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<br /> ' Scvrtal cxam(+Ics of thc computatn� � ��( .
<br /> tunoff flow ratcs and runoff volurnc� .�s .•,�
<br /> ° e Mia
<br /> app;icd to the design of stormwater dclenticm 1 �''�"
<br /> facilities are �vcn in the foliowing nagcs !�� ; �` "O"` c°°•��°� eM��
<br /> illustrate thc use of various mcthods. I "�I
<br /> Although examplc• �rc includcd that cmbody '� � '�' ;; ;
<br /> �hc use of thc basic Rational Fomiula, i' :; ' ; '0"'" ,
<br /> not it�tende� to su�est, encouraFe or
<br /> i�crpctuate the usc of this mrthnd. tiuch �1 _ �z ,•��.
<br /> examplcs are indudcd because they aic ,
<br /> �� ;+ic�] of the dai�n practices of personnel ��( •
<br /> many cnpnccring firms and public agcncics � 2O'"'" •;�
<br /> acro� the country. � ��`�
<br /> Examplcs atc givcn of thc Modificd �!
<br /> .� -�.
<br /> P�tional �'cthod Analysis (uscd in A1adisc�n, 4' ,�. ,,,,, ;a
<br /> 11'isconsiu, and in other r}accs); a refinement " :}
<br /> of the Rational Alcthod developed by Ut. Ven '- .., M,., ,
<br /> 1'c Chow for particul�r application in thc ° ' �
<br /> `��ate of Illinois; and the Colotado Urban r
<br /> :iydrogaph Prucedure. tJ I' �`f
<br /> Afodlfied Rarional Afethad A�u�lysis:' I � •
<br /> 1'hc tcrm Modi.icd ationa ri 1�8IVSI5 �
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<br /> rc(ers to a proccdurc fot nianipulating ihc l ' i ' i •,
<br /> basic Rational ?.'�:thod tcchniqua to rcile�t ���� " �---- � �,�
<br /> ttm fact that storms with duralions greatcr _ � ! �
<br /> ?lian lhe normal timc i�( concentration (or a � !
<br /> ba in will tesult in a largcr volumc of runoff °� �o .o ,o .e ., �
<br /> cven though the peak discharge is reduced. ���� �w M���.0 - �.
<br /> This grcatcr volume of ninoff produccJ by
<br /> lunger duration storms must bc analyzed to � �'
<br /> dctrimine the cortcct sizing for detenlicr� FIGURE9 MOUIFIEDRATIONAL r
<br /> �acilitics. MEfHOD FiYDKOGRAPHS ",�
<br /> h1any limitations and shortcom�ngs in tlic ��
<br /> a�.umption� bchind this mclhod arc c�ident. '�
<br /> The approach becomes more valid on hydroFraph with the peak discharge ';;,
<br /> ��rogressively smaller basins� eventually coinc;ding with the time of concentration for ;,z
<br /> �uulung a size so small that watershed lhe ba�in (T�), is first calculated using the `;,
<br /> modcling ie approachcd. c roccdwc ronnal (onnula Q = CiA. Following this, r 'M�
<br /> _hnuld, therefom, be limitcd to iclativcly. f•rmily of hydrographs rcpresentin� s:�mis of
<br /> tr,_iall arcas such ;is tuofloLc_parkin� lots, or grcater duretion are dcveloped. The peak �; �
<br /> S,thcr upstrcam arcas with lnbutary basinT ninuff ra�e for cach hydroqtaph is equal to � ; �
<br /> than .� a�rrc _l�i� w�,��l�{ m���un�zc man�r CiA whcre (i) is Ihe reinfall intensity for the � ;
<br /> d;imaec which could result from orcatopping stomi Juraiion in qucstion. Thc nsin� limb �
<br /> or lailurc of the proposcd dctention faality. and fallin�limb of the �2ydro ra h are, m �4,
<br /> Figurc 9, hluJificd Rational McthoJ cach c:isc" cqual to { ' or t ic basin. e '►`
<br /> Ilydngrapfu, presents a farnily of curves (or a batic auumntion of this m�thocl is that the ,;� '�
<br /> thcomlical basin. 77�esc hydrographs are �ca _ undcr the assuri ' tranazoiAal i�:
<br /> Jcvclopcd by usiug ihc basic Rational Atcihod �uurraph eyuals thc volume of ninof(from �'
<br /> �csumplions ot constant rainfall intcnsity (i) Ihc Ihcorctical rainfall. Thc arca under thc '�
<br /> timc of Conccntration (T�) [rom lltc moct hydrograpli is also cqual to the pcah diechargt �
<br /> dLtlant �cir,t, lirr�cuis!•, acd thr cccf�ic�^��t �,( tat-� (nr that particular rainfall times lht a
<br /> �unoff (Cl. 71,c t} n�,;il i?.�u:�n:�l 1'���nrl,i di:ration of Ihc iainfalL ��
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