Kennebec River Sand Waves at Fort Popham, Phippsburg, Maine
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Kennebec River Sand Waves, Phippsburg, ME Maine Geological Survey Maine Geologic Facts and Localities April, 2020 Kennebec River Sand Waves at Fort Popham, Phippsburg, Maine 43°45'17"N, 69°47’4"W Text by Stephen M. Dickson Maine Geological Survey, Department of Agriculture, Conservation & Forestry 1 Kennebec River Sand Waves, Phippsburg, ME Maine Geological Survey Introduction For more than 10,000 years, the Kennebec River has carried sand to the sea. Once the riverine sand is released to the coast, ocean waves and currents reshape it into beaches and a large submerged sandy “paleodelta” that extends to a depth of 200 feet and is estimated to have 440 million cubic yards of sediment from the river (Barnhardt et al., 1997; Kelley et al., 1997). To get to Popham Beach State Park and Hunnewell Beach, sand must pass by Fort Popham at the river mouth (Figure 1). Maine Geological Survey Photo by J. T. Kelley, University of Maine, April 3, 2012. 3, April of Maine, University Kelley, T. J. by Photo Figure 1. An aerial photo looking north and showing the location of the Kennebec River at Fort Popham and Popham Beach State Park. Fort Popham is built on bedrock at a narrow constriction on the Kennebec River between Phippsburg (west bank) and Georgetown (east bank). Maine Geological Survey, Department of Agriculture, Conservation & Forestry 2 Kennebec River Sand Waves, Phippsburg, ME Maine Geological Survey Mixing Tides with River Currents From the vantage point of Fort Popham, currents pass by the Fort reversing twice a day as the tides ebb and flood (Figure 2). Hidden beneath the water surface is a complex sandy river bed that is ever changing with the daily tidal flow of salt water and discharge of freshwater. The tide range (from low to high) averages 8.5 feet but can increase to over 12 feet during perigean spring tides, often called King Tides. When the tide range is greatest, tidal currents are strongest and the amount of sand in motion on the river bed increases. King Tide currents can flood (move upstream) at up to 3 knots. Aided by freshwater discharge, the ebb (downstream) current can be up to 4 knots (4.6 miles per hour). Daily, peak tidal currents occur about two hours before high tide and two hours before low tide and the ebb current can be 1 knot faster than the peak flood current (Fenster et al., 2001). Periods of high river discharge, generally in April and May, is when sand is most likely to reach the coast (Fenster and FitzGerald, 1996; FitzGerald et al., 1989) and be reworked onto the beaches (FitzGerald et al., 2000; Goldschmidt et al., 1991). Maine Geological Survey Photo by S. M. Dickson, MGS, October 28, 2011. 28, October MGS, Dickson, M. S. by Photo Figure 2. A view upstream looking at Fort Popham from Riverside Beach. The tide is high. At low tide there is a small pocket beach below the bedrock shoreline. The Fort and parade grounds are open to the public seasonally. From within the Fort it is possible to look both north and south at the river currents. Maine Geological Survey, Department of Agriculture, Conservation & Forestry 3 Kennebec River Sand Waves, Phippsburg, ME Maine Geological Survey Sand Waves by Fort Popham Bathymetric mapping of the river bed by MCMI (2019) in October 2019 revealed that most sand waves had an asymmetry with a steeper slope on the downstream side (Figure 3). This shape, or geomorphology, is an indication of net sand transport toward the steeper slope. While the reversing tides may carry sand both upstream and downstream, the morphology of sand waves indicates the net direction of sand movement over time. So, by examining the shape of the sand waves, it is possible to infer how sand moves along different stretches of the river bed. (MCMI, 2019). (MCMI, Kraun Map created by by Ben Map created Figure 3. A bathymetric map showing depth of the Kennebec River in the vicinity of Fort Popham with the surrounding nautical chart displayed for reference. Sand waves show up as crests and troughs. Some have linear crests while others are arcuate. Sand wave heights vary but can be 6 to 10 feet or more above the adjacent troughs. Between Gilbert Head and Little Cox Head there is a particularly large sand wave crest that crosses about half of the channel at a depth of 30 to 50 feet. Depths on the map are in meters below mean lower low water (1 m is 3.3 feet). Maine Geological Survey, Department of Agriculture, Conservation & Forestry 4 Kennebec River Sand Waves, Phippsburg, ME Maine Geological Survey Sand Waves by Fort Popham In addition, mapping between the Fort and Gilbert Head in Georgetown shows a 120-foot (35 m) deep basin in the river between where the river is narrowest (Figure 3). In constrictions such as this, river currents must accelerate to pass a fixed flow volume through a given river cross section. With faster currents at this narrow spot, no sand waves are present on the river bottom. In effect, no sand can settle for long on the river bed next to the Fort and the channel is deep. Upstream of the Fort, the river banks are farther apart, and the currents can deposit sand in the form of sand waves in various shapes, sizes, and orientations. In fact, where the river is the widest, the sand waves are the shallowest and reach heights 30 to 50 feet below mean lower low water (Figure 3). Since sandwave crests tend to be perpendicular to the flow, their orientation is a clue about the direction of flow. In Figure 3 the crests fan out upstream of the Fort where the river bends around a bedrock shore. Many of the crests bifurcate into multiple smaller crests with shorter wavelengths. Maine Geological Survey, Department of Agriculture, Conservation & Forestry 5 Kennebec River Sand Waves, Phippsburg, ME Maine Geological Survey Sand Waves Under a Gyre Seaward of Fort Popham, the channel widens and is interrupted by the two Sugarloaf Islands that represent emergent bedrock shoals (Figures 1 and 4). Mapping in this area shows flow divergence around the islands creates moats and steep slopes in sand. To the north and east of the islands are ledges that are not covered by sand and show a north-south lineation much sharper in contrast than sand waves. Figure 4. A bathymetric map showing depth of the Kennebec River from Fort Popham at Hunnewell Point south around the Sugarloaf Islands. The surrounding nautical chart is displayed for reference. Freshwater discharge flows from north to south. This stretch of the river is tidal so, on an incoming (flood) tide, the current can reverse and flow north. Sand waves show up as crests and troughs and moats are present around shoals. East of the islands, bedrock ledge shows a different structure from smoother sand waves. In the center of the channel, 2019). (MCMI, sand wave crests bend south in the main ebb flow. On Kraun the west side of the river adjacent to the beach, arcuate crests show a flow north-northwest toward the bedrock supporting Fort Popham. Depths are in meters below mean lower low water (1 m is 3.3 feet). Map created by by Ben Map created Maine Geological Survey, Department of Agriculture, Conservation & Forestry 6 Kennebec River Sand Waves, Phippsburg, ME Maine Geological Survey Sand Waves Under a Gyre Immediately south of the Fort and Hunnewell Point there are arcuate sand wave crests just east of the beach. The curvature of the crests indicates a net northward flow along the bottom. A back-eddy, or circulation gyre, creates flood-oriented sand waves along the river bottom south of Fort Popham and along Riverside Beach (also called River Beach; Figure 4). The bottom flow must be deflected by the bedrock beneath the Fort and sand reinjected back into the center of the river channel. The main channel of the Kennebec River shows a series of ebb-oriented sand waves north of the Sugarloaf Islands. South of Fort Popham, where the outgoing tide forms the gyre, it is sometimes possible to see seals swimming in the eddy (Figure 5). Maine Geological Survey Photo by S. M. Dickson, MGS June 6, 2018. 6, June MGS Dickson, M. S. by Photo Figure 5. A view east across the Kennebec River from the vicinity of Fort Popham and Figure 2 to the Georgetown shoreline framed in bedrock. The black spots in the river are seals that submerge and reappear in the vicinity of the circulation gyre. Maine Geological Survey, Department of Agriculture, Conservation & Forestry 7 Kennebec River Sand Waves, Phippsburg, ME Maine Geological Survey Sand Waves Under a Gyre This gyre can also erode sand along the adjacent beach and result in an arcuate shoreline (Figure 6). Maine Geological Survey Photo by S. M. Dickson, MGS, March 6, 2020. 6, March MGS, Dickson, M. S. by Photo Figure 6. A view looking south along Riverside Beach toward the former Coast Guard station with a red roof and cupola. For reference, the cupola (CUP) is shown in Figure 4. The beach shoreline is not linear because of the interaction of tidal currents, the gyre, and occasional slumping of beach sand into the river (see also Figure 7). Slumping occurs in the foreground to create the first arcuate indentation at the water line. The additional cuspate shoreline features may be related to shore-parallel currents. Maine Geological Survey, Department of Agriculture, Conservation & Forestry 8 Kennebec River Sand Waves, Phippsburg, ME Maine Geological Survey Sand Waves Under a Gyre The shoreline has been observed to be undercut resulting in the upper beach actively slumping into the river.