|
•
<br />
<br />16
<br />Geelobical Surcep ~ and, if possible, confer
<br />tclth the Sur•:c::'s district c^oineer. He
<br />should a1>o make a search of the records of
<br />~;hrr Federal agencies tichich m,a~• hace col-
<br />!(~1~::•l! i::Ca~matton in the rC~lOn, and the rec-
<br />ur i.: l•; ~ta'.c ~•:atcr corscr•::.tion agencies or
<br />~!:ud you;u;ic:l xurt'c;:s: :u:il he would deter-
<br />!::h•.'r tun' infurn:alicr, mac be acail-
<br />ir.t l.r ., ,
<br />alde Iro^.l oi!Irr SiaiC Urpar•^.1ctt!s. f:'ort: coiltl-
<br />tp en,-ir.re_ o~ces, from mtmicipalities in the
<br />•:~cini:~, or from util7t_: companies. \`~here
<br />~trr::n:;lotr records are no; a~•ailable, some
<br />a;•enEic; or irl:.^.b;tints o: t!:e ~•icini;}• ma;
<br />1::1•:c it:forma;ion about high-~•rater marks
<br />c~a:ad by spccif:c historic P,oods.
<br />1~-itll respect to the character of the siream-
<br />f'oti•; data available, floor' fotcs at the damsite
<br />map Lc determined under one of the follotvin;
<br />ccmiitio:u:
<br />(i) S;r~autl,o:c record at or xcar the
<br />do:r,silr.-lf such a record is available and
<br />c•rc~ r~ n period of ^_0 .'cars or more, the
<br />P.oud P.otcs sho~cn b;: t!7e record ma>' be
<br />;foal;: zed to provide !food f: equenc}• caiues
<br />(sec. 59). Iicdrographs of outnt;:nding
<br />fivo•.! cvl~nts rau bu :ula;zed to proridv run-
<br />a.C factor; for use in dcra'n:ir.ire the maxi-
<br />mum prob:lb!c flood (secs. ~1, 53, ~~, and SG).
<br />1( such a rccm'd is acailablc but corers
<br />only a tcw pears, it map not include an}•
<br />flood of ercat maenitudc ~cithin its limits
<br />ar.d, if used alone, it could Fire false indica-
<br />tiun uC flood potential. ~1m..1}'sis ma>-, hotc-
<br />ct:•r, ~i~'e some or all of the runoff' factors
<br />uce~~'d to compute the maximum probable
<br />do•al. rrcquenc;• calucs obtained from a
<br />short record should no; be used ~cithout
<br />:males, of data from nearby ~':atersheds of
<br />cum, ar::blc runoff characteristics.
<br />(^_) Strcr,nLriurz• rrca'd ct•aifablc ou the
<br />e; r,nnL itself, but at e c•o rreiucreble distaxce
<br />f r, ~,n 'the darrrsitc.-Such a record may be
<br />:ucLlp:•.•II to pruv;Jc unL•;:ra17h characteristics
<br />(sets, v°_, SS, and ~~I1, trod frcyucncy data
<br />which n:ac t:c t:::n<fcrred to Lhc dam~itc be
<br />,•;n'oprcac :Irca and basin-rharactcristtc
<br />wr:ilcic::ts (secs. ~: surf ~J). This transfer
<br />• 1'W.L.n•~ I Lf tAe (:o. rnmrnl 1'nn, rr (liL•r nJ ••u JnLv ,e
<br />LL,.r ..i. ~. .b •I .. J. u... ,., ~ ,..~.. norm luel,c wn., ..,..a
<br />~mlvr La,l Lnn., ,rs in lnr (iminJ Yriv• •re m J. uCn4~J.
<br />OESLGN OF Small Dc%'.$
<br />can be made directl>• from ot:e drair.a~e area
<br />to another if t!,e areas hate comparable
<br />characteristics. Often dams:es are Iccated
<br />v:ithi^ the trar.sitior. zor.c :rem most:;a.ns to
<br />p'.ait:s and the StCE•am ~;aen:~ . -^:iocs ::rc
<br />located v:ell out on the ;71ai:•s; in <nch in-
<br />stances, s;7cccLl care mu=; be exrrci~cd t•.h~~n
<br />u<i:7~ the plains mcnrd fur datcrnln:;r;ion of
<br />flood aov: s a[ the d:unsitc.
<br />(3) .\'o cdcquc'c .chca7+Lllo1~ dnfa rrcii-
<br />a61c a7 tnc spcci,nc s:recnt, bLLf r. sl;ti~rcr~~.rp
<br />record %pr c rirr.i7tcge bc.ci7: of si,uiinl cnr.r-
<br />aftErlStLCS ill IILC SC AiC 7~C1J Uj N.-$GCh a I"eC~I'a
<br />ma}' be anahzed for unit;raph citar,:c:eri;-
<br />tics and ireGucncc• data, : rd there L!aia t,.....
<br />fcrred to the danai;c b~: cppropriate area
<br />and basin-cl:aractcri=ticcoc`'icieats.
<br />(4) SU'cau~t7otc rcrorri~ it ;i+r rrgin,l. b+~t
<br />trot satisfr.ctoril:/ usrful Jor r.;~plirr.tiu•~ null
<br />aucl~sis tutdrr' er7c nj tl7r nborr rll.:•~•,•!.:.
<br />-These records mnc be as_ertbled a::d t:n-
<br />alpzcd as rc•fcrrr.cc ir.`arr:::;i;;n lm __:sera!
<br />rur.c5 char:tctcris!:cs.
<br />(~) L'e'e uJ iLigiL-tc'ctrr r~lrnl:?,_;;i~_'n-
<br />tti'ater marks pointed out by inh::bi;a~.:< of
<br />the ca!le;: should be used ~ci;h v::uuon in
<br />estimating flood ma~nuudc>. Hl~•.~ecrt•.
<br />uhcre there arc a number ci hi•_h-crater
<br />marl, in the cicinitr of the p:roi,c:. arii p,r-
<br />ticula:ly if such ma:•;;s are ob:a7n:•<i ire^~ the
<br />records of public uaces (.;ucb ^~ J;:ar ':i~th-
<br />o:ap dcpartmz•nts cr c•ocntr en_inicr>). th~p
<br />ma>• be used as the basis of a separ:r,a sup-
<br />plemental study. These record; me}' ale aced
<br />to determine the orator cross-sectir~na! ;Ina
<br />and the enter ,uriace slope for the tlo~a! to
<br />v.'hich they refer, and from these c!a:a an
<br />estimate of that pa r;icular flood peak n:ar be
<br />prepared using the slope-area n:ethud de-
<br />scribed in :7, pendix P>.
<br />1S'hencvcr it appear that there kill be e.^.e or
<br />mere flood sracon.: benvecn the se!ect:cat et the
<br />damnitc and cun.uuctian of the dam. is:ihtics
<br />for scc•urin;; a s;t'r;:^.I,lutc rccor~ (ur thr prllj.
<br />ec'. should be ,ct up as prun:ptl}• as fn~•sit,ic.
<br />This is of particular :rt:purt::r.ce in nrJcr to Ilb-
<br />t;lin t•.atcc:hcd data dirccL'c a:~;.lirai.lr •u the
<br />comnu;:rtion of the in;lo:•: drsl~n flood .`or thr
<br />d;: m, althou; h u rrcord usaLle for flrl;ccncy
<br />cum~putatiun.; c;nuwt Lc srcurrd. "I'hr tar7li-
<br />
|