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<br />Drainage channels should be designed for che 100-yesr frequency flood <br />event wherever rights-of-way and existing development will allow. Reducing <br />the deaign flood frequency reaults in only samail construction coat .avings for <br />the channels. A 25-year frequency flood design may be used for more restric- <br />ted aress where channel overflow onto adjacent land doea not present a serious <br />ftood ha~srd or potential for loos of life. An easement is required for this <br />opcion on both aides of che channel to contain channel overflows during a 100- <br />year frequency flood. <br /> <br />B. Design Storm Prequenciea - The generally accepted deaign frequency <br />for urban areaa ia thelOOMyearevenc. It iadeairableto prevent O\ajor <br />property damage and loss of life during a 100-year flood aoturante. Thio event <br />" alao commonly used far floodplain regulation and for flood ha~ard analysis <br />1n the Federsl Flood Insurance program. <br /> <br />The design frequency for drainage structures and channel improvements <br />in the Town of Eli~abeth should consider local applicable conditions. Some <br />of the more important tocal conditiona that ahould be considered inClude: <br /> <br />C. Hydraulic DesignStandarcls- <br /> <br />1. The .treets in the Town of Eli~abeth are gellerally gravel surfaced <br />andth"-atreetprofilesprovide..inim<llclear<lncefordraina&e <br />struccure.. <br /> <br />(1) Open Channels <br /> <br />The following minimum design standards are <br />recommended. <br /> <br />Z. The depths of flooding from a lOO-year event arc shallow and the <br />relative magnitude of floods ia small. <br /> <br />Side Slopes: <br />Flow Depth: <br />BoctomWidth: <br /> <br />3. The damage to atreet. due to overtopping has been minimal in the <br />past and occasional regrading of the streets is considered accep- <br />table. <br /> <br />Slope; <br /> <br />4. The low $Creet profites do nocprovide a significanc dafrDllingeffect <br />on flood flows. <br /> <br />Velocity; <br /> <br />:i. Exiating""vet"p_nC, rignl-oi-w"yconstrainta, and street proiit"a <br />limit the level of design improvements which can be economically <br />implemented. <br /> <br />As flat as possible, 3;1 minimum. <br />3.~ feet maximum. <br /> <br />4to6ti"",sthef1<Ndepth. <br />Elizabeth topography ia generally steep. Drop structures <br />may be required to reduce Slopes and prevenc erosion of <br />channels. Cha"n"l slopes in the range of O.St to 1.0% are <br />de.irable. <br />The max~mum velocity should not exceed 4.0 feet per second <br />for unlined channels during the lOO-year "vent. <br /> <br />(2) Culverts - The following standards arc recommended. <br /> <br />Baaed on careful consideration of the items above, it H recommended that <br />the 2S-year frequency evenC be used for design of drainage atructures in the <br />re.idential sleasof th" Town of E!i..abeth. l}rainage structures inbu.iness <br />and commercial areas Iihould be dCli'gned to convey the SO-yeAT fl00d. ~~~ver, <br />che effecta of the l00-ye~r event muat alao be analyzed to aSSure that exces- <br />8~ve or undesirable property damage is .voided. The difference in fLood flow <br />between the design flood "nd the lOO-year flood will be allowed to over top the <br /> <br />Type: <br /> <br />Inlet & Outlet <br />Treatmtmt: <br /> <br />street. <br /> <br />Slopea: <br />Minimum Allow- <br />able Size: <br /> <br />-53- <br /> <br />Culverts may be of an accepcable shape and materi~l 'uitable <br />to the site application. However, the proposed atrutCur.. <br />shall be approved by the designated representative of the <br />town. <br />Flared end ..eccions or he.dwalts with wingwalls are desir- <br />able lu minimize ent...""e ..nd fri"tion l.;uea and C"blend <br />intostopes. Er08ion protection may be required at pipe <br />outlets where velocities are high. <br />Culvert alopes 8hal1 beat least 2.01: to reduce s<'di,..;,ntation. <br />No culvert shall be in.talLed with an e~uiv"lent arc. lea.. <br />than 18 inch diameter round pip,,- <br /> <br />-54- <br />