Waves and currents do most of the moving
Most sand movement on a beach happens in the swash zone, the thin sheet of water that rushes up and slides back down the sand with each wave. High-energy waves tend to pull the shoreline back, while low-energy waves let it build forward, and this pattern holds regardless of how steep the beach is [4]. A gentler slope does reduce how far the shoreline retreats and favors sand building up, which is why the relationship between sand volume change and wave conditions works better when the slope is factored in [4].
Sand also travels along the shore rather than only straight in and out. At Panama City, Florida, where 11.25 million cubic meters of sand were placed over 20 years, about 9 percent of it was lost sideways out of the project area by longshore transport, while 87 percent stayed on the beach profiles [3]. That gives a sense of scale: even a large, well-placed nourishment loses a meaningful share to sideways drift over two decades.
Wind moves sand too, and grain size changes how much
Onshore wind is the main force that builds coastal dunes by carrying sand inland, while offshore wind does the opposite and exports sand seaward, shrinking the beach's sand budget [2]. But how much sand the wind can carry depends heavily on grain size, and grain size itself shifts over time. At Leucate beach in southern France, a microtidal site where the tide range is small, sand ranged from medium to very coarse depending on recent wind and wave conditions [2].
The practical effect is large. At wind speeds of 10 to 14 meters per second, measured sand transport ranged from about 10 kilograms per meter per hour when the beach was coarse to 50–150 kilograms per meter per hour when it was medium-grained [2]. Wind tends to coarsen the surface sand over days to months, while short marine storms mix the beach and leave it finer, so the same wind can move very different amounts of sand depending on what happened in the preceding weeks [2]. The upper beach surface barely changed when sand was coarse, but could drop by 0.5 meters when sand was medium-sized [2].
Rivers, headlands, and harbors redirect the supply
Rivers are a major source of new sand. At El Jadida Beach in Morocco, erosion in the middle of the beach is attributed partly to reduced sediment supply from the Oued Oum Erabia river, combined with exposure to dominant swells and heavy summer use for beach games [1]. The same study found that a harbor breakwater to the northwest and rocky outcrops to the southeast act as barriers that block sediment movement, producing erosion in the middle and accumulation at the ends [1].
Rocky headlands do not always block sand, though. Along a stretch of the Tyrrhenian coast in southern Italy, petrographic analysis of beach sands suggested that the Capo Suvero promontory does not obstruct longshore transport from the north, because storm-wave-driven currents carry sand past the headland [5]. That coastline also shows how human activity can reshape supply: between 1953 and 1983, continuous beach changes including erosion were caused mainly by human activity in coastal areas and inside river basins, even though the long-term trend from 1870 to 2019 was accretion [5].
People also add sand directly. Beach nourishment is the deliberate placement of sand to widen a beach or raise its profile, and at Panama City the measured rise in the beach profile (0.66 meters) closely matched what designers predicted from the volume of sand that remained [3]. That kind of project is one of the few cases where the sand input is known precisely, which is why it offers a clean measure of how much stays and how much leaves.
The same beach can gain and lose sand at the same time
Sand movement is not a single process running in one direction. At El Jadida, beach profile measurements showed sand accumulating on the subaerial beach, the part above the water line, while the lower foreshore eroded over the same two-year period [1]. The long-term shoreline record and the short-term profile record told different parts of the story, which is why the study used both time scales to understand longshore and cross-shore mobility [1].
This matters for anyone trying to read a beach's health from a single visit. A wide, sandy upper beach can coexist with a thinning lower beach, and a coastline that gained ground over 150 years can still have lost it sharply during a few decades of human interference [1][5]. The factors that bring sand in and take it out operate over hours, seasons, and centuries at once, and the net result depends on which of them is dominant at the moment you look.
Sources used in this answer
Periodical Morphodynamic Changes of Sand Movements at El Jadida Beach
At El Jadida Beach, a harbor breakwater and rocky outcrops block sediment movement, producing erosion in the middle and accumulation at the ends, while reduced river supply and heavy summer beach use also contribute to mid-beach erosion.
Aeolian sand transport and beach morphology influenced by temporal beach grain size variability in a microtidal environment
At Leucate beach, offshore wind exports sand seaward and onshore wind builds dunes, and temporal grain-size changes from coarse to medium sand can raise wind-driven transport rates roughly five- to fifteen-fold under the same wind speeds.
Measured Fate of Beach Nourishment Sand
About 87 percent of nourishment sand placed at Panama City over 20 years remained on beach profiles, with roughly 9 percent lost to longshore transport and 4 percent lost to dredging within closure depth.
SHAPING THE BEACH: CROSS-SHORE SAND TRANSPORT IN THE SWASH ZONE
Shoreline retreat and advance in the swash zone are driven by high- versus low-energy waves independent of beach slope, while milder slopes reduce retreat and support accretion.
Shoreline evolution and modern beach sand composition along a coastal stretch of the Tyrrhenian Sea, southern Italy
Along a southern Italy coastline, long-term accretion occurred from 1870 to 2019, but human activity in coastal areas and river basins drove erosion and continuous beach changes between 1953 and 1983, and storm-wave currents carry sand past the Capo Suvero headland.