Laserfiche WebLink
<br />2.2.3 Hydraulic Analysis <br />To examine water surface elevations and delineate flooded areas at various flood return intervals, <br />the u.s. Army Corps of Engineers' Hydraulic Engineering Center River Analysis System (HEC- <br />RAS) model was used. HEC-RAS is a one-dimensional, steady flow model that uses standard- <br />step back-water calculations to determine water surface elevations given a specified channel <br />geometry. HEC-RAS can model mixed flow regimes of sub-critical and super-critical flow as <br />well as bridges, culverts, and weirs. HEC-RAS does not consider channel erosion or deposition. <br /> <br />2.2.3.1 Model Geometry and Setup <br />HEC-RAS utilizes a series of one-dimensional cross-sections to calculate flow-depths. Model <br />cross-sections and reach lengths were first established within Arc View GIS before being entered <br />into the HEC-RAS model. A distance of 3087 feet on Willow Creek from the flume entrance to <br />the confluence, 9629 feet on West Willow Creek, and 3153 on East Willow Creek were modeled <br />in HEC-RAS. Cross-sections were established across the floodplain area perpendicular to <br />anticipated flow lines. One hundred and four cross-sections were established. Fifty-five of these <br />cross-sections were established primarily from the surveyed cross-sections. Slight skews in the <br />surveyed cross-sections were corrected by projecting the geometry in the direction of flow. The <br />geometry for an additional 49 cross-sections was determined from topographic survey points and <br />interpolation from nearby survey points. Figure 2.2.2 displays the overall model schematic with <br />cross-sections that were established in the HEC- RAS model. Actual cross-section numbers can <br />be viewed more closely in the flood delineation maps that are presented later. Cross-section <br />numbers are arbitrary, but increase from downstream to upstream. Cross-section numbers extend <br />from 1 at the Army Corps flume and continue up Willow Creek to the number 45 at the <br />Amethyst Mine. Cross-section numbers on East Willow Creek begin from 1 at the downstream <br />confluence, but are preceded by the letter E in tables and references. Cross-sections numbered <br />with whole numbers (i.e. 6.0) were derived from surveyed cross-sections and this numbering is <br />also present on survey stakes placed in the field, while decimal numbers indicate cross-sections <br />generated from topography/interpolation. <br /> <br />Hydraulic structures such as flumes, culverts, bridges, and weirs within the study reach are also <br />noted in Figure 2.2.2. Culverts, bridges, and weirs were modeled using the hydraulic structure <br />features in HEC-RAS. Four cross-sections are associated with each of the structures. Cross- <br />sections are placed immediately upstream and downstream of the structure, as well as just <br />upstream and downstream of the areas required for flow contraction and expansion into and out <br />of the structure. The geometry of culvert, bridge, or weir openings as well as the roadway <br />approaches is entered separately. <br /> <br />Flow was generally considered to be able to contract at a 1: 1 angle and expand at a 4: 1 (L: W) <br />angle to flow-lines into and out of structure openings and other flow constrictions. The areas <br />outside of these angles were considered to be "ineffective flow" areas that could not contribute to <br />the flow conveyance area. Ineffective flow areas were defined on cross-sections immediately up <br />and downstream of structure openings. The weir at XS 14 demanded that ineffective flow areas <br />be defined on additional cross-sections upstream of the weir. The weir and earthen approach sit <br />at nearly a 45-degree angle to the incoming flow lines. This considerably constricts flow, and <br />much of the western portion of the weir forebay cannot be considered as an effective flow area to <br />convey flow into the weir opening. The flow contraction distances were re-evaluated following <br />the first model run given the methodology described in the HEC-RAS User's Manual and <br /> <br />2-13 <br />