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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />1 <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I. <br />I <br />I <br /> <br />SECTION 4.0 - HYDRAULIC ANALYSIS <br /> <br />4.1 Introduction <br /> <br />The 100-year floodplain and f100dway have been delineated along Ralston Creek from the Arvada/Blunn <br />Reservoir to its confluence with Clear Creek, and along Leyden Creek from Leyden Lake to its confluence <br />with Ralston Creek. The limits of the 100-year floodplain and floodway represent the area in which <br />development is often regulated to prevent increases in water levels and increased damages during a flood. <br />Federal and state agencies such as the Federal Emergency Management Agency (FEMA) , Urban Drainage <br />and Flood Control District, as well as local municipalities of Jefferson County and Arvada, use the <br />floodplain and floodway delineations as a mechanism to identify areas particularly prone to damage from <br />flooding and to oversee development activities in these floodplains. <br /> <br />4.2 Hydrology and Hydraulics <br /> <br />The basis for this floodplain and floodway delineation is the hydrology, which has been summarized in an <br />earlier section of this report. The delineation assumes full basin development, based on land use and zoning <br />projections provided by the affected communities, and existing channel conditions. <br /> <br />A hydraulic analysis has been performed for the 100-year discharge, the results of which were used to plot <br />the 100-year floodplain and profile. Analyses were also performed for the 10-year, SO-year and 500-year <br />discharges and are plotted on the plan and profile sheets in the Appendix. The flood way has been computed <br />for the roO-year event based on a maximum half-foot and one-foot rise in the energy grade line or water <br />surface elevation, whichever was greater. <br /> <br />4.3 Methodology <br /> <br />The delineation of the 100-year floodplain on Ralston Creek and Leyden Creek was accomplished using the <br />U.S. Army Corps of Engineers HEC-RAS computer program (Reference 9) to analyze the hydraulic <br />response of the streams and floodplains using existing channel cross sections. The computational procedure <br />used in the model is the Standard Step Method with the assumptions that the flow is steady, gradually <br />varied, one dimensional, and slopes are relatively small. Major input parameters include digitized cross <br />sections, roughness coefficients, channel lengths between sections, and flow rates. Bridges and culverts <br />were simulated using surveyed field data input into the model. <br /> <br />Flow in the channel was assumed to be sub-critical except in the vicinity of drainage structures; therefore a <br />mixed flow regime was used to calculate the water surface elevations. <br /> <br />Values of Manning's roughness coefficient "n" were estimated for the channel and the left and right <br />overbanks along the channel. Estimates were based on field observations and were computed using the <br />procedures outlined in the Federal Highway Administration publication "Guide for Selecting Manning's <br /> <br />Roughness Coefficient for Natural Channels and Floodplains." This method estimates Manning's "n" by <br />assigning a base value for a particular soil type and applying adjustments for the degree of irregularity of the <br />channel the variations in channel cross-sections, the effects of obstructions, the amount of vegetation, and <br />the deg:ee of meander of the channel. Manning's "n" used in the hydraulic model for channels varied from <br />0.020 to 0.035. The values for the overbanks varied from 0.020 to 0.070. These values appeared to be <br />somewhat higher than the values used in the 1986 HEC-2 model by Write Water Engineering, Inc., perhaps <br />due to mature vegetation, and new fences. At the request of the Urban Drainage and Flood Control District, <br />the maximum overbank "n" value used in the model was changed to 0.050 to more closely match the <br />FEMA-accepted HEC-2 model. <br /> <br />Flow through structures is sometimes partially blocked during high flood flows due to debris accumulation. <br />Most of the structures modeled had fairly large openings, which reduces the amount of debris caught. <br />Based on the size of the structures, and field observations, and District approval, an area reduction <br />coefficient was not used in the model. <br /> <br />4.4 Floodplain Delineation <br /> <br />The 100-year floodplain for the existing channel condition, which includes the detention capacities of the <br />ArvadaJBlunn reservoir and Leyden Lake, has been depicted on the attached plan and profile sheets. The <br />plan depicts the 100-year floodplain, areas of shallow flooding, and areas of split flow, which are <br />hydraulically disconnected from the main flood flows. <br /> <br />The 100-year water surface elevations and floodplain limits determined in the study represent estimates <br />based on updated hydrology, future development and current channel geometry in the basin. As a result, the <br />delineation may differ from the floodplain limits identified in earlier studies. <br /> <br />4.5 Description of Reaches <br /> <br />The following is a description of the 100-year floodplain based on the various reaches. The floodplain of <br />the mainstem of Ralston Creek will be described first, followed by Leyden Creek. The reaches of Ralston <br />Creek below the confluence with Leyden Creek are similar to those outlined in the 1986 Phase B report for <br />Lower RalstonlVan Bibber and Leyden Creeks. <br /> <br />Ralston Creek <br /> <br />Reach R-l (Sheet 13, Stations 100 to 124) <br /> <br />This is the downstream reach of Ralston Creek, which begins at the confluence with Clear Creek and <br />extends upstream to the Pierce Street crossing. The starting water surface for Ralston Creek, at the <br />confluence with Clear Creek, was the lO-year water surface elevation in Clear Creek. <br /> <br />The following roadway crossings occur within this reach: Pierce Street, Nolan Street, Lamar Street, Ralston <br />Road, C&S Railroad, and W. 56th Avenue. The 100-year flood passes under Lamar Street, the railroad <br />crossing, and W. 56th Avenue, but overtops the remaining streets. Downstream of Lamar Street, Ralston <br />Creek has been confined to the channel along the more recently constructed Ralston Road, significantly <br /> <br />10 <br />