Numerical Modelling of Hydrodynamics and Sedimentation in Upland Lakes and Implications for Sediment Focusing

Despite the increasing resolution and precision of palaeolimnological studies these are often founded on assumptions regarding the distribution and completeness of lake deposits that are not always justified. In particular, the assumption that focusing of suspended sediments leads to preferential deposition in the deepest part of a lake is not always supported by observations, especially in upland lakes subject to energetic wind forcing. Surprisingly, very few studies have investigated the hydrodynamic controls on sediment accumulation. This thesis approaches this problem through physically-based numerical modelling of a small oligotrophic upland lake (Llyn Conwy, north Wales, UK). First, a new one-dimensional model is developed to characterise seasonal variation of lake thermal structure and its interaction with meteorological factors. Second, a three-dimensional model, FVCOM, is used to investigate the wind-driven circulation. The existing FVCOM model code is enhanced through provision of a new graphical user interface and a semi-empirical wind wave model. Simulations are performed for varied meteorological conditions and evaluated with respect to observations. Modelled variations in the bottom stress due to currents and wind waves are compared with the observed distribution of bottom sediments. Contrary to the ``sediment focusing'' hypothesis, some of the deepest waters are devoid of recent sediment accumulation and this can be directly attributed to intermittent wind-driven currents that are competent to resuspend material. The processes determining sediment accumulation are investigated further through simulations of suspended sediment dynamics for a set of idealised lake bed configurations and realistic meteorological forcing. Whilst the magnitude and frequency of such resuspension events is a function of the imposed wind climate, their spatial distribution and that of sedimentation within the lake, appears to be strongly influenced by the shape of the basin. Further work is required to extend this ...