Adaptive Management Responses on a Sediment Dynamic Coast in Auckland.
Sam Morgan, Paul Klinac and Jamie Boyle
The majority of beaches along the north-western coast of New Zealand’s North Island are subject to large scale fluctuations or pulses in sediment supply. Little is known of the exact drivers or nature of these fluctuations but they can be in the order of millions of cubic meters and last over several decades. Subsequently there are a number of recognised management issues, ranging from erosion to accretion, water quality, wind borne sand drift and coastal hazard setbacks, right along the extent of this coast. This paper will focus on an example of adaptive management within the Auckland Region as a response to managing the uncertainity around changes in sediment supply.
In December 2014, realignment of the Wekatahi and Marawhara Streams on North Piha Beach and subsequent dune restoration was completed for the third time in 10 years. Specific works included stream realignment, dune reshaping and planting in an effort to restore and stabilise a dune buffer. These works sought to alleviate the risk of undermining of adjacent roading infrastructure as the streams were pushed south by a body of sand being deposited in the wider Piha coastal cell. Future works based on results of monitoring will focus on building the dune between the two streams seaward forcing the confluence point away from critical infrastructure.
This example of an adaptive management response seeks to minimise future risk to infrastructure, whilst addressing some of the outstanding concerns within the local community. These include objections around the requirements for physical management vs. more traditional responses, through to the degree of response which is required.
Little detail is known about the nature of sediment supply and transport along the north-western coastline of New Zealand’s North Island (Figure 1). There are apparent dynamic relationships between sediment supply, coastal processes and large scale geomorphology. Fluctuations in these large scale patterns can result in extended periods of accretion and erosion.
These large scale patterns and behaviours inevitably lead to varying management issues. Erosion of the coast between Manukau Harbour (Karioitahi Beach and Huia) and the Waikato River mouth is threatening coastal infrastructure including community facilities such as Surf Life Saving Clubs. Conversely, large scale accretion between Whatipu and Piha has led to a number of issues, perhaps most notably being the impoundment of streams leading to problems with access and water quality.
This paper will examine management of the Wekatahi and Marawhara Streams on Auckland’s West Coast as an example of adaptive management strategies employed to allow for the uncertainty of larger scale coastal processes. A background to the understanding of the large system is first provided to give context before details around the project are given. A discussion in the application of adaptive management principles will serve as the papers synthesis.
2. Coastal Setting
Sediment transport dynamics along New Zealand’s upper West Coast are not well understood with studies limited by the coastlines scale, remoteness and relative high energy. Sand is thought to be carried back and forth along the coast via interplay between open ocean swell travelling up from the Southern Ocean and locally generated weather patterns, with the predominance of Southwest swell events leading to a net northerly transport of material (Brander et. al., 2003). Sand is thought to be sourced mostly form the Mt. Taranaki volcanic zone to the south, and to a lesser extent localised erosion. The Waikato River (and subsequently the central volcanic zone) is also thought to have been a substantial contributor of sand prior to modification (Hume et al., 1999). Estimates of the quantities being transported are not well understood, and published rates varying between 175,000m3yr-1 and 1-5 million m3yr-1 (Brander et. al., 2003).
The west coast of Auckland, New Zealand, encompasses a littoral cell supplied by abundant sediment supply connecting the embayed and open-ranging beaches (Hamill and Balance, 1985). For the most part, research on sediment dynamics on Auckland’s west coast has been conducted within the multi-decadal time scales, utilising cadastral and hydrographical maps and charts, and aerial photos. Brothers (1954) conducted the first shoreline analysis from Anawhata to South Kaipara, focusing on the Muriwai section of the coast. Historical charts were combined with surficial morphological analysis for the interpretation of successive dune belts and accretion episodes following the Holocene stillstand. Using similar methods, Wright (1969) and Williams (1977) mapped historical shoreline movements at North and South Kaipara, and Whatipu beach respectively. Blue (2009) mapped shoreline positions from aerial photographs within GIS on Whatipu, Karekare, Piha and Te Henga (Bethells) beaches.
Research by Schofield (1975) and Hilton (1982) on South Kaipara barrier has provided the foundations for understanding coastal change over more extensive spatiotemporal scales. Building on the work from Brothers (1954), Schofield (1975) incorporated deep core soil samples, offshore sediment details and land survey contour maps to correlate the succession of five large dune belts with minor fluctuations in sea level. Hilton (1982) also used deep core drilling to determine the subsurface facies, permitting detailed mapping of the stratigraphic sequences.
However, a lack of chronological agreement has left a gap in determining the age of depositional sequences, and thus any meaningful understanding of the patterns of sand storage. Because erosion episodes are also preserved within prograded systems, core interpretation may miss key details of processes that have contributed to coastal change.
With the advent of Ground Penetrating Radar (GPR) technology and lengthening shoreline databases, research from Dougherty (2011) and King (2001) have moved closer to understanding how short-term morphodynamics relate to long-term evolution. Dougherty (2011) used GPR to map stratigraphy of the prograded barrier at South Kaipara, and was able to determine the reactivation of transgressive dunes over a flat lying sand sheet. This provided an additional sequence to Hilton’s (1982) model, suggesting a third evolution stage where transgressive migration contributed to the antecedent topography for successive accumulations. In the context of Large Scale Coastal Behaviour, King (2001) has provided perhaps the most insightful investigation of coastal change across short-term, multi-decadal and late-Holocene timescales at the bedrock embayed beach of Piha. Using beach profile data, historical aerial imagery, and GPR, King (2001) determined both the volumetric storage over the late-Holocene and the styles of deposition that have contributed to shoreline change.
Although somewhat harder to observe beyond contemporary timeframes, these fluctuations in sediment supply have been known to operate in the opposite manner with sediment deficits leading to prolonged periods of erosion. Muriwai Beach is perhaps the best understood of the number of cases, where a loss of up to 65m of seaward dune face has been reported (Dahm, 2002). This is supported by the available monitoring data which shows a dramatic drop in beach volumes between the early 1980’s and the early 1990’s (Figure 2).
3. Retraining of the Marawhara and Wekatahi Streams, North Piha
3.1 Project history and background
Piha Beach is located 30km west from central Auckland on the seaward fringe of the Waitakere Ranges. It is a high energy dissipative beach that is approximately 2.8km in length and separated into two sections by Lion Rock. As of the 2006 census (Statistics NZ, 2006), there are 630 permanent dwellings with 97 of these located within close proximity to the shoreline at North Piha.
The confluence of the Marawhara and Wekatahi Streams is located approximately halfway along North Piha beach (Figure 3). Historic modifications, including stream diversion and reclamation, combined with an estimated ~700,000m3 of sand accumulation within the wider system ( King, 2001) have resulted in the southward migration of the northern dune field (see Figure 4). This in turn influences the directional flow of the Marawhara Stream and ultimately the confluence point of the two streams. Without intervention the combined and reinforced stream flows can ultimately cut toward the road connecting the northern section of the settlement.
Resource consent was obtained in 2003 to redistribute sand on Piha Beach in order to separate the Wekatahi and Marawhara Streams outlets, with the stated intent for the re-alignment being to prevent erosion from destabilising Marine Parade North, the access road to North Piha. Initial physical works were completed in 2004 and involved separating the two streams by constructing new channels and placing the ‘spoil’ sand on the adjacent eroded dunes and old stream beds. Where sand was deposited to repair eroded dunes, planting programmes were established to aid in retention of sand. Future maintenance of the streams was permitted within the resource consent and allowed for up to 5,000m3 to be shifted on an annual basis.
3.2 Project development and community feedback
Detailed monitoring of the project was initiated in 2011 following an observed acceleration in the southward track of the confluence streams mouth. Monitoring involved photo capture from repeated points and profile surveys over the subject area. The intent of the monitoring programme being to investigate causal processes behind the rapid stream migration and the establishment of trigger points to assist with determining the frequency and requirement for future physical works.
Significant erosion has also been documented between the Waikato River and the Manukau Harbour leading to management issues. In particular the seaward location of Surf Life Saving Clubrooms at Port Waikato and Karioitahi has raised debate around the seaward positioning of such high value assets on dynamic coastlines. Further, a change in coastal dynamics at Huia in the outer Manukau Harbour has seen a substantial lowering of beach levels, placing the coastal reserve at risk to increased levels of erosion and inundation. This has subsequently threatened existing infrastructure valued by the community. However, a lack of detailed understanding around larger coastal dynamics has complicated the management decision making process.
Trigger levels were initially set to allow significant retreat of the southern dune bank of the Wekatahi (as a buffer), with the intention of allowing the maximum amount of time for the streams to meander naturally. These triggers were met in early 2012 and physical works were undertaken in May of that year. Deposition of approximately 5,000m3of sand focused on the southern dune bank and infilling of the old stream beds, and dune planting was again undertaken to help stabilise the area. Figures 5a and 5b illustrate the potential risk posed by the southerly migration of the streams and the nature of physical works being undertaken.
A revision of the work programme was undertaken in 2013 following opposition to the acceptance of regular dune loss, promoted as a sacrificial buffer to infrastructure, was raised from within the local community. Through this process further investigation of the site specific processes was undertaken in order to understand the drivers behind the southerly migration of the two streams and subsequent erosion of the southern dune bank.
From this investigation a combination of the following three environmental factors was considered to influence dune stability at the site and induce the joining of the two streams:
- High rainfall events: The amount of rain received within the relatively small and steep catchment influences channel flows at the mouth. Strong flows at the mouth can help to maintain a direct channel to the sea. However, once the streams have joined and trained south, large stream flows tend to cut into to the southern bank/dune and lower adjacent beach levels at the mouth.
- Wave climate: Periods of low wave energy and related processes can induce accretion of sand on the upper beach that then becomes available for wind transportation (see below). Periods of high wave energy will lower sand levels across the beach, leaving the stream beds subject to greater wave run up and the dune toe vulnerable to wave attack.
- Sand drift: Accumulation of sand on the northern side of the dune break assists with pushing the Marawhara Stream south. It is assumed this is driven by Aeolian processes and the predominant south west wind raising the vertical height of the upper beach and making realignment seaward more difficult.
However, there is still some ambiguity over the interplay and timing of interactions between these factors. Trigger levels were reset with this in mind, and consideration given to the scale of reparative work required should the streams be allowed to migrate to far south. The monitoring programme was adjusted to incorporate using RTK GPS to track dune movements, and enable better quantification of changes at the site. The amended monitoring plan sought to provide an increased width of dune retention, with an agreement that trigger levels could then be adjusted following an improved understanding of beach morphology. The rational for revised trigger levels and monitoring was communicated to the wider Piha Community. Evaluation of feedback confirmed (that on balance) the proposed trigger levels satisfied the majority of substantive concerns raised.
3.3 Current setting and future works
Rapid acceleration in stream migration was observed through 2014 and prescribed trigger levels were again met. Physical works were undertaken in December 2014 with approximately 2,500m3 of sand shifted, with deposition of the spoil focusing of the dune area between the two streams. This was based on observations from the photo monitoring record that had noted the seaward migration of the central dune feature between the two streams. The intent of this change in approach was to minimise the scale of physical works and further encourage the seaward growth of the central dune.
Recent monitoring observations have identified a significant accumulation of sand on the upper beach between December 2014 and May 2015. This has effectively impounded both the Marawhara and Wekatahi Streams, apart from some minor movement behind and beyond the high tide berm which has built up. This has raised concerns from within the community with respect to the effectiveness of recent works. However, monitoring data suggests there is an abundance of sand within the wider subject area (Figure 6), and this was able to be effectively communicated to the interested parties.
Future works will continue to focus on building out the central dune area between the two streams, as noted above. This is anticipated to force the confluence point of the two streams seaward and away from southern dune bank. Monitoring will continue to observe changes at the site in order to evaluate the respective success of this strategy, and the results used as a tool for communication with stakeholders.
4. Application of adaptive management practices
The National Institute of Water and Atmosphere (NIWA) have provided high level guidance for adaptive management strategies and pathways in the New Zealand context via the Pathways to Change document (Britton et al., 2011). In the case of managing the Wekatahi and Marawhara Streams, guidelines were able to be applied through the project’s second phase in 2012. Monitoring enabled both the communication of project objectives and challanges. As the body of monitoring data accumulated an analysis of site dynamics was able to be undertaken, which allowed for a review of the projects objectives.
Given the current level of uncertainty around coastal processes of the West Coast of the upper North Island, adaptive management strategies appear to be the most practicable approach to management challanges. Observations and monitoring of environmental changes and coastal processes before and after physical works allow for an evaluation of the respective results and lessons learned. These can then be accounted for and built in to future work programmes.
In the case of the North Piha streams allowance had been made to provide for natural coastal processes and encourage certain aspects of beach behaviour, whilst ensuring the main management objectives are adhered to. A detailed monitoring programme was employed at the Wekatahi and Marawhara Stream in order to better understand the environmental variables operating at the respective sites. The results have been used to provide feedback to the projects respective success and subsequently evolve the projects objectives. Further, data from the programme has been used as a communication tool for engagement with key stakeholders.
There is substantial uncertainty around the exact nature of beach behaviour and coastal change along the western coast of New Zealand’s North Island. Both within historical and contemporary timeframes there have been apparent fluctuations in sediment supply, which in turn have led to respective periods of either erosion or accretion. And recently these trends have provided a range of management challenges along the coast.
Due to this level of uncertainty, coastal managers have looked for flexibility in responses, often whilst detailed monitoring is undertaken. By incorporating targeted monitoring programmes, the success of a respective project can be evaluated and re-evaluated as environmental variables change.
A key lesson learned is through project evolution, monitoring observations and revised objectives should always be communicated clearly amongst key stakeholders and decision makers so there is no confusion around the direction of the project and limitations of outcomes being sought. Informing key stakeholders with this level of information appears to be a critical part of maintaining the momentum of these types of approaches. It is anticipated that the lessons learned through the development of management of Wekatahi and Marawhara Streams will serve as a template for adaptive management within Auckland Council.
- Blue, B. 2009. Multi-Decadal Shoreline Change on Auckland’s West Coast: Whatipu, Karekare, Piha and Te Henga Beaches, Aotearoa/New Zealand. Unpublished honours dissertation, University of Auckland.
- Brander, R.W., Osborne, P.D. and Parnell, K. 2003. High-energy beach and nearshore processes. In Goff, J. R., Nichol, S.L. and Rouse, H.L., (eds.), The New Zealand Coast: Te Tai o Aotearoa. Palmerston North, Dunmore Press. pp.119-142.
- Britton, R., Dahm, J., Rouse, H., Bell, R., Blackett, P. 2011. Coastal adaptation to climate change: Pathways to change. Externally peer-reviewed report prepared as part of the Coastal adaptation to climate change project, NIWA publication. 106 pp.
- Brothers, R.N. 1954. A Physiographical Study of Recent Sand Dunes on the Auckland West Coast. The New Zealand Geographer, 10:47-59.
- Dahm, J. 2002. Coastal Hazard Management Strategy: Muriwai. Report prepared for Auckland Regional Council & Rodney District Council. Coastline Consultants Ltd, Hamilton.
- Dougherty, A.J. 2011. Evolution of Prograded Coastal Barriers in Northern New Zealand. Unpublished PhD Thesis, University of Auckland.
- Hamill, P.F., and Ballance, P.F. 1985. Heavy mineral rich beach sands of the Waitakere coast, Auckland, New Zealand. New Zealand Journal of Geology and Geophysics, 28: 503-511.
- Hilton, M, J. 1982. Post-glacial coastal deposition: South Kaipara peninsula. Unpublished MSc Thesis, University of Auckland.
- Hume, T.M., Smith, R.K., and Ray, D. 1999. Piha Beach: Coastal physical processes, effects of human activities and future management. NIWA Client Report, WTK90201/1.
- King, D.N.T. 2001. Shoreline Change at Piha Beach, Aotearoa, New Zealand. Unpublished MSc Thesis, University of Auckland.
- Schofield, J.C. 1975. Sea-level fluctuations cause periodic post-glacial progradation, South Kaipara Barrier, North Island, New Zealand. New Zealand Journal of Geophysics, 18: 295–316.
- Statistics NZ, 2006. Available at www.stats.govt.nz/census. Accessed on 10/06/2012.
- Williams, P.W. 1977. Progradation of Whatipu Beach 1844-1976, Auckland, New Zealand. New Zealand Geographer, 33: 84-89.
- Wright, L.W. 1969. Coastal changes at the entrance to the Kaipara Harbour 1836-1966. The New Zealand Geographer, 25(1): 58-61