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CHAPTER XIII PROPOSED DRAINACE SYSTE~ IXPROVEMENTS AND ESTIMATED COSTS Streets are placed too high to intercept anything more than trickle flowa. The inlets should be ~dified to depress the grates approximately 3 inches below the gutter flowline. This viII prevent vster from bypaRsing the inlet. and increase interception significantly. The objective 1n providing drainsge system improvements ia to meet the criteria explained 1n Chapter Xl. The propo.ed facilities ore intended to reduce flood damage. to homes within the floodplain and reduce future main- tennnceco.ts. Improvements are ahown in plan On Plates 17 through 19 and are di.cussed separately beginning at the downstrea~ end. Each location is identified by reference line .tationing. The proposed channel improvements include designs for both the 25- and lOO-year events. The IOO-year frequency event design is recommended for implementation where the rights-of-vay and existing development viII permit. (3) Station 10+20 to 11+80 - This area presents a severe flood ha~ard to the cOltDllercial improvements downstream. The potential capacity of the Kiowa ~venue storm sewer is approximately 105 cfs vithout overtopping of Banncr Street. However, the present inlet grate and raised inlet crest limit the capacity to approximately 55 cfs. If brush and debris begin to plug the small open1ngs 1n the inlet grate, the capacity will be reduced even further. Since any overtopping of Banner Street will result in a flood h8~ard to the buildings On both sides of Kiova A~nue, the full capacity of the sto~ Sewer should be utili~ed. Kennett Drainage To reduce the present flood ha~ard through the main commercial area of Elizabeth, the storm sewer inlet area should be improved. We recommend that the inlet crest be lowered approximately eighteen inches and that the parking lot also be regraded and the flowline lovered approximately one foot. The tr..~h guard should he replaced vith a new guard "i.th larger open1ngs (apl'rc>xi- mately 5 inches). The sto~ sewer improv~~nts are the key elements in reducin~ the greatest potential for flood damage in Elizabeth and Should be given high priority for implementation. (1) Station 1+75 to 4+75 - The lover 300 feet of the storm sewer is restricted appruxi""'-tely 40% by sedimentaccunLulation. The remsining leagth elf the storm .ever is clesr. Part of the problem ia caused by the flatter ~lope of the 42" diameter RCF section. The outlat cllannel is slso mild slop- ""g...ndisre.triet..JbyseJi",,,,,tdepoaits. The sediment level in the 42"dia"",ter plpe may become relotivclystabl e at ita present level near the midpoint of the pipe. This is the level of maxi- mum sediment transport for the pipe at its given slope. If sediment continues t"b"i1daftet several significant flo"s, a fl"ahiagorJredgingmaintenanee program will be requir"d. Improvements upstream from the storm sewer consist of increa.ing culvert si~es snd providin& "ell defined and maintained channels. Implementation of the recommended improvements will reduce flood levels and provide better sedi- rhe channel downstre~m from the "ever outlet shouid be modified to Corve ment conveyance. dovnstream tovard the bridge. An additional 6 to 12 inches of (all obtained by this improvement will help avoid sediment accumulation i.n the channel whiCh c~uld affect the culvert. (4) Station 11+80 to 13+10 - A c.hannel should be excavsted vith the following .i~e and properties: (2) Station 4+50 t" 10+00 - The ~torm .ever inlets alollg Ki"w" Aven",' will intercept a minimal :lmount 0:' ".Her. Inlet grates:lt H:lin and banner Option 1; 100-YearChannelDesign Option 2: 25-YcarChanne1Design with Drainage Easement Capacity - QIOO - 285 cfs BottomWidth-gfcet Side Slopes - 4:1 or flatter Estimatcdn",pth-I;9f<1ct Capacity - Q25 ~ 135 efo BOttom Width - 6 fcct Side Slop<1s - 4:1 or fiatter Eatimated Deptb - 1.5 fect -59- -60-