ANNA BAY ENTRANCE, STOCKTON BIGHT Nicolas Pucino¹ª, Sandro
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UAV MONITORING OF DUNE DYNAMICS - ANNA BAY ENTRANCE, STOCKTON BIGHT Nicolas Pucino¹ª, Sandro Condurso² ¹University of Wollongong, NSW ²NSW-National Park and Wildlife Service - OEH, Nelson Bay ªCorresponding author: [email protected] Introduction “When the causes and trends of coastal dune movement are well understood, tailored management strategies can be applied in order to maximize tourist attraction while maintaining a healthy and naturally dynamic geomorphic behaviour ” (Mitasova et al., 2005) This site-specific (Anna Bay entrance) Unmanned Aerial Vehicle (UAV) survey and analysis is part of a broader environmental study conducted at the whole Stockton Bight (NSW) embayment scale. The full-study aims to understand Stockton Bight dune dynamics and migration trends, adding local knowledge to both managers, aboriginal community and visitors. Stockton Bight’s management is challenged by its naturally dynamic geomorphic state, richness in aboriginal sites and artefacts, sand mining activities, high visitation and touristic impact. Its extent is a major issue to deal with. The study site, which is included in the Worimi National Park, features the southern hemisphere's largest mobile coastal sand mass (NPWS). With its 32 km, Stockton Bight is also the longest beach in NSW. Some of its transgressive dunes reach heights of 40m of elevation above sea level and its beach-backdune- Figure 1 Location map of Stockton Bight transgressive dune system covers an area of 2700 ha, which means approximately 3800 football fields. Given the aforementioned conditions, it is evident that the study of such a dynamic and geographically extended area implies trade-offs in spatial scales and analytical techniques. Accordingly, this research features geospatial analysis of both LiDAR (whole- embayment analysis) and UAV (site-specific) digital terrain models. Upon deriving and mapping sand movement trends, the project will also explore sand mitigation measures for targeted locations to best achieve efficient and cost effective sand stabilisation outcomes. 1 The UAV solution at Anna Bay entrance This conference paper describes methods and analysis of a topographical coastal surveys using UAV, Structure From Motion (SfM) and GIS, in Anna Bay entrance, the northern end of Stockton Bight (NSW, Australia) embayment. Here is the area where the most human pressure occurs as it is used as main northern entrance to the national park by tourists, locals, rangers, 4wd vehicles and every kind of recreational or touristic operator activities, including horse and camel riding, 4wd buses and trucks, sand boarding, dogs, etc. As the expected outcome in this area will be an effective and calibrated sand stabilisation strategy, beach morphodynamics and sand movements must be acknowledged and monitored. Thanks to the availability of an UAV system (SenseFly Ebee) and a fully certified, licensed and experienced surveyor (Condurso Sandro), very high spatial resolution (10cm) Digital Surface Models (DSM) and ultra-high (3cm) repetitive orthophotos have been generated and both volumetric change (cut and fill operation) and height differences (DSMs subtraction) analysed over time. These digital products are unevaluable to coastal monitoring projects, especially when the low time-step between acquisition dates allows seasonal variations to be observed. In fact, one of the major limitation of eolian geomorphology has been the relatively short temporal baseline of dune activity observations (Hugenholtz et al., 2012). To fully understand the changing morphology of a dune system, monthly or quaternarly surveys must be conducted in order to correlate data with storm events or weather patterns (Woolard and Colby, 2002; Andrews et al., 2002). Nowadays, UAV systems represent an extremely useful resource with endless potentials in topographic surveys of the coasts, capable of extending the analysis baseline at very short time-periods. This paper presents the details of the most recent UAV survey and the results of the geospatial analysis of all the available datasets (figure 2). Figure 2 Table of the available datasets. Only the datasets labelled as pilot_volume have been used for the geospatial analysis. Structure from Motion Thanks to the recent development of the so called “computer-vision approach”, the fundamental principles that underpin a classic photogrammetric method are combined with robust and well documented image processing algorithms to create DSM models with sub decimetre accuracy, in a (semi) automated and user-friendly way. Classic aerial photogrammetry needs highly specialized personnel, expensive softwares and manned aircraft, perfectly calibrated and virtually distortion-free metric cameras, restrictive image collection requirements and long planning times. Structure from Motion (SfM) approaches allow non-specialized users to align and mosaic aerial photographs acquired 2 by consumer-grade medium resolution camera (though any camera can be potentially used) mounted on many different moving platforms ranging from balloons, UAVs and kites. In this research, aerial photography acquired with a surveyor grade UAV system has been processed with the classic SfM pipeline (figure 3). This consists in placing all the images acquired by the UAV aerial survey into a virtual terrain, aligning and stitching them together, correcting for distortions and georeferencing the mosaic (using Ground Control Points (GCPs) surveyed on the ground), identifying and extracting common features occurring in overlapping images, tying them together and finally computing the elevation of every pixel of the images. Figure 3 Scheme displaying the key steps used in most of the UAV SfM-related studies. 3 Drift Potentials Monitoring dune activity means identifying, quantifying and interpreting dune surface changes related to the eolian sand transport potentials (Hugenholtz et al., 2012). In fact, dune activity is usually regarded as an index of eolian sand transport potential (Fryberger and Dean, 1979; Ash and Wasson, 1983; Lancaster, 1988). Amongst the different indices that exist in eolian geomorphology literature, the Fryberger method (Fryberger and Dean, 1979) for calculating sand Drift Potentials (DPs) and relate them to the wind energetic environment has been used in this study. Fryberger’s method has been validated by many global sand sea (Fryberger and Dean, 1979; Wasson and Hyde, 1983; Bullard et al., 1996) and coastal studies (Miot da Silva and Hesp, 2010; Tsoar, 2005; Levin et al, 2014). The sand Drift Potential (DP) is a parameter that describes the maximum potential amount of sand that could be eroded and transported by wind coming from a determined direction over a period of time (Fryberger and Dean, 1979) and it is widely used all over the world and in Australia (Levin et al, 2014). DPs must be calculated in each of the wind direction component (12 or more Directional Classes; DCs). The assumptions are that the surface is composed by loose quartz grains (0.25-0.30 mm average diameter), no bedforms bigger than ripples are present, it is dry and clear of vegetation cover (Fryberger and Dean, 1979). It is calculated as where U is the wind velocity (m/s, at 10m height from ground level) Ut is the threshold wind velocity (= 5 m/s) and t is the percentage of time that wind exceed Ut. Following the Belly method and valid for the aforementioned assumptions, a threshold velocity (Ut) of 5.97 m/s (11.6 knots) was adopted by Fryberg’s global sand seas analysis. In Stockton Bight transverse dune environment, according to expert local geomorphologist (Thom et al. 1992) and in lack of in situ sand sample, the local threshold has been chosen at 5 m/s (9.72 knots). DPs from all the DCs can be resolved into the Resultant Drift Potential (RDP) and its direction as the Resultant Drift Direction (RDD) via a vector analysis. Following the original Fryberger’s (1979) method sand roses can eventually be produced. Moreover, the RDP/DP is a measure of wind variability, used by Fryberger for classification depositional environments. The wind data utilised for the Fryberger’s indexes have been collected by the Newcastle Nobby’s Signal Station AWS, managed by the Bureau of the Meteorology (BoM). The station is situated at the southern end of Stockton Bight embayment, directly on the headland and facing the ocean. The station is at 33 m above mean sea level and 10m above ground, which is with the 10m assumption of Fryberger’s method. The BoM wind measurements are designed to satisfy weather and climate monitoring needs. Its uncertainty tolerances are 10% of the wind speed when wind speeds are greater than 10 m/s and 1 m/s for wind speeds at or below 10 m/s. 4 Area of study Stockton Bight transgressive sand barrier system (originally “Newcastle Bight”, figure 1) is located on the coast of central New South Wales, Australia, between Port Stephens drowned river valley (north) and the mouth of the Hunter River at Newcastle. Stockton Bight embraces most of the features that characterise coastal NSW, namely, rocky headlands which compartmentalize an embayed and wave-dominated sandy beach or barrier, a steep and relatively narrow inner continental shelf and a limited supply of river sediment to the coast (Thom et al., 1992; Wright, 1996; Davies, 1974; Thom et al, 1981). The climate is temperate, providing minor seasonal variation around a mean warm and humid conditions. This climate highly increases the wave climate variability and sets a moderate to high-energy south-easterly swell (occasionally southerly). The tidal range is less than 2m (micro-tidal system). During the day, sea breezes dominate. Trade winds, swell and regional climate combined, result in the dynamisms that characterised the NSW beaches (Short, 1993). Anna Bay entrance geomorphology is dominated by (inland to seaside): 1) long-walled active ridge 2) mobile sand sheet of migrating transverse and barchanoid dunes (referred as “high dunes”) 3) deflation plain 4) foredune 5) beach Unfortunately, the UAV surveys do not cover the whole shore-normal transect from the long-walled ridge to the beach but only from the upper end of the deflation basin in contact with the transverse sand sheet to the swash zone.