area size that yields the maximum volume of rainfall
<br />within the drainage basin. The optimum storm-area size is "
<br />
<br />, ,
<br />highly dependent on the selected storm-center location, Lc
<br />and the storm orientation, 4>, while the actual volume of
<br />resulting rainfall for the given storm-area size is dependent
<br />on the value for the selected isohyetal axes ratio, P", The
<br />optimum values for the storm-center location, the storm ori-
<br />entation, and the isohyetal elliplical axes ratio (and the tem-
<br />poral rainfall distribulioit; to be discussed later on) are se-
<br />lected with the objective of maximizing the PMF. In other
<br />words, tbe largest PMF may not necessarily result from Ihe
<br />PMS that produces the grealest drainage averaged depth of
<br />, rainfall, Thus;-the spatial distribution of rainfall wilhin Ibe
<br />drainage ba~in, as discussed next, plays an important role in
<br />this regard,' , ,
<br />
<br />Spatial Distribution of Rainfall
<br />
<br />The variation in the spatial distribution of rainfall does not
<br />just result from the different optimum storm~area sizes. The
<br />size depends upon the combinalion of values for Leo 4>, and
<br />PI' Although, a"variation in the storm orientation, 4>>, causes
<br />a variation in the spatial distribution of rainfall. it may not
<br />be significant enough to cause a significant change in the
<br />magnitude of the PMF. The spatial variation of rainfall re-
<br />sulting from a variation in the storm...:center location, Le. in-
<br />teracts with the temporal rainfall -distribution, and causes a
<br />significant variation in the PMF magnitude. A variation in
<br />the isohyetal axes ratio, Ph may decrease the volume of rain~
<br />fall, yet provide a spatial variation of rainfall so as to increase
<br />Ihe magnitude of the PMF; the largest drainage.cenlered con-
<br />centration of rainfall yields the largest estimate for the PMF
<br />even Ihough summing to the smallest volume of total rainfall.
<br />Therefore, the overall effects of the storm center location,
<br />storm orientation, and isohyetal axes ratio result in equally
<br />controlling effects for bolh the volume of rainfall and the
<br />spatial distribution of rainfall in estimating PMF,
<br />
<br />332/ JOURNAL OF IRRIGATION ANO ORAINAGE ENGINEERING / SEPTEMBER/OCTOBER 1995
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