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Guidelines and Specifications for Flood Hazard Mapping Partners [April 2003] coefficients to roughness coefficients published in Federal agencies documents and hydraulic text books. It is extremely important that the Mapping Partner performing the hydraulic analysis select roughness coefficients in overbank areas to carefully represent the effective flow in those areas. There is a general tendency to overestimate the amount of flow occurring in overbank areas, particularly in broad, flat floodplains. The Mapping Partner shall document the use of roughness coefficients to define ineffective-flow areas clearly in the documentation submitted for inclusion in the FIS report. Before preparing work maps, the Mapping Partner performing the hydraulic analysis shall reconcile the 1-percent-annual-chance Flood Profile proposed for the Flood Map Project with all available published or unpublished information. The Mapping Partners shall resolve any identified discrepancies in consultation with the RPO and the Mapping Partner producing the final FIS report and FIRM (in most cases, the reviewing Mapping Partner). Except where a clearly identified change in flooding characteristics or an error in the existing data can be shown, the proposed 1-percent-annual-chance flood elevations must agree with those of other contiguous studies of the same flooding source within 0.5 foot of the contiguous elevation. However, the final l-percent-annual-chance flood elevation or Flood Profile submitted by the Mapping Partner performing the hydraulic analysis for inclusion in the FIS report must match the contiguous elevation or Flood Profile exactly. Where elevations cannot be reconciled within 0.5 foot because of changed flooding conditions or an error in the previous analysis, the Mapping Partner performing the hydraulic analysis shall provide a full explanation and justification for the difference to the RPO in an SPR. The Mapping Partner shall obtain approval for the discrepancy in 1-percent-annual-chance flood elevations from the RPO before proceeding. C.3.2 FEMA-Contracted Flood Map Project for Community That Does Not Have Effective Flood Map [February 2002] The detailed hydraulic analysis for a FEMA-contracted Flood Map Project for a commlmity that does not have an effective FIRM will include a flood elevation determination for the community in accordance with Paragraph 60.3( c) of the NFIP regulations or a flood elevation determination and floodway determination for the community in accordance with Paragraph 60.3(d) of the NFIP regulations. Detailed guidance on the flood elevation determination and the floodway determination are provided in Subsections C.3.2.1 and C.3.2.2. C.3.2.1 Flood Elevation Determination [February 2002] The Mapping Partner performing the hydraulic analysis normally shall determine flood elevations for the 10-, 2-, 1-, and O.2-percent-annual-chance floods, unless otherwise instructed by the RPO. These flood elevations must be referenced to the National Geodetic Vertical Datum of 1929 (NGVD29) or the North American Vertical Datum of 1988 (NA VD88). Flood elevations for riverine areas are normally determined by step-backwater computer models such as the US ACE HEC-RAS Computer Program (USACE, 200l) or the USGS/Federal C-17 Section C.3