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Variation of Beach Morphology along the Saco Bay Littoral Cell
In preparing a 'regional' analysis of beach erosion, several different sources of data were available. Surprisingly little beach profile data was available from the Corps considering the number of reports (Table 4) and the length of Corps involvement with the Saco River navigation project. Therefore, historic shoreline change was documented in this report using aerial photographs from the Maine Geological Survey (MGS). Beach profile data were created using topographic data from a Light Detection and Ranging (LIDAR) flight completed along the Maine shoreline by the National Oceanic and Atmospheric Administration (NOAA) in 2000. Each of the particular methods employed are discussed below.
The analysis of large-scale morphologic variation begins with the determination of general orientations of the shoreline (Gorman and others, 1998; Wijnberg, 1995). Orientation provides an initial indication of the hydrodynamic forces acting on the beach, for example, the shoreline's exposure to incoming wave energy from the Gulf of Maine.
The frequency and alongshore variation of orientations were determined for coastline segments at 100-foot intervals along the Saco Bay shoreline. Orientations were measured at the wet-dry beach intersection using 1995 aerial photograph mosaics (see Estimated Net Erosion and Accretion). All orientations reference the meteorological convention (compass bearing, degrees clockwise from True North).
Sea Level Rise
Sea level data was taken from the NOAA Portland, Maine tide gauge (#8418150). The average annual rates of sea level rise were determined for the overall time period of tide gauge data collection (1912-2001), and for this specific study period (1962-1995). First order linear regression (e.g., "rise over run") was used to determine the average rates of sea level rise for both time periods.
Estimated Net Erosion and Accretion
Aerial photographs were used to estimate net erosion and accretion along the Saco Bay shoreline, from Hills Beach in the south to Pine Point in the north. Photographs from 1962 and 1995 were chosen in order to document shoreline position changes. Photographs were scanned using an Epson ExpressionTM 1600 scanner at 600 dpi and digitized using Corel Photo-PaintTM and CorelDrawTM Version 8. 1962 photographs were standardized to the 1995 photograph scale (1:12,000). Control points, including homes, roadways, and existing structures (e.g., seawalls), were used on each individual photograph during the scaling process. After each photograph was set to the 1995 scale, 1962 and 1995 overlay-mosaics of the Saco Bay shoreline were created. Control points were then checked again for accuracy.
Two Erosion Reference Features (ERFs) were used for local conditions:
These ERFs were digitized along the length of the bay for the 1962 and 1995 photographs. The disadvantage of using the seaward edge of vegetation or existing structure in determining net erosion is that it fails to identify erosion if the structure or vegetation line is held in place by reconstruction or maintenance over the study time period. This may be interpreted as stability, when in actuality, erosion has occurred on the beach. The 1962 and 1995 vegetation lines were overlain on the 1995 aerial photograph. Transects were set at 100-foot intervals along the shoreline (from the southern end of Hills Beach northward to the eastern end of Western Beach at Prouts Neck (a total length of approximately 46,900 ft, roughly 9 miles) and the net erosion or accretion (in ft) was determined between each subsequent shoreline position.
Shoreline Type, Dry Beach Width, and Total Landward Width to Structures
Using the mosaic of 1995 aerial photographs, four different shoreline types were found to exist (from a seaward to landward direction):
For this report, portions of the shoreline comprised of rock outcrops are considered stabilized.
Beach Profiles. Topographic data from a 2000 LIDAR flight (September 28-29, 2000) was available from NOAA for Saco Bay (NOAA, 2000). Data was projected in ArcViewTM GIS 3.2 using the LIDAR Data Handler and Spatial Analyst extensions. Beach profile vertical and horizontal data are referenced to the North American Vertical Datum of 1988 (NAVD88) and the North American Datum of 1983 (NAD83), respectively. Beach profiles were created at each 100-foot alongshore transect used for historical shoreline changes. Profiles generally started near the 13 ft (~4 m) contour (landward of the dune crest, where applicable) and extended seaward to near -3ft (-1 m) NAVD. Beach profile data were exported into MicrosoftTM Excel and MATLABTM for further analysis. Exported images of LIDAR data utilized for this study are in Appendix A.
Maximum Profile Elevations and Base Flood Elevations. The maximum elevations (e.g., dune crest or top of seawall) at each 100-foot interval along the Saco Bay shoreline were determined from beach profiles created using the LIDAR topographic data. These elevations were compared with the Base Flood Elevations (BFE) taken from Federal Emergency Management Agency (FEMA) Flood Insurance Rate Maps (FIRM) to determine possible flood-prone sections of the Saco Bay shoreline.
Estimated Net Volume Changes. Net volumetric changes were estimated using LIDAR profile data in conjunction with net erosion and accretion rates determined from aerial photographs. Due to a lack of profile data from 1995 and 1962, it was assumed that each 2000 LIDAR profile was representative of existing beach conditions during both of those time periods. Therefore, by using net erosion or accretion values, profiles were offset either landward (erosion) or seaward (accretion), and the resulting area of change (A) was determined using the expression:
A = L x H
where L is the length of the rectangle created and H is the height. Assuming that each profile was representative of the beach 50 ft (15 m) in either alongshore direction of its location (for a total of 100 ft), the net volumetric change for each profile transect was determined using the simplified calculation:
V = L x W x H
Last updated on January 9, 2006.
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