Laserfiche WebLink
<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />IV-2 <br /> <br />HYDRAULIC ANALYSIS <br />Methodo logy <br />The HEC-2 computer program was used to analyze the hydrau1 ic response of <br />the streams and floodplains using existing and proposed channel cross sec- <br />tions, Major input parameters include digitized cross section, roughness <br />coefficients, channel lengths between sections, and flow rates. Bridges <br />were simulated using surveyed field data input into the model. <br /> <br />When the improvements proposed were for a recurrence interval less than the <br />lOa-year recurrence interval, that smaller event was also simulated with <br />the model and a profile was developed. An example of this would be Reach <br />R-3 where the improvements are for a la-year event. In this case, the 10- <br />year profi 1 e is a1 so shown on the p1 an sheets. Large bri dges were assumed <br />to be free of debris. These bridges include Lamar, Pierce, Wadsworth, and <br />Carr. Because of the relatively small size of the other bridges, they were <br />assumed to be 20 percent obstructed during floods. <br /> <br />In areas where the hydrau1 ic response was more comp1 ex, such as areas of <br />split flow, hand calculations were performed. In most cases, split flow <br />analyses were performed by developing rating curves for the capacities of <br />various parts of the cross section and developing relationships for the <br />flow between those conveyance areas. Thi s usually requi red est imates of <br />the weir flow capacity between the different conveyance areas. The various <br />flow rates in the different sections were balanced in such a way that the <br />water surfaces and energy grade 1 ines corresponded. In areas where over- <br />bank flow is hydraulically disconnected from the main channel, water sur- <br />face elevations are shown on the plans. <br /> <br />DESCRIPTION OF REACHES <br />Ralston Creek <br />Reach R-l. This is the downstream reach of Ralston Creek which begins at <br />the conf1 uence with C1 ear Creek and extends upstream to the Pi erce Street <br />crossing. The starting water surface, at Clear Creek, was at a stage two <br />feet below the lOa-year water surface elevation on Clear Creek. <br /> <br />Delineation of Improved Floodplains <br />The existing HEC-2 computer model was modified to reflect improvements pro- <br />posed in thi s master p1 an. The lOa-year future flow estimates were then <br />input in the model and the residual lOa-year floodplain was determined. <br /> <br />The downstream portion of this reach, below Lamar Street, is characterized <br />by a broad floodplain with many independent conveyance channels. Flow in <br />the main channel of Ralston Creek proceeds in a general southeasterly <br />direction beneath the Colorado and Southern (C&S) railroad tracks, toward <br />56th Avenue and ultimately through the trailer court to the confluence with <br />Clear Creek. Much of the flow, however, is diverted to the east by the C&S <br />rail road tracks and results in flow sp1 its both over the C&S rai 1 road <br />tracks to the south and the Denver and Rio Grande Western (D&RGW) railroad <br />tracks to the north. This flow is ultimately recombined and either flows <br />overland toward the mainstem of Ralston Creek or proceeds in an easterly <br />direction and passes under Sheridan Boulevard. Upstream of Lamar Street <br />the flow is generally confined between the D&RGW railroad tracks and Ral- <br />ston Road. <br /> <br />Floodplain Delineation <br />The loo~year floodplain for the existing channel condition and lOa-year <br />flood flows based on future basin development with Leyden Lake in place has <br />been depicted on the attached plans. The profile for the same flood condi- <br />tion is also shown on the plans. The plan depicts the loa-year floodplain, <br />areas of shallow flooding hydraulically disconnected from the profile, and <br />areas of split flow with differing hydraulic profiles. <br /> <br />Unique hydraulic features of this reach include the extensive flow splits <br />as a result of the railroad tracks in the downstream portions of this <br />reach. They also include the bridge at Lamar Street. This bridge was <br />improved subsequent to the prior master p1 anning effort, being widened to <br />