Major hazard risk assessment over non-flat terrain. Part II: instantaneous releases

This is the second part of a two-part paper which considers the risk of being exposed to accidentally released toxic gas; part one considered continuous releases and this part considers instantaneous releases. The risk estimated is RI, the probability at any point of receiving a non-zero dose given an instantaneous release (probability language is needed because wind direction is assumed to be a random variable). On flat ground, simple analytical techniques show that for an instantaneous release of buoyancy B with a uniform windrose, at radius r, where is the front Froude number and u the windspeed. In this paper RI is estimated over non-flat terrain using shallow layer modelling for a range of windspeeds and slopes. The source term is identical to that of Thorney Island Trial 08. In a windspeed of 1 m s−1 a slope as little as 2% substantially affects the risk in the sense that the predicted risk contours are far from circularly symmetric; continuous releases showed similar trends. The real terrain data used is from Little Barrier Island, New Zealand (latitude 36°11′ S, longitude 175°4′ E), chosen on account of its being uninhabited, and having steep slopes and complex topography. As the windspeed increases from 1 to 10 m s−1, risk contours become less affected by terrain in both instantaneous and continuous releases.