Deposition in a Hazard Assessment Model of Mass Transport Complexes
Authors: Watts P. 
Published: October 2008

A probabilistic model is proposed to describe the probability distributions of MTC hazards. Existing deposits appear to validate the model to the degree possible. The model reproduces deposit structures, and also identifies model inputs that are most likely to produce hazardous MTCs.

Introduction

A mass transport complex (MTC) can present significant hazards to certain offshore structures and activities. Specifically, the integrity and operations of underwater cables, pipelines, moorings, and other structures can be threatened by MTCs. An effective and manageable use of these structures motivates a study of MTC hazards. In general, MTC hazards are revealed by field studies of existing MTC events (Orange et al., 1999; Tappin et al., 2001, 2003; von Huene et al., 2004). Field studies are complimented by numerical models developed to evaluate MTC hazards. These include various sediment stability models (e.g., Wright and Rathje, 2003), mass transport models (e.g., Imran et al., 2001; Syvitski and Hutton, 2003; Niedoroda et al., 2003), and probabilistic models (e.g., Watts, 2003, 2004). Of these different techniques, probabilistic models have perhaps received the least attention, despite their many advantages. In this work, MTC hazards are found by combining 1) stability analyses and 2) sediment motion into a single hazards assessment model (HAM). The HAM is a probabilistic model that provides probability distributions for most MTC hazards of interest.

Hazard Assessment Model

The HAM presented here is based in part on the probabilistic model of Watts (2003), although the HAM is significantly more sophisticated. HAM inputs include slope morphology, sediment strength, sedimentation rate, water pressures, gas hydrate pressure and temperature, seismic parameters and other slope stability factors. The stability of any given slope may be dominated by only a few model inputs (Watts, 2004). The frequency of MTCs is controlled by the rate of occurrence of storm waves, earthquakes, gas hydrate phase change, oversteepening, sedimentation events and other MTC triggering mechanisms. The HAM performs two distinct computations. Stability analyses of sediment structures evaluate MTC failure planes. Sediment motion post failure describes MTC velocities and deposition.

There are several important differences between our earlier work (Watts, 2003, 2004) and the HAM. First, HAM computations are carried out explicitly on a yearly basis, directly providing return periods of practical interest. Second, HAM outputs can occur at any distance from the initiation of mass failure. Third, HAM outputs focus on deposit hazards rather than tsunami hazards. Fourth, slope stability is treated by a method of slices with a variety of failure plane shapes (Turner and Schuster, 1996). Fifth, gas hydrates influence slope stability in the HAM.

Fig. 1: Region offshore Santa Barabara, CA(Available in full paper)

Uses for Uncertainty

The slope conditions that trigger hazardous MTCs are found by running the HAM multiple times with randomized inputs. The HAM uses probability distribution functions to address geological uncertainty, with the understanding that these uncertainties may have a greater impact on sediment deposits than the errors in the slope stability or sediment motion models used.