Robin Davidson-Arnott

Conceptual Model of the Effects of Sea Level Rise on Sandy Coasts

Abstract Over the past 40 years, consideration of the potential effect of sea level rise on sandy coasts has been dominated by the conceptual model proposed in the Bruun Rule, which is used to predict the horizontal translation of the shoreline associated with a given rise in sea level. A review of the hypotheses that form the basis for this two-dimensional model suggests that the assumption of net sand transfer to the nearshore profile and deposition of a thickness of sediment equal to the rise in sea level is probably incorrect. Moreover, the model omits consideration of a significant component of the coastal sediment budget, namely the dune sediment budget, and the processes associated with beach-dune interaction. An alternative conceptual model is developed on the basis of a two-dimensional equilibrium profile similar to that which forms the basis for the Bruun Model. The proposed model incorporates consideration of the dune sediment budget and foredune dynamics. In contrast to the Bruun Model, it predicts no net transfer of sediment to the nearshore profile and preservation of the foredune through landward migration. It is argued that the model proposed here offers a better starting point for developing more realistic models of shoreline response to sea level rise that incorporate consideration of alongshore sediment transfers and more complex coastal morphology and sediment characteristics. Testing of the validity of the model and its potential use for integrated coastal zone management will require consideration of the volume changes associated with sea level rise on a decadal scale.

Nearshore water motion and mean flows in a multiple parallel bar system

Abstract Measurements of surface waves and nearshore water motion were made with wave staffs and electromagnetic currentmeters across the straight outer bars of a multiple parallel bar system at Wasaga Beach, Georgian Bay, Ontario. Three sets of experiments were carried out between 1983 and 1985, and this paper examines the characteristics of spatial and temporal variations in incident waves and mean flows. Incident wave spectra during storms are typical of fetch-limited conditions, with a prominent peak in the wind wave frequency and a sharp cut-off towards lower frequencies. There is little evidence of significant energy at infragravity frequencies. Mean flows perpendicular to the shoreline during storms, measured at 0.1 m above the bed, are offshore, with speeds of 0.04−0.10 m s−1 during moderate storms and 0.25 m s−1 during intense fall storms. Longshore currents are relatively weak, and their direction closely follows changes in the incident wind angle about shore perpendicular. There is no evidence of rip cell development, and offshore return flow occurs as an undertow uniformly alongshore, forming a simple, two-dimensional net circulation pattern. The presence of the undertow very close to the bed is an important control on the direction of suspended sediment transport, and thus in maintaining the equilibrium of the nearshore bars.

Coastal sediment transport

Synopsis Modification of the coast takes place through the erosion, transport and deposition of material that is either eroded by waves and currents or brought to the coast, e.g., by rivers. Fine sediments in the silt and clay size range do not occur in appreciable amounts in the inner nearshore and surf zones on energetic coasts. They tend to be placed in suspension and diffuse uniformly through the water column. They are then removed offshore or alongshore where they settle out of suspension in deep water in the outer shoreface or further offshore. Fine sediments may also be brought into estuaries, bays and lagoons where they are deposited in quiet water, often promoted by the presence of vegetation such as seagrasses, salt marsh plants and mangroves. Coarser particles of sand and gravel are exchanged readily between the inner nearshore and surf zones and the beach, and may be transported alongshore in appreciable quantities. It is usual to distinguish between processes that lead to the net transport of sediment onshore or offshore (shore normal transport) and those tending to move sediment alongshore (longshore sediment transport), though both processes occur simultaneously. Except in a few locations in the surf zone, such as rip channels and where longshore currents are fastest, unidirectional currents in the nearshore and surf zone are generally not strong enough to erode and transport coarse sediment directly.

Sea level fluctuations and changes

Synopsis While the incident wave climate is the dominant process variable controlling changes in coastal form and evolution, changes in the relative level of the land and the water body, on a variety of time scales, can greatly influence the effect of these processes and the way in which coastal; evolution occurs. Dynamic changes in sea level on the order of hours to a few decades reflect response of the water surface to meteorological and oceanographic processes as well as tides and can be thought of as periodic or episodic deviations about mean sea level. They affect the level at which wave action occurs and may also lead to horizontal movements of water in tidal and other currents. They thus have an effect on coastal erosion, transportation and deposition. In addition to tides produced by the gravitational force of the moon and the sun, short-term fluctuation in sea level occur as a result of storm surge, seasonal variations in pressure and wind patterns and changes in weather patterns on a scale of years to decades such as the El Nino Southern Oscillation (ENSO) cycle in the Pacific. These fluctuations are also extremely significant ecologically, both directly through exposure and coverage of the intertidal zone and indirectly because of the movements of water and nutrients associated with the water level fluctuations.