International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004
3). A 1.5-km-long shoreline in the southern end of Assateague
Island in Section 1 experienced significant deposition from
1996 through 2000 over an area that is 190 meters wide and 1.5
km long from near shore to foreshore. Meanwhile, erosion
200m width in the dune and berm area also occurred (see Fig.
3a), and more significant erosion of 100m width and 980m
length in the berm and foreshore area of the northern end of
Section 1 is evident (see Fig. 3a). A 1700m length of shoreline
facing the Atlantic Ocean in Section 2 has experienced severe
erosion of up to 40m extent in the foreshore and dune portions
of the beach in the period of 4 years (from 1996-2000). A 400m
length on the west side (berm areas) near the middle of Section
2 has experienced deposition over a width of up to 100m (see
Fig. 3b), and from Section 3 to Section 6 the erosion appears to
have slowed from 1996 to 2000. Only a small area experienced
severe erosion, such as Profile | in Section 3 (see Fig. 3c). In
Section 4, the most rapid deposition occurred in a 1200m long
by 85m wide area of the foreshore (see Fig. 3d). In Section 5,
erosion occurred in the near shore, foreshore, and dune areas
facing the Atlantic, and deposition occurred in the berm and
dune areas not facing Atlantic (see Fig. 3e). In contrast, the
foreshore in Section 6 experienced deposition, while the berm
and dune area experienced erosion. Additionally, a 40m wide
by 415m long foreshore area experienced significant deposition
(see Fig. 3f).
In order to analyze in detail the topographic changes in the
study area during the four years, four seasonal (September-
November) data sets were created to investigate topographic
change at a seasonal interval from 1996 through 2000. Through
observation of DEM seasonal changes, we also note that the
coastal area was generally eroded in summer and fall, and most
deposition occurred in winter. In the early spring and late fall,
the coastal topographic change undulates; we think this may be
caused by varying weather. We selected three profiles in each
Section to demonstrate the topographic changes within the
study area, and Figures 4 present the elevation curves of each
profile in different years.
From Fig. 3a and Fig. 4c, we found that a length of about 1500
m in the south end of Section 1 has seen significant deposition
of up to 1.37m in height over a 190m width from the near shore
to the foreshore, and significant erosion up to a maximum 0.8m
in the 200m-wide dune and berm area. This resulted in the
formation of a flat, elevated area of 0.6-0.9m in the south end
as observed in 2000. Most of the 4267m long by 100m wide
berm area along the eastern shoreline of Section 1 experienced
severe erosion in the berm area like that shown in profile 2 of
Section 1 in the period of 1996-2000. In only two years, from
1998 to 2000, a 67m wide berm was eroded 2.4m, resulting in
the coastline shifting 67m inland (see Fig. 3a and Fig. 4b).
Additionally, a 700m long foreshore in Section 1 sustained
deposition like that seen in profile 1 of Section 1 (see Fig. 4a),
and an approximate 67m wide foreshore near shore gained 1m
in height, resulting in shoreline movement outward by 67m,
and an equally wide dune area that experienced erosion,
resulting in the formation of a berm area.
A 1700 m shoreline in Section 2 has experienced severe
erosion of 3.35m in depth for a 40 m width from foreshore to
berm (see Fig. 4f). However, the berm and dune areas with a
400 m length by 100m width at the middle of Section 2 saw
deposition of more than 0.3m height (see Fig. 3b). Most of
foreshore and dune areas have experienced more than 2 m
erosion, resulting in the coastline shifting inland approximately
18-24 m (see Fig. 4d and 4e).
360
Observing Sections 3 through 6, the erosion velocity obviously
was less than that of Sections 1 through 2 in the study period of
1996 through 2000, but a small area in the profile of Section 3
experienced faster erosion at speed of approximate 4m per year
(see Fig. 3c). Since the foreshore slope was eroded, the near
shore was extended to the foreshore by 36m (see Fig. 4g). In
Section 4, the most deposition occurred in a 1220m length by
85m width foreshore area (see Fig. 3d), resulting in a wrack
line movement out by 27-30m (see Fig. 5b, and 5c). On the
other hand, as observed from Profile 2 of Section 4, Profile 1 of
Section 4, and Profile 3 of Section 5, the top of dunes increased
in height some 0.7 to 1.5m (see Fig. 5b, 5a, and 5f). In Section
5 and Section 6, the areas near the shore, the foreshore, and the
dune were all generally eroded, which caused the dune areas
and the shoreline to retreat 6-15m (see Fig. 5d, 5e, 5g, 5h, and
51), and the near shore extended to foreshore by 30-45m (see
Fig. 5e, and 5f). Observing Section 6, the tops of the dune
decreased by 0.8m in the south of Profile 3, by 1.5m in the
middle of Profile 2, and by 1.8m in the north of Profile 1 (see
Fig. 5g, 5h, and 51i).
Based on the DEM analysis above, we can make some overall
Observations: (1) Most of the dune elevations have decreased,
and the dune areas have moved towards the west (retreated).
This observation may be able to explain why the entire island
becomes narrower and narrower from 1996 to 2000. We
illustrated these changes using 18 profiles in Figure 6 through
7. (2) The near shore areas decreased about 0.3-1m in height,
and migrated west into the foreshore from 1996 to 2000. (3)
The berm connected with foreshore experienced serious
erosion, resulting in the shoreline migration inland. Moreover,
the slopes of foreshores in all six sections have grown steeper.
(4) Dune changes differ in topographic profile and morphology
over the course of the study. For example, the dunes from
Section 1 to Section 3 decreased in volume, accreted in Section
4, and retreated west in both Section 5 and Section 6.
Additionally, their rate of change was not the same. For
example, the dunes from Section 1 through Section 3 rapidly
decreased between 1997 and 1998 (sec Fig 4c, 4d, 4e, and 4g),
while the dunes from Section 5 to Section 6 rapidly retreated
between 1998 and 2000 (see Fig 5d, 5e, 5f, 5g, 5h, and 51). In
contrast, the dunes in Section 4 rapidly built up between 1996
and 1997 (see Fig 5a, 5b, and 5c). An approximate 2.6km long
shoreline has retreated inland from 2 - 40m through different
coastal conditions and environments; an example is the
foreshore in Section 2, which retreated about 40m from 1996 to
2000.
4.3 Volumetric Morphologic Changes
To quantify the 4-year topographic change of the study area,
volumetric analysis (deposition, erosion, and net change) of
each section was conducted. Table 1 summarizes the
deposition, erosion, and net change results of the six sections
for the period of 1996 through 2000.
As seen from Table 1, Section 1 had the largest amount of both
deposition (14,647.9 m?), as well as erosion (17,261.3 m)
Section 6 showed the least amount of deposition (2,835.9 m S.
Section 4 experienced a positive net volumetric gain (1,998. 6
m?) Section 4's gain, near 1,998.6 1 n, might largely be
contributed to beach nourishment during the study period. The
entire study area exhibited a net volumetric loss of 26,693.5 m?
from 1996-2000 at an average loss rate of 0.011 m*m? The
total erosion was 67,389.7 m’; Section 1 contributes 17,261.3
m? to this loss at an average loss rate of 0.602 m?/m?. The net
loss of Section 1 is 2,613.4 m at loss rate of 0.005 m?/m?. Over
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