Numerical Analyses of Deep Sea Mass Gravity Flows

Abstract

Abstract A variety of mass gravity flow features have become considerable issues in the design of pipelines, anchors and seafloor facilities in frontier areas. These include deep water areas such as the South China Sea, steep continental slope areas such as those found in the Black Sea, and areas with special features such as mudslides and mud volcanoes such as those found in the Caspian Sea. Such features are known to occur in many other environments as well. As a consequence of the industry's need to understand these features, the methods for surveying and analyzing the hazards related to the seafloor features have undergone considerable advancement in the past several years. This paper discusses a wide range of mass gravity flows and categorizes them into groups based on the flow type, morphological setting, and triggering mechanisms. Data required to characterize the hazard posed to a seafloor facility by mass gravity flows includes: information regarding the source region (scale, sediment physical properties), the runout path (distance, height, lateral constraint, sediment erodability along the path), and the deposit (debris flow and/or turbidity current; lateral dimensions and thickness). Information about previous episode(s) of mass gravity flows can be used to train numerical models which, when combined with the present-day seafloor and sediment physical properties, can be used to evaluate hazards associated with various possible field architectures. The requirements for data acquisition needed to adequately analyze these features are explained. The use of numerical models to diagnose the features, once adequate data are available, is enumerated for a number of different mass gravity flow cases. Introduction Mass gravity flows are a general form of fast near-bottom flows that involve some combination of water and sediment. These flows are very episodic making them difficult to measure, understand and predict. However, because they can be both far traveling and fast they can be an important consideration in the design of seafloor structures. The purpose of this paper is to review and summarize the state-of-practice in analyzing the characteristic behaviors of mass gravity flows and to illustrate methods used in assessing them as geohazards. Definition of Mass Gravity Flows The various types of mass gravity flows are distinguished by their governing mechanics. These flows also differ considerably depending on the physical setting in which they occur. The scale of the flows and the form of the triggering events are also important. Because all of these factors are relevant in controlling the characteristics of mass gravity flows, a rather complex assemblage of flow events occurs. The nature of a flow of one type or scale can be considerably different from another so that it is important to understand them in detail before proper quantitative analyses and predictions can be developed. In the following discussion we examine the governing mechanics, typical settings and triggering mechanisms individually.