Model for the characterisation and design of Passive Fire Protection (PFP) systems for steel structures

High-resolution bathymetric data from the New Jersey and Californian continental margins show a marked depression running along parts of the base of the continental slope. Detailed analysis reveals that the depressions are a series of discrete ‘plunge pools’ with associated downslope topographic ramparts. We have used new bathymetric data to create our own data base (of over 150 examples) and systematically analyse plunge pool morphology and location. Previous observations of plunge pools have been sparse. Plunge pools are up to 1100 m wide and 75 m deep, with a mean diameter of 400 m and a mean depth of 21 m. Plunge pools only occur where there are sharp decreases in slope of more than 4°, and are well developed where changes in slope exceed 15°. We propose plunge pools can be created by two mechanisms. Firstly, they may be due to reduced bed shear stress downstream of hydraulic jumps in submarine sediment-laden density flows that causes the deposition of bedload and the creation of a sediment bar. This bar then defines the downslope margin of a pool. Secondly, the impact of high-momentum sediment-laden density flows can excavate a depression, as has been observed for subaerial snow avalanches. Sediment deposited downslope of these impact pools is very poorly sorted, and partly derived from erosion within the pool. Both mechanisms influence whether turbidity currents are generated from high-density sediment-laden density flows, influence whether depositional flows are channelised, and have implications for base-of-slope facies models