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<br />will be constant, the land-cover distribution may vary with
<br />time? the antecedent soil moisture will be a function of the
<br />antecedent storm, and the infiltration rate will be a function
<br />of the landcover/soil type and antecedent soil moisture. The
<br />shape of the UH is a function of the watershed storage char-
<br />acteristics; in the case of estimating the PMF the shape should
<br />reflect minimum storage conditions (Le., a maximum peak
<br />rate factor), Thestorage flood routing coefficients should also
<br />model minimum storage conditions. "-
<br />Wang and Revell (1983) point out the present state of the
<br />art does not permit precise determination of values for these
<br />meteorological and hydrologic factors. They further point out
<br />that depending on the assumptions and data used in the anal- ,
<br />ysis, the estimated PMF can vary over a significant range of "
<br />values, each associated with a certain degree of conservatism.
<br />
<br />DEVELOPMENT OF PMF GUIDELINES
<br />
<br />Identified Range and Assumed Values for each Factor
<br />
<br />As opposed to the unlimited iterations in the trial-and-error
<br />process to find the opti,mum values for the contributing fac-
<br />tors, the experiment designed herein to perform the sensitiv-
<br />ity analyses included identifying the range of.each factor that
<br />would be expected in typical design situations, The selection
<br />of values for each factor used in the sensitivity analyses was
<br />based on three criteria: (1) The judgmen't of past studies on
<br />the given factor; (2) c~nstr':lints: on the computer run time;
<br />and (3) the adequacy of the initial estimates, While the values
<br />of Some ,of the contributing meteorological and hydrologic
<br />
<br />. Current Approach Used by NWS to Estimate PMF '
<br />
<br />The National Weather Service (NWS) is an authority on
<br />the determination of PMF. According to Ely and Peten; (1984),
<br />the NWS criteria for defining a probable maximum storm
<br />(PMS) require that the magnitude of four meteorological var'
<br />iables be established by the hydrologistJdesigner,(I) The 10"
<br />cation of the storm center; (2) the orientation of the stOlm;
<br />(3) the stonn-area size; and (4) the tempciralrairifall distn'
<br />bution, As for three of the 'remaining fourmeteorologieal'
<br />factors (the PMP depth estimates, the PMP duration, and the
<br />isohyetal pattern), the NWS has suggested specific values
<br />(discussed next) for these factors to he used in estimating a
<br />PMS, Finally, the NWS does not make any recommendations
<br />or suggestions for the value of the antecedent storm :magni-
<br />tude. The NWS recommends that the four PMS variables;:
<br />Ln <1>, S" and Rr> be chosen to produce the maximum peak
<br />'discharge or runoff volume at the point of interest. They
<br />further indicate that'it is neCessary to calculate runoff as part
<br />of the trial and error process of establishing the magnitude
<br />of the four PMS variables (Ely' and Peters 1984),' ,
<br />The NWS has developed a 'computer program called HMR52 '
<br />(HEC 1984) for applying the NWS procedure for defming a
<br />PMS, The HMR52 program calCulateshyetographs for'the
<br />watershed subbasins, The hyetographs are input to the HEC-'
<br />1 precipitation-runoff simulation program to compute a PMF.
<br />Their obk,ctive in calCulating a PMF.is to obtain the largest
<br />flood that can reasonably occur. But, they further, state that,
<br />because of hydrologic characteristics of a watershed, the larg-
<br />est flood may not result from the storm that produces 'the
<br />greatest average depth of precipitation. Ely and Peters (1984)
<br />recommend that a sensitivity analysis consisting of a trial-and-
<br />error process on the four PMS variables should be performed
<br />when using the HMR521HEC-l PMF estimation procedure,
<br />. Their recommended sensitivity analysis procedure does not
<br />cover a wide range of possible values that the factors may
<br />assume. Therefore, a designer may not find- the optimum
<br />values for the PMS factors, Furthermore, there is no consid-
<br />eration made for possible interactions between the factors
<br />that must be considered in a sensitivity,analysis.
<br />While the HMR52 computer program provides an auto-
<br />mation of the required NWS procedure to compute the PMS,
<br />the trial and error process of establishing the magnitude of
<br />the PMS variables may not allow the designer to find the
<br />optimum combination of factors. The use of a trial and error
<br />process can be a result of inadequate knowledge of the effects
<br />of each factor on the PMF and the interactions between the
<br />factors,
<br />The purposes of this study were to provide an assessment
<br />of the effects of each factor on the PMF and to assess the
<br />interactions between the factors. This will provide a designer
<br />with a set of guidelines to use, which is a more rational al-
<br />ternative than a trial and error process.
<br />
<br />3281 JOURNAL OF IRRIGATION ANO DRAINAGE ENGINEERING 1 SEPTEMBER/OCTOBER 1995
<br />
<br />TABLE t. Assumed V~lues for' FaCt~rs In.- sensitivity Analysis
<br /> Function of
<br /> factor in Value of factor as-
<br /> sensitivity sumed in sensitivity
<br />Contributing, faclor Symbol analysis analysis
<br />(1) (2) , (3), , (4)
<br /> (a) Hydrolo~c Fa'ctors
<br />Size of drainage area D. constant 60sq mi (155,5 km ')
<br />Shape of drainage So' constant elliptical with a 2_5. to 1
<br />area axes ratio
<br />LaOd-cover'distribu. "Lt. vari~ble homogeneous fully - de.
<br />tion ',' 'veloped; homogene:
<br /> OllS undeveloped~
<br /> downtream tiall fully
<br /> developed; Lipstream
<br /> h~lf fully deyeloped;
<br /> p' upper middle half
<br /> fuUy develped.
<br />Antecedent soil mois. Am . , constant c?nipletely'saturated
<br />tore' soil conditions
<br />Infiltration rate IR ' constant. <I> ~ 0 in,/hr (0 mmlhr)
<br /> JOL f~lIy -developed
<br /> hind cover
<br /> <I> ~ 1 in,/hr (25,4 mm!
<br /> . hr) for undeveloped
<br /> land cover .
<br />Shape of UH SUH constant subbasin gamma UH
<br /> -with percent in rising
<br />Stoiage flood ro.uting size .approaching 0.5
<br />FR constant subbasin Muskingum
<br /> storage routtng coef-
<br /> ficients k and x
<br /> (b) Meteorological Factors .
<br />PMP depth D, constant' "HMR No, 51" PMP "
<br /> ~epth for Washing-
<br /> ton~ D.C.
<br />Duiatio"n of PMP T, Constant 6 hr
<br />[sohyetal pattern P, variable elliptical axes ratio: 2_5
<br /> to 1 (NWS recom-
<br /> mended), 1.0 to 1;
<br /> 4,0 to 1
<br />Location of storm L, variable upstrean:l subbasins
<br /> centered (0,3,83):
<br /> drainage centered
<br /> (0,0); downstream
<br /> subbasins centered
<br /> (0.---'3,83)
<br />Orientation of storm <I> variable, 1810(along major axis.
<br /> of drainage ar~a);
<br /> 2700
<br />Storm-area size S, HMR52 op- HMR optimum
<br /> timized
<br />Antecedent storms A, constant modeled' indirectly by
<br /> ass~mrl1g completely
<br /> saturated antecedent
<br /> soil moisture condi-
<br /> tions
<br />Temporal rainfall Rr variable early peaked~ center
<br /> peaked; late peaked
<br />
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