T.F. Green Airport Improvement Program - FEIS Chapters - PVD

T.F. Green Airport Improvement ProgramEnvironmental Impact Statement and Final Section 4(f) Evaluation• If any fill or necessary equipment would be placed within the floodplain;• To calculate what the loss of flood storage would be;• If a 100-year flood would lead to the loss of any transportation facility; and• If the loss of flood storage or new obstructions within the floodplain or floodway could result in an increasein depth or duration of flooding, or increase the lateral extent of the flooding.Conceptual design and grading plans of Alternatives B2 and B4 were also used to evaluate the extent of alterationof the floodplain, the amount of fill to be used, and any effects on natural and beneficial floodplain values.According to RIDEM Rules and guidance, all rivers, streams, and intermittent streams have a 100-yearfloodplain. 429 Floodplain mapping completed by FEMA has been used for analysis of floodplain impacts. For thepurposes of the FEIS, floodplain is as described in FEMA’s Flood Insurance Study for the City of Warwick (RevisedDecember 3, 2010) and FIRM Panels 44003C0113G, 44003C0131G, 44003C0129G, and 44003C0127G (RevisedDecember 3, 2010). The area of wetland filling associated with the Runway 34 enhancements is along a portionof the Buckeye Brook watershed where there is currently no detailed cross sections or survey completed byFEMA and no detailed study is available for this area. The floodplain elevation of 14 feet above the NorthAmerican Vertical Datum of 1988 (NAVD 1988) is based on downstream coastal storm surge conditions. ThisFEMA floodplain elevation was used to evaluate the impact of the proposed fill areas and the associatedmitigation on a volumetric basis, by identifying the existing 14 foot contour on the aerial topographic baseprepared for the Airport. Fill volumes placed in FEMA-floodplain were calculated by placing the model surfaceof Alternative B2’s and B4’s 30-percent design grading plan over the existing conditions terrain model andcalculating earthwork volumes below the flood elevation.Additional floodplain mapping described in Appendix L, Floodplains, does not constitute federal floodplain andwas used in the FEIS analysis of floodplain impacts to evaluate flood elevation changes due to fill andmitigation at specific cross section locations along the stream. This Study Area specific floodplain mapping isconsistent with RIDEM guidance. 430 FEMA floodplain impacts are shown in Figure 5-46. Impacts to floodplainmapped for the purposes of this study are shown in Figure L.1-2 of Appendix L, Floodplains.5.14.3.2 Indirect Impacts MethodologyFor the purposes of this FEIS analysis, indirect impacts are those impacts outside the Project area. These can includeincreased or decreased floodplain elevation both upstream and downstream of the floodplain fill area. These impactsare determined by comparing floodplain outside the Project area under existing conditions to floodplain afterconstruction of the proposed alternative based on elevations determined in the additional floodplain mappingcompleted for this study and described in Appendix L, Floodplains. If flood elevations after construction of theproposed alternative are different than under existing conditions, indirect impacts can be said to exist.429 The definition of floodplain is provided in Rhode Island General Law section 2-1-20 (3).430 Floodplain Impacts: Regulatory Provisions Pertaining to Floodplains and Floodways. Rhode Island Department of Environmental Management, Office ofWater Resources. November 16, 2007.Chapter 5 - Environmental Consequences 5-245 July 2011\\mawatr\ev\09228.00\reports\FEIS_Final_July_2011\PVD_CH05_Environmental_Cons_JUL_2011.doc