Coastal Sand Dunes Form and Function

Introduction
Importance of Coastal Sand Dunes
Patterns and Processes
Blowouts
Parabolic Dunes
Transgressive Dunefields

Introduction

Sand dunes are formed at the interface between the sea and land. They are formed from sand which is eroded and ground rock, derived from terrestrial (e.g., glacial or river) and oceanic sources (e.g., coral reefs). The form of dune systems will be dictated by a number of factors, including the shape of the coastline, shape of the beach, currents and swell of the ocean, prevailing wind, frequency of storm events, and particle size of the sand.

One of the most important things to realise with the coastal and dunal environment is that it is dynamic, like the forces that shape it, so these environments are always changing. Accepting change can be a difficult for humans to accept (such as the common perception that damage to established forest in storms is bad) but it is important to acknowledge that change is a natural and health part of this ecosystem, and helps to maintain biodiversity.

Due to the extensive coastal development that characterise much of our beaches now, stabilisation has been a large focus. This has lead to the extensive planting of marram grass that form high densely covered dunes that prevent most sand moving beyond the foredunes, affording protection to developed areas. This form of planting has proven successful for human habitation but has come at the expense of the native sand binders and associated plants. This section intends to outline the values associated with the coastal dune environment as well as the processes associated with dune building and the dynamics of the different forms.

Importance of Coastal Sand Dunes

Natural Character

They are part of the natural character of the coastal environment, which is protected under the Resource Management Act 1991 as a "Matter of National Importance". It is important to note the natural character refers to the unmodified coastal environment. Therefore, human development is generally regarded as a denigration of this natural character.

Maintaining the natural character (including the natural vegetation and morphology) also aids in shielding human development from view, so management and maintenance of coastal dunes is important in continuing this function, while protecting their natural character.

Water Quality

Coastal sand dunes act as filters for water, enhancing and maintaining coastal water quality. The denigration of coastal dunes by draining and pollution of coastal wetlands, as well as the disposal of sewerage. These impacts may be ameliorated by the protection and revegetation of the coastal dune landscape as well as the protection and enhancement of dune wetlands.

Coastal Protection

As mentioned above, coastal dunes gives protection to the land behind, acting as a buffer against eroding wave action. Therefore, dune structural integrity must be maintained to ensure their ability to provide this form of protection. This integrity can be maintained by ensuring vegetation cover is not reduced or damaged and that repair to dunes following erosion or blowouts uses replanting and sand fences to aid natural repair as opposed to using concrete or rocks, which lead to further erosion problems (due to end wall effects).

Cultural and Archaeological Values

The dune environment has significant cultural and archaeological values arising from the occupation of Maori in the past. Those areas where occupation is known provide a valuable source of pre-european records relating to human settlement as well as serving as reminders of our history. These areas can also contain urupa (ancient burial grounds) which are regarded as waahi tapu (sacred). Consequently it is important that these areas are retained, and protection can only be afforded properly if there is an accurate understanding of coastal processes.

Patterns and Processes

There are a number of different kinds of dune system, and specific features of dune systems that can be identified.

Incipient Foredunes

These are the newly developing dunes, initiated by wind blown sand being trapped by vegetation and other obstructing matter such as driftwood.

This is the area where sand-binders such as pikao thrive as they are well adapted to this highly variable environment.

Incipient foredunes are not necessarily permanent features that develop into established foredunes, and there fate will be decided by factors such as wave action.

Established Foredunes

Established foredunes develop from incipient foredunes by steadily growing with sand accretion on the seaward side. The lee (landward side) becomes more stable and protected from salt spray and sand deposition and nutrients increase. This supports the colonisation of intermediate species (and stabilise the foredune further).

The development and morphology of foredunes will depend on a variety of factors and include:

• Sand supply
• Degree of vegetation cover
• Plant species present
• Rate of wind blown sand accretion and erosion
• Frequency and magnitude of wave and wind forces and erosion
• Dune scarping and and overwash processes
• Beach-surfzone type
• The medium to long term beach state (stable, accreting or eroding)
• The extent of human interference and use
• The resultant dunes will range from well vegetated and stable to highly erosional.

Backdunes

Backdunes are those dune located beyond the established foredunes and are typically more stable and covered with intermediate woody and herbaceous species. They are still subject to sand accretion and erosion processes but to a lesser extent. Eventually these dunes will be succeeded and covered with woody shrub and tree species forming coastal forest.

Blowouts

These are depressions in the sand that are cup- saucer- or trough-shaped. They are caused by wind erosion of an existing sandy substrate. Cup/saucer shaped blowouts are shallow dishes and usually occur on flat/low dune terrain, whereas trough shaped blowouts are deeper with steeper erosional walls and are more elongated. See the illustration below.

There are three characteristics of blowouts:

• Depositional Lobe; where sand from the walls and deflation zone is being deposited
• Deflation zone/basin; where sand has been scoured out. Often these reach the water table or the basal material (rocks). Both these factors prevent further erosion and revegetation often occurs.
• Erosional walls; margins of the blowout.

An incipient foredune will often form across the throat of a blowout, reducing sand inundation and allowing revegetation.

Parabolic Dunes

These are inverted U-shaped (parabola) or V-shaped dunes when viewed from the air. They have trailing arms that extend seaward, with a despositional lobe at the bottom of the U or V. They can be formed from blowouts or from the migration of sand at the landward end of a dune field where discrete lobes may form.

The development and migration of parabolic dune will depend on vegetation cover, species and type (woodland or grassland), wind velocities and directional variability of the wind, and dune size.

Stage one: The foredune vegetation is reduced by storm wave erosion.

Stage two: Erosion continues, the deflation basin expands, the depositional lobe advances downwind, and a parabolic dune develops.

Stage three: The foredune reforms across the parabolic dune throat. The parabolic continues to advance downwind forming elongated trailing ridges.

Transgressive Dunefields

Transgressive dunes are also known as mobile or migratory dunes, and sand drifts.

Transgressive dunefields will are generally located where wave energy is high and sand supply is or was moderate to high, or where significant coastal erosion has occurred.

Transgressive dunefields can be unvegetated, partially vegetated or completely vegetated (relict) and the dunes often have a sinuous/fish scale shape.

Good examples of transgressive dunefields can be found in Northland,Farewell spit, Manawatu-Wanganui region and Raglan. Often these have been stabilised with marram grass planting.

Information contained in this page is based on:

• Coastal Sand Dune Form and Function. Patrick A, Hesp. CVDN Technical Bulletin No. 4. Forest Research, Rotorua.
• This is recommended as essential reading and details of how to get a copy can be obtained from Touchwood Books.