Erosion

Coastal erosion & sediment systems

• Contrasting coasts – East and west coast beaches
• Sand movement and storage in embayments
• Sand dispersal along the west coast black sand beaches
• Coastal erosion
• Storms cause a ripple on the sea floor

Contrasting coasts – East and west coast beaches

Open black sand beach on the west coast

Embayed beaches on the east coast

The east coast beaches of the northern North Island of New Zealand are embayed and disconnected from each other while the black sand west coast beaches are open and connected by a river of sand flowing north. Management options are different in the two cases but, without knowledge of how these different sand systems function, decisions on coastal protection and development may be inadequate, premature or simply incorrect.

The main contrasts between the western and eastern coastlines are in the geological setting and exposure to waves. The open sand system of the west coast is characterised by little development and a rugged beauty. Along the 750 km of shoreline between the Patea River mouth and North Cape there are black titanomagnetite sands, intermediate state pocket beaches nestled in between headlands, long (to 90 km) wide dissipative beaches, inland dunes, 100m high coastal cliffs of erodible Pleistocene sands, river mouths and large tidal inlets with associated deltas. Ocean swells from the Tasman Sea and Southern Ocean provide a highly energetic wave climate and drive sand back and forth along this sparsely inhabited shoreline. In contrast, the embayed and disconnected sand systems of the east coast are more developed, with ports and residential subdivisions. This lee shore has a more benign wave climate, disturbed only occasionally by tropical cyclones. Beaches are comprised of while quartzo-feldspathic sands and are intermediate to reflective in nature. Beaches occur mostly as small pocket beaches nestled in between headlands, with a few large embayments.

Important to both open and embayed situations are headlands, which act as valves on coastal sedimentary supply by regulating sand storage and transfers along the coastline. The size of the headlands relative to the surf zone width, the magnitude and frequency of wave-driven currents, the beach sand grain size, and the offshore topography all determine the quantities of sand moving, leaking or stored. In embayed systems the beach sand is largely trapped between the headlands. Studies in these systems can be localised. On the open and connected west coast beaches sand moves between beaches, jumping small headlands as it moves along the shore. and also where it is captured for a time by ebb and flood tidal deltas at inlets. Studies of these systems must considerer regional scale issues.

Sand movement and storage in embayments

We are undertaking detailed studies to quantify the hydrodynamic and sand transport processes across the beach-nearshore and at headland boundaries of embayed systems. To quantify sand movement, we combine detailed measurements of hydrodynamics, bedforms and suspended sediment with numerical modelling. We are focusing on two embayments of differing scale: the Mangawhai-Pakiri system (30 km long) north east of Auckland and the smaller Tairua-Pauanui system (5 km long) on the Coromandel Pennisula. Beach dynamics were measured with our sea sled, boundary layer conditions at the southern headland were measured using our ALICE tripod armed with an IMAGENEX sonar to measure ripple dynamics, the cross and longshore transport of sand were measured by a large tracer experiment and the spatial distribution of bedforms and currents were mapped using side-scan sonar and acoustic doppler current. To study sand transfers around the Pakiri-Tairua headland we have set up our hydrodynamic numerical model to simulate tidal currents, and are using a computer-controlled video monitoring system to monitor the exchange of sand between the beach and nearshore bars.

Map of Study Areas

Sea Sled

The sea sled (pictured to the left) is an instrument system for taking high resolution measurements of beach profiles and ripple geometry. The sled is towed out from the shore by a boat to beyond the surf zone. It then moves in and out across the surf zone by using a truck-mounted winch on the landward side and either an anchored pulley or a boat on the seaward side. The sled’s position and level are exactly surveyed from shore by a survey instrument sighting onto a prism mounted on top of the mast. An inclinometer corrects for the tilt of the sled. Ripples are measured using a wheel that moves up and down over the ripples as the sled is dragged back and forth. Ripples are also monitored using an oblique (sideways-looking) underwater video camera. Click image for an enlarged view.

ALICE Tripod

A photo of Alice deployed in Okura estuary

ALICE is a tripod that is equipped with instruments for making high resolution, synchronous, boundary layer measurements. ALICE comes equipped with four Marsh-McBirney current meters and optical backscatterance sensors (in a vertical stack on the arm cantilevered out to the side), IMAGENEX fan-beam and pencil-beam sonars (the red objects situated on ALICE’s underbelly), acoustic back-scatterance sensors (cylinder on the cantilevered arm), sediment traps, pressure sensors and, finally, batteries and logging systems (all the cylinders at the heart of ALICE).

IMAGENEX Fan- and Pencil-Beam Transducers

The IMAGENEX sonar system is deployed on the underside of ALICE. The transducers emit and receive acoustic beams which scatter off bottom features. The pencil-beam scans along one line (much like a conventional echo sounder) directly below the sensor and gives accurate measurements of ripple height. The fan-beam scans in a circular path, the radius of which the user can select. The figure to the right shows the circular view from the fan-beam transducer. Ripples show up as coloured bands, and the black lines are shadows left by objects on the instrument frame. The inset on the bottom left of the figure shows the profile measured by the pencil-beam transducer.

Numerical Modelling

Numerically predicted currents show a large eddy spinning up south of Cape Rodney on the incoming tide.

Numerical modelling for Coastal Sediments and Storage has been done using 3DD, a three dimensional, baroclinic, finite difference, numerical model. Bathymetry grids for the model were constructed using fairsheets of Royal New Zealand Navy soundings.

GALAI (CIS-100) Stream-scanning Laser Particle Sizer

The Galai set up at NIWA, Hamilton

A GALAI (CIS-100) stream-scanning laser particle sizer is being used to map sand dispersal along the black sand beaches of the west coast of the North Island. It uses laser-based measurements of ‘time of transition’ to determine particle size and image analysis to determine particle shape. Image analysis uses high-resolution digital video camera, microscopic lenses and powerful software to capture and analyse particle images many times a second. Importantly, the technique provides some 60 particle-shape parameters (e.g., particle aspect ratio, sphericity, perimeter length, surface area). The GALAI quickly counts large numbers of particles, providing good statistics on size and shape populations.

Sand dispersal along the west-coast black sand beaches

Beach sand sampling at Piha Beach, West Coast, North Island

The black sand beaches of the west coast of the North Island between the Patea River mouth and North Cape have been sampled as part of a study of sand dispersal along the coast. Here we are interested in sand movement and storage on a regional scale, and how the various contributions to the sand budget from cliff, river and the continental shelf are sorted along the coast. The current understanding is that the titanomagnetite-rich black sand originated mostly from the andesitic cone of Mt Taranaki in the south, with lesser contributions from the Waitakere volcanics in the Auckland region. Other sources of minerals along the shore include the rivers in the south which drain Tertiary rocks and the mighty Waikato River, which drains a largely rhyolitic provenance. South westerly swell originating in the southern ocean drives sand northwards along the shore. Rates of longshore transport are thought to be very large, but no one really knows. A relevant question is whether there is a huge amount of sand just sloshing back and forth along the shore or whether there is a river of sand flowing north along the coast. A total of 150 sand samples were collected from the mid-tide level of beaches every 5 km along the west coast. Preliminary results show a mixed sand grain size and shape population, with a distinctive signature for titanomagnetite, which is relatively spherical compared to other minerals. Regional trends in sand dispersal along the west coast can be differentiated by local effects from the sand size and shape signatures measured using a Galai laser particle sizer (see link to Galai, also see link to Water & Atmosphere article v 9(1) 19-21). Other measurements such as beach slope and orientation were made as well. A helicopter was used as a rapid means of getting to sites and to beat the rising tide along many parts of the coast. Petrological analyses will reveal the origin of the black sands. (Click here to see an example of a black sand beach.)

Coastal erosion

How can we deal with the natural process of coastal erosion when it becomes a problem? And how can research help? Professor Orrin Pilkey and Dr Terry Hume share their views on how to “live by the rules of the sea”

Storms cause a ripple on the sea floor

We were lucky enough to have our boundary layer tripod armed with the IMAGENEX sonar out when Tropical Cyclone Gavin hit the Northland coast in March 1997. This gave us a rare opportunity observe the radical bedform changes that accompany storms. The tripod was positioned on a sandy seabed in 25m water depth as the cyclone passed down the east coast. The sonar scanned the seabed in a radius of 15 m about the tripod every 90 minutes during the storm. The storm had very high (up to 5m), long-period (11s) waves which lasted 130 hours. In addition to measuring waves, currents and suspended sediment, we measured huge changes to the ripple height, spacing and orientation using the sonar. During the storm, ripples evolved from irregular, to long-crested sinuous, to long-crested, then back to long-crested sinuous and finally to short-crested irregular. Ripple height was first to change, followed by steepness and orientation, then crest length, and lastly by ripple spacing. These changes took place in about 8 hours. Relict ripples on the seabed prior to the storm, were replaced by ‘equilibrium-range’ wave orbital ripples early in the storm, followed by orbital ripples in the ‘breakoff range’, coincident with a large increase in ripple steepness. There was no evidence for sheet flow. Because of the requirement for short oscillatory motion, orbital ripples occur most commonly in very shallow water under short-period waves. The occurrence of orbital ripples in deeper water at Cape Rodney can be explained by the fact that the orbital motion of long-period waves is attenuated with depth, resulting in short orbital excursion at the seabed. A sudden increase in ripple steepness was associated with an order-of-magnitude increase in hydraulic roughness and a large increase in sediment suspension.

Ripples measured using Imagenex – before the storm

Ripples measured using Imagenex – after the storm