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<br />v 11-3 <br /> <br />channelization Is necessary In order to. re<lllze the full effect of the <br />bridge opening and to meet road criteria. For thoElt re'ason, the channel <br />Improvements at County LIne Road and at oth,er IOcoEltions within the basin <br />have been consIdered as a highway fund cost rather than as a direct drain- <br />age cost. as indicated in Table VIIJ-2. <br /> <br />Under existing channel conditions and future urbanization, the runoff <br />wou 1 d overtop the Un Ion Pac I f I c embankment .!nd the, County LI ne Road and <br />flow as a sheet north and east Into Weld County. Since this constitutes <br />a trans-basin diversion and it Is poor drainage practice, a "fJow guide," <br />as shown on Sheet 2, Is recommended to retain the flow within Its normal <br />drainage basin. A very Important feature of the I'ecommended design Is <br />that the Union Pacific embankment is sloping from the south downhill to the <br />north. The resuJtlng low point in the embankment is near the flow guIde. <br />When the embankment Is overtopped, the ,..ate,' wi II be captured between the <br />Un Ion Pac I f Ie embankment and Co lorado Sou I evard and channa I ed in a swa I e <br />to the existing thalweg located at the outlet from the embankment. The <br />most Important feature of this concept Is that when overflow occurs, em- <br />bankment washout wll I be 1 imlted to the shal Jowest part of the flood plain <br />and large volumes of water would not be released, in large damaging volumes <br />to Weld County, even for flows substantially greater than the iOO-year <br />design return frequency. <br /> <br />For Ilab 111 ty reasons I tis Important that the governmenta J ent I ties <br />involved (presently the Urban Draln,!lge and Flood Control District, and <br />Adams County) do nothIng that would affect the hycll-aul Ics of the culvert <br />through the embankment. We recommend that the water from the ten-foot <br />diameter culvert be passed under Colorado Boulevard in a concrete chute <br />and that an energy dlss Ipator be located east of Colonldo Boulevard, and <br />that the entire structure Including the dlss ipator, the. Colorado Boulevard <br />bridge, and the ten-foot diameter culvel"t be modeJ.'tested to ascertain the <br />appropriate design. <br /> <br />Although 160th Avenue Is designated to be an oElrterlal, the bridge <br />cross Ing Big Dry Creek has been designed on a 25-ye,ar r'eturn frequency <br />bas J so Because the backwater effect of the Un Ion F'ac i f I c Ra II road embank- <br />ment Inundates 160th Avenue, we do not feei It prudent to design the struc- <br />ture to pass the IOO-year frequency runoff, ,..hlch ",ould Include raising the <br />embankment an addItIonal two feet over the eJevati'J>n shown on Sheet 5, and <br />would Include lengthening the bridge by 30 feet. ,~ppurtenanl: to the 160th <br />Avenue brIdge are spur dikes which are also lJsed at other locations within <br />the study area. The purpose of the spur dikE'S Is to eJ imina!:e the scour <br />next to the br I dge caused by the turbu I ence wh I ch ,::an be not I ced I n one of <br />the pIctures shown at the end of Section I I. In essence, the spur dikes <br />do not eliminate the scour, but the scour Is moved suffIciently far from <br />the bridge to prevent structural damage. <br /> <br />Both 156th Avenue and York Street are de.s I gnated to be (:0 il actors and <br />have less stringent overtopping criteria than do arterials. An appurtenant <br />