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<br /> A sir� ple method for i�eiention basin design
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<br /> By Glen Yrjanainen and Alan W. Warren' �
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<br /> Because the developmenl of land for agrorian, residcn- changing as the head varies. This type of outlet can be
<br /> tial,commercioi ur industrial use continues�o increase, anolyzed by opplying basir calculus to the controlling I �
<br /> the tempowry itorage o(storm runofi in an onsite reten- outFlow equalion. If the outflow is of a wnstant mle,
<br /> tion brsin has become essenlial due fo inodequate out• i.e.,a pump,the analysis is easie�.The volume of storm
<br /> = lets for ihe increased slorm runoff crealed by develop- water info Ihe retention bosin can be d�� ermined by the I
<br /> � ment.lnmostcases,andprimarilyforeconomicreasens, rational formula. The required s�oroge volume is fhe
<br /> �`° adequately designed colledor srorm water rystems (m- volumc of runoff thal Oows in'o Ihe basin minas that
<br /> - poble af handling the slorm runoH from ultimate devel• which Rows ouf.
<br /> opmenl)lag behind the deve!opmenl increases.Because An equation can 6e oblained that relates volume of
<br /> oi ever increasing conitrudion cosls ar.d fne infeasibil• storage to allowoble outflow using the time of fhe storm
<br /> - ity of installing large-diameter storm drains,the concept as a parameter.This equation can then be maaimized by
<br /> of ul�imate design or improvement of colledor storm hasic calculus to find Ihe peak srorage time, wl:ich in I I�
<br /> - waler systems is impradical. The �etention basin that tum can 6e used to cclwlole the moximum volume of I
<br /> =' � meters or restricts flow is here lo ;lay. storage�equired. The one assumption in this melhod is i1 , j i,
<br /> � DESIGN MEfHODS. One meihod of relention basin that sform water rises in the reteNion basin al o con- �
<br /> ' � � slant rote to fill the basin�o the eak volume,and tha� II , I
<br /> design�sed by many design engineert is to use tne ro- P
<br /> tional formula to calculate the peak volume o(retenlion ihe inaximum ollowable.oul0ow is reoched only,al,this , I
<br /> 1 � reyuired.The in0ow�olume for an incremental time of peak volume.� � ,
<br /> � Ihe slorm is chorted,then used lo delermine fhe depfF of e envations of retenfio� equations (or relention I
<br /> refenlion and hence ihe out0ow over Ihe increment of basins wifh a gravity Oow,chonging rale orifice ouflet,
<br /> = time selected. The same procedure is followed for Ihe and a constont rate pump outlet follow. These deriva-
<br /> - next time increment until ihe peak vuiume of retention is tions are for o len year(requency storm in ihe Oakland , j
<br /> !ound. county, Michigan area. Retention equotions for di6 '.
<br /> - One disadvantage of this meihod is that it requires a ferent yoar frequenty s�orms and ofher creas con be ob-
<br /> - Irial volume and areo of relention to be s?lected by ihe tained in ihe same manner. �
<br /> _- designer in order lo delermine the incremental depth of �Connnued on p.al�
<br /> � retention ond ils corresponding outFlow.Another disad-
<br /> --- vontage of this melhod is thar the outllow over the time
<br /> increment is only an average opproximolion, and not DEFItdITION OF TERMS
<br /> ' Ihe instantaneo�nly changing uulflo��that odually dnes
<br /> - occur with on outlet dependenl upon changing head. Fi-
<br /> � nally, this melhod is tin:e consuming, tedious and may A = Contribufing ocreage
<br /> , ir.volve more than one frial if o Fcor:elecfion for ap- o = Cross sedional area of outflow pipe(ft')
<br /> proximale vol�me ond area is made by ihe designer. C = Coefficienf of imperviousness
<br /> - Anolher mefhod of retention basin design used by de- � = Coefficient of confraclion (or an orifice
<br /> ff -- sign engineers is graphicol. If invol.'es obtaining o syn-
<br /> , � the.ic hydrograph for the particular design storm and 9 = A«eleration due to graviy (32.2 ft/sec')
<br /> development in question.The volu�ne of retention is then h = Heod, de(ined as the dislance irom Ihe water
<br /> obtained by calculating the area Lelween Ihe bell surface to Ihe centerlinf�of the oufflow pipe(N)
<br /> � J shaped hydrog�::;ph curve and the superimposed allow• I = Rainfall intensity �175/t t 25) �
<br /> able oufFlow curve,ond then converting Ihis orea+nto a K, = Constant (h/t)
<br /> volume. K, = Constanf(co �, 2gK �
<br /> One disadvanlage of ihis method is fhat 1he designer Ui = Instantaneously chonging oulflow(CFSJ
<br /> � 1^, hos no accuraie melhod of selecting fhe durolion of the �
<br /> storm for which to plol the hydrograph.Another disad- Qo = Mazimum oul(low per acre imperviousnesi
<br /> vantage is thal it is difficul} to obtain an outflow curve (CFS/AC) I
<br /> — � fcr an instanlaneously changing oulFlow lo superimpose T = Srorage time,defined as Ihe instant storoge be• � �
<br /> � , on Ihe hydrograph. Other disadvonfajes are that a gins until Ihe peok sbroge is attained (min� : I
<br /> `_- grephiwl smution is not thuf occurale.und i:somelimes � = Insfantaneous time of storage (min)
<br /> owkward lo abtain. Vn = Volumc of waler flowing inlo ihe relen�ion bo�in � I
<br /> ' �' CAICUWS.Whatisneededisastraighifo:ward,um peracreimperviousness �ft'/AC) i
<br /> : complicated mofhematicol method nf designing a retem Vo = Volume of water powing out of the retenlion � �
<br /> tion basin. If ihc basin wero to hove no oullet,all of�he basin per acre imperviousnesi(Ft'/AC) ��
<br /> s�orm runoff would hove to be ponded.The(act is thol V: = Maximum volume of wa�er s�ored in the relem
<br /> most basins do have an oullet,wlth the outFlow depend• tion basin per acre imperviousness (ft'/AC) � j
<br /> ing upon Ihe amounl of wotcr that is p¢•2)ed and �he � y� � Maaimum volume of woter slored in the reten-
<br /> dcpth of retenlion. The outFlow is mslanlaneously tion basin (ft')
<br /> •C�.1 mg�neerand engnee�ing aid�II.Dmin Commiuioner't OILce,
<br /> Owland Counry,Ponlac,Mich.
<br /> ,� WATER & SEWAGE WORKS, December, 1973 35 �
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