Modelling salt intrusion and nitrate concentrations in the Ythan estuary

A one-dimensional salt intrusion model is used to investigate the hydrograp hy of the Ythan estuary, a small shallow macrotidal estuary in the north-ea st of Scotland. The model simulates the longitudinal distributions of water level, salinity and total oxidized nitrogen (TON) in the estuary. The mode l employs upstream differencing and the Smolarkiewicz anti-diffusion scheme to avoid the numerical difficulties typically encountered when modelling s trong tidal flows using centred differences. The physical mechanisms drivin g the simulations are the tide at the entrance to the estuary and freshwate r discharge at the head. The model was calibrated against measurements of w ater level made at three locations in the estuary, salinity observations ma de at a central platform and axial salinity distributions. At both spring a nd neap tides, the full range of salinity observed at the central platform was simulated. However, at the midway stage between springs and neaps, the simulated peak salinity was less than that observed. This was probably due to the sensitivity of the model to the digitisation of the estuarine bathym etry. The model successfully simulated salinity distributions for periods of high and low river flow, and was used to illustrate how TON concentrations fluc tuated in response to variations in river how. The potential implications o f variations in the bathymetry of the estuary on salinity and nutrient dist ributions were predicted to be slight. However, the four fold increase in r iverine TON concentrations that has occurred over the past 30 years was sho wn to increase TON distributions along the entire length of the estuary. Th e calculated estuary flushing time was strongly dependent on river flow and varied between 11-60 h.