Remediation Strategies and Water Quality of estuarine impoundments

Abstract

The implementation of amenity barrage schemes, such as the recent projects on the Tees Barrage and at Cardiff Bay, has gained popularity as part of urban regeneration projects in the UK. The predecessors of these schemes on the rivers Tawe, Lagan and Wansbeck offer valuable information on the impacts and remediation strategies that maybe required to sustain good water quality in the newly created upstream impoundment. Depending on the design, the stored water body is affected by: density stratification; flow velocity reduction; siltation and contaminant accumulation; all of which can lead to periods of poor water quality following barrage construction. This thesis identifies the principle dynamics leading to low water quality within a partial exclusion system (River Tawe) through numerical and observational analysis of monitoring data recorded over the last nine years. Temporal variation of water quality within total exclusion system (Tees Barrage) is described from continuous monitoring records covering a three-year period. The data for both designs showed significant seasonal variation and flow was identified as a major factor in the process of low water quality development. In addition, the influence of the tidal regime was determined for the oxygen and salinity dynamics in the partial exclusion impoundment. Due to influx of saline water, partial exclusion systems are likely to suffer from saline stratification, which restricts the mixing processes in the water column and often leads to DO depletion in the lower layers of the impoundment. Several remediation strategies have been applied and proposed to prevent this process, including measures to break-up (Mixers), to prevent (Baffles) or to flush out (Sluicing) the stratified water body. Each of these principles is presented and discussed in this work separately, to determine its feasibility for the future management of water quality in partial exclusion impoundments. The installation of a mixing system in the Tawe impoundment was proved to be successful in breaking up saline stratification and recommendations are given for further adjustment. Flushing was ruled out as an effective measure to improve water quality due to its inability to completely expel low dissolved oxygen water. A future installation involving a floating boom/skirt system was tested in laboratory experiments and the general feasibility of concept was confirmed, whilst design criteria for the prototype were established.