Optimizing Shoreface Nourishment Design Using The Concept Of Equilibrium Beach Profiles

Abstract

Introduction and Problem Description: The autonomous nearshore morphodynamics along the Nags Head shoreline are characterized by consistent erosional behavior. The use of shoreface nourishments to counteract this erosion along shorelines has received considerable attention in the past, having the advantage of reduced cost compared to beach nourishments. Although shoreface nourishments are thus an increasingly interesting option for coastal managers, their design is often highly empirical and based on guidelines. A better understanding of the way a shoreface nourishment interacts with the antecedent bathymetry and respective forcing agents (i.e. - waves and tides) may help to reduce the degree of empiricism and possibly optimizing nourishment design in terms of longevity and shoreline sedimentation. Rather than using design guidelines, the present research aims to relate nourishment design to beach profile shaping parameters like wave climate and sediment characteristics.
Representation of beach profiles based on local wave dissipation, wave reflection and sediment characteristics have previously been studied in the form of equilibrium beach profiles (EBP). Recently major improvement in beach profile representations are made by including wave reflection in the energy balance, resulting in a two-sectioned EBP. After local site-specific parameter calibration, the vertical deviation between the initial profile and analogous calibrated EBP (z_{ebp} - z_{ini}) should indicate where a scarcity of sediment along the cross-shore profile is. Shoreface nourishments are then designed by filling in the vertical gaps between these two profiles. A hypothesis was postulated stating that the optimal form for a shoreface nourishment follows the equilibrium beach profile the best. The differences in impact conventional nourishment designs (based on guidelines) have on either cross-shore and longshore transport rates are computed and compared to an EBP-based design in two separate modeling studies.
Effects on cross-shore transport: Beach profile morphology and the response to shoreface nourishments are modeled in the 1D cross-shore profile model UNIBEST-TC. Three distinct conventional designs which have found recent applications are selected and based on either; extension of the outer bar, creating a new outer bar or filling in the trough shoreward of the outer bar. Analysis of the model results show that all four nourishment designs are incorporated well in the cross-shore morphodynamics as compared to the situation prior to construction.
The EBP-design shows strongest reduction of offshore directed transport, followed by designs based on filling in the trough, creating a new outer bar or extending the existing outer bar. Therefore the model simulations suggest that a nourishment design with the largest vertical deviations from the EBP may be the least effective to counteract coastal erosion. This partially confirms the postulated hypothesis that cross-shore transport rates are lowered most efficient while using the concept of EBP to design shoreface nourishments.
Effects on longshore transport: The coastal area flow model Delft3D was utilized for a series of numerical modeling simulations to examine the potential dependencies between nourishment design and post-dredging longshore transport rates and local gradients. Model results show a similar outcome as the cross-shore profile model, where the EBP-design results in the lowest longshore transport rates. Closer examination of the simulations, especially concerning the local gradients and longshore transport rates, show that the EBP-design reduces longshore gradients by a factor two up to four compared to conventional nourishment designs. The EBP-design therefore shows less sediment transport and local gradients because of a divergence of non-linear local sediment transport rates over the coastal zone. Confirming earlier studies that local longshore gradients dominate coastal change at the scale of nourishments and the hypothesis that shoreface nourishment design based on the concept of EBP is a more efficient way compared to conventional designs.
Practical applications: The results and theory demonstrate how the incoming wave climate and sediment characteristics are responsible for both the EBP shape and sediment transport. Since the erosion rates at the project site show local longshore variability, and the EBP shape remains constant even though using the same characteristics, the deviation between the initial- and EBP profile should indicate a gradient in longshore transport rates as well. Knowing this, the overall longshore gradient can be estimated using only one survey dataset. This could lead to a preliminary nourishment design, based on only one bathymetry survey and project budget. Approximation of longshore transport gradients could strongly improve the nourishment lifetime, especially in remote locations.
The constant longshore shape of the EBP with respect to MSL is furthermore efficient in reducing the local longshore transport gradients. Since the EBP-model is used as a template to fill in the vertical deviations between the initial profile and EBP, the longshore variabilities are spread out over the project site creating a less obstructive flow, thereby reducing longshore transport rates as well. Whereas conventional nourishment designs follow the original longshore variabilities and only enhance them, resulting in strong 3D bathymetrical features corresponding to coastal erosion as well.