Appendix 3: The response of different coastline types to coastal hazards
A3.1 Open-coast sand beaches
A sand beach system generally comprises compartments such as dunes, the backshore (normally not reached by tide), the intertidal beach (foreshore) and sand in the shallow nearshore, including the nearshore bar (where most of the wave breaking occurs). Sand is exchanged between these compartments by the wind and waves. Sand exchange between the dunes and the beach is retarded by the growth of vegetation, which traps material. Sand can also be delivered to the beaches by rivers and streams, from cliff erosion, from neighbouring coastal areas by wave-driven littoral drift, from the breakdown of shells, and reworked ashore from offshore seabed sediments.
Prior to human intervention, sand dunes on beaches provided good buffers against both coastal inundation and erosion hazards. Where dunes have been removed by development there is little natural defence (or 'buffer') against coastal erosion and inundation. Developments behind the foredune also prevent the dune from migrating landward (even temporarily); hence on a beach experiencing long-term erosion, the size of the natural buffer is continually reducing.
Removing vegetation leaves bare sand, which is prone to wind erosion. Removing native sand-binding species and replacing them with introduced sand binders, predominantly marram, produces a higher and steeper 'dunescape' which can cause 'blowouts'.
In some open coast locations, the foredune has been removed completely. These 'foreshore only' systems have little natural protection against erosion and inundation. As a consequence, coastal protection measures are often installed, including seawalls or rock revetments. These structures often do not dissipate the energy of wave run up. Instead they cause increased turbulence at the toe of the structure, which in turn causes increased scour and beach lowering. They also prevent the foreshore migrating landward to capture additional sediment, and in some locations have resulted in the total loss of the beach (e.g., Sumner, Christchurch).
Sand spits and sand barriers warrant special consideration from a coastal hazards perspective, particularly when they are narrow. New Zealand has many examples e.g., Ohope/Ohiwa, Omaha, and South Brighton. The end of the sand spit or barrier is very transient, even under normal tidal conditions, but particularly during storms and at times of river floods. Sand spits are among the most dynamic and changeable landforms found on the planet.
Beach and dune erosion is exacerbated locally at stream mouths or stormwater outlets due to both direct erosion as the stream meanders with time, and indirectly because the high water table in the beach sediments makes them more susceptible to erosion.
Long term and storm-induced erosion (sandy beaches)
At longer time scales, slow rates of shoreline change (0 to 1 m/yr) are most common. Rates of long-term erosion greater than 2 m/yr are less common, and long-term rates greater than 5 m/yr are rare (except for sand spits). Although over the long term there may be little change in shoreline position, movements over medium-term timescales (e.g., a couple of years up to decades) can be large and rapid. The most noticeable place where this occurs is on sand spits, where movements of hundreds of metres can occur in a matter of years. For example, the end of the Brighton Spit in Christchurch retreated 500 m in 9 years between 1940 and 1949. Coastal erosion of sandy beaches is also often serious with 'foreshore only' beaches that are tightly squeezed or 'bounded' by development or coastal protection works.
Short-term erosion occurs as a result of individual storm events, or multiple storm events over periods of weeks or a few months. These short-term responses need to be added to the long-term changes to assess total susceptibility to erosion.
Inundation caused by storms
Sand beach foreshores tend to be very flat, with slopes of 1:20 to 1:50 being common. Depending whether the beach has recently been in an erosional or accretion phase, the upper limit of these foreshores tend to be in the order of 2 to 4 m above MSL. Even in an accreted state, moderate storm run-up will run over the foreshore to the sand dunes or what ever lies behind the foreshore. Therefore 'foreshore only' shorelines are very vulnerable to coastal inundation.
Sand dunes are much steeper than the foreshore, so run-up onto dunes is generally less than on the foreshore. Nevertheless, run-up on dunes can reach more than 6 m above MSL. It is also important to determine whether 'blowouts' are present along the dune, since wave run-up can surge through these gaps and flood low-lying areas behind the dunes.
Figure A3.1: Waves overtopping the shingle/sand barrier at East Clive, south of Napier, during a storm in August 1974. [Source: Ministry of Works and Development collection, Napier.]
In situations where beaches are bounded by hard artificial structures, protection from coastal inundation is dependent on the height, slope and type of structure. In general, the lack of dissipation of energy from these structures results in higher wave run-up than would occur on natural beach materials.
Inundation and erosion caused by tsunami
Our understanding of the effects of tsunami is less than that for storm effects, simply because we have had less modern experience with moderate to strong tsunami. Note: small tsunami are relatively frequent (see Appendix 2). However, the greatest impact of tsunami inundation and erosion will be felt on low-lying margins behind sand beaches, gravel ridges and around estuaries and inlets. For sand beaches and gravel ridges, tsunami will run-up the beach to elevations well in excess of the offshore tsunami wave height. Also for a tsunami event with wave heights over 2 to 5 m, the sheer volume in each wave crest and the speed with which the water moves (up to 35-40 km/h) will cause erosion of dunes and beach ridges that will be more extreme than in storm events. Hence the greatest vulnerability is to low lying hinterlands behind narrow coastal barriers of less than 10 m in elevation.
A3.2 Open coast gravel beaches
There are two types of gravel beaches, one where a distinct upper-beach ridge separates the foreshore and the backshore, and the other where the beach consists of only a foreshore slope. The material on gravel beaches is transported only by wave run-up processes, hence these beaches tend to be narrower and lower than sand beaches (where wind processes are also important for beach building). As a consequence, the height of the gravel ridge is limited by the magnitude of past storm wave run-up and the supply of sediment available to build the ridge.
In many locations, there is insufficient material to build a ridge to the full height reached by storm run-up. In these circumstances, storm waves overtop the barrier, resulting in inundation of the low-lying hinterland. Overtopping also results in gravel being 'rolled over' the ridge crest, resulting in the landward retreat of the whole beach profile. In this erosion process, beach volumes are retained, but beach heights are lowered, resulting in the potential for the process to be repeated more frequently.
In locations where stopbanks have been constructed, these have often been buried by the retreating beach ridge, rendering them ineffective. This results in greater offshore sediment losses and increased run-up, accelerating beach retreat and further increasing the risk of inundation. Along the Seadown coast, north of Timaru, three rows of stopbanks have been buried by the retreating gravel barrier over the last 70 years.
Because of the high permeability of gravel beach ridges, flow directly through the beach face can also occur, often causing the crest to 'blow out' as the landward face of the crest is undermined.
Figure A3.2: Attempted armouring of the gravel/sand beach at the mouth of the Orari River, South Canterbury (late 1950s). (Source: D. Todd).
'Foreshore only' gravel beaches occur where the beach has insufficient width for the development of a ridge profile. Where there is insufficient width or elevation to provide the required level of protection, artificial barriers such as seawalls or rock revetments are often constructed at the back of the beach. An example of this is the protection works on State Highway One along parts of the Kaikoura coast. This artificial 'bounding' of the beach often results in decreased dissipation of storm wave run-up and increased turbulence at the toe of the structure, which in turn causes increased scour and beach lowering in front of the structure, further reducing the effective width of the nature buffer system. Ultimately, protection of the shoreline against erosion and inundation becomes totally dependent on the artificial structure.
Gravelly spits enclose shallow elongated lagoons at the mouths of rivers in some locations (e.g., Ashburton River). These low narrow spits are built by wave-driven up-coast drift and are very unstable. They are overtopped in large seas, and during floods the river will burst through the spit (barrier) and straight out to sea.
Because gravel beaches are steeper and coarser than sand beaches, run-up should theoretically be less than for sand beaches. However, there are many examples of gravel ridges up to 6 m high being overtopped by storm wave run-up. The reduction in ridge crest elevation as a result of these failures results in large areas of hinterland being exposed to greater inundation hazards. A storm in South Canterbury in July 2001 resulted in over 1100 hectares of land being inundated by a combination of overtopping and beach failure.
Figure A3.3: Damage caused to a coastal property at Haumoana (Hawke's Bay) by waves overtopping the gravel barrier in the Easter storm of 3-4 April 2002, assisted by high perigean-spring tides. [Source: Hawkes Bay Regional Council].
A3.3 Cliffed coastline
Where the rocks of cliffs are hard and strong, such as the metamorphic cliffs of Fiordland or the hard volcanic rocks of the Banks Peninsula and parts of the Auckland coast, the susceptibility to erosion at management timescales is very low. However, with softer sedimentary rocks the rates of cliff retreat are often up to 1 m/yr, and where the rocks are poorly compacted, badly weathered, closely jointed, sheared, or faulted, long-term retreat can be 2 m/yr.
Erosion generally occurs during storms when elevated sea levels and large waves attack the base of the cliff, resulting in undermining and failure. Cliffs also erode slowly under wetting and drying processes.
Many sea cliffs, particularly those formed in harder rocks, have either inter-tidal wave cut shore platforms at the base of the cliff or nearshore reefs. These platforms can modify vulnerability to erosion by either reducing wave heights at the shore, or possibility focusing wave energy to one spot. Soft coast cliffs, particularly alluvial outwash fans, may have beach deposits at the base of the cliff, which can reduce the frequency and intensity of wave run-up attacking the cliff face, hence reducing their vulnerability to erosion.
Other processes that can affect the stability of cliffs are vegetation cover (positively or negatively), rainfall runoff, stormwater discharges and seismic instability.
A3.4 Estuaries
In many situations, rural and urban development has occurred right up to the edges of estuaries, and the shore have been bunded, removing the important transitional area at the margin of the estuary. As a result, coastal hazards exist around many estuaries.
In estuaries where processes are driven largely by the tides and river inflows, the hazards of flooding and erosion will be greatest at the mouth and in the headwaters. Inundation is maximised when the estuary is surrounded by low-lying land and extreme tides combine with river floods. While wave energy is inhibited inside most estuaries, erosion of estuary banks still occurs, particularly in the soft sediments often found around the margins of low-lying estuaries. Changes in the location of channels can also result in significant erosion of estuary banks. In many cases, urban and farming development of the margins of these shallow estuaries has resulted in artificial barriers being placed around the edges of estuaries, to provide protection against both inundation and erosion. Large estuaries (e.g., Manukau Harbour) have fetches great enough that winds generate sizeable waves that attack the shoreline. Ocean swell can enter the mouths of large estuaries at high tide and erode the shore (e.g., Kaipara Harbour).
Erosion
The long-term movements of estuary shorelines are generally poorly recorded and difficult to quantify. However, estuary shorelines are generally less vulnerable to erosion than open coast shorelines, due to the low energy of the erosion drivers present. Also, in general, sedimentation rates in the main basins of estuaries have been keeping pace, or surpassing, the contemporary rates of sea level rise of about 2 mm/yr. As a consequence, estuaries do not have sediment deficit. Therefore significant long-term erosion of estuary shorelines is mainly limited to changes in channel patterns in the estuary, the causes of which are complex.
Inundation
Flooding of low-lying coastal land about the shores of estuaries is common, particularly where wetlands have been reclaimed for farmland and the ability to accommodate additional water in the shallow basins is limited. Although wave heights inside estuaries are often limited due to the shallow water depths and short fetch lengths, waves can make a significant contribution to inundation when strong winds combine with extreme water levels. Ocean waves overtopping narrow sand or gravel barriers can also significantly increase water levels in lagoons.
Tsunami
For large inlets, the topography may increase the amplitude of the incoming tsunami waves, due to resonance. For example, it is believed that amplification in Lyttelton Harbour during a 1960 tsunami resulted in higher tsunami elevations that on the open coast. Tsunami can also scour the entrances of tidal inlets, causing long-term changes in the tidal compartment.
