Coastal Structure and Dune Crest Inventory and Overtopping Potential - Frequently Asked Questions

Q1. How do I use this viewer?     Back

To use the viewer, either zoom in on the map to your area of interest, either manually click the plus minus buttons keys on the left of the viewer tab), or by typing in an address or place and hitting the search tab search button. You can also zoom in or out by using the scrolling wheel on your mouse, if you have one. You can convert the Viewer into Full Screen mode by clicking on “View Map Full Screen” tab directly below the viewer home page.

To turn certain specific data layers on or off, simply click the data layers tab layers button and place a checkmark in the appropriate data layers you want to view.

When zooming, available data layers will automatically zoom with you. In the far-away view (outside of a certain scale), the viewer will only display line features for dune crests and coastal engineering structures. At a certain scale, the viewer will add point features representing the elevations of the structures and dune crests in reference to the FEMA BFEs, along with additional information. Once inside a certain scale, the viewer will only show the point features (the line features will stop drawing). To get more information on a feature (such as a line or point), simply click on that feature to extract information as described below in What data is visualized in this viewer?

Data descriptions, data disclaimers, and frequently asked questions are also available on the webpage below the viewer.

Q2. What data is visualized in this viewer?     Back

Several different features are visualized, including line features representing coastal engineering structures (rip-rap, bulkheads, etc.) and dune crests, and point features representing comparisons between preliminary FEMA BFEs and the elevation of the engineering structures or dune crests.

Line Feature Classes

Structures Lines – a black line representing the digitized linear extent of coastal engineering structures, including rip-rap seawalls, bulkheads, jetties, breakwaters, geotubes, etc. for coastal communities in York and Cumberland Counties, from Kittery to South Portland. Note that in some cases there can be more than one line of structures along a section of shoreline, and that a structure may be seaward (or landward) of a dune crest. If you click on the structures line, attributes include:

  • Structure_Type – seaward shoreline habitat type (beach, bedrock, mudflat, etc., if determined in the analysis), followed by the landward shoreline structure type (rip-rap, bulkhead, etc.), and if present, the more landward structure type (rip-rap, bulkhead, etc.). Note that the structure_type was developed at the time of data analysis and may not reflect current conditions.
  • HAT_Contact – whether or not it appears that the highest astronomical tide and/or water action is in contact with the structure (qualified by a Y, N). Note that HAT_Contact is dependent upon the topography at the time of the LiDAR data collection and may not reflect current conditions.
  • Town – the municipality in which the structure is located.
  • County – the county in which the structure is located.
  • Dist_mi – the distance of the structure, in miles.

Dune Crest Lines – a dashed brown line representing the digitized linear extent of the crest of primary undeveloped (with armoring structures) coastal sand dune crests in York and Cumberland County coastal communities. Note that in some cases there can be more than one dune crest along a section of shoreline, or a dune may be seaward (or landward) of a coastal engineering structure. If you click on the line, attributes include:

  • Dune Type – whether not in a dune (0), in a frontal dune (1), or back dune (2). Note that dunes change over time and the mapped dune crest is dependent upon the topography at the time of analysis and many not reflect current conditions.
  • Dist_mi – the distance of the dune crest, in miles.
  • Town – the municipality in which the structure is located.
  • County – the county in which the structure is located.

Point Feature Classes

Structures Points – a color-coded point file representing points (approximately 5 meter spacing) along the digitized structure line where elevations were extracted from available LiDAR data and compared with available preliminary FEMA 100-year BFE values. The data is symbolized by color-coded squares which represent the relationship between the engineering structure elevation and the FEMA BFE. Yellow to red colors indicate that the coastal engineering structure is below the FEMA BFE, while light to dark shades of green indicate that the coastal engineering structure is at or above the FEMA BFE. Coloring is as follows:

legend structure points

If you click on an individual structure point, attributes include:

  • SEAWARD_HAB (descriptor of the seaward shoreline habitat type)
  • STRUCTURE1 (descriptor of the seawardmost structure, if appropriate)
  • STRUCTURE2 (descriptor of the landwardmost structure, if appropriate)
  • DUNE (descriptor of the dune system the structure is located within, if appropriate)
  • HAT_CONTACT (whether or not the structure contacts HAT)
  • Elev_ft (the elevation, in feet, of the wall crest from LIDAR)
  • FLD_ZONE (the flood zone in which the point falls, either A, AE, AO, VE, or OPEN WATER)
  • STATIC_BFE (the BFE in which the point falls referenced to feet, NAVD88; if no published value, -9999 is used)
  • NEAR_DIST (the nearest distance, in meters, to the flood zone, if the point is not in a flood zone; if not used)
  • NEAR_FLD_ZONE (the nearest flood zone to the point, if not in a flood zone; if not used)
  • NEAR_STATIC_BFE (the nearest static BFE, if point not in a flood zone; if not used)
  • BFE_ELEV_COMPARE (comparison between BFE and structure maximum elevation)

Dune Points – point file representing points (approximately 5 meter spacing) along the digitized dune crest line where elevations were extracted from available LiDAR data and compared with available preliminary FEMA 100-year BFE values. The data is symbolized by color-coded circles which represent the relationship between the dune crest elevation and the FEMA BFE. Yellow to red colors indicate that the dune crest is below the FEMA BFE, while light to dark shades of green indicate that the dune crest is at or above the FEMA BFE. Coloring is as follows:

legend dune crest points

If you click on an individual structure point, attributes include:

  • DUNE (descriptor of the dune system the structure is located within, either 0, 1 or 2)
  • Dune_Elev_ft (the elevation, in feet, of the dune crest from LIDAR)
  • FLD_ZONE (the flood zone in which the point falls, either A, AE, AO, VE, or OPEN WATER)
  • STATIC_BFE (the BFE in which the point falls referenced to feet, NAVD88; if no published value, -9999 is used)
  • NEAR_DIST (the nearest distance, in meters, to the flood zone, if the point is not in a flood zone; if not used)
  • NEAR_FLD_ZONE (the nearest flood zone to the point, if not in a flood zone; if not used)
  • NEAR_STATIC_BFE (the nearest static BFE, if point not in a flood zone; if not used)
  • BFE_ELEV_COMPARE (comparison between BFE and the dune crest maximum elevation)

Coastal Elevations (m) – raster file depicting color-coded (blue to green, low to high) land-surface elevations from the 0 m NAVD contour up to the 10 m NAVD contour. LiDAR elevations are included so that users of the Viewer can inspect whether elevations landward of a shoreline engineering structure or dune crest get higher, stays about the same, or get lower in topography. This helps users investigate the elevations of their infrastructure (homes, etc.) in reference to the structure or dune crest and the relationship with the FEMA Base Flood Elevation (BFE).

Q3. What datasets were used to compile this information?     Back

Several different data sources were used to compile this dataset, including:

  • NOAA Environmental Sensitivity Index (ESI) Office of Response and Restoration GIS data layers representing different shoreline engineering structures and habitat types;
  • Available Maine DEP GIS-data relating to issued permits for seawall construction or reconstruction in Maine;
  • Available aerial orthoimagery from Maine GeoLibrary;
  • Shoreline engineering structures from work by Rice (2015) which mapped hard stabilization structures in support of beach habitat mapping; and
  • MGS Highest Astronomical Tide (HAT) layer.
  • Available LiDAR topographic data for York and Cumberland Counties, including data from 2006, 2010, 2013, and 2014 from NOAA’s Data Access Viewer;
  • Mapped boundaries of the coastal sand dune system from Coastal Sand Dune Geology Maps (Slovinsky and Dickson, 2011);
  • GIS layers of additional mapped coastal sand dunes not originally mapped by Slovinsky and Dickson (2011); and
  • Preliminary FEMA Digital Flood Insurance Rate Map (DFIRM) GIS layers from the FEMA Map Service Center and the National Flood Hazard Layer database.

Q4. Are there any quick data summaries you can share?     Back

In the York and Cumberland County coastal study area (communities from Kittery north to South Portland) there is approximately 511 miles of tidally influenced shoreline. Coastal engineering structures front 68.4 miles – or about 13.4% - of the overall tidally influenced shoreline. Undeveloped dune crests front approximately 31.6 miles – or about 6.2% of the tidally influenced shoreline.

About 96.4 miles of York and Cumberland County are considered “open coast” – that is, for the most part, exposed to dynamic wave action. About 28.3 miles (or 29.3%) of this shoreline has mapped dune crests. About 38 miles (or 39.4%) of this shoreline has coastal engineering structures.

The majority of coastal engineering structures – over 44% - are located within the FEMA VE (Velocity) Zone, as shown in the table below.

table 1
Table 1. Percentage of coastal engineering structures located in different FEMA flood zones.

A large majority (over 85%) of coastal engineering structures are below preliminary FEMA BFEs, with only about 14% above the 100-year BFE. Most (almost 23%) are between 2 and 4 feet below the FEMA BFEs. Only about 10.7% of existing structures are at least 1 foot above the BFE.

table 2
Table 2. Comparison of coastal engineering structure elevations with 100-year FEMA BFEs.

Coastal Sand Dune Crests vs. FEMA 100-year BFEs

Similar to coastal engineering structures, the majority of protective sand dune crests – over 80% - are located within VE-zones.

table 3
Table 3. Percentage of coastal sand dune crests located in different FEMA flood zones.

A large majority (over 68%) of dune crests are below preliminary FEMA BFEs, with about 32% above the 100-year BFE. Only about 25% of current dune crests are at the recommended level of 1 foot or more above the BFE.

table 4
Table 4. Comparison of dune crest elevations with 100-year FEMA BFEs.

All of this indicates that the majority of the shoreline along Maine’s York and Cumberland County coastline fronted by either shoreline engineering structures or coastal sand dunes are potentially not adequately protected to withstand the water levels associated with a 100-year storm event.

Q5. What are the limitations or assumptions of this data?     Back

The supporting datasets were developed at different times and each only represents a static (or “snapshot”) of conditions at the time the data was developed. For example, new seawalls or bulkheads could have been constructed, or altered, since the data representing the extent of coastal engineering structures was developed. Secondly, the extracted elevation data was developed using Light Detection and Ranging (LiDAR) data from several different time periods, including 2006, 2010, 2013, and 2014, as data was available. Elevations and locations of dune crests and seawall crests may have changed since the data was extracted. It is also possible that the digitized locations of either seawall or dune crests may not capture all of the maximum elevations of the features. It is also possible that our analysis missed areas of coastal engineering structures which were not readily visible on aerial orthoimagery, or part of our analysis of permit data, etc.

In terms of comparing BFEs with maximum elevations of dunes or coastal engineering structures, the FEMA BFEs extracted were from most-recently available (2017-2018) preliminary FEMA Digital Insurance Rate Maps. The extent of mapped flood zones and elevation of the BFEs may change from when this information was extracted. Also, it is important to note that just because a seawall or dune crest is below a BFE that structural damage would necessarily occur for infrastructure (homes, etc.) behind that dune or coastal engineering structure. In some cases, topography behind the wall or dune upon which infrastructure may be situated is well above a BFE. Our analysis also doesn’t account for dissipation of waves by either structures or sand dunes, which could further protect infrastructure behind these structures. Similarly, just because a dune or coastal engineering structure may be above the BFE, this does not necessarily mean that landward infrastructure may not be damaged in a large storm event. In order to help investigate this the Viewer includes a raster depicting LIDAR-derived elevations which show how a wall or dune crest relates to the land surface elevations landward of the wall or dune crest.

References

Rice, T., 2015. Beach Armor, 2012-2015, North and Mid-Atlantic, U.S., Terwilliger Consulting, https://www.sciencebase.gov/catalog/item/5b8561c1e4b05f6e321d03e7

Slovinsky, P.A., and Dickson, S.M. 2011. Coastal Sand Dune Geology 1:4800 scale Online Maps. Maine Geological Survey. Augusta, ME. https://www.maine.gov/dacf/mgs/pubs/online/dunes/dunes.htm

Last updated on February 27, 2020