Morphological sustainability of barrage impoundments
Barrages built in estuaries fundamentally alter the dynamics of the river with regard to both flow and sedimentation patterns. Therefore it is essential to ensure that these structures do not affect the sustainability of the systems in which they are built. In recent years there has been increased emphasis on assessing the effect of climate change on river flows and the impact that this has on watercourses. Therefore, to investigate morphological sustainability of barrage impoundments, the effect of climate change must be included. An assessment of the morphological sustainability of the River Tees impoundment is presented. The predictions were completed using the l-dimensional software package ISIS, which modelled flow and sediment movement within the impoundment. Fifty-year simulations were completed to predict the sediment distribution through the system under differing future scenarios. A method is proposed for extending the flow boundary for the numerical model, which uses a generic statistical modelling technique. It uses the historical flow data recorded on the Tees and forward predicts the series based on its statistical properties. Firstly, the Markov Chain method was used to predict a 50 year flow series which assumes a stable climate. The predicted series showed good correlation with the measured series in terms of both statistical properties and structure. Secondly, the method was further developed to enable climate change predictions to be incorporated. This means that the generated series can be modified to directly account for the possible influence of climate change on discharge. This technique uses a Markov model fitted in the framework of a multinomiallogit model, enabling catchment precipitation and temperature values to be linked to the discharge. Climate change predictions available for the period 2070 to 2100 were then used to create 50-year modified flow series for the River Tees under a medium\high and medium\low emissions scenario. During the period of sediment monitoring on the Tees a change to the sediment supply was noticed as a result of the high flows experienced in October/November 2000. Unfortunately, it is unclear whether the sediment supply will return to its original levels or if, as a consequence of higher flows resulting from climate change, the supply will remain at present levels. Hence three different sediment rating curves were created from the field data to deal with this uncertainty; representing high, medium and low sediment supply conditions. Using the data generated for the flow and sediment boundaries, simulations were undertaken to assess the morphological sustainability of the Tees impoundment. Simulations using a flow boundary, which assumed both a stable climate and a changed climate, as well as three different sediment supply options for each, were considered. The results show that the impoundment reaches a dynamic equilibrium during the modelled period, irrespective of the sediment supply. From this it is possible to state that the Tees Impoundment is morphologically sustainable over the next 50-80 years. Climate change, while increasing the sediment supply, actually appears to improve the sustainability of the impoundment with regards to sediment. The increased number of high flows cause more steep water surface slopes which reentrain sediments and partially flush the system. In conclusion this thesis presents an assessment of the morphological sustainability of the Tees impoundment under differing future climate scenarios for both the fluvial and sediment inputs. Within the course of the work a different technique for extending flow series assuming both a stable and changed climate has been proposed. It is hoped that these methods will be of use in future sustainability assessments; however further investigations into these methods would be beneficial.