Contents

• Introduction
• Methodology
• Results
• Discussion
• Findings and Recommendations
• Conclusions
• References

Impacts of Future Sea Level Rise on the Coastal Floodplain

Results

Generalized results for the simulated changes in sea level are shown in Table 2 and Table 3, and in Figures 3 through 17. These results are described below for each scenario, including existing conditions.

Marsh Habitat Changes

This section presents the results of the different sea level change scenarios and their impacts on the existing marsh habitat(s) within the study area.

Existing Conditions

Figure 3

Approximately 48% of the study area is considered upland (elevations above highest annual tide (HAT), 1.8 m). High marsh currently dominates the salt marsh, accounting for 24% of the study area (Table 2). Open water (14%) and low marsh (14%) account for the remainder of the study area (Figure 3).

1-ft Sea Level Rise

Figure 4

After a simulated static 1-ft rise in sea level, 44% of the study area is upland, 27% is low marsh, 17% is open water, and only 12% is high marsh (Table 2). Most notable is a dramatic 101% increase in low marsh area from existing conditions, and a substantial decrease of -49% in high marsh area (see Table 3, Figure 4). There also is a 15% increase in open water area, and a -8% decrease in the upland area. This indicates that, even after only a 1-ft rise in sea level, high marsh area will most likely be pinched out between the low marsh and the uplands since it is not able to convert enough upland to high marsh due to steeper slopes.

Low marsh areas expand dramatically along the main tributaries of the Webhannet River and marsh system, along the Little River, and east and south of the Drakes Island Road causeway. High marsh areas expand only slightly, mostly at relatively flat, low-lying areas (i.e., along Drakes Island Road), but are overtaken by low marsh along many areas. It appears that the high marsh area capacity within the study area is near its maximum.

2-ft Sea Level Rise

Figure 5

After a simulated static 2-ft rise in sea level, about 40% of the study area is upland, 33% is low marsh, 19% is open water, and less than 9% is high marsh (Table 2). The low marsh has expanded by over 144% (21% more from 1 ft conditions) from existing conditions, while open water areas have grown by over 33% since existing conditions. The high marsh area continues to decrease by an additional -30% (from +1 ft numbers), and uplands decrease by 10% (Table 3 and Figure 5). A total of 65% of the existing condition high marsh has been lost after 2-ft of sea level rise.

The majority of low marsh expansion continues to encroach on previous high-marsh areas around the Webhannet and Little River main tributaries. High marsh, in general, continues to lose ground to low marsh. However, high marsh and highest annual tide expansion continues around Drakes Island and in the Little River tributaries.

3-ft Sea Level Rise

Figure 6

After a simulated static 3-ft rise in sea level, only about 36% of the study area remains upland. Low marsh areas account for 34% of the study area, while 22% is open water area. Only 8% of the study area remains as high marsh (Table 2). The low marsh area increased over 153% from existing conditions, and the open water area grew by 54%. The high marsh decreased by a total of about 67% from existing conditions, while the upland area lost about 25% of existing conditions total upland area (Table 3 and Figure 6). Low marsh only grew an additional 4% from 2-ft-to-3-ft conditions, indicating its expansion is reaching steeper slopes.

Figures 7 through 9 display overall changes for Open Water (OW), Mean High Water (MHW, assumed to be "Low Marsh"), and Highest Annual Tide (HAT, assumed to be "High Marsh") for the 1, 2, and 3-ft rises in sea level scenarios.

Figure 7

Figure 8

Figure 9

Flooding Implications

Changes in sea level may also have major implications for developed areas (both private and public infrastructure) in terms of flooding. This section describes the results seen from 1, 2, and 3-ft sea level rise scenarios for the developed areas of Drakes Island and Wells Beach. Refer to Figures 10-16 as referenced below. Note that only figures for MHW and HAT conditions have been created for further investigation of each scenario, as no flooding results directly from changes to OW conditions.

1-ft Sea Level Rise

It does not appear that any developed areas on Drakes Island or Wells Beach would be threatened as a result of +1ft OW sea level conditions (no figure shown). +1 ft MHW conditions result in encroachment (but not inundation) on numerous properties along the marsh side (west side) of Island Beach Road on Drakes Island (Figure 10). There do not appear to be any impacts along Wells Beach.

Figure 10

Figure 11

Along Drakes Island, +1 ft HAT conditions will cause isolated flooding concentrated on the north side of Drakes Island Road, and southwest of the Eaton Ave/Drakes Island Road intersection towards Fern Street (Figure 11). In this area, close to 30 private properties may become flooded, as do substantial portions of Drakes Island Road, Eaton Avenue, and Grove Street. Additionally, portions of Island Beach Road leading southwest to the public parking lot, and a portion of Shady Lane would undergo flooding. North of the Drakes Island Road/Island Beach Road intersection, isolated lots on the northwest side of Island Beach Road would also incur flooding.

2-ft Sea Level Rise

+2ft OW conditions do not appear to result in any apparent flooding of Drakes Island or Wells Beach. +2 ft MHW brings the water level directly against all of the bulkheads along Wells Beach, though it does not appear that any are overtopped. Along Drakes Island, flooding appears at the unpaved public parking lot and adjoining properties northeast of the intersection of Eaton Ave and Drakes Island Road, and at isolated lots west of Island Beach Road. No public roadways appear to be compromised (Figure 12).

Figure 12

Figure 13

Figure 14

The areas of possible flooding along Drakes Island increase dramatically with 2-ft of sea level rise under HAT conditions (Figure 13). Flooding of Island Beach Road nearest the parking lot (that adjoins the Wells jetties) is more extensive, as is flooding along developed lots along Shady Lane, Eaton Ave, Grove, and Fern Streets, and both sides of Drakes Island Road. Additionally, it appears that the causeway (Drakes Island Road) crossing the marsh may become flooded under +2 HAT conditions. Along Island Beach Road north of the intersection with Drakes Island Road, flooding of individual private lots becomes much more extensive. Only isolated pockets of minor flooding appear along the Webhannet River side of Wells Beach after +2 HAT conditions (Figure 14).

3-ft Sea Level Rise

+3ft OW conditions still do not appear to result in any apparent flooding of Wells Beach or Drakes Island private or public property. +3ft MHW results in possible minor flooding along Wells Beach at the southern end of the study area, and ponding of water at the northern end of Wells Beach, near the jetty. Along Drakes Island, +3ft MHW inundates portions of Shady Lane (due to an excavated canal running parallel to it), and flooding becomes more extensive along Eaton Ave and Drakes Island Road. Several lots along the west side of Island Beach Road also become inundated (Figure 15).

Figure 15

Figure 16

Figure 17

As expected, +3ft HAT conditions are the most dramatic, especially along Drakes Island. The Drakes Island Road causeway (Figure 16) becomes completely flooded, and results in extensive flooding west, east, and north of Eaton Road. Numerous lots are flooded all the way along Drakes Island Road. Additionally, the majority of lots west of Island Beach Road undergo inundation. Along Wells Beach, flooding occurs along the marsh-side at several isolated spots within the study area, ponding just south of the northern jetty, and inundation of several low-lying areas towards the oceanfront (Figure 17).

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Contents   Introduction   Methodology   Results   Discussion   Findings and Recommendations   Conclusions   References

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Last updated on April 24, 2006

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