Estuarine shoreline processes in a dynamic low-energy system

Abstract

Estuarine shorelines are often classified as low-energy coasts and are, therefore, expected to undergo little variation. Port Stephens (SE Australia) is a ria-like drowned river valley microtidal estuary located on a wave-dominated coast. The outer part of the estuary is tide-dominated and has a large shallow flood-tide delta, which is also affected by waves. The northern (predominantly low-energy) shoreline of outer Port Stephens is a continuous stretch of sand comprising areas of high mobility and areas of relative stability terminating in a western extending sand spit. This paper investigates the effects of periodic high-energy conditions during which waves penetrate into the estuary by analysing two types of storms, low to moderate (more frequent type) storms and severe to extreme (low frequency) storms. It is established that low to moderate storms cause generalised erosion over the northern shoreline. On the contrary, severe to extreme storms, while causing erosion on parts of the beach, can transport new sediment across the flood-tide delta and deposit it to build a mobile shore attached sandwave. Long-term (decadal) trends identified in the study area are in agreement with short- and medium-term results. Moving into the estuary are four complementary zones of sediment transport which include: (1) sandwave formation and westward migration; (2) a relatively stable area between the sandwave and an erosion zone; (3) an erosion zone undergoing shoreline retreat and finally (4) a depositional terminus causing westward extension of the sand spit.