Modelling the potential impact of groundwater dynamics on gully erosion and drainage basin evolution

In river basins, groundwater dynamics often condition the spatio-temporal patterns ofrunoff generation and discharge. Hence, water table level controls the patterns of"Dunne" runoff generation or saturation excess overland flow. In this study geomorphicchanges of a complex gully system in Swaziland's Middleveld were traced from 1947 to1998. For this purpose, High-resolution Digital Terrain Models (HDTMs) weregenerated, based on aerial photographs. The gully system shows a complex history, thatinvolves fluvial erosion and mass movement processes. During the observation period,104,500 m3 of material were eroded from an 11 ha gully contributing area. The evolutionof gully growth is strictly depending on the deepening of gully bottom at the mouth (localerosion base level), that in turns is influenced by the temporary base level (regionalerosion base level). Consequently, the longitudinal profile of the stream generates lateralsubsurface head distributions. To investigate the escarpment retreat by groundwatererosion we developed a landscape evolution model with an integrated hydrologic module.Groundwater flow is simulated by a dynamic two-dimensional equation for anunconfined aquifer. This model was subsequently applied to evaluate the effect ofgroundwater fluctuations on the development of gullies and hence, on landscapeevolution. In this paper we show that gully genesis and growth are closely related to thehydrological properties of a complex aquifer system. © 2009 by Nova Science Publishers, Inc. All rights reserved.