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

T.F. Green Airport Improvement ProgramEnvironmental Impact Statement and Final Section 4(f) Evaluationabove NAVD 88 was determined by calculating the stillwater elevation at Connecticut Point where the MillRiver discharges to Narragansett Bay. This elevation was then projected up Buckeye Brook to a point where thestream elevation is above 14 feet NAVD 88.The RIDEM Rules and Regulations Governing the Administration and Enforcement of the Freshwater Wetlands Actincludes a definition of floodplain, 243 which in some circumstances extends state jurisdiction beyond the limits ofthe FEMA 1% Annual Chance Flood boundary (base flood boundary). The limits of state-regulated floodplainsassociated with Buckeye Brook and its tributaries were estimated and mapped based on accepted engineeringmethodology for floodplain elevation estimation according to FEMA.Since the FEMA base flood elevation was determined by projecting the stillwater elevation from a coastal waverunup analysis up Buckeye Brook, there were previously no detailed cross-sections or modeling of BuckeyeBrook to identify its flooding characteristics in response to rainfall events. For the purposes of this FEIS, in orderto more accurately evaluate state-regulated floodplain impacts to downstream properties, a more detailedanalysis was completed. The supplemental floodplain analysis completed for the Improvement Programinvolved the development of a site-specific floodplain model for the Buckeye Brook Watershed upstream ofWarwick Avenue. As described in Appendix L, Floodplains, the model was developed using data from the recentsurvey of wetlands and topography, one-foot contour information provided by the RIAC, local rainfall records,and available pertinent reports and field studies. This information was used to determine the appropriate inputparameters for the development of a Hydrologic Engineering Center – River Analysis System (HEC-RAS) modelthat was used to calculate the estimated floodplain elevation for a 100-year storm recurrence with a rainfallvolume of 8.7 inches over a 24-hour period. Details of the HEC-RAS modeling are included in Appendix L,Floodplains. The HEC-RAS model was used to determine if impacts to downstream properties would occur as aresult of Improvement Program. See Chapter 5, Environmental Consequences, Section 5.14, Floodplains.4.13.1.1 Floodplain Study AreaThe floodplain Study Area includes that area which the FEMA’s Flood Insurance Study Kent County (AllJurisdictions) (effective December 3, 2010) and FIRM Numbers 44003C0131G and 44003C0133G (both witheffective dates of December 3, 2010) show flooding physically contiguous to Buckeye Brook. The floodplainStudy Area is shown on Figure 4-30, 1% Annual Chance Floodplain. The 1% Annual Chance Flood is equivalentto a flood which inundates an area with a one percent likelihood of occurrence in any given year. The floodplainarea includes:• Buckeye Brook south of Airport Road through Warwick Pond, extending south and downstream to OldMill Creek;• Wetland A8 west of Warwick Pond extending across a portion of Runway 34; and• Wetlands A13 and A14 south of Runway 34.243 As defined in section 2-1-20(3) of the Act, that land area adjacent to a river or stream or other flowing body of water that is, on average, likely to becovered with flood waters resulting from a one hundred (100) year frequency storm. A storm of this nature is one that is to be expected to be equaled orexceeded once in one hundred (100) years, and hence may be said to have a one percent (1%) probability of being equaled or exceeded in any givenyear. Rainfall intensity data for such a storm are those established for New England locations by the National Weather Service (formerly the U.S. WeatherBureau).Chapter 4 – Affected Environment 4-68 July 2011\\mawatr\ev\09228.00\reports\FEIS_Final_July_2011\PVD_CH04_Affected_Env_JUL_2011.doc