Future Shoreline Changes Research Articles

Beach erosion under rising sea‐level modulated by coastal geomorphology and sediment availability on carbonate reef‐fringed island coasts

AbstractThis study addresses gaps in understanding the relative roles of sea‐level change, coastal geomorphology and sediment availability in driving beach erosion at the scale of individual beaches. Patterns of historical shoreline change are examined for spatial relationships to geomorphology and for temporal relationships to late‐Holocene and modern sea‐level change. The study area shoreline on the north‐east coast of Oahu, Hawaii, is characterized by a series of kilometre‐long beaches with repeated headland‐embayed morphology fronted by a carbonate fringing reef. The beaches are the seaward edge of a carbonate sand‐rich coastal strand plain, a common morphological setting in tectonically stable tropical island coasts. Multiple lines of geological evidence indicate that the strand plain prograded atop a fringing reef platform during a period of late‐Holocene sea‐level fall. Analysis of historical shoreline changes indicates an overall trend of erosion (shoreline recession) along headland sections of beach and an overall trend of stable to accreting beaches along adjoining embayed sections. Eighty‐eight per cent of headland beaches eroded over the past century at an average rate of −0·12 ± 0·03 m yr−1. In contrast, 56% of embayed beaches accreted at an average rate of 0·04 ± 0·03 m yr−1. Given over a century of global (and local) sea‐level rise, the data indicate that embayed beaches are showing remarkable resiliency. The pattern of headland beach erosion and stable to accreting embayments suggests a shift from accretion to erosion particular to the headland beaches with the initiation of modern sea‐level rise. These results emphasize the need to account for localized variations in beach erosion related to geomorphology and alongshore sediment transport in attempting to forecast future shoreline change under increasing sea‐level rise.

Determining shoreline response to sea level rise

The Bruun rule is the most widely used method for determining shoreline response to sea level rise. It assumes that the active portion of an offshore profile rises with rising sea level, and the sand required to raise the profile is transported from the shoreline. It is difficult to evaluate the efficacy of the Bruun rule because sea level rise often has a lesser effect on shoreline change than that produced by sand sources, sinks, and longshore transport gradients. In addition, some shorelines have advanced seaward with rising sea level. Dean (1987) showed that equilibrium profile theory predicts that rising sea levels produce landward sand movement forced by nonlinear waves. This paper presents an equation with terms representing all phenomena affecting shoreline change including Bruun-rule recession, onshore sand transport, sand sources and sinks, and longshore transport gradients. As an example of its use, rates of onshore transport are determined along the 275-km Florida southwest coast, USA, and a 19-km portion of this coast using known values for sand sources, sinks, and longshore transport gradients. Then future shoreline changes are projected for both coasts from 2015 to 2065 and for the southwest coast from 2015 to 2100, using sea level rise projections from the Intergovernmental Panel on Climate Change. Beach nourishment is shown to be a very effective adaptation strategy for sea level rise with shoreline change projections useful to estimate required rates of beach nourishment to counter sea level rise.

Coastal Erosion in Ghana: Causes, Policies, and Management

ABSTRACTCoastal erosion is a serious problem that affects the safety and livelihoods of many coastal dwellers along Ghana's coast. Despite the fact that coastal erosion is a natural phenomenon, erosion trends have been largely aggravated by human-induced factors. This study analyzed shoreline change rates for three neighbouring coastal communities in the Central region of Ghana; Elmina, Cape Coast and Moree. Two epochs were analyzed, 1974–2012 (medium-term) and 2005–2012 (short-term), using ArcGIS and Digital Shoreline Analysis System. Overall, the entire study area recorded average shoreline change rates of −1.24 myear−1 and −0.85 myear−1 in the medium-term and short-term period respectively. Less consolidated shoreline segments recorded higher erosion rates in both periods while cliffs and rocky segments experienced very little erosion or high stability. Because shorelines undergoing chronic erosion do not fully recover after short-term erosion events such as storms, facilities located close to such shorelines are threatened. Taking a proactive approach to coastal erosion management, such as coastal sand mining prevention, inter-sectoral land use management and adopting a construction setback approach may be prudent for the long-term management of the coast since this recognizes future shoreline changes and safeguards coastal landscape for other uses.

On the significance of incorporating shoreline changes for evaluating coastal hydrodynamics under sea level rise scenarios

The influence of including the dynamic effects of future shoreline changes associated with sea level rise into hydrodynamic modeling is evaluated for the coast of the Northern Gulf of Mexico from Mobile Bay, AL to St. Andrew Bay, FL. A two-dimensional, depth-integrated hydrodynamic model forced by astronomic tides and hurricane winds and pressures representative of Hurricanes Ivan (2004), Dennis (2005) and Katrina (2005) is used to simulate present conditions, 2050 projected sea level (0.46 m rise) with present-day shorelines, and 2050 sea level with projected 2050 shorelines. The 2050 shoreline and nearshore morphology are projected using Coastal Vulnerability Index shoreline change rates to determine the position of the new Gulf and bay shorelines, while the active beach profile is shifted horizontally according to the amount of erosion or accretion, and vertically to keep pace with rising seas. Hydrodynamic model results show that taking a dynamic approach to modeling sea level rise (as opposed to a static, or “bathtub” approach) increases tidal ranges and tidal prisms within the bay systems. Incorporating the projected shoreline changes does not alter tidal ranges, but some bays experience changes in tidal prisms depending on whether the planform area of the bay increases or decreases with the projected erosion or accretion. Barrier islands with projected erosion are vulnerable to increased overtopping from storm surge inundation, which impels more water into the back-bays and increases the inland inundation extent and magnitude. Inundation along barrier islands with projected accretion remains relatively the same as inundation under present-day shorelines, which prevents additional overtopping and limits more water from entering back-bays. Results demonstrate that although modeling sea level rise as a dynamic process is necessary, the incorporation of shoreline changes has variable impacts when evaluating future hydrodynamics and the response of the coastal system to sea level rise.

Shoreline change at an infinite jetty for wave time series

Future shoreline changes on a sandy beach with a structure such as a jetty or groin can be estimated when wave time series is known (i.e. sequence of wave height, period, and direction). This paper presents an extension of an existing solution ( Pelnard-Considere, 1956) for the linearized partial differential equation for shoreline change at an infinite jetty where waves are time varying and when the angle of the shoreline is small with respect to the waves breaking at the shoreline. The novel solution provided in this paper allows the previous constant wave condition solution to be extended to the case where wave properties (i.e. wave direction, wave height, and wave period) are time varying. Example usage of the method presented shows that shorelines may be of different final plan form shape for time varying wave conditions even though the sediment transport along adjacent beaches is not spatially varying (i.e. spatially constant) from time step to time step. Although this difference in shape may have been known previously using numerical models, it could not be proved analytically. Reversals of wave height, period, and direction time series are shown to provide different final shoreline shapes even though the time series consists of the same waves although in different ordered time. The solution provided will allow one line numerical shoreline models to be tested using an analytic solution.