Water renewal and risk assessment of water pollution in semi-enclosed domains: Application to Bilbao Harbour (Bay of Biscay)

The inclusion of water renewal behaviour in the risk assessment of water pollution for Bilbao harbour (located within the Nervión estuary, southeastern Bay of Biscay) is analysed. Several parameterisations of water renewal (integral and local perspectives), obtained from the numerical evolution analysis of a Eulerian tracer, were used. The local characterisation of water renewal confirmed a strong horizontal spatial variability. In addition, vertical variability on water renewal patterns, due to the 3-dimensional structure of the flow was also appreciated. For the period analysed (August 2008) the water renewal was less effective at the surface, than in deeper waters, because marine breezes counteracted water renewal at the surface, during the observed period. The incidence of this vertical variability, on harbour environmental management, was analysed in implementing a recently developed methodology to evaluate the risk of water pollution. Since this methodology takes into account water renewal, the values of risk of water pollution in Bilbao harbour differed considerably, as a function of the renewal pattern at the surface or within deeper waters. In conclusion, it was found that the water renewal should be parameterised properly, in order to improve environmental management strategies in semi-enclosed water bodies in coastal areas. The findings permitted knowledge to be gained on water renewal behaviour within Bilbao harbour, advancing the framework for the environmental management of harbour waters.

► A method to evaluate the risk of water pollution was applied at Bilbao harbour. ► This method comprises the use of hydrodynamic numerical modelling. ► A marked difference in the renewal pattern of the water column was appreciated. ► It was found that marine breezes can affect negatively the water renewal at surface. ► 3-D patterns of water renewal are needed to implement risk assessment of pollution.