Inception of stream incision by seepage erosion

Abstract

[1] Because seepage erosion is generated by complex interactions with other processes, associated stream incision process is not well understood. In this study, some fundamental characteristics of incipient incision by seepage erosion were investigated by laboratory experiments and linear stability analysis. The experiments were conducted with various sediment layer depths and gradients. With similar discharges in the experiments, incision spacing decreases with increasing depth of the sediment layer and with increasing gradient, whereas incision width increases with increasing sediment layer depth. A linear stability analysis was performed using the Dupuit-Forchheimer equation and an expression of the planimetric retreat of the scarp. The retreat velocity of the scarp consists of two terms: (1) a power law function that describes the specific discharge in excess of a critical discharge and (2) a diffusion-like function that describes the incision edge shapes, in which the retreat rate is enhanced or reduced by the convexity and concavity of the edges, respectively. This analysis shows that the characteristic incision spacing becomes infinitely small when the effect of the edge shapes is excluded. Using the experimental data of incision spacings, the values of the diffusion-like coefficient in the second term were estimated. Since the weight of a failure block and hydraulic pressure are the driving forces in the slope stability analysis, a relationship was found between the diffusion-like coefficient and the combination of the two forces.

Experiment of Channelization Due to Seepage Erosion

“…As the scale of mass failure is related to sediment size (larger failure when the size is smaller), we hypothesize that the larger scale stabilizes the scarp from channelization and bifurcation. This idea is similar to the previous theory of channelization [3] that the mass failure represents the diffusion-like function that tries to smoothen the perturbations along the scarps. Finally, we have introduced the concept of network circularity to investigate the development of channel network.…”

“…New channels were always initiated when S = 3%. According to [3], the channel spacing is related to groundwater depth. If the depth is shallower, the spacing is shorter.…”

“…In this study, we have extended the experiments by [3] & [4]. In their experiments, there were no initial channels at the downstream scarp.…”

“…The experiments conducted by [3] show that under the constant water discharges, a steeper chamber slope causes a shallower groundwater depth and results in an increase in the number of incisions at scarps. As the sediment layer depth increases, the channel width also increased.…”

“…In [3] and [4], water fed to the experimental chambers at the far upstream end. Channel bifurcation by uniform rain on the sediment surface has been studied by [5].…”