|
<br />SENSITIVITY TO PROBABLE MAXIMUM FLOOD
<br />
<br />By Ablam I. Sbalaby,1 Member, ASCE
<br />
<br />ABSTRAC1: The probable" maximum flood (PMF) is a function of numerous factors, both meteorological,and
<br />hydrologic. An accurate estimate of the PMF, required for high-hazard design problems, requires finding the
<br />optimum combination of i.actors. The PMPJUH (probable maximum precipitation/ul1it hydrograph) method
<br />is the currently accepted method to estimate the PMF. Difficulties with the current trial-and-error process
<br />used to estimate the PMF include: (1) Finding :li.e optimum combjul.1tion of. factors; and (2) 11::'::,~:::,::'}J}g. lhe
<br />rel20tive effect and precision of the underlying assumptions. The first difficulty may lead to an inade.quate
<br />design. and the second difficulty may result in an inaccurate estimate of the risk involved in the design. All
<br />experiment was designed to perform a sensitivity analysis of the PMF to its contributing factors. A set of
<br />guidelines were developed for assessing the effects of causative factors on the PMF, so that: (1) The optimum
<br />value of the PMF is more likely to be computed; and (2) the effort to find the optimum PMF is reduced.
<br />Preliminary verification of the results was conducted using Seneca Creek, Maryland, as a case study.
<br />
<br />INTRODUCTION
<br />
<br />The planning and design of large hydrologic or hydrolog-
<br />ically affected facilities often requires the assessmenl of max-
<br />imum flood potentials. Consideration of maximum flood po-
<br />tential is used for types of engineering tasks such as the
<br />following: (]) The design of high-hazard dams that are con;
<br />structed in populated areas, where dam failure could cause
<br />a major loss of life; (2) Ihe planning of reservoir flood op-
<br />erating procedures; and (3) the siting of nuclear power plants,
<br />Furthertnore, Ihe maximum flood potenlial is used when
<br />flooding cannot be toleraled, The most common term applied
<br />to this upper limit on flooding is the probable maximum flood
<br />(PMF).
<br />Corresponding to Ihe PMF is the probable maximum pre-
<br />cipiiation (Pill), The PMP is defined as the theoretically
<br />greatest depth of precipitation for a given duration that is
<br />physically possible over a given size storm area at a particular
<br />geographical location at a certain time of the year (NWS
<br />]982). For design, the computed PMP is spalially andlem-
<br />porally distributed over the drainage area in ,order 10 maxi-
<br />mize runoff, The resulting synthetic design storm, which pro-
<br />duces the PMF from a particular drainage area, is called Ihe
<br />probable maximum storm (PMS). The NationalWealher Ser-
<br />vice (NWS) has provided PMP estimates for the entire United
<br />States in "Hydrometeorological Report (HMR) Nos, 36, 43,
<br />49,51, and 55" (NWS, 1961, 1966, W77, 1978, 1983), Of
<br />special interest inlhis study is "HMR No. 51,"whicb contains
<br />generalized (for any storm area) all-season PMP estimate,s
<br />for ihe United States, east of Ihe -105th meridian, The NWS
<br />has provided criteria and a stejrby-slep procedure, for con-
<br />figuJing a PMS using PMP estimates from "HMR No, 51."
<br />This procedure is presented in "HMR No, 52" (NWS 1982),
<br />with a computerized version being available (HEC 1984). The
<br />PMF is computed as the runoff from the PMS using an ap-
<br />propriale ,precipitation-runoff model such as BEC-I (Hy-
<br />drologic 1981). " .
<br />The PMF is a function of nUmerous faClolS, bolh mete-
<br />orological and hydrologic, and for any walersbed Ihe PMF
<br />requires finding the oplimum combination of, faclolS. Al-
<br />though Ihere are a number of methods that may be used to
<br />estimate the PMF, at present, Ihe optimum combination 'of,
<br />
<br />IAssr. Prof.. Dept. of Civ. Engrg., School of Engrg., Howard Univ.,
<br />Washington. DC 20059. . . .
<br />Note. Discussion open until March 1, 1996. 'To extend the closing
<br />date one month, a wriuen request JJlQst be filed with the ASCE Manager
<br />of Journals. The manuscript for this paper was submitted for review and
<br />possible publication on June 1, 1993. This paper is part of the Jounwl
<br />of ImgaJion and Drain/lge Engineering. Vol. 121, No.5, Septemberl
<br />October 1995, @ASCE, ISSN 0733"9437/95/0005-0327-0337/SZ,()() +
<br />$,25 per' page, Paper No, 6263, '
<br />
<br />faclors must currently be determined by a trial.and-error pro-
<br />cess because there are no guidelines for assessing the effect
<br />of each factor on the PMF. Wilhout guidelines for making
<br />the PMF estimat~, inexperienced engineers andlor policy-
<br />makers are faced wilh two basic problems: (1) Finding Ihe
<br />optimum combination of factors; and (2) assessing the relative
<br />effect and precision of the underlying assumptions. The first
<br />problem may result in an inadequate design, whkh could lead
<br />10 Ihe failure of a project while Ihe second problem may result
<br />in an inaccurate estimate of the risk involved in the design;
<br />Ibe latter may be a significant faClOr in economic analyses,
<br />Giyen that the PMPIUH (unil hydrograph melhod) is Ihe
<br />currently accepted method to estimate the PMF, the objective
<br />here, was 10 develop a sel ot'guidelines for assessing the effecls
<br />of causa live faclors on the PMF, so that tbe optimum value
<br />of Ihe PMF is more likely 10 be compuled. The proposed set
<br />of guidelines are intended to eliminate the trial~and-error
<br />process in selecting factors to estimate the Pt.iF thus mini-
<br />mizing the. chance of obtaining. an inaccurate and underesti-
<br />mated value,oflhe PMF. Furthermore, tbese gniddines allow
<br />the design engineer andfo! the policymaker to assess the con-
<br />fidencei" Iheir assumptions about tbe contrihuling faclors,
<br />
<br />METEOROLOGICAL AND HYDROLOGIC FACTORS IN
<br />PMF ESTIMATION
<br />
<br />, ,
<br />.'. . . . .
<br />.. '., .
<br />'The PMF determination procedure usually considers Ihe
<br />following meteorological and hydrologic factors. The mete-
<br />orological factors are: (1) The PMP depth; Dp; (2) the du-
<br />ration of PMP, Tp; (3) tbe.isohyetal pattern; P,; (4) the lo-
<br />cation of the storm Genter, L,; (5) the orientation of the
<br />storm, <\>; (6) the storm-area size, S,; (7) antecedent storms,
<br />A,; and (8) the temporal rainfall' dislribution, RT. The hy-
<br />drologic faclors are: (1) The size of Ihe drainage area, D,;
<br />(2) Ihe shape of the drainage area, SD; (3) Ihe land cover
<br />distribution, LD; (4) the antecedent soil moisture AM; (5) the
<br />infiltration rate 1 R; (6) the shape of the unil hydrograpb, S UH;
<br />and (7) storage flood routing; FR'
<br />The PMF, varies with both meleorologi~al and hydrologic
<br />fact~Ts, so .it is necessary in design pract.ice. for the designer
<br />to vary these factors until the optimum comb~nation of factors
<br />is found, While Ihe values for the PMP deplh and its duration,
<br />Ihe isohyetal pattern, the location of storm center, Ihe slOrm
<br />orientation, and the temporal rainfall distribution are selected
<br />with tbe objective of maximizing the peak flow, Ihe storm-
<br />area size is selected based on the values for P" Ln and tP
<br />with Ihe objeclive of maximizing Ihe tOlal rainfall volume,
<br />The antecedent storms may be modeled indireclly by assum-
<br />ing completely saturated soil moisture conditions. As for the
<br />hydrologic factors, the size and shape of the drainage area
<br />
<br />JOURNAL OF IRRIGATION ANO DRAINAGE ENGINEERING I SEPTEMBERlOCTOBER 19951327
<br />
|