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<br />have been used as a base for defining the D1S. The 10 and 25-year floods
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<br />""tct'.f... "",":" "H'" '
<br />ft-. '::-F:.l:..!i+~:S-t: .~ '1"":-'~ t 1;:YR-~ ~1
<br />I,,,,""',,,,E'.. i,. ,i:
<br />f:~: r;:::-1r:+..: l;:~: .. ,,~. d.'.. ".
<br />'", !.:l1".Il." . !.'i" "" '"i,..'i :~,'
<br />s.:.:~j"'''":~, . ' "..'
<br />i" ':';; :,i":: = ,,,,
<br />Fe: "" Hi, .." i;:', T'" tr:
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<br />ii,.. TI~T 'i' .." :;
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<br />were picked because they are low frequency floods which can be eatimated
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<br />by widely used regional flood frequency studies and for other reasons
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<br />elaborated below.
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<br />Normalized discharge curves, plots of Eq. 4, are a function only of
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<br />~~~ ~~j::-n:t
<br />".,. ,', ~~' :-,1,. p; i:W. rt: .i','.,~ 1;(:"::1
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<br />.." ,..," """-Wi ''''It ....... "'.. ~
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<br />..".... '~ '.$;"."'7" ~.. ,', ;tF', =
<br />_ . C"l --:,", . _ t-:-l~' _........N ~... ~
<br />'t;"- 4+'I;i-i~ -:-l. + ,,+ . 0 ~H :tt:+- . , t?~ Er !H-' l:td
<br />r.:~~ ilt. '-+.l-. ., 1\ ;: II l:f 11 ~.:...;. H" t II ~.., tT.~ 4t..:- II .,. ~
<br />__, .." ....CI I' '""C "'" = 1,:;1=0=
<br />t:t"".",,,,,,,, '.,:, ;-,~~. ..~~..", .. ~~';~" ~
<br />4-t-:'. ..~.. ;:.;t.::i:.~;..:-.j7,_~t_... ~
<br />Hit ~;- -he "';J.f ....r.;:. -..t:: I~- ".. H-'
<br />_:~f:.t--:-';:....__'Ii= ~"1f :-:""E::-r::-'-_ H
<br />.. ~il':::... 1m: i.fi~~: ;" r"" :::", '+'.
<br />
<br />skew (which can be estimated from regional studies (2)) snd flood frequency.
<br />
<br />The normalized discharge can either be plotted against return period with
<br />
<br />the lines lsbeled by skew (Fig. 1) or plotted against skew with the lines
<br />
<br />labeled by return period (Fig. 2).
<br />
<br />Use of the "Discharge Index Slope" Method
<br />
<br />Use of the normalized discharge curves begins with estimation of
<br />
<br />the 10-year and 25-year flood discharges to determine the "discharge
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<br />index slope", the denominator in Eq. 4. The two discharges are usually
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<br />t::-: ::~: s: .'" .... '-t~ :.t: h :
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<br />....' .t:-:
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<br />best estimated from regional flood frequency studies. The nationwide
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<br />..' ""HI
<br />.".;-':"';'~
<br />,_h ____ ,.... _..~;-
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<br />~t.: ;:~
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<br />series of regionsl studies completed by the USGS in the 1960's is listed
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<br />eO -"c','" ~ $....=
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<br />....,...., -- ..- ~ ~_..-
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<br />'1 ",F=. ..: t, . "-., c:~U~:==
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<br />,,, "'~ .. "', ,'F .;tt,,-,,,,--
<br />II... ,~ . B ., . ,",,'
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<br />"", _ '''i8j;~\;i "" ""_.;=::::c ',..
<br />"::1,'" "" .. ~,"L''''' ,," = L= 'C'-' "" ;,..
<br />: :.: I: :': .,. :~ ~_o+ .,._1;:.. :.:."1 .,., _~: _.1.::-:-: ::..::
<br />;j:: .., "V".. h ",J\r;...; .. E ~i=.
<br />E '.., i"I:i'" "-',"\1\;:.";: .._. ,",
<br />" ;i" i'it ,.:: ,,'I ,,,1 il'];I+, ....
<br />'[: ""~ ""T. ~~:;;: ~~c,"~~~.., ...'CHF
<br />::: .... _q ~:,: ..,. :;:; -i-;L.::..:::t: :t~;';::r ... ..,- ..'- -- 0
<br />.... ..,. r~~"".~F'i1~ H:E-"-"~:;::
<br />ii, ~~.~ ::1-: ~~:~ ~::i \:1": ~~: :-:7. .'"> h." __. ~__ .~: :~: :~.
<br />"",,,,, ..' "" i'" ,:I: I'" to" ,'le' " ",':...'.
<br />''''['1"' I'"'' .." .t. +',...,.....r; 'l:\:: -:--
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<br />in table I, and some more recent studies may be available. Other sources
<br />
<br />of regional flood frequency studies are State Highway Departments, the
<br />
<br />U. S. Army Corps of Engineers, and State Geological Surveys.
<br />
<br />A study by Hardison (2) can be used to estimate the regional skew
<br />
<br />,I:'
<br />
<br />at any location in the coterminous U. S. By knowing the skew, the nor-
<br />
<br />, .
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<br />malized discharge for a rarer return period can be read from Fig. 1. If
<br />
<br />the 50, 100, 200. or SOO.year flood is desired, Fig. 2 is essier to use.
<br />
<br />The normalized discharge (NO) and the "discharge index slope" (DIS) can
<br />
<br />then be substituted in Eq. 4 to estimate the discharge for a flood of de-
<br />
<br />sired frequency (f) as
<br />
<br />Qf = 10 (ND)(DIS) + log Q10......,....,..,.....,(5)
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