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URS Greiner Woodward Ctyde Hydraulic Modeling Options to Complete the Lower Amazon Creek LOMR Study Based on specific FEMA requirements, URS evaluated three options for completing the detailed hydraulic analysis for the Lower Amazon Creek LOMR study. Each option involves using a different hydraulic model to perform the required hydraulic analyses. The three options are: 1. XP-SWMM: XP-SWMM is a proprietary version of EPA's SWMM and is currently being used by the City to develop comprehensive stormwater master plans for the major basins in the City. An XP-SWMM model has already been developed by URS for the Amazon Creek Basin, including the Lower Amazon Creek. However, XP- SWMM is not currently a FEMA accepted model. It may take years before FEMA approves the use of the model for conducting flood insurance studies. Other major disadvantages include the significant effort that would likely be required to re- produce the required effective tie-in and XP-SWMM's inability to perform automated floodway analyses. Although the XP-SWMM model could be converted to EPA's SWMM, which is a FEMA approved program (see Appendix), additional calculations on culverts and bridge crossings would be required. Major disadvantages in using EPA's SWMM also include the difficulties in producing the required effective tie-in and performing floodway analyses. 2. HEC-RAS: HEC-RAS is the Windows version of the HEC-2 model. HEC-2 was used in the 1979-1986 FIS study for the upper and middle portions of the Amazon Creek. HEC-RAS is FEMA's preferred FIS model. Therefore, the effective tie-in required by FEMA would be relatively easy to achieve by using the same model. HEC-RAS also has automated floodway analysis capabilities. One major i ~ disadvantage of this option is that HEC-RAS is not currently used as a hydraulic analysis tool for most active City projects. Also HEC-RAS, a steady state model, would be less robust in analyzing the complex system of split flow and storage on the project site than unsteady state models, such as XP-SWMM or UNET. 3. UNET: UNET is an unsteady state model and currently used by the Corp for analyzing the 1135 project. UNET is a FEMA approved hydraulic model. The major disadvantage of this option is that it would likely require a significant effort to achieve the FEMA required effective tie-in. Also an automated floodway analysis option is not available in UNET. The advantages and disadvantages for each of the three options are summarized in Table 1. Among the three options, URS recommends Option #2: using HEC-RAS to perform the hydraulic analyses. This recommendation is mainly based on the fact that HEC-RAS is well accepted by FEMA, the effective tie-in would be most easily achieved, and HEC- RAS is capable of performing automated floodway analyses. Although XP-SWMM (Option #1) is currently used in the City's comprehensive stormwater master plan studies, it is not recommended due to the uncertainty involved in FEMA's approval of the model. UNET (option #3) is not recommended mainly due to its inability to conduct automated \\Por2\PROD-WC~945042na1TASK030(MMAZOMFEMAUema.doc 4:08 PM 02/25/00 Page 3