Bedform movement in a marine tidal delta: air photo interpretations

Abstract

AbstractMorphological changes in bedforms composing a tidal delta at the northern entrance to Moreton Bay, Queensland have been studied by examining aerial photographs spanning a 26-year time period. The aerial photographs show the movements of 51 different sand-bank crestlines, and the morphological characteristics of both sand banks and sandwaves. From the orientation of sandwave crests to the sand-bank crestlines, zones of ebb- and flood-dominance in sand-transport direction are distinguished. The migration directions of the sand banks are predicted by considering the cross-sectional asymmetry of the sand banks together with their adjacent ebb/flood zones of net sand transport. The reliability of the predictions is tested by comparisons with sequential air photo data. When applied to 53 bedforms, the predictions achieved a high success rate, with 45 predicted migration directions matching those observed on the sequential aerial photographs. Bedform movement can be predicted, therefore, for any water depths in which submarine bedforms can be clearly seen on aerial photographs (< 10 m).Based upon their mobility, sand banks are classified into three categories: dynamic sand banks, which change quickly (within 2 years) and which have migration rates that are non-uniform along the bank crestline; progressive sand banks, which change slowly (from 2–10 years) and have migration rates that are uniform along their crestlines; and immobile sand banks, which change only over time intervals which exceed 10 years. Changes in sand-bank morphology occur by migration of the crestline together with growth and decay, and are considered to be linked with changes of larger ebb- and flood-dominant zones of net sand transport. The three different sand-bank types are characterized by distinctive heights, crestline lengths and wavelengths. They occur in different locations within Moreton Bay, possibly related to distance from external sand supplies and to relative tidal current and surface wave energy levels.