Storm and Channel Dynamics at Popham Beach State Park, Phippsburg, Maine

Introduction

This site revisits Popham Beach State Park to examine three aspects of this highly dynamic beach, dune, and tidal inlet system. The images and information provided here supplement other MGS web pages (Dickson 2008a, b) that together attempt to track geological changes to the shoreline, dunes, and maritime forest at the state park and adjacent areas.

Figure 1

Here we examine the sand bar that connects the Center Beach at Popham Beach State Park with the Fox Islands (Figure 1). We also inspect the complex system of sand bars within the meandering channel of the Morse River along West Beach. Finally, we review storm action offshore of West Beach on the Seawall Beach side of the Morse River. The easterly growth of Seawall Beach toward the state park is beginning to show signs of ending. This hint of change could be an early sign that the cycle severe erosion at Popham Beach State Park is coming to an end.

A Closer Look at the Fox Islands Tombolo

This broad sand bar is called a tombolo by geologists since it runs roughly perpendicular to the shoreline and connects to an offshore island. Most sand bars on a beach are aligned parallel to the shoreline and are shifted onshore and offshore by surf. Wave action on a tombolo comes from two sides and is a result of waves bending (refracting) around a headland such as the rocky islands offshore. On a tombolo, sand moves onto and off the bar primarily by wave action. At Popham Beach State Park the tombolo has also been strongly affected in the last few years by flooding and ebbing tidal currents of the nearby Morse River.

The Fox Islands tombolo is a broad expanse of sand that becomes exposed as the tide falls and submerged as the tide rises. During a period of a few hours around low tide it is possible to walk across the sand bar to the Fox Islands, climb to the summit and look back at the mainland (Figure 2). For more than the last year, during falling (ebb) tides the Morse River has cut across the tombolo and limited passage to the Fox Islands. In addition, the river has eroded (lowered) the elevation of the tombolo by washing sand in an easterly direction. This lowering is visible as one walks the tombolo at low tide (Figure 3). Fortunately, sand that is pushed east across the tombolo is reworked by surf into a sand bar system on the easterly side of the tombolo. The wave action helps push sand back up onto the tombolo and restore erosion caused by the Morse River (Figure 4). Closer to the mainland, however, the river flow has eroded and lowered the beach so that there is little dry beach width in front of the dunes. In fact, there is a considerable amount of dune loss at Center Beach due to the action of the Morse River (Figure 5). This dune erosion has led to a dune scarp (or vertical face) that exposes many layers of dune building that has taken place over decades (Figure 6). This scarp is likely to remain a prominent feature of the park landscape for years to come.

Morse River Meandering

When the tide is high or at mid-levels, the Morse River can cross over the tombolo (Figure 7) and make passage to the Fox Islands difficult and dangerous. During the lower half of the tidal cycle, however, the Morse River outflow moves to sea along the west side of the tombolo (Figure 8). This broad and shallow channel is also where the first rising tides will flow inland toward the salt marshes behind the beach and dunes of West Beach. The Morse River channel along West Beach has had a series of channels and bars for the last several years. Some of the tidal flow has been right against West Beach and led to the loss of the maritime forest and resulted in pitch pine trees on the beach (Dickson, 2008a,b). Erosion by the Morse River has exposed old layers of salt marsh peat that were once behind the dunes. There are two channels that the river has used in the last few years (Figure 9). In April 2009 the channel nearest West Beach appears to be smaller than the one offshore (Figure 10). This is a good indication that the river has moved seaward slightly along part of the beach and thus has directed eroding currents away from some of the dunes. This minor channel also shows signs of infilling in the form of a small flood-tidal delta within the channel and shoreward migration of sandbars in wave action may close off the channel further (Figure 11). The dominant channel of the Morse River is now in a more seaward location which is good for potential future migration away from the state park.

Storm Action Almost Causes an Avulsion

A broad sand bar or beach spit system connected to Seawall Beach has been a primary block to the Morse River flowing in a more southerly and direct course to the ocean (Dickson, 2008a,b). This spit has built in an easterly direction due to currents and wave action along the shore that have carried sand along Seawall Beach toward the state park (Cary, 2005; Chandler, 2009a,b; Oster, 2009a,b).

On April 7, 2009 Bates College geology professor Mike Retelle and some of his students were at Seawall Beach to measure the sand bar that is seaward (south) of the Morse River's tidal inlet. Their field survey was shortly after a storm that came during a period of high tides and coastal flooding to a level of over 11 feet above Mean Lower Low Water for two sequential high tides recorded at the Portland tide gauge (Figure 12; Dickson, 2007). The combined predicted tide plus the storm surge was 2 feet above mean high water but not exceptional among annual winter storms on the Maine coast. Storm surges of 3 to 4 feet are possible on top 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). Waves at the Casco Bay buoy (NOAA buoy 44007) showed wave heights of 10 feet with a dominant period of 8 seconds (NDBC, 2009), rather common storm wave characteristics.

The Bates College survey discovered a recently formed channel across the spit that is the first sign of an avulsion or course change in the river (Figure 13 and Figure 14). This channel shows signs of flow in a north-south direction that could have been cut by (a) flooding driven by surf and wave overtopping of the spit or (b) ebbing currents just after the time of high tide when water levels exceeded the top of the spit and led to a continuous seaward flow. As the tide fell, scour and erosion into the spit appears able to have cut down farther into the spit and removed a few additional inches of sand (Figure 15).

It is likely that, as the tide fell to lower levels during the storm, wave overtopping, down cutting, and sand removal were slower than the rate the tide fell so incision ceased and the Morse River resumed the regular course of ebbing in the main channel. It is also likely that, as the storm passed, further episodes of channel cutting around the time of high tide were not possible due to (a) reduced wave heights and (b) less flooding and release of water from the back-barrier marsh and upland watershed including Spirit Pond. 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 that will end the severe erosion cycle that has affected Popham Beach State Park for the last several years.

Signs of Spring

Frontal dunes experience erosion by waves and flooding in storms that arrive primarily in the winter. In addition to storms, daily tidal currents near inlets can erode both frontal and back dunes through tidal channel migration or meandering as seen over the last few years at Popham Beach State Park. Eroded dunes can naturally rebuild through periods of less extreme weather. Small surf and waves tend to work sand ashore from nearshore bars and push it up on the beach. Waves can even carry sand into the frontal dune to build a ridge. This process of washover can lead to several inches of vertical dune building in a single storm event (Figure 16). As the tide falls, sand dries on the beach and can be picked up by wind and blown into the dunes. American beach grass that dominates the vegetation in frontal dunes thrives by being buried by sand deposited by wind and waves. In spring as the growing season begins in April for American beach grass the plants will grow up through the winter accumulation of sand and extend leaves upward (Figure 17 and Figure 18). The beach grass then slows wind carrying sand and this sheltering leads to further sand accumulation in the dunes. The process of recovery from winter storms and flooding begins in spring and lasts through the summer and continues into early fall. While this process of dune rebuilding can be slow, it is a natural process and part of a longer seasonal and multi-year cycle that can be observed on a field trip to the beach in summer.

Figure 16

Figure 17

Figure 18

References

Cary, C. L., 2005, Modeling of sedimentation trends on Seawall Beach in Phippsburg, Maine and their variations over daily, weekly, and seasonal timeframes: Honors Thesis, Department of Geology, Bates College, Lewiston, Maine.

Chandler, E. A., 2009a, Seasonal processes affecting the Seawall barrier system and the evolution of the southwestern barrier spit and Sprague River Inlet, Phippsburg, Maine: Honors Thesis, Department of Geology, Bates College, Lewiston, Maine, 120 p.

Chandler, E. A., 2009b, Modern and historical development of Seawall Beach: Phippsburg, Maine: Geological Society of America, Abstracts with Programs, v. 41, no. 3, p. 41.

Dickson, S. M., 2007, Portland tide gauge and waterfront.

Dickson, S. M., 2008a, Tombolo breach at Popham Beach State Park, Phippsburg, Maine.

Dickson, S. M., 2008b, Seawall and Popham Beach Dynamics, Phippsburg, Maine.

National Data Buoy Center, 2009, Station 44007 - PORTLAND 12 NM southeast of Portland, ME.

Oster, D. J., 2009a, Mapping short term barrier beach processes at Seawall Beach, Phippsburg, Maine to model transgressive shorelines in 2100 from sea level rise: Honors Thesis, Department of Geology, Bates College, Lewiston, Maine, 132 p.

Oster, D. J., 2009b, Mapping Seawall Beach to model future sea level rise: Geological Society of America, Abstracts with Programs, v. 41, no. 3, p. 41.

Slovinsky, P. A., 2007, The Patriots' Day storm at Willard Beach, April 2007.

Popham Beach State Park

Please visit the Department of Conservation's web site to learn more about Popham Beach State Park.

Text and photos by Stephen M. Dickson

Last updated on April 23, 2012