Predicting the effect of beach nourishment and cross-shore sediment variation on beach morphodynamic assessment

Studies of coastal morphodynamics are becoming increasingly more focused on quantification of relationships between processes, form and function of dynamic landform systems because wave climates (e.g., wave height, wave period, seasonality, cyclical patterns) and sediments (i.e., composition, size, and shape) interact in various ways to collectively produce distinctive types of beaches. This paper identifies criteria and boundary conditions that characterize beaches in terms of morphodynamic states (environmental conditions or energetic stages of development) that produce discrete beach types. Long-term hindcast wave statistics (U.S. Army Corps of Engineers, Wave Information Study), small-scale aerial photography, cross-shore beach profiles, and beach sediment data from Florida's Atlantic (Delray Beach) and Gulf (Longboat Key) coasts were used as a basis for linking beach morphology with coastal processes. The role of cross-shore sediment variation in classification of beaches is related to prediction of beach types using the dimensionless fall velocity (Q), the results of which are compared with field observations and morphological interpretation of aerial photographs. A new curve-fitted equation, the morphodynamic boundary condition (MBC), identifies parametric limits that force dissipative or reflective beach conditions when new sediments are artificially placed on the beach (e.g., during beach nourishment programs). Results indicate that the 92 parameter of the new model is strongly influenced by cross-shore selective sorting of bimodal sediments, temporal changes in beach grain size, seasonal wave patterns, and inputs of new sediments to the littoral system. Calculation of Omega using beachface samples produces a bias toward reflective states (decrease in Omega). Sediment samples from bar systems, on the other hand, produce a bias toward dissipative states (increase in Omega). Composite grain size of the active profile is recommended because it more accurately predicts beach states based on Omega (compared to field observation of beach type). The addition of new sediment to beaches requires an understanding of form-process continuums and the MBC equation is a step in that direction. The potential impacts of beach nourishment on beach morphology can be thus determined from local wave data by calculating hypothetical Omega values as a function of changes in beach composite grain size and a constant K value. The MBC equation (Q=Kd(f)(-2)), as applied in planning phases of renourishment projects, can predict changes in beach morphodynamic states, as conditioned by the grain size of the placed fill. (C) 2004 Published by Elsevier B.V.