Determining Sediment Budgets and Coastal Morphodynamics Based on 3-Dimensional Terrain Modelling Using GIS

Colin Woodroffe, Bongkoch Samosorn, Ava Simms, David Kennedy, John Marthick

Spatial Analysis Laboratories, School of Geosciences, University of Wollongong (AU)

Geographical Information Systems (GIS) offer potential to refine the estimation of sediment budgets and therefore provide insights into coastal morphodynamics, the co-adjustment of form and process as landforms evolve. Generally the pattern of sedimentation in coastal systems has been examined in 2 dimensions, including planimetric trajectories of change based on superimposition of shoreline position from surveys or remote sensing at decadal time scales, and cross-sectional stratigraphy and radiometric dating at century to millennial time scales. GIS enables the 3D or quasi-3D representation of coastal environments, and this paper explores the prospect of reconstructing mass budgets in three contrasting coastal settings, each of which appears to have experienced a relatively simple pattern of Holocene sediment accumulation. The three examples comprise a reef island on a coral reef platform (Warraber Island, Torres Strait), a coastal lagoon on an embayed coast (Lake Wollumboola, southern New South Wales), and a discrete coral reef and lagoon (Lord Howe Island, southwestern Pacific). The topography of Warraber Island, perched on a platform reef in central Torres Strait, has been determined on the basis of photogrammetry, which reveals a pattern of progradation of successive ridges. The depositional history of the island is inferred on the basis of pits excavated across the island and radiocarbon dating on the skeletal sands recovered from those pits. Lake Wollumboola in southern New South Wales, is one of a series of coastal lagoons each of which is at a different stage of infill. It has a small coastal catchment from which sediment loss has been estimated using a raster overlay procedure based on the Revised Universal Soil Loss Equation. The mass budget of accumulated sediment in the lagoon was determined using 3D terrain modelling on the basis of shoreline surveys, bathymetric cross-sections, probes into the lake floor to point-of-refusal, and coring and radiocarbon dating of the Holocene muds. The fringing coral reef on Lord Howe Island occurs at, or near, the latitudinal limit to reef growth, and the reef that has developed during the Holocene along the western shore, and deposition of lagoonal sediments, have been discrete in both time and space. Extensive drilling, seismic reflection profiling and vibrocoring, together with a sequence of radiocarbon dates, offers the opportunity to reconstruct reef growth as well as the accumulation of associated sediments in detail. Despite good topographic and geochronological control on the pattern of accretion in each of these different settings, there are several constraints on the effectiveness with which a mass balance can be determined. Horizontal cell resolution at which each system is represented is a constraint, particularly in relation to the determination of the boundaries of each depositional system. Although quasi-3D modelling appears to enable greater flexibility in representing the vertical dimension, errors associated with radiometric dating complicate mass budget estimation. The sensitivity of modelling of each of these systems to such constraints is examined.