Shoreline autoretreat substantiated in flume experiments

Under conditions of constant sediment supply (S) to the basin and a steady rate of rise (A) in relative sea level, the autoretreat theory predicts that shorelines can retreat landwards at a relatively early stage of delta growth and attain an "autobreak" state, after which the existing subaqueous slope begins to be starved of sediment and thereby lose its clear delta-front configuration. A numerical model based on this theory further suggests that shoreline migration depends on the inclination of basin's basement slope (phi) and subaqueous depositional slope (beta) and, to a lesser degree, on the inclination of subaerial depositional slope (alpha). Two-dimensional flume experiments in which a miniature delta is subject to steady forcing (A = const > 0, S = const > 0), were conducted to test the expectations of the autoretreat theory. The experiments consisted of two different series of runs: one was of "phi modulation" associated with a narrow range of the ratio of water discharge to sediment discharge (q/S), the other was of "q/S modulation'' with phi kept nearly constant, The q/S ratio was mostly responsible for changes in alpha but also affected beta to some extent. All experimental runs substantiated the landward turnaround feature of shoreline accretion (i.e., autoretreat) and the geometrical break of the delta-front shape during its retreat phase (i.e., autobreak). The obtained shoreline trajectories hold their geometrical patterns even after they were nondimensionalized in terms of the S/A ratio. A key hypothesis confirmed in the experiments is that A and S can never balance each other. Both of the experimental series brought some systematic change of shoreline trajectories, but the variations of shoreline trajectory due to phi modulation were much more striking than those related to q/S, Even these contrasting results are consistent with the numerical predictions. The present experimental results support the applicability of the autoretreat concept to natural environments and help clarify the processes governing the regression and transgression of a shoreline on a constructional margin.