Beach fill, or artificial nourishment, involves placing sand on an eroding beach to provide a wider beach. As time passes, however the beach fill will erode as (1) the original erosion mechanisms still prevail, and (2) the fill represents a perturbation of the shoreline.
This applet shows the behavior of a rectangular beach fill with time. You provide the initial length (along the beach) and the width of the fill. Further you need to provide the shoreline diffusivity. Then after pushing the Calculate button, the erosion of the fill begins. The sketch shows a planform (aerial) view of the beach, with the ocean to the top and the yellow denoting the original beach. The fill is colored with magenta. The figure shows only half of the fill; the other side is a mirror image. The length of the beach shown in the panel is 3 (fill length)/2.
The figures output to the panel are the elapsed time since the fill placement, the maximum distance the fill still protrudes into the water (the distance from shore to the furtherest offshore fill point--at the left in the panel), the percent of fill remaining in the original placement area, and the half-life (defined to be the time for one-half of the fill to depart the original fill area.) .
By experimentation, you will find that the longer the original fill (typically on the order of kilometers), the longer it remains in the original fill area. Further the greater the shoreline diffusivity (found from field measurements to vary between 0.002 and 0.14 m^2/s), the shorter the half-life of the fill.
The panel area is very distorted. The panel height is a little more than the original fill width and its length (as mentioned before) is 1.5 *(fill length). This leads to distortions on the order of 10, and the appearance that the sand is not conserved--don't worry, it is.
Comments: Robert Dalrymple
Center for Applied Coastal Research
University of Delaware, Newark DE 19716