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Coastal Erosion Assessment for Maine FIRMs and Map Modernization Plan
Spatial analysis of large beach systems that may have experienced shoreline changes significant enough to result in FIRMs being out of date
Ranking FIRMs using Shoreline Change
Large beach systems in Maine have undergone significant shoreline excursions in the past. Extrapolating from Nelson's (1979) work tabulated in the section above, 87% of beach areas are dynamic enough to result in an alteration of the beach profiles and hence the location and elevation of coastal flood hazard areas. By this estimate, there are 26 miles of Maine shoreline that could have FIRM panels made out of date by shoreline change.
The most important question about the effects of shoreline change on a FIRM are related to the rate at which physical changes are taking place versus the age of the FIRM panel itself. If, for example, the FIRM is only 2 years old, then shoreline change of 1 foot per year is unlikely to have made the map obsolete. However, if the map is 15 to 20 years old and the erosion or accretion rate is 1 foot per year, then flood hazards are likely to be very different today than when the map was generated.
Using shoreline change data from Nelson (1979) and the Maine Geological Survey's archive, FIRM panels with beaches in York, Cumberland, and Sagadahoc Counties were examined to calculate which areas have experienced the most change since the maps were created. In this analysis, it was assumed that the FIRM date was close to the age of the physical conditions used to determine FHAs. It is possible that some of these dates are much more recent than the actual data used to make the map since some panels may have been updated in just one geographic area. Consequently, this analysis may underestimate the need to update some FIRMs.
In Table 3, the greatest absolute amount of shoreline change is used to rank the results. In terms of coastal flood hazards, however, it is most likely that only those eroding (negative value) shorelines are areas where the flood hazard is likely to increase. Over time, the flood hazard in accreting areas should be declining, all other factors being equal. In order to evaluate a particular panel (as mentioned above) the greatest amount of shoreline change was used. In some cases, however there are portions of the panel that are eroding and areas that are accreting. A high erosion or accretion rate should be taken as a sign of a large disruption in the coastal sediment budget. A panel with a large change should be recognized as having had significant changes to coastal topography that will affect wave approach and runup during a 100-year storm. Consequently, even if a panel is identified as accreting (more than eroding) remapping is justified based on the potential for significant hydrodynamic changes along the shoreline since the map was first produced.
Shoreline Change over 40 Feet
The greatest amount of shoreline change was due to accretion, not erosion, along Hills Beach in the City of Biddeford. Hills Beach was also the location with the greatest negative amount of shoreline change. The history of shoreline change and relationship of the dune elevations to flood elevations has been examined for this area and all of Saco Bay by Slovinsky and Dickson (2003). In this study, both Camp Ellis Beach (City of Saco) and Hills Beach were found to be areas with significant risk to property due to erosion, flood elevation, dune topography, coastal engineering, and distance of habitable structures from the high tide line. Changes on FIRM 230145, Panel 5 are immediately adjacent to the south jetty of the Saco River. Disruption to the sediment budget is due to the federal jetties at the mouth of the Saco River and their influence on waves and sand transport. Movement of sand within the area sheltered by the jetty is expected to continue in the next decade at the same rate.
Shoreline Change Over 30 Feet
Adjacent to the panel described above is a portion of Hills Beach that has a slower accretion rate as well as an erosion rate as high as 2 feet per year. FIRM 230145, Panel 6 has a 1984 date and thus an age of 19 years. Erosion in this panel has appeared to be chronic. Homes along the frontal dune have been armored with high seawalls. The beach profile at this erosion-prone area is steep through the upper half of the intertidal profile and thus waves can have a large force upon the seawalls during storms and under conditions of high tide.
Mile Stretch Beach
Parts of this beach are on FIRM 230145, Panel 6 that includes Hills Beach. Mile Stretch Beach has a slower erosion rate, on the order of 1 foot per year and a history of shoreline armoring in response to chronic shoreline recession. The sand supply to Mile Stretch Beach is limited both offshore and from any upland sources and there are exposures of back-barrier salt marsh peat on the beach in some locations due to the beach and dune migrating inland over the last few thousand years (Hulmes, 1980; 1981).
Pine Point Beach
At the northern end of Saco Bay, Pine Point Beach is the northern terminus of a seven-mile beach system that begins at the mouth of the Saco River. The northern end of the bay is the recipient of longshore drift (sand transport by waves and currents along the beach). FIRM 230052, Panel 23 in the Town of Scarborough has undergone as much as 33 feet of accretion since the area was mapped. The positive sand budget in this panel is due to the accumulation of sand adjacent to the federal jetty along the south side of the Scarborough River tidal inlet (Dickson et al., 1993; Farrell, 1972; Kelley et al, 1995; Nelson, 1979; Slovinsky and Dickson, 2003). Shoreline progradation has filled an area adjacent to the jetty to such an extent that the shoreline may be becoming less progradational in the future. In fact, over the last decade there have been episodes of significant short-term erosion of the frontal dune at Pine Point. These recent changes are most likely due to wave shoaling and refraction on the ebb-tidal delta of the Scarborough River, as sand has accumulated offshore. Consequently, the pattern of wave runup and flooding may have significantly changed since the panel was mapped in 1992.
Shoreline Change Over 20 Feet
Popham and Hunnewell Beaches
Goose Rocks Beach
Another isolated beach and dune system is located in Goosefare Bay in Kennebunkport. Goose Rocks Beach is a sandy pocket beach system with an arcuate shoreline due to wave refraction around offshore islands and shoals. At either end of the beach are tidal inlets (Little and Batson Rivers) with dynamic beach spits and back barrier salt marshes. Southwest of the sand beach is a headland with mixed sand and gravel beaches on Nessler and Marshall Points. Sand along Goose Rocks Beach is very dynamic and moves from one end of the beach to another. Nelson (1979) documented shoreline change within the bay and relict shorelines within the dunes along about 75% of the length of the beach. In parts of the dunes, the historical erosion limit is landward of houses. About 70% of the shoreline is armored with a riprap seawall (Nelson, 1979). The sand budget for the bay has not been studied. However, there are no new significant sources of sand to replace sediment transported into the tidal inlets or eroded from the beach and carried to offshore sand bars. This beach system has a tendency to erode and accrete over a decade, but to a lesser extent than at Popham and Hunnewell Beaches because the bay is more sheltered and there is less influence from the river systems. FIRM 230170, Panels 7 and 8 were last mapped in 1983. These are the oldest FIRMS evaluated in this report.
Shoreline Change Over 10 Feet
Fortunes Rocks Beach
In Biddeford, Fortunes Rocks Beach fronts The Pool and uplands. The beach continues northeast to become Mile Stretch Beach (mentioned above with Hills Beach). South of the long strand of sandy beach are three pocket sand and gravel beaches: Horseshoe Cove, New Barn Cove, and Curtis Cove. All of the pocket beaches have gravel overwash deposits on and behind the frontal dune ridge. Duffy et al. (1989) documented exposed salt marsh peat on the beach profile of Horseshoe Cove. Hulmes (1980) documented long-term erosion along Fortunes Rocks and Mile Stretch Beach (mentioned above). Seawalls front most of the southern portion of Fortunes Rocks Beach and, consequently, Nelson (1979) was unable to determine a rate for shoreline change. Along the natural shoreline, Nelson measured about 1.6 feet per year recession. This beach appears to have chronic sand loss and net shoreline recession and or active seawalls along the frontal dune ridge. FIRM 230145, Panel 11 was last mapped 19 years ago in 1984. The present beach profile and conditions for wave runup are probably quite different from when the FIRM was last mapped.
Old Orchard Beach and Ocean Park
The Ocean Park section of Old Orchard Beach is flanked by the Goosefare Brook tidal inlet to the south and continues with the long strand of central Old Orchard Beach to the north. The shoreline along the north shore of Goosefare Brook is stabilized with a riprap wall to reduce channel meandering. The ocean shoreline is relatively stable to accretional (Nelson, 1979; Slovinsky and Dickson, 2003). Beneath the dunes at Ocean Park is buried a pipeline that extends offshore to an outfall for the Old Orchard Beach sewage treatment facility. Along the central section of Old Orchard Beach there are artificial frontal dunes that contain buried sewer pipelines that service the dune neighborhoods. A dune management plan was established in the 1980s (Timson and Denison, 1986) and has succeeded in stabilizing much of the Old Orchard Beach dunes. The apparent shoreline accretion in the segment of Saco Bay is due, in part, to dune restoration and management so the apparent shoreline change used in this report is a function of the management action. FIRM 230153, Panel 3 was last mapped 19 years ago in 1984.
Gerrish and Cutts Islands
Cape Elizabeth Beaches
Along the Cape Elizabeth shoreline are a series of small beaches sheltered by Richmond Island. Crescent Beach is the largest beach and dune system and part of a state park and makes up the Coastal Barrier Resources System Unit E-19/19P. Strawberry Hill Beach is a small double tombolo beach and back barrier marsh system with a cuspate foreland connected to a rock breakwater that extends to Richmond Island. Main Beach is a small pocket beach with a freshwater back barrier wetland west of Strawberry Hill beach and sheltered by Ram Island (Nelson and Fink, 1980). These latter beaches are Unit A06 of the Coastal Barrier Resources System. Little direct erosion rate data exists for these beaches so the natural background rate determined by Nelson (1979) of 1 foot per year was used in this analysis. FIRM 230043, Panels 3 and 11 were last mapped in 1992. These dune systems are relatively undeveloped so shoreline change here is less significant than at other locations listed above.
Great Chebeague Island
The Jenks Landing Coastal Barrier Resources System Unit A05C is located on Great Chebeague Island in the Town of Cumberland. This Casco Bay location has three separate parts to the CBRS on Great Chebeague Island, each a mixed sand and gravel deposit related to accumulations of sediment from coastal bluff erosion (Bryant et al., 2002a, b; Timson, 1976a, b). On the south western side of the island is Indian Point. This CBRS has a road out to the point that is vulnerable to shoreline change. The Jenks Landing CBRS area between Coleman and Johnson Coves is undeveloped and relatively sheltered by nearby islands. However, there are shoreline properties immediately adjacent to the south that could have flood hazard areas altered by shoreline recession. The Waldo Point portion of the CBRS is also undeveloped but faces northeast into the dominant storm wind and wave approach. Little direct erosion rate data exists for these beaches so the natural background rate determined by Nelson (1979) of 1 foot per year was used in this analysis. FIRM 230162, Panels 21 and 23 were last mapped in 1992.
Mile and Half Mile Beaches
Reid State Park in Georgetown is the location of two large beach and dune systems: Mile and Half Mile Beaches. These natural dunes are some of the largest in Maine and the beaches are relatively linear compared to most systems bound by bedrock headlands. These beaches are part of the Coastal Barrier Resources System ME-15P called the Little River Unit after the tidal inlet adjacent to Half Mile Beach. Over centuries, these beach systems are transgressive and back barrier peat and tree stumps can be found on the intertidal portion of Mile Beach after winter erosion (Dickson, 2002a). Nelson (1979) estimated an erosion rate up to 1 foot per year for Mile Beach between 1940 and 1972. FIRM 230201, Panel 9 was last mapped in 1992. Minimal coastal development exists along this public beach and it serves as a good reference for the rates of shoreline change in an area with little human influence on the sand budget.
Crescent Surf and Parsons Beaches
At the northern end of the Wells Embayment, Crescent Surf and Parsons Beaches in the Town of Kennebunk are two adjacent beaches that each has a dynamic spit end at a tidal inlet and each fronts a back barrier salt marsh system. These beaches are relatively natural with only a short segment of seawalls fronting Parsons Beach. At both beaches the spit ends tend to be more unstable and have had more shoreline recession than the middle portions that are anchored to a bedrock headland (Nelson, 1979). Erosion rates were determined by Nelson (1979) to range up to from near zero to over 2 feet per year (the latter from 1940 to 1953). Nelson found the most common rate of recession on Crescent Surf Beach to be 1 foot per year. There has been little interference with the local sand budget in the last 50 years along these shorelines, so the variability seen in Nelson's data may be a good indication of decadal variability in natural rates of shoreline recession in the Wells Embayment. There appears to have been episodes of rapid erosion followed by reduced recession or minor amounts of accretion in the last half century at these beaches. These beaches are part of the Coastal Barrier Resources System Unit A08. FIRM 230151, Panels 14 and 15 were last mapped in 1992.
Goochs and Middle Beaches
Adjacent to the Kennebunk River, the Town of Kennebunk has Goochs Beach, a pocket beach, with a small active frontal dune adjacent to the federal jetty at the river mouth. The beach is primarily fronted by a wooden seawall that extends most of its length. The beach profile is low and, due to the lack of sand exchange with most of the dune system and repeated wave action on the seawall, has a minimal summer berm. Beach Avenue runs parallel to the beach on the frontal dune and sand is sealed beneath the road. This highly engineered beach has little opportunity to buffer storm flooding and waves can overtop the seawall and pass into Beach Avenue. Middle Beach is west of Goochs Beach and a mix of sand and gravel sediment. This beach also functions as a coarse-grained coastal barrier system. However, it too has an enormous seawall and Beach Avenue continues along the frontal dune. This area has also had many episodes of coastal flooding and seawall damage. Gravel from the beach tends to wash onto and across Beach Avenue and attempt to build a higher dune ridge where residential development now exists. No historical shoreline change measurements are available since this beach has been engineered since air photos were first taken. For this study it was assumed that the system seaward of the seawall would respond with an erosion rate of 1 foot per year. FIRM 230151, Panel 15 was last mapped in 1992.
Head Beach and Small Point Beach
Camp Ellis and Ferry Beach
The City of Saco has one long beach strand from the Goosefare Brook to the federal jetty/breakwater at the mouth of the Saco River. The residential and commercial community next to the jetty is called Camp Ellis. The beach adjacent to Camp Ellis has undergone significant land loss since early 1900 (Figure 17). Over the last century as many as 36 properties have been lost to shoreline recession. Erosion rates are on the order of 2 to 3 feet per year next to the jetty (Figure 18) and 1 to 2 feet per year north of the riprap along Surf Street (Duffy and Dickson, 1995, Kelley et al., 1995, Slovinsky and Dickson, 2003).
This area is currently under investigation by the U.S. Army Corps of Engineers, New England District, as part of a Section 111 (Rivers and Harbors Act) study for mitigation of erosion caused by the federal jetty. Details about the history of the area can be found in Slovinsky and Dickson (2003). FIRM 230155, Panel 29 was last mapped in 1998. The Kinney Shores section of Saco (FIRM 230155, Panel 27) has a lower erosion rate of about 1 foot per year with a net shoreline change of about 5 feet since the last FIRM was produced. Anticipated federal action to modify the jetty and to nourish the beach with sand may result in a significant alteration of the shoreline configuration on Panel 29 in the next 5 years.
Kennebunkport Pocket Beach
A small pocket beach is located just north of the federal jetty on the north side of the Kennbunk River tidal inlet in the Town of Kennebunkport. This small beach experiences storm wash over and may be erosional, in part, to a negative sand budget associated with dredging the Kennebunk River. For this analysis an erosion rate of just half a foot per year was assumed. However, it has been 20 years since FIRM 230170, Panel 3 was produced so there is a potential for 10 feet of shoreline change in and around this location at the river mouth.
Scarborough and Western Beaches
North of Prouts Neck is Scarborough Beach. This relatively linear beach system is Unit A07 of the Coastal Barrier Resources System. The beach fronts a freshwater wetland in the back barrier environment. The shoreline is primarily sandy with a natural frontal dune. Along the southern end of the beach there are low-lying wooden seawalls. These seawalls are regularly overtopped by storm flooding and currents carry gravel cobbles up and over the walls to form a gravel ridge. Erosion on Scarborough Beach was examined by Nelson (1979) and there have been several episodes of erosion and accretion from 25 to 50 feet in either direction. There is some variability from year to year of the location of the widest portions of the beach suggesting that some of the sand is moved alongshore over a period of several years and then it returns.
Western Beach is on the western side of Prouts Neck and adjacent to the tidal inlet of the Scarborough River. The Scarborough River drains the largest back barrier salt marsh in southern Maine and there are strong tidal currents that pass through the inlet throat and near Western Beach. Western Beach has experienced surprisingly large variations in erosion and accretion. The beach is located in the down drift end of the Saco Bay littoral cell. Despite this fact, sand has been lost from the beach and dunes of Western Beach in the last decade. From 1953 to 1970 (Nelson, 1979) and from 1962 to 1995 (Slovinsky and Dickson, 2003) the shoreline moved seaward at a rate of a foot per year or more. The accretion that took place over these four decades may have been in response to (a) the construction of the federal jetty on the south side of the Scarborough River inlet at Pine Point and to (b) the regular maintenance dredging of the main channel of the ebb-tidal delta seaward of Western Beach. During a period after construction of the jetty sand was trapped next to the jetty on Pine Point and was unable to continue movement toward Western Beach. During the same interval of time, maintenance dredging may have prevented sand from bypassing the inlet channel to reach Western Beach. Also during these decades the ebb shoals expanded offshore and the main channel and associated bars extended farther east so longshore drift deposited sand in a new area away from Western Beach. For this analysis, a 1 foot per year accretion was used. However, the rates and direction of shoreline change at Western Beach are far from uniform and will likely change significantly in the near future as the sediment budget continues to adjust to natural processes and human activity. FIRM 230052, Panel 24 was last mapped in 1992 and significant change has taken place since the FIRM was made.
Shoreline Change Less than 10 Feet
Last updated on February 8, 2006.
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