Appendix A River Hydraulics Analysis

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Fairview Street Improvements HYDRAULIC MODEL Existing Conditions A baseline HEC-RAS (Reference 4) model was provided by the Corps of Engineers. This model was used to represent the existing conditions. An existing analysis by HNTB Corporation in June 2017 prepared for the OC Streetcar, Santa Ana / Garden Grove Project (Reference 2). The report analyses a proposed bridge, downstream of the Fairview Street Bridge, just upstream of the existing OCTA Railroad Bridge. Review of the results of this analysis shows that the proposed Streetcar Bridge does not impact the hydraulics of the river near the Fairview Bridge. Vertical Datum The vertical datum used by the Corps of Engineers` baseline model is based on the National Geodetic Vertical Datum of 1929 (NGVD29). The mapping and survey information used for the Fairview project is based on the North American Vertical Datum of 1988 (NAVD88) which is approximately 2.4 feet higher. Therefore, 2.4 feet has to be added to the hydraulic model results to correlate with the project plans. Proposed Hydraulic Model The US Army Corps of Engineers, Hydrologic Engineering Center, River Analysis System, HEC-RAS 5.0.6. November 2018 (Reference 1) was used to model the proposed improvements to replace the existing bridge. Four new piers will replace the eight existing piers. The bridge will be wider and, therefore, the piers longer. The pier modeling includes debris loading of 2 feet on both sides of each pier, to a depth of 6 feet (see cross sections in appendix). Modeling Parameters • Flow Regime - Mixed flow was modeled as both subcritical and supercritical flows are anticipated within the study reach. • Design Discharge - The design discharge of 46,000 cubic feet per second (cfs) is used at the proposed crossing as provided in USACE HEC-RAS model. • Roughness values - Roughness values used in the Baseline Model; are 0.014 for the portion of the reach that is a concrete channel. • Boundary Conditions — The Baseline Model uses critical depth at the upstream and downstream boundary conditions. • Coefficients of Contraction/Expansion — no change in channel shape occurs within the reach of interest; therefore the current baseline model contraction and expansion coefficients of 0.1 and 0.3 are used. • Bridge Modeling Method — Highest Energy solution between (1) Energy Only (Standard Step) and (2) Momentum. Summary of Results The hydraulic analyses were performed using the mixed flow computation scheme. The boundary conditions of the models and design flow rates were set the same as USACE HEC-RAS model (Baseline) with critical depth at the upstream and downstream boundary conditions and design flow of 46,000 cfs at the project site. The hydraulic model under both existing and proposed condition shows a supercritical flow regime, in this reach of the river, downstream of Fairview Avenue Bridge. The regime becomes subcritical upstream of the bridge. A hydraulic jump occurs further upstream and flow transitions to supercritical regime. As shown on the profile provided in appendix, the new bridge improves the river hydraulics upstream of the bridge by lowering the water surface elevation and reducing the length of subcritical regime by J:\356.07.17\RIVER HYDRAULICS\RIVER HYDRAULICS REPORT TEXT.DOC Page 2