Models

Following an intensive year-long water quality survey in the Conwy, together with combined sewer overflow (CSO) data retained from Dwr Cymru/Welsh Water, our modelling objective is to simulate this period to better understand the controls of, and natural seasonal variability in, viral dispersal through the river-estuary-coastal continuum. We can then use the model to investigate specific extreme events during other periods, or realistic-but-idealised events, in more detail; hence, generate spatial ‘risk maps’ that can be interpreted back to water quality managers.

In this video, we have developed an estuarine model, which is also coupled to a catchment water quality model, to simulate these processes in the Conwy estuary and the surrounding waters of northwest Wales. The model has so far helped us understand that small or steep catchment-estuary systems like the Conwy – of which there are many throughout the UK – are sensitive to short-duration heavy rainfall events, whereas larger systems will respond to long-term and sustained frontal weather systems. Importantly for up-scaling nationally, these contrasting systems require different modelling techniques and data resolutions.

This model was created using Telemac-2D (an open-source, finite-element, free-surface, ocean model) which simulates two theoretical viral peaks released from Llanrwst and Betws-y-coed during a realistic high flow river event (data from 20th October 2013). The model is set-up at high-resolution spatially (~20 m) and also with high-resolution temporal forcing (e.g. 15-minute flow/viral forcing). The model works well in very shallow and inter-tidal regions. Tides are forced form the TPXO global tidal database. Bathymetry is derived from a variety of sources, including Admiralty charts, multibeam surveys, and LIDAR surveys.

The video shows the theoretical release and dispersal of a pathogenic virus in the coastal zone. The viral dispersal commences at a wastewater discharge point upstream in the river Conwy and flows out into the Conwy Bay. The animation illustrates a dispersal using realistic hydrodynamics but theoretical viral dispersal data (e.g. position, timing and concentration). As fieldwork progresses, and with data inputs from Welsh Water, this theoretical data will be replaced with actual values.

The video shows the simulated transit of virus particles down the river Conwy from waste water treatment works (WwTWs) in Betws-y-Coed and Llanrwst to the tidal limit. The model assumes a 30 minute pulse of virus particles into a ‘virus free’ river. The input virus concentrations are estimated from measurements made at each WwTW discharge point and all concentrations are in GC L-1. Discharge flows are set at 0.01 and 0.025 m3 s-1 respectively and virus concentrations at 200,000 GC L-1. River flows are computed from historic data, with discharge at Llanrwst of ~6 m3 s-1, which is a little below average flow.

The transport of viruses shown is based on advection alone. The model is being developed to account for dispersion. In-stream processing will be included once estimated rates have been established from field and laboratory experiments.