Beach monitoring to better plan for the future

06 Apr 2021

Changes to the shoreline of 17 beaches along the east coast of the Coromandel Peninsula are being regularly monitored in a bid to help local authorities better plan for the future. Recently a Waikato Regional Council survey team was busy measuring the dunes along Buffalo Beach in Whitianga, one of the beaches forming part of the monitoring programme. The information will be fed into a long-standing database to see what changes have occurred over the years.

In a recent indicator update report, WRC said that its database of shoreline change helped planners considering the impact of developments near the coast. It noted that many of the Coromandel’s coastal housing developments had been built close to the shoreline.

Sandy beaches were naturally prone to changes in shoreline position due to major storms and changes in climate patterns over longer time periods. “Houses and infrastructure that have been built close to the sea are vulnerable to these natural fluctuations,” the report said. “Engineered structures such as seawalls are sometimes placed on the beach to protect at-risk assets, but these can damage natural and recreational values of the beach and can transfer the erosion risk to the adjacent coastline.”

It said that local councils needed to understand the extent of natural beach change to “sustainably guide” existing and new developments in these areas.

A key focus of New Zealand’s Coastal Policy Statement was reducing and, where possible, avoiding coastal hazard risk and this was an important part of regional and district council plans. “Shoreline change information is one tool that can be used to understand which areas of coastline are at risk from erosion due to natural processes over decades,” the report said.

While the data showed that, in general, the position of the dune toe along the Coromandel’s east coast beaches fluctuated, there was “no clear widespread long-term (permanent) shoreline change.” “While most beaches seem quite stable now, a long-term trend for erosion may occur in response to climate change, due either to accelerated sea level rise or a modified wave climate,” the report said.

Monitoring along the east coast of the Coromandel began at some beaches in 1979 and there are now 46 monitoring points spread over the 17 beaches, though some of them no longer required to be monitored.

WRC’s coastal and marine team leader, Michael Townsend, told the Informer, “Understanding how shorelines are changing is a key component of our coastal monitoring. We expect the need for this type of information to grow in the future with sea level rise and changing climatic conditions.”

Mr Townsend said WRC staff used various methods to measure shoreline change, including GPS technology to measure beach height. “The recorded beach height can then be compared with previous surveys to see how the shape of the beach changes at that location,” he said.

Other techniques included operating cameras at beaches, including Tairua, in conjunction with NIWA, to compare how the beach changed over the years. Historic aerial photographs also provided an important record of coastal change.

The next indicator report is expected to be released later this year or early next year.

The 17 beaches along the east coast of the Coromandel that form part of the monitoring programme are Whiritoa, Whangamata, Onemana, Opoutere, Pauanui, Tairua, Hot Water Beach, Hahei, Cooks Beach, Maramaratotara (Front Beach), Buffalo Beach (including Brophy’s Beach), Wharekaho, Opito Bay, Kuaotunu East, Kuaotunu West, Matarangi and Whangapoua.

In Whitianga, the recent indicator report said that Brophy’s Beach had been influenced by human activities, including stormwater drainage and more recently by the construction of a geotextile seawall.

At the northern end of Buffalo Beach, severe erosion occurred in the winter of 2000. “The shoreline in this area was then artificially rebuilt and seawalls have prevented any significant erosion since,” the report said.

The shoreline had been able to fluctuate more naturally further south, though this area had experienced erosion partly caused by effects of the seawalls. “This site is located north of the Taputapuatea Stream (“Mother Brown’s Creek”) and has experienced over 25m of erosion since the late 1990s,” the report said. “Historic aerial photographs taken in the 1940s show this shoreline has fluctuated significantly over periods of many decades.

“While the shoreline at Buffalo Beach has not recovered since the erosion in the late 1990s and in 2000, the beach has been relatively stable at most sites in the last 20 years, with fluctuations of no more than 10m.”

No monitoring point was located at the southern end of Buffalo Beach close to the harbour entrance, but other records showed that there had been significant accretion (sand build-up) over the last 40 years.

Pictured: Beach art created over Easter at the southern end of Buffalo Beach in Whitianga, close to the Whitianga Harbour Entrance.  Records showed that there had been a significant build-up of sand at this part of the beach over the last 40 years.