Catchment-scale sediment delivery : the development of a tool for the simulation of in-stream concentrations

Fine sediment acts as a critical transport vector for nutrients, trace metals and organic contaminants, and can have a severe detrimental effect on aquatic ecology and the viability of riverine habitats. An understanding of the II!.oyement of fme sediment within river catchments is therefore important. Modelling is an important tool for understanding and managing the catchmetn sediment delivery system. Existing modelling tools fall broadly into two categories: simple empirical models based on statistical relationships in observed data; and highly complex process-based models based on physical modelling of the mechanical processes involved in sediment delivery. De Roo (1998) expressd the need for a new type of model that makes a compromise between the catchment and process representation of complex models and the simplicity and wide-ranging applicability of empirical models. The aim of this thesis is to develop a new model that fulfils this need. The development of the model is guided by three criteria: • the model must require only readily available and significant data; • the model can use only a limited number of key variables and parameters; • the model should only include the dominant key processes controlling sediment delivery. In order to meet these criteria, new process-informed equations representing the key processes of sediment delivery are derived based on a review of existing modelling approaches. These equations simulate catchment hydrology and the processes of sediment erosion, transport and deposition within a semi-distributed model structure. The model is applied to three lowland catchments in Berkshire, England: the Upper Kennet, the Lambourn and the Enborne. Model output is shown to successfully reproduce observed river flows and suspended sediment concentrations, although there is a consistent underprediction ofthe highest flows and concentrations in all three catchments. The model successfully simulates the contrast between the hYdrology and sediment delivery of the Upper Kennet and Lambourn Chalk catchments and the clalbtchment of the Enborne. The model is subjected to a Monte Carlo based General Sensitivity Analysis. This analysis confirms the efficacy ofthe hydrological model and identifies the transport capacity of direct surface runoff as the critical process in the determination of in-stream suspended sediment concentrations. The results of the model applications and the sensitivity analysis are used to assess the success of the model in meeting the stated criteria. This assessment shows the model to be efficiently modelling the key elements of the catchment sediment delivery system.EThOS - Electronic Theses Online ServiceGBUnited Kingdo