Flume Experiments

The second phase involved five joint experiments (Table 1),  using the high capacity flume installed at the Hydroenvironmental Research Centre in the School of Engineering, Cardiff University. The 17 m long flume was modified to accommodate a model estuary (Fig.1) and the system filled with fine commercial sand to a depth of 10 cm, with the exception of a plug (30 wide by 10 cm long). In some experiments the region 2 m<x<6 m (x=distance downstream) was filled with sieved, untagged Mersey sediment to investigate the effect of more natural sediment on the partitioning of exchangeable metals. The tagged sediments were transported to Cardiff and inserted in the flume as a plug located at x=4.5 m (Fig. 1). The flume had a water depth of 40 cm and was run at a constant flow rate of 0.5 m s-1(see video clip).

Figure 1: Plan view of the high capacity flume fitted with an idealised funnel-shaped estuary model (defined by the blue area). The key regions of the model estuary are the upstream channel (2 m<x<5 m), the expansion zone (5 m<x<11 m) and the downstream channel (11 m<x<15 m).  The flow is from left to right. The plug of Plym or Mersey sediment (i.e. the brown bar at 4.5 m) is tagged with tracers Rh and Pt, and exchangeable metals, Ni and Zn.

Water samples were taken, at 6 strategic sites along the axis of the flume, both at near the bed (NB) and at 40% of the water column height above the bed (0.4H), using a specifically designed pumped, sample-acquisition system. Water samples were obtained for 3 intervals during an experiment, i.e. at 0.5 h, 4 h and 8 h and at some locations samples were taken in triplicate to assess the reproducibility of the overall method. Monitoring of water velocities, at the same heights above the bed and at the same times as the water sampling, were undertaken using acoustic Doppler velocimetry. The bed levels were also measured. Each water sample was filtered, within 3 h of collection through pre-weighed GF/F filters and the filters holding the SPM were stored frozen in individual Petri dishes. The filtrates were stored in plastic bottles and acidified to pH=2 using ultra-pure concentrated HCl. After 8 h the flow was stopped and the water drained from the flume to allow sediments cores to be taken. A syringe-type corer was deployed, to a depth of about 6 cm, at various intervals along the flume and the cores were sectioned on retrieval. The sediment and SPM samples were returned to Plymouth for metal analysis by ICP-MS.