7.3 What is the state of coastal hazards?

 

Natural shoreline dynamics

Shoreline ‘state', that is the trend in the location of the shoreline, can generally be classified into three categories:

· Eroding (shoreline is retreating landward)

· Equilibrium (shoreline is generally stable)

· Accreting (shoreline is extending seaward)

These three states are related to five key factors, namely, sediment supply to the beach, wave energy, sea-level change, location of shoreline and man-made modification to shoreline (i.e. dune system). Twenty-four beaches throughout Northland are monitored on a regular basis, as shown in figure 1 (below). An estimation of their long-term shoreline state is presented in table 3 (below).

Northland's west coast beaches generally display a long-term trend for equilibrium or accretion, with high to moderate sediment supply, high wave energy and largely unmodified shorelines. There are exceptions to this, such as embayed harbour beaches like Omapere which display a long-term trend for erosion, due to variability in sediment supply and extensive man-made modification to the shoreline.

Map of Northland showing beach monitoring sites.The state of Northland's east coast beaches is more diverse, due to the greater shoreline complexity and more extensive shoreline modification. The majority of east coast beaches tend to be in an equilibrium state, for example Oakura, Taipa and Taupo Bay. There are also a number of eroding east coast beaches, including Tauranga Bay and Pataua South. Erosion of these systems appears to be largely a function of natural fluctuations in sediment supply coupled with extensive modification of the foredune system.

Figure 1: Beaches included in the beach profile monitoring programme.


 

Table 3: Beaches monitored as part of the beach profile monitoring programme, with their estimated long-term shoreline state. Note: Numbers correspond to those in figure 1.

Number in figure 1 Beach Record commenced Generalised shoreline state
1 Mangawhai spit 2000 Equilibrium
2 Bream Bay 1976 Accreting
3 Pataua South 1998 Eroding
4 Wellingtons (Whangaumu) 1998 Eroding
5 Ngunguru spit 1998 Equilibrium
6 Matapouri 1998 Eroding
7 Whananaki spit 1998 Equilibrium
8 Te Mimiha (Helena) 1999 Equilibrium
9 Teal (Ngawai) 1999 Equilibrium
10 Oakura 1998 Equilibrium
11 Ohawini 1998 Equilibrium
12 Te Ngaire 2002 Equilibrium
13 Tauranga 2002 Eroding
14 Taupo 1999 Equilibrium
15 Hihi 1999 Eroding
16 Coopers Beach 2003 Equilibrium
17 Cable Bay 2003 Equilibrium
18 Taipa 1990 Equilibrium
19 Tokerau 1990 Equilibrium
20 Rangiputa 1999 Eroding
21 Ahipara 1999 Equilibrium
22 Omapere 2002 Eroding
23 Glinks 1994 Accreting
24 Pouto 1989 Equilibrium

Note: The above are indicative observations only. Shorelines may display several states (i.e. erosion and accretion) in different sections of the shoreline.

Recent shoreline dynamics

In the period following the damaging easterly storms of July 2000, the majority of Northland's east coast beaches have been in an accretionary phase. This is consistent with the moderate to weak El Nino conditions that have predominated over the past five years. This phase has enabled dune systems to rebuild and grow seaward under the existence of favourable ‘dune building' conditions, as shown for Ruakaka Beach in figure 2 (below).

Those areas where the foredune complex and native vegetation have been extensively modified are an exception to this trend. In these areas modification of the foredune significantly reduces the ability of the dune to retain sand and grow seaward during accretion phases. As a result the erosion dominates over accretion, causing a long-term trend for shoreline erosion.

Erosion at Whananaki North beach in July 2000.Whananaki North beach in March 2006 showing how dunes have been rebuilt by nature.

Erosion scarp to Whananaki North beach in July 2000 following erosion (left) and same location in March 2006 following the predominance of weak El Nino conditions demonstrating beach and dune rebuilding (right).

Figure 2: Beach profile survey data for Ruakaka Beach, Bream Bay. The extent of beach rebuilding is apparent in the surveys following the damaging easterly storms of July 2000.

Coastal flooding

Wave-assisted coastal flooding occurs when high tides coincide with intense low-pressure weather systems causing storm surge and wave run-up to temporarily inundate low-lying coastal hinterlands.

Many factors contribute to the potential risk of coastal inundation, including extent of low-lying land, degree of exposure, beach slope, near-shore sea-bed shape among others. A number of Northland coastal communities are exposed to coastal flooding. Projected sea-level rise associated with climate change to year 2100 is likely to substantially increase the extent of areas subject to inundation. This is particularly the case for locations with low topography, particularly low-lying barrier-spit systems and progradational coastal lowlands.

Storm surge at Ngunguru Estuary in July 2007.Debris on the beachfront at Teal bay after a storm in 1989.

Moderate storm surge event in Ngunguru Estuary in July 2007 (left), photo courtesy of Gloria Bruni, and debris from storm wave run-up causing temporary inundation of low-lying properties at Teal Bay in August 1989 (right).

Tsunami Hazard

A tsunami is a natural phenomenon that results when a large volume of water is displaced causing a series of waves to be generated, most commonly due to earthquakes causing seafloor displacement. Tsunami can devastate coastal communities causing inundation, strong currents, contamination and other effects. Tsunami generally occur as a large, broken low wave or mass of water.

The post-historic record shows that New Zealand has been affected by more than 40 tsunami in the last 150 years. Of these, four moderate tsunami inundated Northland's east coast. One of these events is documented in figure 3 (below). The prehistoric record indicates at least one large event, or a series of large closely-spaced events, have affected Northland's coast in the last 600 years, plus others.

Figure 3: Newspaper article following the moderate 1960 Chile tsunami, sourced from The Northern Advocate.

Tsunami hazard is considered a significant risk for Northland and could affect a number of Northland's coastal communities. A generalised tsunami hazard-risk model for Northland indicates that a moderate hazard and risk exists for most of the northwest and east coast, a high hazard and moderate risk for the north, and a low hazard and risk for the west. The hazard is largely a function of tsunami source, intensity and return period.

Sources from near the South American coast are the most likely to cause a moderate event that would affect most of Northland's east coast, as shown in figure 4 (below), with an annual exceedance probability of around 1 – 2 %.

Figure 4: Maximum water surface elevations modelled for a tsunami generated from a magnitude 9.0 earthquake located near the Chilean coast (Image courtesy of NIWA).

Hazard zones

Coastal hazard zones (CHZ) identify areas of coast that may be subject to coastal hazards, usually erosion and/or coastal flooding. CHZ enable local authorities to better plan and manage coastal development in areas susceptible to coastal hazards by providing for building set back and/or restrictions on the type of development that is permitted. CHZ have been derived for more than 24 coastal settlements in Northland and incorporated in the district planning maps.

To assess the state of coastal hazards, based on the amount of property and development within CHZ, a GIS-based analysis was undertaken of coastal communities within the zones to determine:

· The number of properties and dwellings in coastal hazard zones.

· The capital values of properties in coastal hazard zones.

· The setback distance from the coastline of each dwelling located in a coastal hazard zone.

Approximately 638 dwellings are located within or partially within identified coastal hazard zones, indicating a probability of exposure to risk from coastal hazards (i.e. erosion and/or coastal flooding) within a timeframe of the next 50 to 100 years.

Mean setback distances of dwellings within hazard zones range from 11 metres at Matapouri Beach to a maximum of 89 metres at Tokerau Bay shown in table 4 and figure 5 (below).

Table 4: Summary of setback distances and number of dwellings within identified coastal hazard zones.

Community Mean setback of dwellings from shoreline (m) Number of dwellings in CHZ 1 Number of dwellings in CHZ 2 Total number of dwellings in CHZ's
Teal Bay 11 8 10 18
Matapouri 12 18 1 19
Tauranga Bay 13 4 2 6
Cable Bay 14 2 0 2
Taiharuru 17 7 1 8
Ohawini 18 7 8 15
Pataua Estuary 20 22 28 50
Taupo Bay 21 30 5 35
Te Ngaire 21 13 3 16
Oakura 23 48 33 81
Whangaumu 25 3 12 15
Omapere 26 17 15 32
Pataua North 35 22 15 37
Taipa 36 0 28 28
Langs Beach 38 4 0 4
Rangiputa 38 16 25 41
Coopers Beach 39 21 14 35
Opononi 41 0 21 21
Ahipara 45 25 11 36
Bland Bay 53 0 4 4
Helena Bay 59 2 2 4
Hihi 68 1 16 17
Ruakaka 77 1 5 6
Tokerau 89 1 107 108
Totals 272 366 638

 

Figure 5: Mean setback distances of dwellings from high tide within designated coastal hazard zones and total number of dwellings within coastal hazard zones.

Approximately 1,092 properties are located within, or partially within, identified coastal hazard zones. The total capital value of these properties is approximately $453 million. The market value of these properties is likely to be much higher.

Total capital values of properties for communities within identified coastal hazard zones ranged from $1.2 million at Helena Bay to $49.9 million at Oakura Bay shown in figure 6 and table 5 (below).

Figure 6: Total capital values (based on 2006 data) of properties within hazard zones and number of properties within coastal hazard zones.


Table 5: Capital value of property located within coastal hazard zones.

Community Property count Total capital value ($) Mean capital value ($)
Helena Bay 5 1,181,500 236,300
Tauranga Bay 14 2,480,000 177,143
Taiharuru 8 3,590,000 448,750
Cable Bay 8 3,929,400 491,175
Ruakaka 9 4,254,000 472,667
Ohawini 18 8,422,000 467,889
Opononi 39 9,401,000 241,051
Teal Bay 22 10,020,000 455,455
Te Ngaire 20 12,817,000 640,850
Hihi 58 13,729,500 236,716
Bland Bay 28 16,475,000 588,393
Wellingtons 33 18,459,000 559,364
Langs 24 19,136,000 797,333
Omapere 74 19,471,000 263,122
Rangiputa 54 20,891,200 386,874
Pataua Estuary 63 21,270,000 337,619
Taipa 56 21,827,000 389,768
Matapouri 24 25,054,000 715,829
Ahipara 89 25,576,900 287,381
Pataua North 46 28,545,000 620,543
Taupo Bay 55 30,506,500 554,664
Coopers Beach 71 38,203,000 538,070
Tokerau 181 47,612,500 263,052
Oakura 93 49,860,000 536,129
Sum 1092 $452,711,500