Terminal groin study : final report. - Page 59 |
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NC TERMINAL GROIN STUDY
FINAL REPORT
______________________________________________________________________________________
March 2010 II- 16
Bodie Island spit into the channel and a shift of the channel thalweg southward.
Bathymetric changes in the ebb- tidal delta region reflect the narrowing and seaward
extension of the main ebb channel, which resulted in a growth and seaward displacement
of the terminal lobe ( outer bar of the ebb- tidal delta). The point to emphasize here is that
the longshore transport system, Bodie spit evolution, tidal inlet hydraulics, ebb- delta
sedimentation trends, and erosional- depositional changes to the northern tip of Pea Island
( terminal groin region) are all intimately interconnected. A perturbation to one part of the
system affects the processes and morphology of others.
As discussed in the previous section on geological framework and physical processes, the
configuration of the ebb- tidal delta at Oregon Inlet strongly controls sedimentation
processes in the vicinity of the terminal groin. The orientation of the main ebb channel
dictates the asymmetry of the ebb- tidal delta and overlap of the updrift or downdrift inlet
shorelines. As seen in Figure II- 10, in 1959 the main ebb channel was oriented straight
out the inlet and the ebb- tidal delta fronted the downdrift northern end of Pea Island. In
this configuration, swash bars migrated onshore, adding sand to the northern shoreline.
Conversely, in 1975 the main channel was situated along the updrift Bodie Island Shore
and Bodie Island was the beneficiary of landward bar- welding events and the northern of
Pea Island was exposed to storm waves and erosion.
e) Northern Pea Island
Wave refraction around the ebb- tidal delta is another important process at Oregon Inlet
as shown in Figure II- 11. An aerial view of Pea Island in 1991 shows the terminal groin
extending into the inlet and the fillet region containing little sand. However, swash bars
can be seen immediately offshore of the groin and these may have moved onshore and
contributed sand to the beach. By 1993, the groin had trapped sufficient sediment
( through beach nourishment and natural processes) so that the fillet region was mostly
filled with sand. The 1993 photograph reveals a relatively wide tidal inlet and an ebb
delta that is pushed close to the inlet mouth. Note that waves are breaking at a steep angle
to the beach, indicating that at this time sand was moving northward along the beach
toward the groin ( Figure II- 11). Currents generated by the flooding tides would have
enhanced northerly sand transport along the tip of Pea Island.
This same morphology is observed in a 2001 photograph of the region ( Figure II- 12).
This photograph demonstrates that after the beach accretes to the end of the groin,
additional longshore transport of sand toward the inlet moves around the groin ( as well as
over and through the groin during elevated tides and high wave energy events) and is
deposited along the inlet shoreline. It should also be noted that sand is also sequestered at
the northern end of Pea Island as a consequence of storm overwash into the fillet region.
Beach sand blown into the back dunal area also adds to the sand reservoir in this region.
Object Description
Description
| Title | Terminal groin study : final report. - Page 59 |
| Full Text | NC TERMINAL GROIN STUDY FINAL REPORT ______________________________________________________________________________________ March 2010 II- 16 Bodie Island spit into the channel and a shift of the channel thalweg southward. Bathymetric changes in the ebb- tidal delta region reflect the narrowing and seaward extension of the main ebb channel, which resulted in a growth and seaward displacement of the terminal lobe ( outer bar of the ebb- tidal delta). The point to emphasize here is that the longshore transport system, Bodie spit evolution, tidal inlet hydraulics, ebb- delta sedimentation trends, and erosional- depositional changes to the northern tip of Pea Island ( terminal groin region) are all intimately interconnected. A perturbation to one part of the system affects the processes and morphology of others. As discussed in the previous section on geological framework and physical processes, the configuration of the ebb- tidal delta at Oregon Inlet strongly controls sedimentation processes in the vicinity of the terminal groin. The orientation of the main ebb channel dictates the asymmetry of the ebb- tidal delta and overlap of the updrift or downdrift inlet shorelines. As seen in Figure II- 10, in 1959 the main ebb channel was oriented straight out the inlet and the ebb- tidal delta fronted the downdrift northern end of Pea Island. In this configuration, swash bars migrated onshore, adding sand to the northern shoreline. Conversely, in 1975 the main channel was situated along the updrift Bodie Island Shore and Bodie Island was the beneficiary of landward bar- welding events and the northern of Pea Island was exposed to storm waves and erosion. e) Northern Pea Island Wave refraction around the ebb- tidal delta is another important process at Oregon Inlet as shown in Figure II- 11. An aerial view of Pea Island in 1991 shows the terminal groin extending into the inlet and the fillet region containing little sand. However, swash bars can be seen immediately offshore of the groin and these may have moved onshore and contributed sand to the beach. By 1993, the groin had trapped sufficient sediment ( through beach nourishment and natural processes) so that the fillet region was mostly filled with sand. The 1993 photograph reveals a relatively wide tidal inlet and an ebb delta that is pushed close to the inlet mouth. Note that waves are breaking at a steep angle to the beach, indicating that at this time sand was moving northward along the beach toward the groin ( Figure II- 11). Currents generated by the flooding tides would have enhanced northerly sand transport along the tip of Pea Island. This same morphology is observed in a 2001 photograph of the region ( Figure II- 12). This photograph demonstrates that after the beach accretes to the end of the groin, additional longshore transport of sand toward the inlet moves around the groin ( as well as over and through the groin during elevated tides and high wave energy events) and is deposited along the inlet shoreline. It should also be noted that sand is also sequestered at the northern end of Pea Island as a consequence of storm overwash into the fillet region. Beach sand blown into the back dunal area also adds to the sand reservoir in this region. |
