Sampling of the debris
Another sampling method is to enclose the string with a square of one meter from the side and collect all the shells found in it. You can then say how many shells are found per square meter in that area.
We are used to inspect the sand with magnets: in Palo are less than 150 grams per pound of ferromagnetic material! As these pictures show, there are areas with finer detritus and sand and other debris with larger, is appropriate to collect samples at various "stations" selected at regular distances from each other, to draw statistical conclusions.