Because sandy beaches are often totally devoid of primary producers, food webs based on autochthonous production are very rare. The accumulation of wrack along the shorelines has been shown to allow spatially subsidised food webs in these environments. A method for quantifying and describing the spatial distribution of beach-cast macrophyte material was developed based on line transects set up perpendicular to the shore. The method combined rapid visual techniques with measurements of detritus patch area, patch thickness, and patch distance from the shore. Average agreement between field observations and measurements was good: R2 = 0.93, p < 0.05, N = 53, with R2 ranging from 0.65 to 0.98. The method was used to quantify beach-cast Cymodocea nodosa leaf litter (dry weight, carbon, nitrogen, and phosphorus) along the 29 km shoreline of a semienclosed estuarine bay that is part of the delta arrow of an estuarine system (Alfacs Bay, Ebro River delta, Tarragona, Spain). The total amount of beach-cast leaf detritus varied from 0.07 to 3.02 kg dry wt m−1 of shoreline. The largest amounts of detritus accumulated on gentle slopes exposed to the dominant winds in the area. Total beach-cast leaf litter stocks in the bay amounted to 22.1 t dry wt, 7.1 t of carbon, 0.54 t of nitrogen, and 0.013 t of phosphorus. Export rates to the beach were estimated at 55.5 to 97.3 t y−1, which represented 4.1% to 7.4% of the C. nodosa leaf production in the bay and 5.8% to 10.3% of the total meadow leaf export.