Row-Boat-Row Appeal Memo_FINAL.PDF
MEMORANDUM TO:
Chris McCall
CC:
Erik J. Olsen, P.E. Chris Creed, P.E.
FROM:
Patrick J. Snyder, P.E.
DATE:
30 November 2015
RE:
FEMA PFIS Appeal Village of Bald Head Island, NC – Row Boat Row
This memorandum describes a coastal engineering analysis conducted in support of a proposed appeal of the FEMA Preliminary Flood Insurance Rates Map (PFIRM) flood zone designations for a portion of the Village of Bald Head Island’s shoreline. The subject shoreline is approximately 1,500 feet in length and located at the northwest corner of the Island, just north of the entrance to the Bald Head Island Marina (see Figure 1). The shoreline faces the Cape Fear River and fronts homes along Row Boat Row and Turks Head Court, and will herein be referred to as ‘Row Boat Row.’ This area falls within North Carolina Panel 3004 of the North Carolina Floodplain Mapping Program and FEMA National Flood Insurance Program’s Flood Insurance Rate Map (FIRM). Currently, the subject area is mapped within VE and AE zones on the effective FIRM (Map Number 3720300400K, revised 10/16/08) (Figure 2). The PFIRM (Map Number 3720300400L, revised 08/29/14) maintains the study area within VE and AE zones but proposes changes to the location and limits of the VE and AE zones as well as the Base Flood Elevations (BFEs) across the area (Figures 2 and 3). Specifically, changes to the flood zone designations across the Row Boat Row area include a significant eastward shift in the landward location of the VE zone. As such, almost the entire land mass along Row Boat Row is proposed to be incorporated into a VE zone. Review of the engineering analyses in the Coastal Flood Hazard Study Report (AECOM, 2014), the report upon which the PFIRM is based, suggests that the principal contributor to this change is the assumed location of the landward heel of the Primary Frontal Dune (PFD). Herein, an argument is made that the proposed landward heel of the
PDF
is
incorrect
and
the
PFIRM
should
be
revised
accordingly.
olsen associates, inc. | 2618 Herschel Street | Jacksonville, FL 32204 | 904.387.6114 | FAX 904.384.7368 www.olsen-associates.com.
Figure 1:
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Location of the Row Boat Row shoreline area of Bald Head Island, NC.
Existing PFIRM Mapping Review of the Coastal Flood Hazard Study Report (AECOM, 2014) reveals that the proposed VE Zone designations are based upon analyses of the site with anticipated erosion and flooding conditions at two transects: 79 and 80. Figures 2 and 3 depict the location of Transects 79 and 80 as well as the proposed VE and AE Zones across the study area, as reflected in the Effective FIRM and the PFIRM1, respectively. Also shown in the figures are the corresponding BFEs and, in Figure 3, the location of the landward heel of the PFD as identified by AECOM (2014). VE zones are coastal high hazard areas where wave action and/or high-velocity water can cause structural damage during the base flood. They are subdivided into elevation zones with BFE’s assigned. VE zones are identified using one or more of the following criteria for the base flood conditions: 1. The wave runup zone occurs where the (eroded) ground profile is 3.0 feet or more below the 2-percent wave run-up elevation. 2. The wave overtopping splash zone is the area landward of the crest of an overtopped barrier, in cases where the potential 2-percent wave runup exceeds the barrier crest elevation by 3.0 feet or more (ΔR>3.0 feet). 3. The breaking wave height zone occurs where 3-foot or greater wave heights could occur (this is the area where the wave crest profile [elevation] is 2.1 feet or more above the total stillwater level). 4. The primary frontal dune zone, as defined in 44 CFR Section 59.1 of the NFIP regulations, is “a continuous or nearly continuous mound or ridge of sand with relatively steep seaward and landward slopes immediately landward and adjacent to the beach and subject to erosion and overtopping from high tides and waves during major coastal storms. The inland limit of the primary frontal dune occurs at the point where there is a distinct change from a relatively steep slope to a relatively mild slope. 1
“Effective FIRM” is the current version of the FIRM used by FEMA, while the PFIRM is the proposed revised FIRM being appealed herein.
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Figure 2: Effective FIRM designations in the vicinity of the Study Area. Aerials: March 2015
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Figure 3: PFIRM designations in the vicinity of the Study Area. The landward heel of the PFD as identified by AECOM is represented by the yellow dashed line. Aerials: March 2015
The landward limit of the VE zone is defined as the farthest inland extent of any of the four criteria listed above. The Coastal Flood Hazard Study Report indicates that the newly proposed landward extent of the VE Zone within the Row Boat Row study area was determined to be controlled by the primary frontal dune zone criteria (Criteria 4, above). Criteria 1 and 2 were determined in The Coastal Flood Hazard Study to not be applicable to Transects 79 and 80 (and indeed, most of the Brunswick County shoreline). Due to the “540 rule,” these transects were modeled using dune removal, which resulted in very mild slopes (1:50) across the upper beach with only small profile emergence above the Total 1% Stillwater Elevation (TSWEL, SWEL + wave setup). The breaking wave height (Criteria 3) was found to be the dominant hazard at these transects, as “the overall profile slope was determined to be too mild for run-up to apply” (AECOM, 2014). Criteria 3 is based on the potential occurrence of 3-foot or greater wave heights determined through use of the Wave Height Analysis for Flood Insurance Studies (WHAFIS) model. In practice, this is the area where the wave crest elevation is 2.1 feet or more above the TSWEL. The AECOM WHAFIS results indicate that the landward limit of the breaking wave height zone is located on the beach face, as depicted in Figures 4 and 5. As shown in Figures 4 and 5, AECOM (2014) identifies the location of the landward heel of the Primary Frontal Dune (marked “Preliminary PFD”) as significantly landward of the landward extent of the breaking wave height zone. Therefore, the AECOM results suggest that Criteria 3 is superseded by Criteria 4 at both transects and the landward limit of the VE Zone is proposed to be located at the assumed landward heel of the PFD. This gives a VE Zone designation across most of the Row Boat Row upland area. The result is a VE Zone that extends 200 to 300 feet landward of the WHAFIS 3-foot Breaking Wave Height Zone, and covers nearly the entirety of the landform between the shoreline and the marina, including Row Boat Row and homes landward thereof.
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Figure 4:
WHAFIS modeling results at Transect 79 from the Brunswick County, North Carolina Coastal Flood Hazard Study Report.
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Figure 5:
WHAFIS modeling results at Transect 80 from the Brunswick County, North Carolina Coastal Flood Hazard Study Report.
Comparing the location of the seaward toe of the dune2 to the landward heel of the PFD, it is noted that the existing analysis indicates Primary Frontal Dune widths of nearly 200 ft and nearly 300 ft at Transects 79 and 80, respectively. AECOM (2014) identified the landward heel of the PFD based upon “1.) beach/dune surveys conducted in November 2010, 2.) 2001 Lidar data, and 3.) aerial imagery. The primary dataset at the modeling transect was the beach surveys.” AECOM notes, however, that “because PFD delineation uses a regional approach, if the survey is in contrast to the DEM [Digital Elevation Model] and aerials in the surrounding areas, the latter two sources may supersede. Delineation of the PFD line is based on a regional approach, meaning that the engineer drawing the line is at a scale no smaller than 1:1,500 or 1:2,000.” The DEM used by AECOM was developed by splicing 2010 survey data at the transects into the 2001 Lidar data. It is shown, in Figure 7, that the two data sets represent very different topographic conditions along the Row Boat Row shoreline. Further, these data do not represent well the configuration and details of existing topographic features along the Row Boat Row shoreline. The “appeal” analysis presented herein will focus on two issues. First, it will utilize more recent Lidar data and aerial imagery that better represents existing conditions. Second, it will delineate the heel of the Primary Frontal Dune in a way that better follows guidance presented in Section 59.1 of the National Flood Insurance Program (NFIP) Regulations to more accurately characterize the frontal dune that is subject to impacts during major coastal storms. Appeal Analysis The AECOM analysis outlined in the previous section is largely reliant on Lidar data collected in 2001 and topographic data collected along the transects in 2010. These data were combined to create the ‘Study DEM”, or a representative three-dimensional model of the Row Boat Row topographic features. Figure 6 shows the topographic contours created with the Study DEM (left) and a more recent, high-resolution Lidar survey conducted in 2014 by the National Geodetic Survey (NGS) (right). 2
In WHAFIS modeling, the seaward toe of the dune is identified by the modeler and used as the base point for profile erosion in the case of Dune Removal (dune failing the “540 rule). The eroded profile is given an upward slope of 1 on 50 landward from the toe.
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Figure 6: Elevation contours in the vicinity of the Row Boat Row Study Area. The left panel shows the Study DEM, while the right shows the 2014 NGS Lidar. Elevations are relative to NAVD88. Aerials: March 2015.
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Inspection of the left panel (see expanded view in Figure 7) reveals significant perturbations in the data where the 2010 transect data has been spliced into the 2001 Lidar survey, such that the zero foot contour at Transect 80 is up to 50 feet seaward of its location to the immediate north. The 2014 Lidar set (right panel in Figure 6), on the other hand, offers a more detailed characterization of beach, dune, and other upland topographic features. Dune ridges and swales are more evident and pronounced, and identification of a Primary Frontal Dune is much more straightforward. Additionally, the significant changes that have occurred over the intervening 13 years are captured by the more recent data.
Typical shoreline perturbation caused by merging of 2001 and 2010 topographic data sets
Figure 7: Close-up view of perturbation in elevation contours for the Study DEM in the vicinity of the Row Boat Row Study Area. Elevations are relative to NAVD88.
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Figure 8 compares the 2001/2010 and 2014 topographic data sets in cross sectional view. The 2014 data corroborates the horizontal location of the dune ridges as represented by the 2001/2010 data, but gives more detail that differentiates a large frontal dune and several distinct dune features across the profile.
Figure 8: Cross sections of CHAMP model elevation data (initial and eroded) for Transects 79 and 80 compared to corresponding data extracted from the 2014 NGS Lidar. Also shown for reference are the TSWEL, Wave Crest Elevation, and Preliminary PFD location.
Using these data, Figures 9 and 10 identify the distinct topographic feature immediately landward of the beach face that is a clear frontal dune feature. Comparing Transects 79 and 80, this feature is generally consistent in size, shape, and distance from the shoreline. The estimated landward heel of this feature is also depicted in the figures. This landward heel of the dune is argued to be a more appropriate location of the PFD than that proposed by AECOM.
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Figure 9:
WHAFIS modeling results at Transect 79 compared with elevations extracted from 2014 NGS Lidar, annotated with location of Preliminary and Recommened PFD.
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Figure 10:
WHAFIS modeling results at Transect 80 compared with elevations extracted from 2014 NGS Lidar, annotated with location of Preliminary and REcommended PFD.
Applying the identified landward heel of the PFD from both transects to the 2014 Lidar data and aerial photographs (Figures 11 and 12, respectively), a recommended landward heel of the PFD is delineated alongshore. The feature is consistent with NFIP Regulations (Section 59.1), which describe the PFD as a “continuous or nearly continuous mound or ridge of sand with relatively steep seaward and landward slopes immediately landward and adjacent to the beach and subject to erosion and overtopping from high tides and waves during major coastal storms.” As shown on both figures, the proposed BFE for the AE area removed from the VE Zone is 10, which is consistent with WHAFIS Part 6 modeling results from the Coastal Flood Hazard Study Report (AECOM, 2014).
Summary The Coastal Flood Hazard Study Report (AECOM, 2014) found that the landward extent of the PFIRM VE Zone along Row Boat Row is controlled by the location and extent of the PFD. Analyses herein, however, found that the AECOM study sited the landward heel of the PFD, and thereby the landward limit of the VE Zone, too far inland based upon existing topographic conditions along Row Boat Row. In this analysis, a high resolution 2014 Lidar survey is used to identify and delineate the landward heel of the PFD at a location that is significantly more seaward than that represented in the PFIRM. It is recommended that the PFIRM be amended to consider this new information and the proposed extent of the VE Zone along Row Boat Row be revised accordingly.
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Figure 11: Proposed location of PFD line (pink curve), V/A Zone boundary, and proposed BFEs in the Row Boat Row Study Area. Contour elevations: 2014 NGS Lidar, NAVD88. Aerials: March 2015.
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Figure 12: Proposed location of PFD line (pink curve), V/A Zone boundary, and proposed BFEs in the Row Boat Row Study Area. Aerials: March 2015.
Chris McCall
CC:
Erik J. Olsen, P.E. Chris Creed, P.E.
FROM:
Patrick J. Snyder, P.E.
DATE:
30 November 2015
RE:
FEMA PFIS Appeal Village of Bald Head Island, NC – Row Boat Row
This memorandum describes a coastal engineering analysis conducted in support of a proposed appeal of the FEMA Preliminary Flood Insurance Rates Map (PFIRM) flood zone designations for a portion of the Village of Bald Head Island’s shoreline. The subject shoreline is approximately 1,500 feet in length and located at the northwest corner of the Island, just north of the entrance to the Bald Head Island Marina (see Figure 1). The shoreline faces the Cape Fear River and fronts homes along Row Boat Row and Turks Head Court, and will herein be referred to as ‘Row Boat Row.’ This area falls within North Carolina Panel 3004 of the North Carolina Floodplain Mapping Program and FEMA National Flood Insurance Program’s Flood Insurance Rate Map (FIRM). Currently, the subject area is mapped within VE and AE zones on the effective FIRM (Map Number 3720300400K, revised 10/16/08) (Figure 2). The PFIRM (Map Number 3720300400L, revised 08/29/14) maintains the study area within VE and AE zones but proposes changes to the location and limits of the VE and AE zones as well as the Base Flood Elevations (BFEs) across the area (Figures 2 and 3). Specifically, changes to the flood zone designations across the Row Boat Row area include a significant eastward shift in the landward location of the VE zone. As such, almost the entire land mass along Row Boat Row is proposed to be incorporated into a VE zone. Review of the engineering analyses in the Coastal Flood Hazard Study Report (AECOM, 2014), the report upon which the PFIRM is based, suggests that the principal contributor to this change is the assumed location of the landward heel of the Primary Frontal Dune (PFD). Herein, an argument is made that the proposed landward heel of the
is
incorrect
and
the
PFIRM
should
be
revised
accordingly.
olsen associates, inc. | 2618 Herschel Street | Jacksonville, FL 32204 | 904.387.6114 | FAX 904.384.7368 www.olsen-associates.com.
Figure 1:
-2-
Location of the Row Boat Row shoreline area of Bald Head Island, NC.
Existing PFIRM Mapping Review of the Coastal Flood Hazard Study Report (AECOM, 2014) reveals that the proposed VE Zone designations are based upon analyses of the site with anticipated erosion and flooding conditions at two transects: 79 and 80. Figures 2 and 3 depict the location of Transects 79 and 80 as well as the proposed VE and AE Zones across the study area, as reflected in the Effective FIRM and the PFIRM1, respectively. Also shown in the figures are the corresponding BFEs and, in Figure 3, the location of the landward heel of the PFD as identified by AECOM (2014). VE zones are coastal high hazard areas where wave action and/or high-velocity water can cause structural damage during the base flood. They are subdivided into elevation zones with BFE’s assigned. VE zones are identified using one or more of the following criteria for the base flood conditions: 1. The wave runup zone occurs where the (eroded) ground profile is 3.0 feet or more below the 2-percent wave run-up elevation. 2. The wave overtopping splash zone is the area landward of the crest of an overtopped barrier, in cases where the potential 2-percent wave runup exceeds the barrier crest elevation by 3.0 feet or more (ΔR>3.0 feet). 3. The breaking wave height zone occurs where 3-foot or greater wave heights could occur (this is the area where the wave crest profile [elevation] is 2.1 feet or more above the total stillwater level). 4. The primary frontal dune zone, as defined in 44 CFR Section 59.1 of the NFIP regulations, is “a continuous or nearly continuous mound or ridge of sand with relatively steep seaward and landward slopes immediately landward and adjacent to the beach and subject to erosion and overtopping from high tides and waves during major coastal storms. The inland limit of the primary frontal dune occurs at the point where there is a distinct change from a relatively steep slope to a relatively mild slope. 1
“Effective FIRM” is the current version of the FIRM used by FEMA, while the PFIRM is the proposed revised FIRM being appealed herein.
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Figure 2: Effective FIRM designations in the vicinity of the Study Area. Aerials: March 2015
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Figure 3: PFIRM designations in the vicinity of the Study Area. The landward heel of the PFD as identified by AECOM is represented by the yellow dashed line. Aerials: March 2015
The landward limit of the VE zone is defined as the farthest inland extent of any of the four criteria listed above. The Coastal Flood Hazard Study Report indicates that the newly proposed landward extent of the VE Zone within the Row Boat Row study area was determined to be controlled by the primary frontal dune zone criteria (Criteria 4, above). Criteria 1 and 2 were determined in The Coastal Flood Hazard Study to not be applicable to Transects 79 and 80 (and indeed, most of the Brunswick County shoreline). Due to the “540 rule,” these transects were modeled using dune removal, which resulted in very mild slopes (1:50) across the upper beach with only small profile emergence above the Total 1% Stillwater Elevation (TSWEL, SWEL + wave setup). The breaking wave height (Criteria 3) was found to be the dominant hazard at these transects, as “the overall profile slope was determined to be too mild for run-up to apply” (AECOM, 2014). Criteria 3 is based on the potential occurrence of 3-foot or greater wave heights determined through use of the Wave Height Analysis for Flood Insurance Studies (WHAFIS) model. In practice, this is the area where the wave crest elevation is 2.1 feet or more above the TSWEL. The AECOM WHAFIS results indicate that the landward limit of the breaking wave height zone is located on the beach face, as depicted in Figures 4 and 5. As shown in Figures 4 and 5, AECOM (2014) identifies the location of the landward heel of the Primary Frontal Dune (marked “Preliminary PFD”) as significantly landward of the landward extent of the breaking wave height zone. Therefore, the AECOM results suggest that Criteria 3 is superseded by Criteria 4 at both transects and the landward limit of the VE Zone is proposed to be located at the assumed landward heel of the PFD. This gives a VE Zone designation across most of the Row Boat Row upland area. The result is a VE Zone that extends 200 to 300 feet landward of the WHAFIS 3-foot Breaking Wave Height Zone, and covers nearly the entirety of the landform between the shoreline and the marina, including Row Boat Row and homes landward thereof.
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Figure 4:
WHAFIS modeling results at Transect 79 from the Brunswick County, North Carolina Coastal Flood Hazard Study Report.
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Figure 5:
WHAFIS modeling results at Transect 80 from the Brunswick County, North Carolina Coastal Flood Hazard Study Report.
Comparing the location of the seaward toe of the dune2 to the landward heel of the PFD, it is noted that the existing analysis indicates Primary Frontal Dune widths of nearly 200 ft and nearly 300 ft at Transects 79 and 80, respectively. AECOM (2014) identified the landward heel of the PFD based upon “1.) beach/dune surveys conducted in November 2010, 2.) 2001 Lidar data, and 3.) aerial imagery. The primary dataset at the modeling transect was the beach surveys.” AECOM notes, however, that “because PFD delineation uses a regional approach, if the survey is in contrast to the DEM [Digital Elevation Model] and aerials in the surrounding areas, the latter two sources may supersede. Delineation of the PFD line is based on a regional approach, meaning that the engineer drawing the line is at a scale no smaller than 1:1,500 or 1:2,000.” The DEM used by AECOM was developed by splicing 2010 survey data at the transects into the 2001 Lidar data. It is shown, in Figure 7, that the two data sets represent very different topographic conditions along the Row Boat Row shoreline. Further, these data do not represent well the configuration and details of existing topographic features along the Row Boat Row shoreline. The “appeal” analysis presented herein will focus on two issues. First, it will utilize more recent Lidar data and aerial imagery that better represents existing conditions. Second, it will delineate the heel of the Primary Frontal Dune in a way that better follows guidance presented in Section 59.1 of the National Flood Insurance Program (NFIP) Regulations to more accurately characterize the frontal dune that is subject to impacts during major coastal storms. Appeal Analysis The AECOM analysis outlined in the previous section is largely reliant on Lidar data collected in 2001 and topographic data collected along the transects in 2010. These data were combined to create the ‘Study DEM”, or a representative three-dimensional model of the Row Boat Row topographic features. Figure 6 shows the topographic contours created with the Study DEM (left) and a more recent, high-resolution Lidar survey conducted in 2014 by the National Geodetic Survey (NGS) (right). 2
In WHAFIS modeling, the seaward toe of the dune is identified by the modeler and used as the base point for profile erosion in the case of Dune Removal (dune failing the “540 rule). The eroded profile is given an upward slope of 1 on 50 landward from the toe.
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Figure 6: Elevation contours in the vicinity of the Row Boat Row Study Area. The left panel shows the Study DEM, while the right shows the 2014 NGS Lidar. Elevations are relative to NAVD88. Aerials: March 2015.
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Inspection of the left panel (see expanded view in Figure 7) reveals significant perturbations in the data where the 2010 transect data has been spliced into the 2001 Lidar survey, such that the zero foot contour at Transect 80 is up to 50 feet seaward of its location to the immediate north. The 2014 Lidar set (right panel in Figure 6), on the other hand, offers a more detailed characterization of beach, dune, and other upland topographic features. Dune ridges and swales are more evident and pronounced, and identification of a Primary Frontal Dune is much more straightforward. Additionally, the significant changes that have occurred over the intervening 13 years are captured by the more recent data.
Typical shoreline perturbation caused by merging of 2001 and 2010 topographic data sets
Figure 7: Close-up view of perturbation in elevation contours for the Study DEM in the vicinity of the Row Boat Row Study Area. Elevations are relative to NAVD88.
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Figure 8 compares the 2001/2010 and 2014 topographic data sets in cross sectional view. The 2014 data corroborates the horizontal location of the dune ridges as represented by the 2001/2010 data, but gives more detail that differentiates a large frontal dune and several distinct dune features across the profile.
Figure 8: Cross sections of CHAMP model elevation data (initial and eroded) for Transects 79 and 80 compared to corresponding data extracted from the 2014 NGS Lidar. Also shown for reference are the TSWEL, Wave Crest Elevation, and Preliminary PFD location.
Using these data, Figures 9 and 10 identify the distinct topographic feature immediately landward of the beach face that is a clear frontal dune feature. Comparing Transects 79 and 80, this feature is generally consistent in size, shape, and distance from the shoreline. The estimated landward heel of this feature is also depicted in the figures. This landward heel of the dune is argued to be a more appropriate location of the PFD than that proposed by AECOM.
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Figure 9:
WHAFIS modeling results at Transect 79 compared with elevations extracted from 2014 NGS Lidar, annotated with location of Preliminary and Recommened PFD.
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Figure 10:
WHAFIS modeling results at Transect 80 compared with elevations extracted from 2014 NGS Lidar, annotated with location of Preliminary and REcommended PFD.
Applying the identified landward heel of the PFD from both transects to the 2014 Lidar data and aerial photographs (Figures 11 and 12, respectively), a recommended landward heel of the PFD is delineated alongshore. The feature is consistent with NFIP Regulations (Section 59.1), which describe the PFD as a “continuous or nearly continuous mound or ridge of sand with relatively steep seaward and landward slopes immediately landward and adjacent to the beach and subject to erosion and overtopping from high tides and waves during major coastal storms.” As shown on both figures, the proposed BFE for the AE area removed from the VE Zone is 10, which is consistent with WHAFIS Part 6 modeling results from the Coastal Flood Hazard Study Report (AECOM, 2014).
Summary The Coastal Flood Hazard Study Report (AECOM, 2014) found that the landward extent of the PFIRM VE Zone along Row Boat Row is controlled by the location and extent of the PFD. Analyses herein, however, found that the AECOM study sited the landward heel of the PFD, and thereby the landward limit of the VE Zone, too far inland based upon existing topographic conditions along Row Boat Row. In this analysis, a high resolution 2014 Lidar survey is used to identify and delineate the landward heel of the PFD at a location that is significantly more seaward than that represented in the PFIRM. It is recommended that the PFIRM be amended to consider this new information and the proposed extent of the VE Zone along Row Boat Row be revised accordingly.
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Figure 11: Proposed location of PFD line (pink curve), V/A Zone boundary, and proposed BFEs in the Row Boat Row Study Area. Contour elevations: 2014 NGS Lidar, NAVD88. Aerials: March 2015.
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Figure 12: Proposed location of PFD line (pink curve), V/A Zone boundary, and proposed BFEs in the Row Boat Row Study Area. Aerials: March 2015.
