Impacts of coastal protection structures take place over decades

The Tweed River and Letitia Spit, where the coastal intervention impacts were studied.

Griffith University researchers have analysed decades of surveys documenting theenvironmentalresponse to coastal protection structures at an iconic stretch ofthenorthern New South Wales beaches,findingthat someimpacts can take years to eventuate.

Published inMarine Geology, researchers from theCoastal & Marine Research Centreand theSchool of Engineering & Built Environmentassessedsixsurveys throughout 1967 to 2020 to observe the morphologicaland sand volumechanges totheLetitia Spit -south of the Tweed River-in response to the construction of therivertraining walls around1962-1964 and an artificial sand bypassing systemimplementedin 2001.

This coastal area inNew South Wales,that alsobordersthe southern Gold Coast,has been under the influence of management actions over the last 100 years, with the first rudimentary rock wall built in theTweed River in the1890s.

The beaches north and south of the Tweed River are popular locations, particularlyfor surfers,and the Tweed Riverentranceis a busy thoroughfare for recreational and commercial vessels.

PhD candidate AnaPauladaSilva andtheresearch team foundthatwhilethebeach in theimmediate updrift(south)of the Tweed Rivercoastal interventionsrespondedwithin monthstoacouple of yearstothe introduction ofthe entrance trainingstructures, the extension of those impacts furthersouthalongtheLetitia Spitcoastlinewasgradual and tookdecades.

Overall, for Fingal Beach—atthe southern end of Letitia Spit — the impacts were largely reduced and there is no evidence over the period of the study ofmorphologicalchangesextendingsouth of Fingal Head.

About two to three decades were necessaryforLetitia Spittoreachthemaximumcapacity forsand accumulation on the beachcaused by the training wall obstruction of the littoral(nearshore zone)drift, whereas thesubsequenterosional stateoccurred followingthecommencement of the artificial bypassingandcontinued foronlyabout one decade before the newbeachequilibrium was reached.

“The beaches are normally under what we call dynamic equilibrium: they fluctuate around anaveragepositionin response to the variability in hydrodynamic factors— liketidal cyclesandwave climate and sediment supply.”

These oscillations will lead to the natural phases of erosion and accretion in a beach, which might happen seasonally and/or interannually – that will vary from place to place. But the important thingis:inthe same way the beach erodesat times, it accretesat others. It isa natural changethatis mostly naturally reversible.

“So, when something external interactswith the natural beach dynamics, likenewcoastal structures,thecomplex and integrated system that sustains the beach equilibrium needs to adapt.

In the case ofLetitia Spit,construction of thetraining wallsin the 1960sblocked the sand movement to the north and caused Letitia Spit to go into a persistent accretion trend that lasted until the 1990s.

If the beach is in a constant trend for so long, it is not in dynamic equilibrium and it cannotreverse the situation by itself.

“It is interesting that decades are a long time for us, for coastal management and for community needs; but it is actually a short time for some natural processes,” Silva said.

“It is easier for us to understand if we think about the erosion trend, which was happeningon the southernGold Coast while Letitia Spit was accreting. By itself, it would be very hard for the Southern Gold Coast beaches to accrete again and find an ideal dynamic equilibrium,at least on a timescale relevant for the community needs.

“It isalsorelevant to understand that despite Letitia Spitbeing solargely accreted (whichmightseem good from one’s perspective), thiswasnot a naturalcircumstance, it happened because of thenewtraining walls.

“Therefore, with any change in the beach conditionsthe accretion trend couldeasily flip to an erosion trend.”

The introduction of the artificial sand bypassing restarted the littoral driftto the north, and consequentlysomeupper beach erosionoccurredas the excess of sand accumulated on Letitia Spitwas transported away. Currently, the beach has returned to its natural dynamicequilibriumand it is inasimilar condition to the firstfewyearsfollowingthe training wallsconstruction.”

This study raises attention to the value of long-term beach monitoringonboth sides of the implemented coastal structure.

Our findingsoffer valuable scientific information to share withcoastal managers andthelocalcommunityshowingthat there isa strongneed to keep monitoringthesebeachesfor decades to followtheirevolutionafter implementing any intervention,”Silva said.

The research‘Updrift morphological impacts of a coastal protection strategy. How far and for how long?’has been published inMarine Geology.