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CHAPTER XIII PROPOSED DRAINAGE SYSTEM IMPROVEMENTS AND ESTlMAIED COSTS Streets are placed too high to intercept anything more than trickle flows. The intets should be modified to depress the grates approxi~ately 3 inches below the gutter flowline. This will prevent water from bypusing the inlets and increase interceptioa significantly. The objectiv" ln providing drainage syste~ improvements ,. to meet the criteria explained in Chapter XI. The proposed facilities are intended to reduce flood damages to homes within the floodplain ..nd reduce future "",in- tenance coats. Improvements are shown in plan on Plates 17 through 19 and are discussed separately beginning at the downstream end. Each location is identified by reference line stationing. The proposed channel improve~nts include designs for both the 25- and 100-year events. The 100-year frequency event design ia recommended for implementation where the rights-of-way and existing development will permit. (3) Station 10+20 to 11+80 - This area presents a Severe flood hazard to the commercial improvements downstream. The potenrial capacity of the Kiowa Avenue storm sewer is approximately 105 cfs wirhout overtopping of Banner Street. However, the preaent inlet grate and raiaed inlet crest limit the capacity to approximately S5cfs. If brush and debris begin to plug the small openings ln the inlet grate, the capacity will be reduced even further. Since any overtopping of Banner Street vill result in a flood hazard to the buildings on both sides of Kiowa Avenue, the full capacity of the storm sewer should be utilized. ~nnett Draina~e (1) Station 1+75 to 4+75 - The lower 300 feet of the storm sewer is re~tricted approximately 40% by "ediment accumulation. Ihe remaining length uf the storm SeWer is cl~ar. Part of the problem is caused by the flatter slope of the 42" diameter RCPaection. TheoutletchannetisalsoOlildslop- lng and is restricted by sediment deposita. To reduce the present flood hazard through the main commercial arCa of Elizabeth, the storm sewer inlet area ahould be improved. We recommend that the inlet creat be lowered approximately eighteen inches and that the parking lot also be regraded and the flowline lowered approximately one foot. The trssh guard should be replaced with a new guard with larger open.nga (approxi- mat"ly S inch.,,,). The storm sewer improvements are the key elemenU in reducing the greatest potential for flood d~ge in Elizabeth and should be given high priority for implementation. The sediment level in the 42" diameter pipe may becoIllC relatively stable at its present level near the midpoint of the pipe. This is the levelofllll1xi- IlIUm sedimcnt tr:lnSport for the pipe at its gi.vcn slope. If sediment continues to build after scvcral significant flows, a flushing or dredging maintenance program will be required. Improvements upatream from the storm sewer consist of increasing culvert sizes and providing well defined snd maintained channels. the recommended improvements wilt reduce flood levels and Implementation of provide better sedi- The channel downstream from the sewer outlet should be modified to curv~ dowostream toward the bridge. Anadditionalbtol2inChesottallobtained by this improvement w,ll help avoid sediment accumuLation in the channel which could affect the culvert. ment conveyance. (4) Station 11+80 to 13+10 - A channel should be excavated with the following size and properties: (2) Station 4+50 to 10+00 - Ihe storm Sewer inlets along Kiowa Avenue will int~rcept a mbi"",l amount of water. Inlet gut"" at Main and Banner Option 1: 100-YearChsnnelDesign Option 2: 25-YesrCha<u,elDesign with Drainage Easement -59- Capacity - Q100 . 285 Cf8 Botto~ Width - B feet Side stop"s - 4,1 or flatter Estimsted Depth - 1:9 feet Capaeity-Q2S,"135cf. BottQ~ Width - 6 feet Side Slopes - 4:1 or flatter Estimated o.'pth-l.5 feet -60-