Variation of Beach Morphology along the Saco Bay Littoral Cell

Methods

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.

Coastline Orientation

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 Expression^TM 1600 scanner at 600 dpi and digitized using Corel Photo-Paint^TM and CorelDraw^TM 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:

the seaward edge of vegetation; and
in developed areas with no vegetation, the seaward edge of shoreline structures.

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):

seawall to structure;
vegetated dune to structure;
vegetated dune and seawall to structure; and
seawall and vegetated dune to structure.

For this report, portions of the shoreline comprised of rock outcrops are considered stabilized.

Figure 3

Aerial photographs were also used to determine the distance (x) from the seaward edge of an existing structure or vegetated dune to the first habitable dwelling structure (i.e., home, motel, etc.) at each 100-foot interval. Next, dry beach widths (dbw) along Saco Bay were mapped during field investigations conducted on August 21 and August 26, 2002. A Garmin® GPS 12 Map handheld was utilized to map the high water line along the beaches, in addition to the seaward edges of vegetated dunes and stabilization structures. The GPS was set to record track points at an interval of every second in the WGS84 datum. Dry beach width data was projected using ArcView^TM GIS 3.2. Dry beach width (dbw) and distance from seaward edge of structure (x) data were combined to create the Total Landward Width (Z) criteria

Total Landward Width (Z) = x + dbw
An example is provided (Figure 3) for a vegetated dune to structure shoreline type.

LIDAR Analysis

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 ArcView^TM 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 Microsoft^TM Excel and MATLAB^TM 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.

Figure 4

Beach Slopes. For each beach profile created, beach slope values were calculated using a first order polynomial (linear) expression for profiles from their maximum to minimum elevations

h = Sx + c
where S is profile slope, x and h are associated horizontal and vertical values, and c is the y-intercept (Figure 4).

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

Figure 5

where the W is width of shoreline represented by the beach profile (100 ft, 30.5 m). Because the LIDAR beach profiles only extend to a vertical elevation of approximately -3 ft (-1 m) NAVD, volumes calculated only include the dune, berm, and a portion of the intertidal segment of the beach, and in no way represent volume changes undergone by the entire beach profile. Figure 5 is a schematic of this calculation. Volumes were summed for different regions in order to determine overall volume changes for each region from 1962 to 1995 (see regional delineations under Results).

Contents Introduction Historical Background Methods Results Discussion Recommendations Conclusions Additional Study and Research References Appendix A

Last updated on January 9, 2006.