Hillslope evolution by nonlinear, slope-dependent transport: Steady state morphology and equilibrium adjustment timescales

Soil-mantled hillslopes are typically convex near the crest and become incr easingly planar downslope, consistent with nonlinear, slope-dependent sedim ent transport models. In contrast to the widely used linear transport model (in which sediment flux is proportional to slope angle), nonlinear models imply that sediment flux should increase rapidly as hillslope gradient appr oaches a critical value. Here we explore how nonlinear transport influences hillslope evolution and introduce a dimensionless parameter TL to express the relative importance of nonlinear transport. For steady state hillslopes , with increasing YL (i.e., as slope angles approach the threshold angle an d the relative magnitude of nonlinear transport increases), the zone of hil lslope convexity becomes focused at the hilltop and side slopes become incr easingly planar. On steep slopes, rapid increases in sediment flux near the critical gradient limit further steepening, such that hillslope relief and slope angle are not sensitive indicators of erosion rate. Using a one-dime nsional finite difference model, we quantify hillslope response to changes in baselevel lowering and/or climate-related transport efficiency and use a n exponential decay function to describe how rapidly sediment flux and eros ion rate approach equilibrium. The exponential timescale for hillslope adju stment decreases rapidly with increasing TL. Our results demonstrate that t he adjustment timescale for hillslopes characteristic of the Oregon Coast R ange and similar steep, soil-mantled landscapes is relatively rapid (less t han or equal to 50 kyr), less than one quarter of the timescale predicted b y the linear transport model.