Sediment Transport On The River Bandon, Co. Cork, Ireland

This thesis analyses sediment transport on the River Bandon, Co. Cork, Ireland. Bedload transport and suspended sediment transport were monitored on the River Bandon over an extended period to determine rates of transport over varying flow conditions. A literature review of sediment properties (individual and bulk sediment), sediment transport, and numerical modelling was undertaken. A manual field sampling programme was undertaken at different locations on the River Bandon to collect bed sediment and suspended sediment for laboratory testing. This data, and data collected using an automatic water sampler and bed sediment traps also, was used to determine baseline conditions for the river. Sediment transport during high flow events and the impact of instream works (dredging) on sediment transport was then assessed to compare with baseline conditions to determine the impact on sediment transport on the river. A numerical model (HECRAS) was applied to different combinations of sediment transport functions and sorting and armouring methods to determine the combination that best represents sediment transport on the River Bandon. Baseline suspended sediment conditions yielded results for suspended sediment concentration (SSC) levels in the range 1-10mg/l at varying locations along the river. Suspended sediment concentration (SSC) levels were observed to increase to 45mg/l during trial dredging undertaken in September 2016 and in excess of 350mg/l during dredging in Bandon Town in Summer 2017. In general, SSC levels increased to between 20-45mg/l during high flow events but on occasion rose to in excess of 100mg/l and the maximum SSC observed during high flow events was in excess of 330mg/l. A hysteretic loop analysis for high flow events during the data collection phase revealed that 65% of suspended sediment was transported on the rising limb of the hydrograph. Baseline bed sediment conditions yielded results for median grain size (D50) in the range of 12- 35mm. Bed sediment was observed to move for flow rates as low as 18m3/s. Using bed sediment traps, as much as 7 tonnes of bed sediment was estimated to move during a single high flow event. Bed sediment was estimated to comprise between 2-19% of total sediment load during the high flow events analysed. A numerical model, previously calibrated for hydraulic conditions on the River Bandon, was used to determine the best combination of sediment transport function and sorting and armouring method for the river. The Meyer Peter Muller function with Wong Parker correction and the Active Layer settling method combination was found to most closely simulate the observed field data.