New Zealand Capstone 2008
Day 1- South Brighton Spit, Christchurch, NZ
March 8th, 2008
After a 13 hour flight across the Pacific Ocean we arrive in Christchurch New Zealand. The group (below) gathers their bags and heads straight to Brighton Spit to conduct a field investigation on Brighton Spit using GPR.
Background to research project
The field research project began by walking around the spit (below) to understand the coastal landscape and look for potential transect lines. The South Brighton Spit have very active aeolian and coastal processes occurring, which allows for the southward migration of the spit. Ground penetrating radar (GPR) will aid in understanding the subsurface stratigraphy and the formation of South Brighton Spit. There are two outstanding issues which fostered the field study. First, there is a major concern that the growth of the spit is closing the Avon/Heathcote estuary off from the Pacific Ocean, which has happened in the past resulting in a costly dredging project. Second, storm erosion along the spit has been the major contributing factor to the damage of homes that have been built on the spit. Lack in understanding basic coastal geomorphic processes is now affecting the development along the spit.
Along the eastern coast of New Zealand’s South Island sediment is eroded from the Southern Alps and is then transported east to the oceans by numerous rivers (below). Once the sediment reaches the sea it is carried northward by the ocean currents and longshore drift. The northward flow is interrupted near the city of Christchurch by Banks Peninsula, an ancient volcanic island connected to the mainland. Some of the northward flowing sediment, along with a contribution from the local braided rivers, is trapped by a back-eddy that forms in Pegasus Bay (Pacific Ocean). The change in direction of the current circulates water and sediment southward near the Avon-Heathcote Estuary.
The accumulated sediment is presently forming South Brighton spit which continues to build southward across the mouth of the estuary. Over the past several decades the relative size and physical location of the spit has fluctuated due to changing coastal and aeolian geomorphic processes.
Winds, ocean currents, sediment transport, wave action, and beach drift, have all caused the size of the South Brighton Spit to grow as well as to migrate in a southerly direction (below). Waves lift the sediment into suspension while the current transports the sediment. Wave action lifts sediment from the near shore area and also transports the sediment to the beach. This constant supply of sediment and transport allows for the progradation and aggradation of the South Brighton Spit over time.
To begin collecting data, we laid out four-100m tapes. The first two lines were run in an east to west direction, and the second two lines were run in a west to east direction. The lines were not run consecutively since an international school group were planting trees for an environmental project. Next, the GPR system was set up with a rope tied to the sled so that it could be dragged more easily; while the fiber optic cable, connected to the transmitter antennae was attached to the laptop for collecting data (below). The transmitter and receiver antennae were mounted on a sled and dragged at a steady pace for each 100m transect. It took four people per transect to operate the GPR unit efficiently; one to drag the sled, one to hold the fiber-optic cable, and two to carry the computer equipment; we rotated duties in between each line. The data was collected in a continuous mode and the trace positions were noted at 20m intervals to ensure the spatial accuracy of the collected data. As we started collecting data one person would wait at each 20m increment, and when the sled reached the mark, the trace position was noted and recorded (below).
Several data collection problems were encountered on South Brighton Spit. No topographic information was collected in the field. Topographic changes are present along the transect, leveling would provide necessary elevation changes that would help in understanding the subsurface stratigraphy on the GPR profile. Secondly, disturbances were not accounted for in the data collection. Some of these disturbances include tree roots and materials buried in the sand (i.e. a wooden pathway buried by the dunes). Some materials (i.e. wood, rubbish) scatter the EM waves causing the final results to be skewed. The speed and velocity of the sled varied due to other factors (i.e. pace of each individual and the materials in which the sled pulled over) and were not accounted for in the data collection. Lastly, is that each line run should have been run consecutively in a row versus the first two in one direction and the last two lines in the other.
On South Brighton Spit (Christchurch, New Zealand) four GPR transects were collected to image coastal and aeolian sedimentary deposits. A Sensors and Software pulseEKKO 100 GPR system using a 200 MHz antenna was pulled on a sled at a constant rate across the Spit (above). Radar stratigraphic analysis was used to interpret sedimentary structures from the observed reflection patterns in the GPR profiles.
Data Processing and Plotting
Topographic correction was also used to integrate the elevation changes of the landscape along the profile. This information was imported from an ASCII file with the positions co-located with elevation.
The continuous to semi-continuous eastward dipping reflections(blue) are interpreted as layers within aeolian sand dunes formed from predominantly offshore easterly winds that blow the beach sand further inland by saltation and surface creep (below). Typically beach sand is blown up the windward side of a sand dune and then slumps down the dune’s leeward side (slip face) where eventually the sand grains come to rest. At South Brighton Spit the sand surface is often “wet” so when sand grains are transported they “stick” to the windward side of the sand dunes creating eastward dipping layers. The observed reflection patterns show that as deposition is actively occurring these coastal sand dunes are prograding oceanward (east).
The depth of signal penetration in the GPR profiles collected on South Brighton Spit is limited to approximately 3.5-4.0 meters. This depth is probably due to saltwater intrusion, which occurs when saltwater flow mixes with the spit’s freshwater aquifer. The saltwater is conductive and would therefore attenuate the electromagnetic signal.
The GPR profile below is a representative section of the GPR transects collected on South Brighton Spit. The continuous to semi-continuous slightly dipping reflections are traced in red. The eastward dipping reflections are interpreted as prograding sandy beach faces that result from coastal wave action. Progradation occurs when sand is moved up and down the shore of South Brighton Spit by beach drift.
Continuous to semi-continuous horizontal to sub-horizontal reflections (green) are also observed. These reflections are interpreted as aggradation (vertical buildup) of the coastal spit. The aggradation of the coastal spit is due to a combination of wave action, beach drift, and windblown processes. This portion of the collected GPR transects intersect with the historical delineation of the 1948-1950 South Brighton Spit. One can observe that the beach has both prograded and aggraded to displace (fill in) the former inlet. As a result of the actual geomorphic processes the growth of South Brighton Spit after 1948-1950 (interpreted from the historical map) has resulted in numerous measurable changes in this dynamic coastal landscape.
Sediment deposited onshore by wave action and beach drift is transported further inland by aeolian processes, such as saltation and surface creep, forming costal sand dunes (below).
Written by: Pat Dryer
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