COASTAL RESEARCH The shoreline erosion problem: lessons

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Erosion on an undevelopedshoreline at Pakiri on the eastcoast of Auckland. (Photo:Keith Smith)

AROUND THE WORLD there are somespectacular examples of the damage caused byretreating shorelines. And there are equallyspectacular examples of the expense to whichsome governments will go to hold their shorelinesin place. More than 80% of the world’s shorelinesare eroding at rates varying from centimetres tometres per year. In undeveloped areas, of course,a retreating shoreline is no problem. Usually weare not even aware that it is happening, thoughoften there are signs of erosion such as fresh cliffsin sand dunes or trees that have fallen onto a beach.Erosion only becomes a problem when a human-made structure is threatened.

Why are shorelines retreating?Beaches exist in a dynamic equilibrium involvingfour factors:1. the supply of sand to a beach;2. the wave energy (related to wave height);3. sea-level change;4. location of the shoreline.

Sand suppliesSand is food for beaches. Sand sources includerivers, eroding bluffs, adjacent beaches and thecontinental shelf (seabed sands are combed ashoreby fair-weather waves). The amount of sandavailable can vary a lot. On the west coast of theNorth Island rivers feed black sand to the coastfrom the rocks of Mt Taranaki. ToweringPleistocene sand cliffs crumble into the sea andthis light-coloured sand is driven far up the coast

so that the beaches are less black to thenorth. In contrast, on the east coast ofNorthland and Auckland, rivers and cliffsprovide only a trickle of sand to the coast.Headlands cause the sand to get lockedup in small bays.

Human activities can affect the sandsupply. Extracting sand near the shore canupset the balance of erosion andaccumulation in bays where sand suppliesare meagre. Damming rivers andextracting water for irrigation changes theflow condition in the rivers and suppliesof gravel and sand to the Canterburycoast. Construction of ports which project

offshore, such as Taranaki, alter the patterns ofsand transport along the shore.

WavesWaves, combined with tides and wind, are criticalcomponents of beach evolution. The higher thewaves and the greater the angle at which they strikea beach, the larger the volume of sand carried.During storms, waves move sand offshore andflatten the beach. Most often shorelines retreat injumps, each jump corresponding to a storm.Typically, after a storm sand builds up again,though recovery is much more gradual thanerosion. However, when viewed over time spansof a few decades, storms are a constant and reallyare not the fundamental cause of the world’seroding shorelines.

Sea-level riseSea-level rise may underlie much of the world’scoastal erosion. In New Zealand sea level iscurrently rising at a rate of about 15 cm percentury. The rising sea brings each storm a tinyincrement farther inland than the preceding storm.A huge debate is underway about the origin ofsea-level rise and likely movement in the future.Sea-level rise is due primarily to the thermalexpansion of sea water and melting of glaciers andice caps. (New Zealand’s glaciers are almost theonly ones that are advancing at present.) BecauseNew Zealand is an area with lots of volcanic andearthquake activity, it is a certainty that the sea-level rise is not the same on all beaches here.

Shoreline locationThis is the visible part of the problem. Shorelinesmove back and forth between storms and periodsof fair weather. It is when the shoreline movementimpacts on human-made structures that it becomesa beach erosion problem.

Managing shoreline erosionThere are three approaches to erosionmanagement.

Hard stabilisationThis is holding the shoreline in place using fixedhard structures, such as walls perpendicular to theshoreline (groynes, as at Hokitika), or sea wallsparallel to the shoreline (for example, Milford

COASTAL RESEARCH

The shoreline erosion problem:lessons from the pastOrrin H. Pilkey

Terry Hume

How can we dealwith the naturalprocess of coastalerosion when itbecomes aproblem? Andhow can researchhelp? Anoverseas expertpresents his viewof the issues inrelation to NewZealand.

House undermined by erosionof the sand dune at NagsHead, North Carolina, USA.

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Beach, Auckland). Hard shoreline stabilisation isthe best way to protect beachfront property, andthe bigger the wall the better. However, walls arecostly, lead to the loss of sand on the beach in front,restrict access to the beach and are ugly.

Soft stabilisationThis is fighting erosion at one location by bringingin sand from another. At Omaha and Orewa sandwas dredged from the estuary and piped to thebeach, at Napier sand and gravel is trucked toWestshore Beach, while sand from Pakiri Bay wasbarged south to nourish Mission Bay. This so-called beach nourishment “improves” the beachand also protects buildings while the beach is inplace. However, the procedure is costly and onlytemporary.

RelocationSometimes referred to as retreat, this is the do-nothing (and let houses fall in) or move-’em-backapproach (e.g., at Omaha during the 1978 storms).However it is done, this alternative allows natureand the sea-level rise to roll on. Relocation savesthe beach and saves shoreline stabilisation costs.However, it can be politically very difficult and itcould be financially costly if government isrequired to purchase land. Also, land is lost.

No solution?There are heavily developed shorelines in NorthAmerica and Japan, from which hard lessons maybe taken and applied to the New Zealand situation.

Lesson 1. Usually the obvious answer to an erosionproblem is to build a hard structure. But it is bestto start soft and then go hard, especially ifpreservation of the beach is a priority.

Lesson 2. The soft solution can become very costly.On the major nourished beaches of the east coastof the USA the cost is about US$10,000 perbeachfront lot per year.

Lesson 3. Take a hard look at what is beingpreserved. Does it really create problems to let parkbeaches retreat? Can the buildings be moved? Arethey worth the cost of nourishment?

Lesson 4. Once erosion control has been started, ithas to continue. Hard stabilisation is generallynever removed; it is just made bigger. Even more

critical, hard structures often cause erosion onadjacent beaches leading to even more hardstructures.

In New Zealand we have perhaps been fortunatethat circumstances have conspired to preserve thecoast from large engineering works andarmoured shores. A relatively longcoastline (11,000 km) and sufficientnatural harbours, combined with a smallpopulation, have ensured little coastaldevelopment. Apart from ports, fewdevelopments demand protection fromcoastal erosion. Funds for protectionhave been small because coastalcommunities provide only a modestrating base. Furthermore, there are nocentral government subsidies for coastalprotection, territorial local authoritiesfund protection only of public assets andsubsidies for works to protect private property aremuch more difficult to obtain, and regionalcouncils serve a regulatory and advisory role andprovide no funding. And, unlike in some othercountries, there are no powerful lobby groups forengineering the coast.

Unfortunately the lack of funding for coastal worksand poor understanding of coastal processes in thepast have left the coast peppered in places withsmall ad-hoc structures, now in various states ofdisrepair and usefulness. There are 400 structureson the Environment Waikato coast. Auckland’sNorth Shore beaches, now lined with veryexpensive real estate, are fronted in places byancient and ugly protection works.

While it is still not easy to solve the erosionproblem, we can conclude on a bright note. TheNew Zealand circumstances, our much-improvedscientific knowledge of coastal processes, and theuptake of this knowledge into coastal hazard anderosion management initiatives by regionalcouncils, mean there should be no excuses for not“living by the rules of the sea” and getting it rightfrom now on. ■

Orrin Pilkey is based at Duke University, NorthCarolina, USA; Terry Hume is at NIWA inHamilton.

Orrin Pilkey is theJames B. Duke Professorof Geology and Directorof the Program for theStudy of DevelopedShorelines (PSDS)within the Division ofEarth and Ocean Scienceat Duke University,North Carolina, USA.Orrin is well known forhis outspoken criticismof engineeringintervention on thecoast, a philosophy speltout in his books “Livingby the Rules of the Sea”and “The Corps and theShore”. Orrin was inNew Zealand in April2000, when he was akeynote speaker at theICS2000, aninternational coastalsymposium held inRotorua.

Shore protection by massiveconcrete armour unitsfronting a resort in Japan.(Photo: Jess Walker)

Research can helpNIWA’s research programme “Natural physical hazards affecting coastalmargins and the continental shelf” provides improved knowledge, toolsand models to assist the development of strategies to plan for andmitigate against hazards associated with storms, coastal flooding,inundation, erosion and tsunami events.

A seawall left battered by theJuly 1978 storm at Omaha onthe east coast of Auckland.

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