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<br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />C. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />V. <br /> <br />ALTERNATIVES DEVELOPMENT AND SELECTION <br /> <br />conveyance through problem reaches. Alternatives included various combinations of conveyance <br />improvements, roadway crossing improvements and rcgional detention basins. Each of these <br />components is discussed briefly below. <br /> <br />A. <br /> <br />GENERAL <br /> <br />The alternativc cvaluation phase of this study began with a formulation and assessment of possible <br />solutions to the drainage and flooding problems stemming from the lack of an adequate drainagc <br />systcm within the lower First Creek watershed. Those alternatives that were deemed feasible by <br />TC&8 and the project sponsors were further evaluated. The peak flow rates from the base model <br />(i.e. future land use condition with master planned improvements upstream ofRMA) were used for <br />the alternatives evaluation. Evaluation of the alternatives included identifying: peak flows and peak <br />flow reductions resulting from the implementation of an alternative; ROW requirements; potential <br />impacts to existing wetlands; conflicts with existing utilities; and potential impacts to ground water. <br />An opinion of probable cost was prcpared for each alternative." <br /> <br />Convevance Improvements <br /> <br />Conveyance improvements include an improved channel and floodplain management. The <br />improved channel consists of a low flow channel and overbank areas on either side of thc low flow <br />channel. The channcl side slopes will be at a 4: 1 slope. The banks of the low flow channels will be <br />lined with riprap to prevent erosion. A 12.foot-wide maintenance/recreation trail will be provided, <br />per Commerce City criteria. Where possible, the trail will be located in the overbank areas. A <br />suggested channel scction is shown bclow in Figure V-I. <br /> <br />The following text summarizes the process of identification, evaluation, and selection utilized in this <br />study. A complete discussion of the alternative evaluation process, as well as a discussion of <br />alternatives that were eliminated before the evaluation process, can be found in the Alternatives <br />Report (Reference 22). <br /> <br />GRASSED CHANNEL WITH LOW FLOW CHANNEL <br /> <br />MINIMUM EASEt.4ENT/RO W. WIDTH <br /> <br />B. <br /> <br />PROBLEM AREA IDENTIFICATION <br /> <br /> <br />OVERBANK <br /> <br />l.4AIN CHANNEL <br /> <br />OVERBANK <br /> <br />r FREE80ARO <br /> <br />[NORMAL <br />OEPTH <br /> <br />Several types of problems exist along the stretch of First Creek within the study area. These <br />problems can be summarized as: flooding created due to a lack of channel capacity; flooding created <br />upstream of channel crossings with inadequate conveyance facilities, resulting in the potential <br />inundation of property upstream of these crossings; flooding of roads crossing the channel resulting <br />in traffic interruption and potential danger to drivers; washout of the Fulton Ditch, O'Brian Canal <br />and Burlington Ditch; and erosion of the channel and bridges. <br /> <br /> <br />,T <br /> <br />LOW FLOW CH"NNEL BOTTOM <br /> <br />OPTlONAllOCATlON OF <br />RIPRAP TOE PROTECTION <br />FOR FREE.MEANOER <br />lOW FLOW CHANNEL <br /> <br />Channel crossings determined to be inadequate to pass thc frcquent flood events are: <br /> <br />. <br /> <br />O'Brian Canal <br />Burlington Ditch <br />US-85 <br />1-76 <br />SH-2 <br />Union Pacific and Burlington Northern Santa Fe railroads <br />96'h Avenue <br /> <br />HOTES: <br />1 LOW FLOW CHANNEL: CAPIl.C1TY TO BE eQUIVALENT TO 113 TO 112 OF THE 2.YEAR FLOW FOR THE <br />FUllY DEVELOPED, UNDETAlNEDCONOlTION. <br />2 NORMAL DEPTH: FLOW OEPTH FOR lQO-YEAR FLOW SHALL NO EXCEED 5 FEET. NOT INCLUOING <br />THE LOW flOW CHANNEL DEPTH. 10Q.YEAR FLOW VELOCITY AT NORMAL OEPTH SHALL NOT ExceeD <br />1 FTIS FOR. CHANNELS WITH EROSIOH RESISTANT SOIt.S OR 5 FTIS FOR CHANNELS WITH EROSIVE SOILS <br />3 FREEBOARD: FREEBOARD TO SEA MINI"'UM OF ONE FOOT. <br />. ......INTENANCE ...CCESS ROAD; MIN1UU... STABLE WIDTH TO BE a FEET WITH A CLEAR WIDTH OF 12 FEET. <br />5. R.D.W. WIDTH: MINIMUM WIOTH TO INCLUDE FREE80...RO "'NO M"'INTEN...NCE ACCESS ROAD <br />a OVERa"'NK. FLOW IN EXCESS OF MAtN CH"'NNEL TO BE CARRIED IN THIS "'REA. AREA MAY BE <br />USED fOR RECRE"'TIONPURPOSES. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />ALTERNATIVES DEVELOPMENT <br /> <br />Using the CUHP and UDSWMM models, peak flows along the proposed channel alignments werc <br />identified at several locations within the study area for the 2-, 5-, 10-, and 100-year runoff events. <br />Based on these observations, numerous alternatives were developed to provide adequate stormwater <br /> <br />Figure V-I <br /> <br />.32 - <br />