Buzzards Bay NEP / MCZM Study of Flood Zone Expansion with Sea Level Rise
Sub-pages: Quality Assurance Plan (pdf file)
IntroductionThe Buzzards Bay National Estuary Program (BBNEP) and the Massachusetts Office of Coastal Zone Management are evaluating the potential expansion of the existing Federal Emergency Management Agency (FEMA) 100-year floodplain using Flood Insurance Rate Map (FIRM) base flood elevations for Buzzards Bay municipalities. The floodplain will be expanded with 1-foot, 2-foot, and 4-foot increases in sea level. The existing floodplain will be extrapolated upward based on the FIRM base flood elevations. Using a recent assessor's data set, the number of buildings, their assessed values, and municipal structures are being enumerated within these various sea level rise expansion scenarios.
For this study, we considered only the landward most extent of the FIRM base flood (1% storm floodplain), and the published base flood elevation, to define the baseline flood zone used for this analysis. We then expanded this baseline flood zone by adding 1, 2, and 4-ft to the base flood elevation (whether A or V zone). The extrapolations were based on a digital data set of estimated bare earth elevations established by a 2007 aerial survey using LiDAR technology that was obtained from FEMA and which was used in part to prepare the 2009 updated Bristol and Plymouth County FIRMs (CDM, 2008).
The selected 1, 2, and 4-ft elevation increases in this study were chosen as convenient management elevation markers. The relative sea level rise in southern New England has been somewhat less than 1 foot during the past century. The IPCC (2007) consensus range for sea level rise, applied to this region, is 1 to 4.5 feet by year 2100. However, some other studies with alternative scenarios with more expanded Greenland and Antarctic glacier melting, or changes in the North Atlantic gyre may result in higher local sea level rise rates. We thus leave open ended how quickly the 1, 2, and 4-ft elevation increases may occur.
MethodsArcMap GIS software (ArcView license) was used to manipulate the various existing digital data sets, with some additional analysis completed in spreadsheets using pivot table functions. No field collection of data or ground truthing was required for this analysis. We used a 2007 LiDAR study contracted by FEMA, and described in detail in CDM (2008). These LiDAR data were provided to the Buzzards Bay NEP as both 2-ft contour lines, and as digital elevation models (DEMs) in the form of Triangular Irregular Network (TIN) raster files. To a limited degree, for certain flood zone expansion areas we also used 2011 Northeast National Map LiDAR project data (unpublished ). In general, the precision of the LiDAR data is 1 cm, but the accuracy is approximately 6 inches over the entire southeast study area, and the relative accuracy over a small geographic area along the same flight path is considerably better (USGS, 2004).
The base flood elevations from the Flood Insurance Rate Maps (FIRMs) released by FEMA for Fairhaven in 2009 were overlain on the detailed LiDAR contour data (Fig. 1) and digital elevation models (Fig. 2). Because the flood zone delineations on the FIRMs do not exactly follow LiDAR contours for a number of reasons, this step was necessary to establish a detailed baseline floodplain that could be consistently expanded by the projected sea level rise scenarios. Typically, the LiDAR 2-ft elevation contour lines were adequate to estimate expansion or adjustments of the boundaries each of the sea level rise scenario. However, where land slopes were slight, and the base flood elevation was set to an odd-number value, then TIN raster images were often used to visually estimate the respective new flood zone boundaries, as in Fig. 2. In this way a baseline floodplain zone was defined which was used as the initial conditions for the purposes of this study and allow for more meaningful and precise comparisons among the sea level rise scenarios.
This baseline floodplain was then expanded to account for 1, 2, and 4-foot sea level rises by adjusting the boundaries to the LiDAR elevations that corresponded to the base flood elevations identified on the FIRMS (as in Figs. 1 and 2). Thus, if the base flood elevation on the FEMA FIRM was specified as 14 feet for a site, the boundary of the baseline floodplain would be expanded to the 18-ft LiDAR contour in the + 4-ft sea level rise scenario. This is a simplified approach, and a more accurate approach would have involved detailed engineering analyses to determine how much the flood elevations would rise along the coast given the submergence of land in the 0-4-ft zone, but such an effort was beyond the scope of this study.
After the floodplain boundaries were created, assessor's data were applied to the GIS parcel data set. Because a building footprint coverage for Fairhaven was not available, centroid label points were created for each parcel to represent the location of each house. The position of these points was carefully examined along the boundary of each flood zone scenario and moved to coincide with the house footprint. If a house was crossed by several floodplain scenarios, the point, representing the vulnerability of the structure to sea level rise, was placed in the lowest elevation scenario as illustrated in Fig. 3 (note house near Indian Way). Secondary or ancillary detached structures were ignored, and the property building value was assigned to the main structure, typically the primary residential structure. On some parcels there are multiple detached dwellings units, but no parcels of this particular type were bisected by a flood zone.
Once the position of structures was set relative to the sea level rise scenarios, and assigned assessed values for total building structures on the parcel linked to these points, this digital point coverage was intersected with flood zone sea level rise scenario polygons. The resulting data set was processed in an Excel spreadsheet, and a pivot table was used to quantify building data using various classifications of structures by flood zone and inside or outside of the hurricane barrier.
ResultsMost of the reports are 7-12 MB pdfs, the appendixes are mostly 10-15 MB pdfs. Some data files have yet to be posted. Report a problem or make suggestions toJoe Costa
|Municipality||Report||Appendix A||Appendix B||GIS Shapefile||Google Earth: Scenarios||Google Earth: Structure Pts|
|Fairhaven||Final Report||Appendix A Maps||NA||GIS Shapefile||Google Earth: Flood Scenario||Google Earth: Built Points|
|Westport||Draft Report||Appendix A Maps||Appendix B||Google Earth: Flood Scenario||Google Earth: Built Points|
|Dartmouth||Draft Report||Appendix A Maps||NA||GIS Shapefiles||Google Earth: Flood Scenario||Google Earth: Built Points|
|New Bedford||Draft Report||Appendix A Maps||NA||GIS Shapefile||Google Earth: Flood Scenario||Google Earth: Built Points|
|Mattapoisett||Draft Report||Appendix A Maps||NA||Google Earth: Flood Scenario||Google Earth: Built Points|
|Marion||Draft Report||Appendix A Maps||NA||Google Earth: Flood Scenario||Google Earth: Built Points|
|Wareham||Draft Report||Appendix A Maps||NA||Google Earth: Flood Scenario||Google Earth: Built Points|