The development of small-scale sandy alluvial fans at the base of a modern coastal cliff: process, observations and implications

The development of a series of small-scale alluvial fans has been studied at the base of a sandy coastal cliff on the Torre del Loro Beach near Huelva, southwestern Spain. These fans are small enough to permit the experimental modification of flow, sediment load and morphology. Elongate sandy debris flow deposits are also present. ns are related to sources with perennial discharges and are fed by solitary, shifting distributary channels. The fans have radial morphologies and grow by localized lobe-building at the intersection point, in a mid-fan or fan-toe position, and/or by bulk aggradation due to overflow (sheet flow). Sediment dispersal is by channelized or unconfined water flow, but also involves slugs of “hyperconcentrated” flows and occasionally sandy debris-flows. The radial fans (channelized flow dominated) have surface slopes of up to 20°, their medial segments are often the steepest, and sediment at their toes and lateral flanks can be coarser than in the inner zones. The elongated sandy debris flow deposits are related to sources with ephemeral discharges and are mass flow-dominated. These deposits are smaller and are built chiefly by sandflows (finger-like sandy debris-flows). Their surface slopes are rather uniform, but only up to 6°, and there is no obvious downfan variation in grain size. egarded as small-scale analogues for larger alluvial fans, these fans deviate from the textbook models, particularly in grain-size distribution, simultaneity of “quiet” and “energetic” processes, and longitudinal gradient profile. Our study indicates that the relationship between fan morphometry and depositional processes still requires a better knowledge.