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<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
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