Variation of Beach Morphology along the Saco Bay Littoral Cell

Discussion

Saco Bay is generally considered to be an enclosed littoral cell. Previous work by others (e.g., van Heteren and others, 1996; Kelley and others, 1995; Barber, 1995) have estimated sand budgets and inferred sediment transport directions along the Saco Bay shoreline. Such work has established a general northerly-directed sediment transport direction along the Bay, with the source of the majority of the sediment being the Saco River. Stabilization of the river entrance has undoubtedly influenced the sediment budget of the beach system.

Results of the current study suggest that the Saco Bay system can be further divided into four distinctly different geographic regions (regions 1 to 4) based on general beach morphology (Figure 1). Both regions 1 and 2 appear to be influenced by the stabilized Saco River channel. Region 3 seems to be outside of the direct influence of the Saco River jetties; however, it does appear to be influenced by the Scarborough River and its jetty. Tidal currents of the Scarborough River heavily influence the morphology of Region 4 (Western Beach).

The southern jetty of the Saco River positively influences the updrift Hills Beach region for approximately 2,000 ft, while the northern jetty negatively influences approximately 5,000 ft of the downdrift shoreline. These trends are evidenced by dramatic changes in overall beach profile shapes, slopes, and erosion/accretion trends. Region 3 as a whole is highly accretive, receiving the majority of sediment within the Saco Bay system and resulting in the largest positive net volume changes along the bay's shoreline.

Because Saco Bay can be divided into four different regions, the remainder of this section will be separated into a regional discussion of the parameters and characteristics investigated.

Region 1 (Hills Beach)

Estimated Net Erosion and Accretion (1962-1995). The majority of region 1 is generally stable to accretive (Figure 9). These trends may be attributed to the following factors: the sheltering of Hills Beach from incoming wave energy from the south by Fletchers Neck and Biddeford Pool and several offshore islands, and from the north/northeast by the northern Saco River jetty), and the trapping of sediment by the southern jetty.

The southern portion of this region, nearest Fletcher Neck, exhibits little shoreline change due to a shoreline stabilized by natural rocky outcrops and seawalls. Pocket accretion exists from 700-2,000 ft, to just south of Surf Avenue, probably attributable to wave shadowing by Stage and Basket Islands.

Net accretion near the northern end of the beach (4,000-6,000 ft), adjacent to the jetty, is on the order of ~ 130 ft with an average rate of 4 ft per year. This indicates that over the time period of study (33 years), sediment has accumulated and substantially widened the dunes. Hulmes (1980) suggested that no new sediment (aside from beach nourishment by the Corps, although amount unspecified) has been introduced into the Hills Beach system since the Saco River jetties were emplaced. Therefore, a distinct deficit in the sediment budget exists; the actual source of sediment for such shoreline progradation is unknown, however several possibilities exist.

It is possible that the accretion seen is a result of the slow, landward migration of ebb-tidal delta shoals due to wave action. This process most likely occurred several years to several decades following construction of the southern jetty in the late 1890's (USACE, 1961).

However, some ebb-tidal shoals may continue to periodically nourish the system. It is also possible that the accretion seen in the northern portion of Hills Beach is a result of material reworked within the Hills Beach littoral cell. Portions of Hills Beach that appear to be relatively stable (e.g., the central portion, 2,000-4,000 ft) are in actuality eroding; the erosion signal is simply not present due to the reconstruction of seawalls along this stretch of shoreline in order to "hold the line". Over time, sediment lost in front of these seawalls may be diverted northward by currents due to wave refraction around the Hills Beach cell. A Corps study conducted in 1961 focused on erosion problems at Hills Beach. Part of the study utilized shoreline positions from 1859, 1871, 1913, 1954, and 1960 in order to describe accretion and recession along the Hills Beach shoreline. The largest net accretion along Hills Beach occurred between 1871-1913. The Corps concluded that this was "...attributable to impounding of material by the south jetty following initiation of construction in 1891" (USACE, 1961).

It is important to note that the central portion of the shoreline (near 4,000 ft) that is fully stabilized by a continuous seawall corresponds with the thinnest portion of the entire beach, and may have been previously breached and connected with a tidal inlet to a back-barrier tidal creek. This area, because of its narrow width and relatively low topography, remains extremely susceptible to washover and erosion. This segment of shoreline probably has eroded; however, due to the erosion reference feature utilized in this study (seaward extent of seawall), the shoreline exhibits relative stability.

The dramatic increase in net accretion within 2,000 ft adjacent to the southern jetty (0 at 4,000 ft to almost 320 ft adjacent to the southern jetty) may also relate to elevation variations along the length of the southern jetty. The 4,800-ft long jetty is constructed to +11 ft MLW for 1,760 ft, then at +5.5 ft MLW for the remainder of its length. The continuously exposed portion of the jetty (1,760 ft) influences the adjacent beach for a distance of its approximate length by interrupting the northward flow of sediment.

Shoreline Type, Dry Beach Width, and Total Landward Width. The region 1 shoreline is comprised of rocky outcrops, seawalls, or vegetated dunes. Approximately 38% of the shoreline is stabilized, while the remaining 62% is vegetated dune. Dry beach widths along region 1 are consistently less than 50 ft. Minimum widths (0 ft) are adjacent to stabilized (seawalls) portions of the shoreline. Total distances to habitable structures are widest for naturally vegetated dune portions of the shoreline. It is clear that segments of the shoreline fronted by seawalls and rocky outcrops are those with the shortest total distances to habitable structures. Along the southernmost stretch, habitable structures are constructed close to the rocky coastline. A pocket of relatively wide vegetated dune (~ 1,000-1,800 ft) exists due to wave sheltering and protective sandy shoals associated with Stage and Basket Islands. The seawalled shoreline and subsequent short distances to habitable structures from 2,700-4,200 ft may exist because of shoreline erosion caused by wave focusing within the Hills Beach area. Although dry beach widths are narrow at the northern end, the overall dune system is wide, leading to a relatively wide total width from HWL to habitable structures.

Alongshore Variation of Beach Profile Shapes. Beach profiles within the Hills Beach region are highly variable due to the influences of sand shoals, rock outcrops, seawalls, and the southern Saco River jetty (Figures 11 and 12). Profiles within the southern compartment of Hills Beach (0-2,000 ft) have dune crests that are situated above the BFE, which ranges from near 13 ft to about 15 ft (Figure 21). The relatively flat slopes in the southern section may be attributed to sheltering from wave action by large shoals and Basket Island, just offshore. This southern compartment also exhibits a stretch of exposed rock outcrops in the surf zone. Beachfront homes within this stretch are not only above the BFE, but also have a horizontal buffer (see Shoreline Type, Dry Beach Width, and Total Landward Width to Structures) that serves to dissipate floodwater energy.

Profiles within the central compartment are generally below BFE, and beachfront structures are generally less than 100 ft from the seaward edge of dune or structure. The stretch of steepest slopes (1,700-2,500 ft) corresponds with a change in shoreline orientation that relates to the loss of the protective shoals associated with Basket Island (Figures 6, 7, and 22). The steeper slopes in this segment may be the result of wave energy focusing on this stretch of beach due to deeper water offshore. Profile slopes tend to flatten near 2,600 ft, and steepen from 3,000-3,700 ft. This trend may relate to a stretch of seawalls that have been constructed along the beach from 3,000 to near 4,000 ft.

Dune crests are generally below BFE along the central to northern stretch of Hills Beach (2,000-5,500 ft), although the width of dunes seaward of structures substantially increases from near 100 ft at 4,300 ft, to 300 ft at 6,000 ft, adjacent to the jetty. Profile slopes tend to flatten corresponding with substantial beach progradation up to about 5,100 ft. Nearest the jetty, profile slopes steepen, though the beach exhibits an accretive trend.

Estimated Net Volume Changes (1962-1995). Beach profiles within region 1 exhibit a net overall positive volume change of approximately 232,200 yd³ over the 33-year period of study, and appear to support three separate compartments based on volume change (Figure 23, Table 7). The southern compartment, with net positive volume change on the order of 76,600 yd³, corresponds with the wave-sheltered portion of the coastline. The relatively small positive net volume change of about 18,700 yd³ over the 33-year study period seen within the central compartment may be due to the presence of seawalls, most of which have been maintained, and may not reflect actual shoreline changes since the shoreline has been artificially held in place. In all likelihood, erosion of the submerged beach has occurred along this stretch. Also, several Corps nourishment projects have added sediment to Hills Beach as beneficial use of dredged material from the Saco River, although the exact amount is unknown. The northern compartment, as discussed under Estimated Net Erosion and Accretion, has undergone substantial accretion with a positive net volume change on the order of 136,900 yd³.

Region 2 (Camp Ellis Beach, Ferry Beach, Bay View, and Kinney Shores)

Estimated Net Erosion and Accretion (1962-1995). Net erosion dominates the initial 5,000 ft of the shoreline with the highest net erosion (40-100 ft) confined to an approximate 1,000-foot (7,700-8,700 ft) stretch of beach near the northern jetty (Figure 9). This stretch corresponds with a section of shoreline that has been heavily stabilized with seawalls, with unstabilized pockets experiencing more net erosion than those that have been continually maintained. A pocket of slight net accretion (20 ft) occurs from 8,900-9,300 ft, corresponding with a stabilized stretch of beach between Lower Beach Avenue and Sunrise Avenue. North of this, the shoreline is erosive from Sunrise Avenue north to Morris Avenue (9,400-10,600 ft), and relatively stable to slightly erosive from Morris Avenue north to near Long Pond in Ferry Beach (10,600-12,700 ft). The different patterns along this stretch may relate to the shoreline types; vegetated dune and seawall along the stretch of beach from Sunrise to Morris Avenues, and natural dune with no seawall from Morris Avenue to near Long Pond.

The pattern becomes accretive near Long Pond (12,700 ft), and extends north through Bay View and Kinney Shores to Goosefare Brook, although some short lengths of shoreline display erosive trends. These pockets appear to coincide with several small yet abrupt changes in shoreline orientation. Pockets of accretion (noted as distinct bulges in the shoreline) near 13,700 and 15,700 ft (north of Bay View Avenue) correspond with wave shadowing associated with bedrock outcrops in the subtidal beach.

Shoreline Type, Dry Beach Width, and Total Landward Width. Approximately 25% of this region is stabilized by seawalls alone; while some combination of seawalls and vegetated dunes account for an additional 31% of the shoreline. The remaining 44% is vegetated dune only. Seawalls are concentrated along a stretch closest to the Saco River jetty (7,700-9,700 ft), with distances to habitable structures generally less than 100 ft and dry beach widths less than 25 ft. North of this, a mixture of naturally-vegetated dune and seawall/dune combinations extend from Fairhaven Avenue north to Outlook Avenue (9,800-17,000 ft), with distances to habitable structures generally greater than 100 ft except for an oceanfront condominium south of Bay View Avenue (near 15,000 ft), which is located just landward of the seawall/dune. The remaining shoreline within the region includes some form of stabilization. The northern end exhibits relatively short distances to structures, generally fifty feet or less due to proximity of Goosefare Brook, which has a high tendency to shift position on its south side. It is apparent that homes in this area are constructed right at the seawall.

Alongshore Variation of Beach Profile Shapes. Beach profiles within this region appear to be heavily influenced by both the northern Saco River jetty and stretches of privately constructed seawalls (Figure 13). The stabilized shoreline along Camp Ellis Beach, which extends roughly 2,000 ft northwards from the jetty, constitutes the steepest profile slopes along Saco Bay (Figure 22). The slopes flatten from south to north as a result of fewer shorefront structures and a decrease in the influence of reflected waves from the northern jetty. The jetty, in conjunction with numerous seawalls, influences beach profile slopes for a distance of almost 6,000 ft, from the jetty to north of Long Pond (7,700-13,700 ft), assuming that the slope of the 'natural' beach is roughly 0.050. This is also apparent in the maximum profile elevations, especially for those profiles within approximately 2,000 ft of the jetty.

Beach profiles approach a constant slope value of 0.050 near 13,700 ft, which corresponds with a bulge in the coastline, probably resulting from wave shadowing by an exposed bedrock outcrop. Aside from abrupt profile flattening near 15,700 ft, attributed to another bedrock outcrop that has trapped sediment, slopes remain at values near 0.050 north to Goosefare Brook. This general alongshore stability in profile slope, and distinct increase in maximum profile elevations may be attributed to:

the fact that profiles north of 13,700 ft appear to be outside the influence of the northern jetty of the Saco River; and
shoreline stabilization structures are not as abundant along this stretch of beach.

Estimated Net Volume Changes (1962-1995). Although region 2 exhibits a relatively small positive net volume change for the study, it includes a compartment that shows the largest negative volume change along the Saco Bay shoreline, on the order of 55,400 yd³ (Figure 23). This stretch of beach, roughly from Bay Avenue north to near Long Pond (7,700-12,700 ft), corresponds with highly erosive portions of Camp Ellis and Ferry Beaches and is heavily stabilized with sea walls. In reality, the reconstruction and maintenance of seawalls along Camp Ellis Beach, especially along Surf Street, has probably decreased the calculated net volume and net erosion of the shoreline. This is a result of using the seaward extent of a seawall as the erosion reference feature in determining net shoreline erosion or accretion. If a seawall is maintained, apparent horizontal erosion of the immediate seaward shoreline may be abated; however, vertical erosion of the profile may be accelerated, and is immeasurable using techniques employed herein. Therefore, net volume changes may be grossly underestimated in this section.

The distinct shift from negative net volume change to positive net volume change occurs at the same location as a sudden increase in coastline orientation associated with a small bulge in the shoreline. This bulge is located in line with Eagle Island to the east, and may result from wave shadowing by that island.

Region 3 (Ocean Park, Old Orchard Beach, Surfside, Grand Beach, Pine Point)

Net Accretion/Erosion (1962-1995). Adjacent to Goosefare Brook, frontal dune growth and retreat varied within Ocean Park, from roughly 19,600-20,800 ft, probably associated with periodic migration and shoaling of Goosefare Brook (Nelson, 1979; Farrell, 1972). The shoreline underwent little change near 21,000 ft, and becomes accretive along Old Orchard Beach, from 21,400-25,400 ft (Figure 9). The shift from variable, to stable, to accretive coincides with small changes in coastline orientation. The progradation of the frontal dune along 21,400-25,400 ft may be attributed to artificial dune construction for emplacement of a sewer pipeline (within the frontal dune) along portions of Ocean Park and Old Orchard Beach.

A stretch of unvegetated, stabilized shoreline extends from near Fernald Avenue north to Brisson Street (25,500-28,000 ft), coinciding with a heavily developed section of Old Orchard Beach. Accretion is relatively minimal, on the order of 20 ft. Accretion slightly increases from Brisson Street to near Rosewood Street, associated with small artificial dunes constructed as part of the northern extension of the sewer pipeline. From near Rosewood Street to near Parcher Avenue (approximately 31,000-34,000 ft), the shoreline has been continuously stabilized with seawalls and has undergone little change. From just north of Parcher Avenue northeast to just north of Little River Road (34,000-36,000 ft), small dunes have grown seaward of the 1962 line (established by a seawall), with net accretion near 50 ft. Net accretion increases to over 100 ft near 36,400 ft and continues to increase to a peak of near 250 ft near Oak Street in Grand Beach (40,000 ft), decreasing slightly northwards to the Scarborough River jetty. The accretion in this region corresponds with the distinct crescent shape of the shoreline, changing abruptly from 50-60^°, then increasing to 90^° at the peak of the accretion.

$wave refraction patterns$
Figure 24

This distinct increase in accretion may relate to the reworking of sediments trapped in the Scarborough River ebb-tidal delta. Sediment most likely migrates northwards into the Pine Point area and becomes trapped within the Scarborough River ebb tidal delta. Over time, waves refracting around Prouts Neck rework the sediment and spread shoals in a landward (and slightly southern) direction until they weld onto the beach at Pine Point and Old Orchard Beach. It seems that the southern limit of this accretion is just north of the old Little River Inlet, near Grandview Avenue in Surfside (~ 32,500 ft); this southern limit corresponds with wave refraction patterns illustrated in an aerial photograph from Goldsmith (1976) (Figure 24).

Shoreline Type, Dry Beach Width, and Total Landward Width. Approximately 75% of region 3 (including Western Beach, 69%) is fronted by either stabilization structures, or structures combined with vegetated dunes. Portions of the shoreline that are solely stabilized (25,500-27,100 ft in Old Orchard Beach) are generally those with habitable structures constructed right at the landward extent of the dry beach/seawall (Figure 10). Although this stretch has no dune whatsoever, it does coincide with the highest dry beach widths along the bay, ranging from 150 to over 200 ft. This stretch is part of the heavily developed "downtown" section of Old Orchard Beach. Shoreline along 27,300-29,500 ft is fronted by vegetated dune and seawall, yet has dry beach widths near 50 ft and total widths approaching 100 ft. The shoreline adjacent to the former location of the Little River Inlet (near 32,500 ft) is fronted by seawalls, yet the total width (and subsequent dry beach width) is between 150 and 200 ft. This is a result of the former migratory nature of the Little River Inlet, which incised paleochannels through the beach during migration. Due to this historical instability, such areas typically become erosion prone (i.e., erosion 'hot spots'). Such is the case for other historically unstable areas along the east coast such as the 'Washout' at the northern end of Folly Beach, South Carolina (Gayes and others, 1998).

The northern portion of the region exhibits extremely large distances to habitable structures and no shoreline structures are located at the landward edge of the beach. This stretch is the most naturally protected shoreline along Saco Bay, even though dry beach widths tend to decrease from a point at 35,100 ft northwards. This stretch, however, exhibits a wide vegetated dune (and subsequent distances to structures ranging from 150 ft to near 500 ft) and maximum profile elevations that exceed BFE values.

Alongshore Variation of Beach Profile Shapes. In general, region 3 beach profiles (Figures 11 and 14) exhibit flat slopes generally on the order of 0.050 or less, except for along the western portion of Western Beach (Figure 22). This may be attributed to the general accretive nature of the shoreline. From 19,700-25,400 ft, the shoreline supports a vegetated dune and a constant slope on the order of 0.050, with high maximum profile elevations (from just below to over 20 ft). These elevations are the result of an artificially constructed dune that covers a sewer line for portions of Ocean Park and Old Orchard Beach.

A stabilized portion of Old Orchard Beach exists from 25,500-28,500 ft, and beach profile slopes tend to flatten slightly in front of this stretch, in addition to dramatically decreasing in maximum profile elevations. The crests of the seawalls that front the beach along this stretch are sometimes below BFE (Figure 21). It is interesting that beach profile slopes decrease in this stretch, considering the influence of structures on the beach profile slopes adjacent to Camp Ellis (dramatic increase in profile slopes). This difference in beach response may be attributed to sediment supply: Old Orchard Beach is relatively sand-rich, while Camp Ellis is starved of sediment. Profile flattening may also be attributed to wave action, in which waves reflected off of the seawall transport sediment in a seaward direction, thereby decreasing overall profile slope.

North of this stretch, small dunes are developed and beach profiles gently flatten to 0.025 near 32,000 ft. The flattest profiles of all exist from near Rosewood Street to just south of Parcher Avenue (32,000-33,700 ft); this stretch corresponds with a slight change in shoreline orientation and the location of another seawall. An increase in shoreline compass-bearing occurs near 32,500 ft, and seems to correspond with the former location of the Little River Inlet, which was closed in the 1870's by the construction of a railway through Old Orchard Beach (Farrell, 1972). Closure of the inlet is probably partly responsible for the abrupt jump in net accretion seen north of this area due to the abandoning of ebb tidal shoals (Kelley and others, 1995). This location also coincides with the lowest profile elevations (near 10 ft), well below BFE. The substantial decrease in maximum profile elevations along this entire stretch may be a result of the historical migratory nature of the Little River Inlet, prior to its artificial closure. Evidence of the inlet still exists in the aerial photographs, and existing geomorphology of the coastline indicates a northward transport of sediment. The shoreline has subsequently been heavily developed and stabilized with seawalls. In addition, the thickness of dune sand in this area is also the thinnest of any between the Saco and Scarborough Rivers (van Heteren and others, 1996).

North of this section, slopes slightly steepen, but maximum profile elevations increase to above 15 ft and stay at or above BFE values (Figure 21). This stretch includes an old seawall that has been fronted by a large, accretive dune that stretches from northeast of Little River Road to about Oak Street (36,500-38,100 ft). The growth of this dune may be attributed to the welding of historic sediment shoals associated with the old Little River Inlet.

Beach profiles continue to steepen from Oak Street east into Pine Point (38,300-41,200 ft), corresponding with a curvature of the coastline towards 90° from North (Figure 6). An abrupt change in shoreline orientation and distinctly steeper beach profile slopes near 41,300 ft may be the result of a shore perpendicular trough between shoals of the Scarborough River ebb delta. This shoal structure may cause wave focusing, resulting in the steeper profiles and change in orientation. Profile flattening north of this is attributed to shoaling adjacent to the Scarborough River jetty.

Estimated Net Volume Changes (1962-1995). Beach profiles within region 3 exhibit the highest positive volume changes along Saco Bay (Figure 23). Stretches with very little volumetric change over the period of study are concentrated between 25,500-36,000 ft, corresponding with the heavily stabilized sections of the Old Orchard Beach, Surfside, and Grand Beach shorelines. This makes sense based on the technique used to determine erosion/accretion and subsequent volume changes, since the seaward edges of structures were measured in areas with no frontal dune vegetation.

The largest positive net volume changes within region 3 occur within compartment 3C, downdrift (northeast) of Goosefare Brook, and updrift (southwest) of the Scarborough River. Both Goosefare Brook and Scarborough River act as sediment sinks (van Heteren and others, 1996). Goosefare Brook has actually closed several times in recent history, most recently in 1992, when dredged material used to nourish the beaches at Camp Ellis migrated northwards. The inlet of the brook also has a history of meandering periodically to the north and south, which is evident in both the 1962 and 1995 photographs. These variations may be attributed to wave refraction patterns and currents in the vicinity of Goosefare Brook (Kelley and others, 1995), and probably cause the instability seen at the immediately adjacent shorelines.

The Scarborough River also acts as a sediment trap, and receives the majority of the sediment moving northwards along the Saco Bay shoreline, supplied mostly by the Saco River. As mentioned previously, Kelley and others (1995) estimate that the Saco River supplies approximately 10,500-21,000 yd³ (8,000-16,000 m³) of sediment annually to Saco Bay (Kelley and others, 1995; Barber, 1995). According to their work, from 1955 to 1991, Pine Point received a minimum of 11,117 yd³/yr (8,500 m³/yr) of sediment from both erosion of the shoreface and alongshore drift.

Based on volume changes within the subaerial-intertidal beaches of Grand Beach northeast into Pine Point (approximately 36,000-42,300 ft), there was an overall net volume gain of near 705,900 yd³. This equates to approximately 21,000 yd³/yr volumetric gain from 1962-1995. Barber (1995) estimated that the total volume of accretion at Pine Point over a 96-year period from 1859-1955 was on the order of 5.8 x 106 yd³ (4.46 x 106 m³), with approximately 31% (1.798 x 106 yd³) of this going into spit accretion at Pine Point. If the annual volumetric gain of 21,000 yd³/yr from the current study (1962-1995) is used to compute a net 96-year period for comparison, a net volume change of +2.016 x 106 yd³ is recorded, similar in magnitude to Barber's (1995) net volume gain at Pine Point.

Region 4 (Western Beach)

Estimated Net Erosion and Accretion (1962-1995). Shoreline changes indicate, in general, substantial amounts of accretion along Western Beach. Aside from some small pockets of little change or slight erosion (e.g., nearest the Scarborough River jetty, and near 44,600 ft at a tee location of the golf course), net accretion averages around 60 ft, with the stretch of beach from 43,700-44,800 ft having an average net accretion of 47 ft, while the remainder of the beach has an average of 70 feet. This difference may be attributed to the influence of the Scarborough River channel on this section of beach nearest the Scarborough River jetty. This area is proximal to the main channel of the river; because of this, current velocities are most likely higher in this area, making sediment accumulation and subsequent accretion more difficult.

Shoreline Type, Dry Beach Width, and Total Landward Width. Since Western Beach borders the Prouts Neck Country Club, there are very few habitable structures along the shoreline, and hence, very few stabilization structures to protect them, although a clubhouse and several smaller structures are located at the southeastern end of Western Beach. The majority of the shoreline type is therefore vegetated dune. Western Beach, however, does border several holes of the Country Club. Therefore, distances were measured from the vegetation line to the nearest green or fairway, since shoreline changes may impact course locations. Distances to the golf course, in general, are well over 100 feet aside from several pockets near 44,500 ft that are near 50 feet (near Tee #2). No dry beach widths were measured for this section.

Alongshore Variation of Beach Profile Shapes. Along Western Beach, beach profiles appear to be impacted by the Scarborough River, especially those most proximal to the river's main channel. Beach profiles along the first 1,100 ft stretch adjacent to the Scarborough River jetty are relatively high in elevation (near 15 ft) and quite steep (slopes on the order 0.100), achieving depths of -3 feet within 200 feet of the dune line. Beach slopes tend to flatten with distance away from the jetty, as the main channel migrates farther offshore. This trend may be due to the influence of tidal currents on beach profile shape. In general, currents within the main channel move at greater velocities than those outside the main channel, prohibiting sediment accumulation that permits the formation of a wide, flat beach. Farther east, beach profiles flatten and extend much farther offshore (greater than 500 feet) in order to achieve depths of -3 feet.

Estimated Net Volume Changes (1962-1995). As a whole, the shoreline along Western Beach has gained approximately 141,200 yd³ of sand over the 33-yr study period, or approximately 4,300 yd³/yr. However, the 2 separate compartments within the region have undergone different net changes. Only 28% (40,000 yd³) of the net volume change is accounted for in the first 1,100-ft stretch (compartment 4A) of beach east of the Scarborough River. The morphology of this compartment is heavily influenced by tidal currents associated with the Scarborough River. Ebb and flood currents probably divert sediments into the ebb and flood deltas, leaving little sand for accumulation adjacent to the main channel. The majority of positive net volume change occurs in the easternmost 1,900-ft (compartment 4B) of shoreline. This is probably due to sediment entrapment by Prouts Neck and a weakening in the impacts of tidal currents from the river.

General Discussion

An imbalance exists in the sediment budget along the Saco Bay shoreline. Part of this may be a result of the method employed to determine erosion/accretion and subsequent volumetric gains and losses. By using the visible seaward edge of vegetation, or a stabilization structure in the absence of vegetation, false stability may be represented when in fact erosion has occurred. This is most likely the case along the Camp Ellis Beach and portions of the Hills Beach shoreline, which are heavily engineered with seawalls; these stabilized sections, especially in the case of Camp Ellis Beach, have been rebuilt several times between 1962 and 1995. Therefore, the seaward edge of structure, for the overall time period of 1962-1995, appears to not have changed even though it has most likely undergone episodes of substantial erosion.

It is unlikely, however, that the sole source of material for the accretion seen at Pine Point is eroded and reworked material from the Camp Ellis shoreline. Sediment originating from the Saco River may be migrating northwards and feeding the beaches to make up for the apparent sediment budget deficit. It is also possible that sediment from the subtidal portion of the beach along Camp Ellis is being eroded and reworked northwards, although this trend is impossible to decipher given that existing profile data only extends to near the low water mark. Kelley and others (1995) and Barber (1995) estimated an annual range of 10,500-21,000 yd³ of sediment input from the River. This accounts for 346,500-693,000 yd³ of sediment over the 33-year period of this study. The upper limit of this range is comparatively close to the approximate 705,900 yd³ gain seen at the subaerial-intertidal portions of beaches at Pine Point during the current study, although it is unlikely that all sediment introduced to Saco Bay by the Saco River ends up at Pine Point.

Interestingly, if the amount of sediment dredged and used for either beach nourishment (at Camp Ellis Beach) or disposed of inshore/offshore (Tables 2, 3) during the study period is taken into account, the numbers relate well. Approximately 473,700 yd³ of sediment was used for nourishment at Camp Ellis Beach, and approximately 427,500 yd³ of sediment was dredged from the Scarborough River and disposed of in either the nearshore or the offshore, equaling 901,200 yd³ of sediment added to the Saco Bay nearshore system. Assuming that the overall gain in sediment volume for region 3 (at Pine Point) was 1,125,300 yd³, this leaves a deficit of about 224,000 yd³ of sand, well below the range for total input of sediment from the Saco River over a 33-year period (using Kelley and others, and Barber's numbers, above). Since the current study only accounts for sediment volumes incorporated in the subaerial-intertidal portion of the beach profile, this variation may be justified. The process of upwelling may also rework some of this sand onshore during westerly gales (Dickson, 1999) through a regular exchange with the nearshore (Heinze, 2001).

As a whole, Saco Bay beach profiles (from Hills Beach north to Western Beach on Prouts Neck) have experienced an overall net volumetric gain of approximately 1,526,400 yd³ for the 33-year study period. This summed value falls in between values for total sediment entering the nearshore system (including volume of dredged materials added), which range from 1,247,700-1,594,200 yd³.

Identification of Regions of Potential Concern

Net erosion or accretion values were used to estimate annual rates for the time period of study. By multiplying calculated annual rates by a time period of 100 years (a 100-year time frame was selected since Maine's Coastal Sand Dune Rules, Chapter 355, reference a 100-year erosion standard), future estimates of shoreline position for each 100-foot alongshore transect were made and areas with potential future erosion problems were identified (Figures 25-28). Areas where existing structures may be threatened by projected erosion of the shoreline were identified. As expected, areas with existing erosion problems (e.g., Camp Ellis Beach), exhibit the farthest landward shift in shoreline position. It is important to note that future projected shoreline changes are based solely on shoreline change data (erosion and accretion rates) calculated from the 1962 and 1995 data (see Data and Method Limitations below).

100-year projection of shoreline in region 1

Based on this method, the future shoreline of region 1 (Figure 25) will be relatively stable to highly accretive, with only one small pocket of projected erosion that threatens existing structures. The sandy shoreline of Hills Beach, from the entrance to The Pool north to near Surf Avenue, may accrete several hundred feet seaward as a result of wave shadowing by numerous shoals and a tombolo that stretches seaward to Basket Island. Extensive accretion is also projected to occur from 4,200-6,000 ft along the shoreline, associated with continued shoaling along the southern Saco River jetty. Shoreline stability, albeit artificial stabilization-induced, is projected for the central section of the shoreline (2,200-4,200 ft), with one pocket of erosion threatening a structure near 2,100 ft, north of Surf Avenue. However, historically documented erosion (e.g., Hulmes, 1980; USACE, 1961) of this portion of the shoreline will probably continue unless stabilization structures are continually rehabilitated into the future. The actual source of sediment for projected shoreline accretion is not known; however, based on existing data, it appears that sediment in front of the central portion of the shoreline may be reworked by waves to the north and south, and the existing nearshore tombolo and shoals may continue to accrete (evident in a 2002 GPS survey of the Hills Beach dry beach widths).

In region 2 (Figure 26) segments of the shoreline within 1,000 ft of the northern jetty of the Saco River, from Bay Avenue north to Lower Beach Avenue, may experience up to 400 ft of erosion over the next 100 years. Such changes to the shoreline may jeopardize numerous existing habitable structures. Minimal shoreline accretion may occur for a short stretch of shoreline between Lower Beach Avenue and Sunrise Avenue (8,800-9,300 ft) within Camp Ellis Beach. This projected accretion is probably inaccurate, resulting from the reconstructed seawall that fronts this portion of the shoreline along Surf Street. Shoreline stability may be maintained through the maintenance of the existing seawalls. Substantial shoreline erosion may occur north of this stretch, from Sunrise Avenue in Camp Ellis north to the shoreline adjacent to Long Pond in Ferry Beach (9,300-12,700 ft), although no existing structures appear to be affected. Within Kinney Shores, several structures along the southern seawalled side of Goosefare Brook appear to be threatened by future erosion.

Future shoreline positions within region 3 show dominant accretion and stretches of general stability for the majority of the region (Figures 27A, B). The shoreline adjacent to Goosefare Brook within Ocean Park, from 19,700-21,700 ft, is fairly stable, while future shoreline positions show projected accretion from Tioga Avenue northeast to Fernald Avenue (21,800-25,400 ft). It is imperative to note that the projected future accretion along this stretch is a direct result of artificial dune construction associated with the sewer line installation, and therefore may not reflect any natural accretion rate or a sustainable trend. Shoreline stability and slight accretion is projected for the seawalled shoreline along Old Orchard Beach and Surfside, from 25,500-33,700 ft. A small pocket of erosion that may threaten several oceanfront structures is projected within Surfside (near 32,000 ft), corresponding with the old location of the Little River Inlet. North of this stretch, future shoreline positions project high accretion from Grand Beach into Pine Point (34,200-42,300 ft), on the order of several hundred ft to over 700 ft of accretion over the next 100 years assuming recent accretion rates and a continuing sand supply from the southern part of the Bay.

Based on existing trends used herein, large portions of Western Beach may also accrete over 200 feet in the next 100 years (Figure 28). Areas with projected accretion of 50 feet or less are located directly adjacent to the Scarborough River, adjacent to a tee of the golf course, and in the vicinity of the practice green. The cause of lower projected accretion along compartment 4A of Western Beach may be an old abandoned tidal channel situated adjacent to the shoreline (Figure 28). This channel is slightly deeper than adjoining shoals to the east, and inhibits beach accretion due to higher currents. As a result, this stretch of beach coincides with the steepest beach profiles along Western Beach, and very low accretion rates over the study-period. This lends to the relatively small accretion projected along this stretch. The remaining portions of Western Beach appear to be fed by the adjacent shoal. It is important to note that estimating future shorelines along a stretch of beach heavily influenced by a tidal channel is difficult (see Data and Method Limitations below).

Areas of projected substantial potential accretion or erosion are also identified in aerial photographs of the different regions in the context of other physical characteristics that correspond with the Recommendations section (Figures 33-38). In addition to projected erosion or accretion areas, these figures identify beach areas that may be vulnerable to shoreline change due to any of the following characteristics: maximum elevations below base flood elevations (BFE); dry beach widths of less than 25 feet; total widths of less than 100 feet; areas stabilized with shoreline structures; and areas proximal to tidal inlets. Refer to the Recommendations section for further discussion of these characteristics in relation to each region.

Data and Method Limitations

Estimated Net Erosion and Accretion (1962-1995). There are several limitations to the methods employed within this study in determining net erosion and accretion. In general, there are three types of aberrations that should be corrected, when possible, if aerial photographs are used for shoreline change: variable scale, tilt, and radial lens distortion. To account for scale, common control points (e.g. homes, road corners, etc.) were set in each photograph, and the 1962 photograph was "rubbersheeted" (stretched) linearly to assigned control points of the 1995 photograph. However, rubbersheeting does not account for displacements due to tilt and radial lens distortion. Therefore, errors may be introduced into the shoreline data by tilt and radial lens distortion. Because of this, the shoreline change values should be used for overall trend determination and guidance only, and not taken as exact values.

The method used to determine net erosion or accretion is called the "end point" method since it only uses shorelines from two years (1962 and 1995). Although this method is widely utilized by state coastal management agencies, it cannot take into account yearly or decadal fluctuations in shoreline position as a result of different erosion or accretion rates (Crowell and others, 1991). It is, however, less complicated and time consuming, and by some researchers (Leatherman and Anders, 1999) has resulted in the same shoreline change rate as the "linear regression" method. Therefore, results should be used for general planning only, and cannot indicate exact erosion or accretion rates, which may vary from year-to-year or more frequently due to storm events. Future technical reports on shoreline change (data permitting) will utilize the "linear regression" method rather than the "end point" method.

The seaward edge of vegetation was chosen as the erosion reference feature (ERF) because it is an indicator of the first point of longer-term stability than high water or wrack lines (Leatherman and Anders, 1999). However, using seaward edge of vegetation may be inaccurate at times due to unnatural (human) impacts to vegetation, such as planting, artificial construction or beach scraping. Such is the case with the stretch of beach along Ocean Park and Old Orchard Beach that underwent sewer line emplacement within an artificially constructed frontal dune planted with beach grass. This gives a false signal of natural accretion that must be either discounted, or at a minimum, duly noted.

Using the seaward edge of a seawall as an ERF may also cause errors in net erosion or accretion calculations. Shorelines may be naturally eroding, yet "stabilized" by a seawall (e.g., compartment 2A, Camp Ellis, and compartment 1B of Hills Beach). The seawall, if maintained over the time period of study, provides a signal of shoreline stability, while an adjacent unprotected shoreline may be undergoing erosion. In addition, along erosive shorelines fronted by seawalls (where the HWL is at the seawall), wave reflection can erode the beach vertically, not horizontally, since the beach cannot retreat in a landward direction.

Estimation of Net Volume Changes. The method for estimating net volumetric changes for 1962-1995 also has limitations. Since Saco Bay lacks adequate beach profile data, beach profiles from a 2000 LIDAR flight were utilized. For calculating volume changes, it was assumed that the 2000 profiles represented beach profile conditions from both 1962 and 1995. Other authors have used a similar technique to estimate volumetric changes along the South Carolina shoreline, lacking present beach profile data (Kana and Gaudiano, 2001). Although this assumption is largely inaccurate, lacking beach profile data from these time periods, it was one that had to be made. However, it must be noted that beach profiles are highly variable (Heinze, 2001) and one single snapshot from 2000 may not accurately represent beach profile shapes from either 1962 or 1995. In addition, the current study only looks at variations in the subaerial-intertidal beach, not the subtidal beach where the majority of sediment is located.

Projection of 100-yr Shoreline Positions. Projection of future 100-yr shoreline positions is based on average annual erosion rates (AAER) calculated from net erosion/accretion using the end point method. Therefore, yearly or decadal trends in shoreline change are not taken into account. For example, in the 33-yr period of study, a shoreline may have accreted 50 ft in 10 years, eroded 70 ft the next 10, then accreted 20 ft over the next 13 yrs. However, the only data available from the 1962 and 1995 photos are the beginning and ending shorelines, yielding a "stable" net shoreline change of 0 ft even though the shoreline underwent large fluctuations and was anything but stable. Using an AAER of 0 ft projected 100 yrs from now indicates a very stable shoreline. In areas prone to highly variable shoreline conditions (e.g., directly adjacent to a highly migratory tidal channel), this method may yield inaccuracies. Such may be the case with the shoreline at Western Beach (region 4), which historically has been highly variable due to its proximity to and the influence of the Scarborough River tidal channel. However, using 1962-1995 data, projected shorelines indicate accretion over the next 100 yrs. Therefore, future shoreline projections should be used for guidance purposes only.

Correlations

This section presents and discusses notable correlations, or a lack thereof, between certain beach profile characteristics and overall shoreline trends. For example, an erosive beach should exhibit different beach profile characteristics than an accretive beach. Such relationships were investigated along the Saco Bay shoreline.

Coastline Orientation. Coastline orientation (Figures 6, 7) is an important characteristic of shoreline morphology since it dictates those portions of the shoreline that receive direct, incoming wave energy, and those portions that may be protected from dominant wave conditions. Wave energy in many cases is a driving factor controlling beach profile shape and dominant alongshore geomorphic characteristics (e.g., erosion or accretion). Therefore, the relationships between coastline orientation and different beach and shoreline characteristics were investigated.

Figure 29

Figure 29 illustrates the inverse relationship between coastline orientation and beach profile slope for the general regions identified. In general, beach profile slope generally decreases with an increase in coastline orientation. This trend is most evident in region 2, and barring several outliers, also within region 1. Slopes within region 1 are highest from 100^°-120^° orientations probably due to the fact that these profiles are located at the southern end of Hills Beach where they are more exposed to waves arriving from the northeast, around the southern jetty of the Saco River. The other slopes in region 1 generally decrease with increasing compass bearing. This relates to wave sheltering from the northeast by the jetty, and may permit smaller, longer-period, summer wave conditions to help build the beach.

Profile slopes are highest within region 2 at orientations between 150^°-180^° from TN. This may be a result of several factors. First, summer wave conditions (approaching from the S and SE) that generally move sand onto the beach are completely blocked by the northern jetty. This orientation exposes the shoreline to erosive, large, short-period storm waves that approach from the E and NE.

This relationship within region 3 is harder to discern due to the general flatness of beach profiles along the stretch from Goosefare Brook north to the Scarborough River. However, a distinct trend does emerge. As coastline orientation increases, there is a negative relationship from 200^°-220^°, with a slight decrease in profile slopes. For profiles located at orientations of 220^°-250^°, there is a consistent slope. A large portion of profiles with these orientations falls within the heavily seawalled section of Old Orchard Beach. There is then a direct relationship between orientation and slope for orientations ranging from 250^°-275^° from TN. This may relate to an increase in maximum profile elevations (dune crests) at the northern end of the bay where the sand dunes are relatively high. In addition, a distinct negative relationship is apparent for region 4 beach profile slope values from along Western Beach (centrally located on Figure 29 between 100-160^° from TN).

There is a direct relationship between coastline orientation and net erosion/accretion (Figure 30). This is partly due to the steady increase in orientation of the coastline from south to north, combined with the general increase in accretion from south to north. This relationship is also positive for each region, signifying the dominant northerly transport of sediment throughout the Saco Bay area. A relatively strong relationship is seen within region 3, where the highest net accretion occurs at Pine Point (linear regression coefficient, R² = 0.66). The weakest relationship is for region 2 (R² = 0.11). Region 1 has a relatively strong positive relationship due to the shoaling that occurs on the southern side of the Saco River jetties. The relationship for region 2 is weakest due to the fact that there is no littoral barrier that aids in trapping sediment, aside from the natural sink at Goosefare Brook.

coastline orientation versus erosion or accretion

Figure 30

increase in net accretion with distance from jetty

Figure 31

Estimated Net Erosion and Accretion (1962-1995). The relationship between the distance from the northern Saco River jetty (jetty being a distance of 0 ft) and net erosion/accretion along the Saco Bay shoreline is illustrated in Figure 31. Regions 1 (Hills Beach) and 4 (Western Beach) have been removed because they have very different trends, and act as their own littoral cells. Although a direct linear relationship exists, it is not overly strong (R²=0.49). If artificial accretion, caused by the construction of dunes to protect a sewer line within Ocean Park/Old Orchard Beach, is removed (points circled), the relationship becomes slightly more direct (R²=0.53). However, the overall trend of net erosion/accretion along Saco Bay does not appear to be linear; it is best represented by a third order polynomial expression (R²=0.80). This expression represents well the distinct variations in shoreline changes of the shoreline from the Saco River northwards to the Scarborough River. The shoreline is generally erosive from the Saco River jetty northwards approximately 5,000 feet. From 5,000-30,000, the shoreline is generally stable to mildly accretive. This section represents a "conveyor belt" of sand, where sediment is moving along the beach on its journey northwards. Subsequently, the northern end of the bay, nearest the Scarborough River jetty (30,000-35,000 feet), is highly accretive. High accretion in this section is associated with the trapping of sediments by the Scarborough River ebb tidal delta, which generally forms the northern boundary of the Saco Bay littoral cell.

The relationship shows several other important features. First, the Pine Point shoreline has accreted much more than the Camp Ellis/Ferry Beach shoreline has eroded, illustrating an imbalance in the sediment budget (see General Discussion above). This clearly indicates sediment input other than eroded sediments from compartment 2A being reworked and transported northwards along the bay. Other sources of sediment may include the Saco River, reworked sediments from the subtidal beach of compartment 2A, and a sand shoal south of Prouts Neck. Second, the different slopes of the polynomial may provide some insight into the gradients of alongshore drift and may relate to different forcings that cause the three different trends seen. These topics are not within the scope of this report, and will be discussed further in different papers.

Figure 32

Beach Profile Slope. An exponential relationship consistently exists between beach profile slope and distance from an updrift and downdrift jetty (Figure 32). The relationships are closest between portions of region 1 (Hills Beach, denoted as 'S of Saco River') and region 4 (Western Beach, 'E of Scarborough River'). This is unexpected, as one is considered an updrift beach (region 1), while the other is a downdrift beach (region 4). This relationship may be explained by the fact that both regions may be considered their own enclosed littoral cells found at the ends (south and north, respectively) of the larger, log-spiral shaped Saco Bay littoral cell. Also, both appear to be influenced more by tidal currents than regions 2 or 3, which are located along the spine of the log-spiral shoreline.

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

Last updated on January 10, 2006.