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April 2009 - An Overwash Channel Forms
On April 7, 2009 Bates College geology professor Mike Retelle and his students were at Seawall Beach to measure the sand bar that is seaward (south) of the Morse River's tidal inlet. This sand bar, also called a beach spit by geologists, had grown and extended eastward across the mouth of the Morse River for several years. Their field survey was just after a storm on April 6 that came during a period of high tides and coastal flooding to a level of over 11 feet above Mean Lower Low Water (MLLW) for two sequential high tides recorded at the Portland tide gauge (Dickson, 2007; Dickson, 2009 - Figure 12). During their survey, the Bates College team found evidence of storm washover and downward erosion of the sand bar (Dickson, 2009 - Figure 13, Figure 14, Figure 15). This was the first sign that an avulsion, or course change, might occur in the Morse River.
That particular storm had a combined tide and storm surge 2 feet above mean high water. That water level is not exceptional among annual winter storms on the Maine coast and it is possible that similar episodes of erosion occurred in prior winter storms in 2009. To put this storm in perspective, storm surges of 3 to 4 feet are possible adding to the normal elevation of the tides (Dickson, 2007). The 2007 Patriots' Day Storm had a surge of up to 2.5 feet over 7 high tides during a period of astronomically high tides (Slovinsky, 2007; Slovinsky and Dickson, 2009) and resulted in sand deposition into Popham Beach's dunes (Dickson, 2008 - Figure 7). The early April 2009 storm had rather common storm wave characteristics with wave heights of 10 feet with a dominant period of 8 seconds at the Casco Bay buoy (NDBC, 2011). It was concluded that it may take a larger storm event and continued seaward erosion into the spit by the Morse River channel to result in a lasting breach (Dickson, 2009).
November 2009 - The Forest Falls
By the fall of 2009, the Morse River had cut deeply into Popham Beach State Park causing erosion that removed primary frontal dunes and carved into back dunes to fell mature trees from a pitch pine (Pinus rigida) maritime forest (Dickson, 2010 - Figure 12, Figure 16). The climax forest here is the largest, most northern such stand of trees in the Gulf of Maine (Nelson and Fink, 1980). Based on the presence of a mapped forest on an 1879 nautical chart (Survey of the Coast, From Seguin I. to Cape Elizabeth, Maine, scale 1:80,000, Maine State Archives), the forest and possibly these trees may be as much as 100 years old. This 2009 shoreline may be the farthest inland the Morse River has meandered since the mid 1800s or earlier.
An aerial reconnaissance flight on November 10, 2009 by John Picher of the Department of Conservation showed a thin neck in the Seawall Beach spit at the outside (erosional) bend of the Morse River channel (Dickson, 2010 - Figure 17). The overwash channel was visually estimated to be 10 to 11 feet above MLLW and perhaps 200 feet wide (Dickson, 2010 - Figure 18). The presence of the narrow spit neck and the overwash channel led the Maine Geological Survey to predict a river course change, or avulsion, at that location sometime in the near future.
December 2009 - The Beach Moves in on the Bath House
Storms continued to erode Popham Beach State Park in December. A prolonged series of strong fall and winter storms allowed the Morse River to become repeatedly elevated and sweep away large amounts of sand in a matter of hours from the edge of the forest at the state park (Figure A3 and Figure A4). A storm on December 12 elevated the tide with a 1.5-foot storm surge and resulted in minor coastal flooding over the 12-foot level. Offshore waves in this storm were 12 feet high. Surf broke on sand shoals offshore of Popham Beach State Park and moved ashore as smaller waves that still managed to erode the forested dunes and cause flooding in low areas (Dickson, 2010 - Figure 9, Figure 10, Figure 11).
There was a general, but undocumented, consensus from eye witnesses that erosion was reduced by the trees temporarily bundled with ropes in front of the bath house - just 75 feet from the building (Figure A5; Dickson, 2010 - Figure 14).
February 2010 - Setup for a Breach
On February 27, after the peak waves of the storm passed, the Maine Geological Survey determined that only a shallow and wide overwash channel had formed on the Seawall Beach spit (Figure A7, and Figure A8). It was clear that the storm did not open a permanent new channel that the Morse River could pass through at all stages of the tidal cycle. An Emery Method beach profile survey on February 27 across the channel and parallel to the beach showed a broad lowering of the spit relative to surrounding areas (Figure A9 and Figure A10). The cut in the spit was about 300 feet wide at the spring high tide level and about 200 feet across at the mean high tide level. So elevated storm tides were able let the Morse River flood and ebb on a more direct path to the sea. After the water level fell below a normal high tide elevation, the river still was confined to its old path along Popham Beach State Park. At this point, it remained uncertain if additional surf would (a) deposit more sand in the overwash channel and seal it up or if (b) overtopping by waves and tides would keep this section of the beach spit low and allow for more downward erosion. These competing coastal processes were quite dependent on the nature of waves coming ashore, the strength of tidal currents, and water levels from both tides and storms.
A unique set of meteorological and oceanographic conditions followed the February 26 storm. There were 7 days of high "spring" tides that allowed the river to ebb and flood across the sand spit. For a period of 9 days, from February 24 through March 4, there was also coastal flooding from a persistent storm surge of 1 to 2 feet. These conditions resulted in 15 extra high tides (Figure A11). With each exceptionally high tide, more water was able to flow up the Morse River, enter the back-barrier salt marshes, and elevate Spirit Pond (Figure A1). This increased volume of water needed to exit over a fixed number of hours on a falling tide, so tidal currents were stronger than normal. Faster flood and ebb currents are capable of moving more sand off the top of the spit, and deposit it either into the deeper Morse River or offshore into the surf zone. During this period of stronger tidal flow, moderate waves, in the 5-foot range, continued to create surf, wave action, and turbulent flow across the spit around the time of high tide.
With each successive tidal cycle, we expect that more sand was removed from the spit and lowered its peak elevation in the overwash channel. Lowering would have led to even longer tidal flow in and out of the Morse River and thus more duration of erosion on the spit. Since a straight path to and from the sea is hydraulically more efficient, the Morse River would have preferentially flowed across the spit when it had the opportunity during this week.
March 2010 - Final Breach
By March 6, 2010 the 9 days of stormy weather were over. A reconnaissance survey by Laura Sewall, the Director of the Bates-Morse Mountain Conservation Area, documented active Morse River flow through the new channel. This was the same location that was progressively lowered by storms in the previous year and more deeply incised in late February and early March (Figure A12 and Figure A13). Once this new connection to the sea had been started, tidal ebbing and flooding of the Morse River rapidly led to additional channel deepening and widening. Sand was deposited into the old course of the Morse River resulting in shoaling that made the former channel hydraulically less efficient and helped to confine more flow in the new channel (Figure A14, Figure A15, and Figure A16).
August 2010 - Closing the Old Morse River Channel
The breach resulted in the separation of the spit from Seawall Beach. This process led to the formation of a low-relief sand island that was surveyed by the Maine Geological Survey on March 27, 2010 (Figure A17). By August 20, 2010 the former channel of the Morse River was blocked by sand swept northward off the western end of the island to form a small spit platform upstream (Figure A18).
November 2010 - The Island Moves Landward
Through the summer and fall of 2010, sand continued to be reworked by waves and tides. From March through November 2010, the sand island migrated 500 to 700 feet east-northeast toward the state park while retaining its overall subaerial exposure. Concurrently, the Morse River's new channel began to migrate easterly (Figure A19). This pattern of channel movement is consistent with what it has done in the past (Goldschmidt and FitzGerald, 1991).
The dramatic change in shoreline positions is best seen by overlaying the 2010 island perimeter in relation to the dunes in a 2003 air photo (Figure A20). Remarkably, the landward edge (approximately the high tide line) of the island now overlaps the area where the seaward edge of the park dunes was back in 2003.
Shoreline Change at Popham Beach State Park
Historical shoreline change has been previously documented (FitzGerald and others, 2000; Nelson, 1979) as has the meandering of the Morse River (Goldschmidt and others, 1991). Nelson and Fink (1980, p. 110) described the shoreline at Popham Beach State Park as "...among the most unstable in Maine." FitzGerald and others (2000) estimated that the state park lost an average of 74,000 m3/yr from 1942-1953 and gained 15,000 m3/yr from 1953 to 1979. The last time the shoreline was close to the pitch pine forest was in 1953 (Dickson, 2008 - Figure 12).
Recent shoreline change along the dune edge shows the pattern of erosion at Popham Beach State Park that led to unprecedented dune and forest loss since 2005. It is clear that the Morse River began to encroach on the state park dune field before the 2007 Patriots' Day Storm and that a considerable area of open dunes were lost from 2005 to 2007. The pattern of erosional loss is consistent with easterly migration of the outer bend, or cut bank, of the Morse River (Dickson 2010 - Figure 7). From 2007 through January 2011 the erosion trend at the state park continued with a progressively easterly loss of dunes (Figure A21).
From 2007 to 2011, erosion took place on East Beach and severely compromised the height of the frontal dune, particularly on the eastern end. The lower dune ridge is now more susceptible to wave overtopping and additional sand deposition on the back slope of the frontal dune ridge (Figure A22). Based on shoreline positions from 1980 to present, the area of vegetated dunes at the state park is now about the lowest on record (Figure A23).
The last time the Morse River breached the sand bar was November 23, 1986 (Goldschmidt and others, 1991). Goldschmidt and others estimated that about 100,000 m3 of sand migrated onto Popham Beach State Park as a result of that course change in the river. Based on partial submergence of the sand bar in an air photograph taken about 18 months before the breach (May 9, 1985; Goldschmidt and others, Figure 3) the sand bar was significantly lower than it was in 2009 when tides could not routinely overtop it. The 1986 breach was followed by breaching of the Fox Island tombolo whereas in 2008, the tombolo was breached prior to channel avulsion (Dickson, 2008). It remains to be seen if the onshore migration of the island leads to another tombolo breach in the near future.
In the next few years the sand island should migrate ashore and weld onto Popham Beach State Park. As sand moves ashore, the intertidal beach will become much wider and the height of the tombolo to the Fox Islands should increase providing additional recreation space and protection for the remaining dunes at Center Beach. Based on past trends, the next several years should see additional dune building from wave and wind action. Once the upper dry beach, or berm, reaches an elevation above the spring high tides, American beach grass should colonize the area. Vegetation, if kept from excessive foot traffic, will trap additional sand and lead to further dune building. Given the large size of the island, the area of dunes built over the next decade may reach an equivalent area to those removed in the last decade - possibly completing a natural cycle that began about 20 years ago.
Popham Beach at the mouth of the Kennebec River is one of Maine's premier public beaches. The beach and dunes have been highly dynamic based on historical evidence. The geology of the system is complex and affected by bedrock islands that affect wave shoaling and by the ebb and flood of currents in the Kennebec and Morse Rivers. Geological evidence of impending spit breaching by the Morse River was observed for about a year. It took a trifecta of large storm waves, coastal flooding, and extreme tides for an avulsion to abruptly alter the channel of the Morse River away from the state park and a new bath house. Erosion reached much farther inland than ever recorded in the last century, but conditions are favorable for the beach and dunes to return naturally in the next several years.
Dickson, S. M., 2008, Tombolo breach at Popham Beach State Park, Phippsburg, Maine.
Dickson, S.M., 2009, Storm and channel dynamics at Popham Beach State Park, Phippsburg, Maine.
FitzGerald, D. M., Buynevich, I. M., Fenster, M. S., and McKinley, P. A., 2000, Sand dynamics at the mouth of a rock-bound tide-dominated estuary: Sedimentary Geology, v. 131, p. 25-49.
Goldschmidt, P. A., FitzGerald, D. M., and Fink, L. K., Jr., 1991, Processes affecting shoreline changes at Morse River inlet, central Maine coast: Shore and Beach, v. 59, p. 33-40.
National Data Buoy Center, 2011, Station 44007 - PORTLAND 12 NM southeast of Portland, ME.
Nelson, B. W., 1979, Shoreline changes and physiography of Maine's sandy coastal beaches: M.S. thesis, University of Maine, Orono, Maine, 302 p.
Nelson, B. W. and Fink, L. K., Jr., 1980, Geological and botanical features of sand beach systems in Maine: Maine Sea Grant, Bulletin 14, 163 p.
Slovinsky, P. A., 2007, The Patriots' Day storm at Willard Beach, April 2007.
Slovinsky, P.A. and Dickson, S.M., 2009, State of Maine's beaches in 2009: Maine Geological Survey, Open-File Report 09-57.
Last updated on July 6, 2011
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