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<br />IV. HYDRAULIC ANALYSIS
<br />
<br />A. GENERAL
<br />
<br />The main purpose of hydraulic analysis is to determine the hydraulic conditions and associated
<br />capacities of the channel reaches and drainage structures, This determination requires physical measurements,'
<br />of channel sections and structures as well as selection of appropriate and reasonable hydraulic parameters.
<br />
<br />The United States Army Corps of Engineers, HEC-RAS, River Analysis System, Version 3,1.1,
<br />(Reference 8) was used to determine the capacities of the existing channel reaches for streams within the Flood
<br />Hazard Area Delineation study limits, All HEC-RAS models wen~ prepared based on steady flow conditions to
<br />estimate the water surface profiles and associated floodplain boundaries,
<br />
<br />B.
<br />
<br />HYDRAULIC PARAMETERS
<br />
<br />The HEC-RAS computer program requires the following information for hydraulic computations:
<br />
<br />1, Channel Cross-Sections
<br />
<br />The majority of the cross-sections for input into the HEC-RAS models were developed with
<br />HEC-GeoRAS using the topographic mapping produced by Map Works, Inc, All cross-sections
<br />are stationed from left to right looking downstream and starting with a station designation of
<br />0+00, unless noted differently on the plan drawings, All cross section labels correspond to the
<br />channel centerline stations,
<br />
<br />2,
<br />
<br />Mannina's Rouahness Coefficients
<br />
<br />The estimates of Manning's "n" values were derived, from a review of the field channel
<br />conditions, as compared to the generally accepted values based upon standard references and
<br />the UD&FCD Criteria (Reference 1), Generally, the "n" values used for this study fall within the
<br />following descriptive ranges:
<br />
<br />IV-I
<br />
<br />Description
<br />
<br />Maintained grass-lined channels with hard low flow channel
<br />
<br />Un-maintained grassed areas, with unlined channel
<br />
<br />, Riprap/Gabions
<br />
<br />Heavy brush/shrubs
<br />
<br />Tall grassy vegetation with trees
<br />
<br />an" value
<br />
<br />,030-,035
<br />
<br />,035-,045
<br />
<br />,040-,050
<br />
<br />,050-.070
<br />
<br />,040-,050
<br />
<br />Composite Manning's "n" values were used forchannel reaches that exhibited a high degree of
<br />roughness variability across the channel section,
<br />
<br />3, Bridae/Culvert Parameters
<br />
<br />Most of the bridges and culverts in the study reaches were analyzed using the bridge or culvert
<br />routines available in the HEC-RAS program, Forthis purpose, Map Works prepared a structure
<br />inventory which included measurements of the bridge and culvert sizes, shapes, upstream and
<br />, downstream invert elevations, and overflow elevations at the structure centerline, Bridge pier
<br />coefficients were estimated from the HEC-RAS Users Manual based upon the observed pier
<br />shape, For roadway overtopping analysis, a weir coefficient of 2,6 to 3,0 was used, Expansion
<br />and contraction coefficients of 0,3-0,5 or 0,1-0,3 were typically used near the bridges and
<br />culverts, No "plugging factors" were used to artificially reduce the structure;s hydraulic capacity
<br />to account for existing or future accumulation of sediments within the structures.
<br />
<br />C. ANALYZED FLOOD FREQUENCIES
<br />
<br />For the purpose oflhis FHAD study, the hydraulic capacities of channels and structures were analyzed
<br />for the 10-, 50-, 100-, and 500-year flood events based upon existing channel conditions. Please refer to
<br />Section III for detailed discussions on the estimation of peak flow rates,
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