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The Patriots' Day Storm at Willard Beach, April 2007
The recent Patriots' Day storm battered numerous sections of the Maine coastline for several days with high waves and strong winds. Offshore wave heights reached over 8 meters (26 feet) on April 16 (Figure 1). The storm also coincided with astronomically high tides; predicted tides were exceeded by approximately 0.75 m on April 16, indicating that a storm surge of about 2.5 feet occurred at the height of the storm (Figure 2).
Many beaches along the southern and mid-coast, especially those that face east to northeast, underwent relatively substantial erosion from this storm. One such beach is Willard Beach, a small, crescent-shaped pocket beach located in suburban South Portland.
Willard Beach has been the site of a proactive community-based dune management program. Volunteers participate in monitoring the beach by conducting beach profiles on a monthly basis as part of the State of Maine Beach Profiling Project (SMBPP). A total of six beach profiles are recorded at Willard Beach each month (WI01-WI06). The Maine Geological Survey supplements this beach profile data with alongshore Real Time Kinematic Global Positioning System (RTK-GPS) terrestrial surveys of the edge of vegetation in order to accurately monitor horizontal changes in the dune and beach system.
A pre-storm survey of the vegetation line was completed by MGS on November 3, 2006. Subsequent beach profiles were collected along the beach on November 11, 2006 by the Southern Maine Community College, which participates in the SMBPP volunteer monitoring program.
Following the Patriots' Day storm, MGS resurveyed the edge of vegetation on April 19, 2007 in order to calculate the horizontal erosion that occurred along the edge of the dune. This was followed by beach profiling, completed on April 20, 2007, at the six set locations. The results of these observations are shown in Figure 3.
Transects were cast at 30 meter (~100-foot) intervals along the beach, resulting in a total of 19 transects (see light blue lines, Figure 3). These transects were used to measure horizontal shoreline change along the beach. Results are shown in Table 1. Up to about 12 meters (~40 feet) of erosion occurred, with the areas of highest erosion between transects 7-13 and 15-17. The averaged erosion that occurred was 7.1 meters or about 23 feet.
Beach profile changes were determined from the SMBPP collected profiles. The differences between the November 2006 and April 2007 profiles can be interactively viewed by clicking on each beach profile location (WI01-WI06 in Figure 3). With these two data sets, both horizontal and vertical changes can be observed and estimated volumetric changes quantified.
Overall estimated volumetric changes at each beach profile location are shown in Table 2. This volumetric change estimate assumes that the beach profile is representative of a width of only one meter, which allows for the volumetric calculation of cubic meters. It is apparent that accretion actually occurred in the lower portions of the beach profiles at WI04-WI05. This is likely a result of erosion at the upper portion of the profile (the frontal dune), and accretion at the lower portion of the profile in the form of a protective sand bar. Alternatively, sediment eroded from adjacent areas of the beach could be stored within the lower portion of these profiles as it is moved offshore in response to the storm.
Based on this analysis and physical characteristics, we can break Willard Beach into three different cells due to littoral barriers (Figure 4). If we then assume that the volumetric changes observed at each beach profile apply to certain distances of beach, we can determine a rough estimate of the volumetric changes that occurred from time 1 (November 2006) to time 2 (April 2007).
The beach along the southeastern end of Willard Beach - from WI01 to WI03 - is confined by an outfall pipe adjacent to WI03 (Figure 4). Combining this stretch of beach (into Cell 1) provides for an alongshore distance of approximately 220 meters (Figure 4). The averaged volumetric change rate (per meter of beach) is -5.2 m3. Assuming this change is representative of the 220 meters of beach contained in Cell 1, the overall volumetric change of this section of beach is -1143 m3. Cell 1 lost a large amount of sediment from within its boundaries.
Profiles WI04-WI05 are located within a separate littoral cell, defined by a pipe that serves as a littoral barrier (Cell 2, Figure 4). This stretch of beach measures approximately 137 m. The averaged volumetric change (per meter of beach) was +5.4 m3. This cell actually gained approximately +740 m3 of sediment along the overall recorded profile shape.
Conversely, the final cell (Cell 3, Figure 4), from WI05 northwards to WI06 had an averaged negative volumetric change rate of -1.9 m3 per meter of beach. The cell measures 250 m in length, and approximately -468 m3 of material was removed from the beach profile within this cell.
Combining this data, we see a net change of -871 m3 or -1139 cubic yards of material that was lost from the overall nearshore beach profile along Willard Beach. However, a gain in the beach profile volume occurred within Cell 2, even though the horizontal erosion along the dune appeared to be similar within this cell to the other cells.
Even though the dunes were eroded up to 30 feet in some areas, the active community dune management program at Willard Beach most likely helped protect many residential structures and public infrastructure from extensive flooding and damage from the Patriots' Day Storm. Sediment was not fully lost from the beach profiles, as evidenced by changes observed at WI04-WI05 within Cell 2.
The response of Willard Beach to the Patriots' Day storm, though it appears dramatic, is a natural seasonal shift of sediment that occurs at many beaches. The shift of sediment offshore during stormy months, followed by the onshore movement of sediment to build back the beach during calmer summer months, is a natural cycle. We expect the beach to begin recovery from this storm, and sand to return to the upper portion of the profiles, over the calmer summer months.
Related sites of interest
Site by Peter A. Slovinsky
Originally published on the web as the May 2007 Site of the Month.
Last updated on April 23, 2012
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