Sea
Beach (baseline width)
Accretion (above baseline)
Erosion (below baseline)
Groynes
Waves rarely meet a beach head-on. When they break at an angle, the swash of each wave surges obliquely up the sand while the backwash drains straight back down the slope under gravity — and that asymmetry drags sediment steadily along the coast, a process called longshore drift. This top-down 2D model renders that mechanism directly: the along-coast transport rate at every cell boundary is computed each frame from the same sin(2θ) CERC-style law used in the 3D version, then integrated as a real 1D sediment-conservation ODE across the coastline. Drop a field of groynes into the surf zone — click the shoreline to place or remove them — and watch the physical consequence play out as a live beach-width profile: sand piles up in bright bars on the updrift face of each wall while the beach immediately downdrift is starved of supply and erodes toward exposed rock, exactly the trade-off coastal engineers weigh when they build one.