Abstract

Although current landscape evolution models can predict landscapes with specific concave-convex slopes, regolith thicknesses, drainage densities and relief, these models rarely include realistic groundwater and overland flows, and channel-hillslope interactions.  To overcome the potential drawbacks, the study in Chapter 2 couples hydrologic processes with hillslope and channel sediment transport processes to form a new hydrologic-morphodynamic model (LE-PIHM) for regolith formation and landscape evolution.  In Chapter 3 two scenarios with and without groundwater flow are presented to demonstrate the importance of this coupling. Comparison of the steady state landforms indicates that hillslopes are steeper and relief is higher with groundwater flow. Unlike many previous studies which simplified the hydrological processes, and thereby neglected the influence of landscape evolution on hydrology, this study emphasizes the importance of the co-evolution of the hydrological-morphological landscape system. The sensitivity of the numerical solution to mesh geometry is tested and it is shown that the model simulations maintain the characteristic features of a landscape over a reasonable range of maximum triangle area and minimum interior angle which are key parameters to generate
unstructured triangular mesh for the model domain. To predict long-term landscape change, a morphological acceleration technique is presented and a method for choosing an optimal morphological scale factor is introduced. In the fourth study, the spatial distributions of the evolution of topography, regolith transport, and bedrock weathering of the Susquehanna Shale Hills Watershed (SSHW) at the watershed scale are estimated using LE-PIHM. The channel spacing and elevation relief of the simulated steady state landscape are consistent with the current SSHW, implying that the SSHW is close to morphological equilibrium, and the parameters derived from field measurements are representative of the current SSHW at the watershed scale. In the fifth Chapter a one dimensional steady state soil thickness profile derived from LE-PIHM is very similar to the soil thickness profiles of SSHW where the soil thickness thickens downslope. The spatial variation of morphological diffusivity in the Shale Hills watershed is also estimated by using a 1-D analytic model framework. In the sixth Chapter, LE-PIHM is also applied to a groundwater sapping landscape where groundwater dominates channel erosion. Unlike runoff dominated landscapes which develop fan-shaped channels, groundwater sapping landscapes evolve relatively straight main channel and very few tributaries. This study uses a 3-D modeling approach to understand the control of soil properties on the channel evolution. The numerical experiments reveal that the soil vertical and horizontal hydraulic conductivity (K_v and K_h) exerts the first order control on the channel pattern.
Although sediment particle size was varied by two orders of magnitude, it exerted only a weak effect on channel pattern. With different combination of K_v and K_h, the channel evolution switches strong control from strong K_v control to strong K_h control or from strong K_h control to strong K_v control accordingly. The study also classifies three regions where surface runoff erodes landscape, groundwater sapping controls erosion and landscape with very mild fluvial erosion.

Citation

Zhang, Y. (2016): FULLY-COUPLED HYDROLOGICAL AND MORPHOLOGICAL PROCESSES FOR MODELING LANDSCAPE EVOLUTION . Doctor of Philosophy, Civil and Environmental Engineering, The Pennsylvania State University, p. 196.

This Paper/Book acknowledges NSF CZO grant support.