In part I of this paper, near-bottom fluid velocity and sediment concentration measurements from the shoreface of a middle Atlantic barrier (Long Island coast) are analyzed to provide insight into the dynamics of erosional shoreface retreat. In Part II ∗ ∗ Part II is on pp. 363–396 of this volume. , these data are combined with observations of coastal stratigraphy in order to construct a model for the evolution and behavior of middle Atlantic barriers. Wave motions on the Long Island shoreface tend to drive sediment onshore. Calculations of the onshore sediment flux driven by asymmetrical, shoaling waves show that for the wave states commonly encountered on the middle Atlantic shelf, values become strongly positive landward of approximately 10 m water depth (upper shoreface) but drop to very low values shortly seaward of that isobath. On the lower shoreface and inner shelf floor, fair-weather wave-current interaction tends to cause a landward creep of sediment, even though the wave-orbital component of the near-bottom velocity field is nearly symmetrical. This movement occurs because the wave approach direction generally lies within 90° of the direction (up-coast or down-coast) of the alongshore wind-driven current. Therefore, fluid shear stress acting on the sediments tends to be reinforced during the shoreward stroke of the wave orbital motion and partially cancelled during the seaward stroke. During major storms, the rate of sediment transport increases by at least an order of magnitude and the role of wind-driven currents becomes important. Much more sediment is resuspended by the wave orbital current component because storm waves are more powerful than fair-weather swells. But the wind-driven flow component is also much stronger and is now available for transporting the resuspended sediment. Storm flows over the shoreface occur in distinct dynamic zones (surf zone, friction-dominated zone, transition zone, geostrophic zone). The zones are defined by dynamical considerations and expand or contract with the intensity of the causative wind stress. During peak flow events, the characteristic length scales of the friction-dominated and geostrophic zones tend to correspond with the morphologic zones of the shoreface and inner shelf. The storm-intensified, wind-driven, alongshore flows of the lower shoreface are frequently jet-like in nature and may be upwelling or downwelling. Wind-driven coastal flows with a strong downwelling component occur during most storms on the Long Island coast and are particularly important to the coastal sand budget. During these events, sand is entrained by the storm-intensified upper shoreface circulation system of wave-driven alongshore currents and rip currents, and is fed into the main wind-driven coastal current. Because of the offshore component of bottom flow, sand is swept down the lower shoreface and onto the adjacent inner shelf. Fair weather processes may be unable to return storm-deposited sand to the beach from such an offshore position. The shoreface transport regime of the Long Island Barrier coast thus consists of long periods of time (months) during which sand moves slowly toward the beach, punctuated by short intense periods (hours or days) during which sand is transferred from the shoreface to the adjacent inner shelf. The long-term sense of movement of the shoreface, whether retreating (the Long Island case) or prograding, must depend on the loss or gain of sand by shoreface with respect to the inner shelf. The data shows that the coastal sand budget is controlled not only by the upper shoreface cycle (withdrawl of sand from the beach prism, storage in the breakpoint bar and its subsequent return to the beach), but by a cycle of larger spatial and temporal scale, in which sand is exchanged between the shoreface as a whole and the adjacent inner shelf.