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Protect the Natural Dune Seawall - page 5 / 11

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If we look at a typical profile across a barrier island like ours, we can see the relationship between the beach and the dunes and how they change in storms and the recovery period after storms. This diagram from the Texas General Land Office (GLO) is very similar to the actual situation here in Port Aransas. Cross Section of a Texas Barrier Island (Dune Protection and Improvement Manual, fourth edition, Texas General Land Office.

Cross Section of a Texas Barrier Island (Dune Protection and Improvement Manual, fourth edition, Texas General Land Office. http://www.glo.state.tx.us/coastal/pdf/DuneManual.pdf

Note the location of the coppice dunes with vegetation is in the transition zone between the beach and the dunes at the upper backshore (dry beach). They are miniature dunes and their correct name is coppice dunes. These coppice dunes form when dry sand blows up from the foreshore and the seaward part of the backshore and begins to accumulate around patches of vegetation or in small dunes with no vegetation. With time, the vegetation causes more sand to accumulate and the coppice dunes grow larger and eventually weld onto the foredune ridge or begin to form a new dune ridge seaward of the existing one. In this way, the line of vegetation and the vegetated dunes slowly grow seaward in the interim between hurricanes. The GLO paper defines “critical dunes as all dunes (coppice dunes, foredunes, foredune ridge, and some backdunes) that store sand to replenish eroding public beaches.” Clearly the dunes that form on the backshore of Port Aransas beaches are critical dunes and need to be protected.

This

GLO

illustration,

shows

how

the

beach-dune system changes during a storm. The higher the storm surge water level, the more important the protection by a strong natural dune seawall. Sand from the beach, the coppice dune area and the foredunes is eroded and is transported offshore into the

profile.

This

has

the

very

useful

benefit

of

nearshore

bar zone,

causing

the waves

to expend

flattening the beach more energy offshore

and reduce the rate time to pass before

of attack on the main dune destroying the entire natural

line, buying precious dune seawall and the

time so that the storm structures behind it. If

has the

coppice

dunes

and

the

foredunes

are

well

vegetated,

the

root

system

slows

the

rate

of

erosion

of

these critical dunes during wave attack by beach in front of the main foredune line

storms. protect

Well vegetated coppice dunes on the upper the foredunes from rapid undercutting and

5

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