Progress Reports: December 2005 June 2005 December 2004 June 2004 December 2003
Risk Assessment for Submarine Slope Stability
OBJECTIVE: This project seeks to develop the tools needed to assess the risk of submarine slope failures. One of the greatest sources of uncertainty, and thus risk, is the distance that submarine landslides can travel once they are initiated. Accordingly, this project is focusing on the mechanisms that lead to large movements of seafloor slides caused by hydroplaning.
APPROACH: Existing analytical and numerical models of slope failure and post-initiation slide movements have been reviewed. Hydroplaning has been identified as the most probable mechanism explaining large post-initiation slide movements.
A simple analytical model for hydroplaning has been evaluated and limitations of the model have been identified. Several important assumptions have been identified including: (1) all resistance to post-hydroplaning slide movement is produced by viscous shear at the slide mass-parent material interface; (2) movement is steady state and does not describe either the initiation or cessation processes for hydroplaning; (3) movements and flow are one-dimensional (4) the moving slide mass is a rigid rectangular block, and does not allow the slide to spread out laterally or change in thickness; and (5) the flow is laminar. Preliminary studies showed that the fluid resistance at the leading and trailing edges of a moving slide mass during hydroplaning could be appreciable and greatly affect the movement.
Numerical models are being used to examine more closely the interaction between a slide mass that is hydroplaning and the surrounding water with particular emphasis on the leading and trailing edges of the slide mass. The goal of the numerical modeling is to provide a basis for better characterizing the soil-fluid interaction at the boundaries of the slide mass. Once a better understanding of the slide mass-fluid interaction has been established attention will be given to the processes whereby hydroplaning is initiated and eventually terminated.
DEPLOYMENT OF RESULTS: Results of the research will be a model for predicting when hydroplaning may occur and how far a slide mass may move once hydroplaning is initiated. Ultimately it is expected that the model will serve as a basis for prediction of slide movements as part of an overall assessment of risk of submarine slope failures. Much of the risk associated with a seafloor slope failure is based on the distance a slide may travel once it is initiated. Slides which occur far from an area of interest and do not travel far pose far less risk than slides that can travel large distances.ANTICIPATED PROJECT DURATION: 2 years
PROJECT PLAN FOR YEAR 2 (2004-2005):
Scope of Work: During this year work will be focused on numerical analyses to:
(1) Better understand the mechanism of hydroplaning and the interaction between a moving slide mass and the surrounding water.
(2) Verify the assumptions currently made in analytical models and in particular the use of boundary layer theory to characterize the fluid behavior at the slide mass-parent soil interface during hydroplaning.
(3) Develop suitable representations for the fluid pressures at the leading and trailing edge of a slide mass during hydroplaning with the objective of improving the existing analytical models.Anticipated Results: We anticipate developing a suitable model for predication slide movements where hydroplaning occurs.
PRINCIPAL INVESTIGATOR (S) & OTHERS INVOLVED IN PROJECT:
PI(s): Dr. Stephen G. Wright, OTRC/University of Texas at Austin
Others: Ph.D. level graduate student - Hongrui Hu.
Date: June 2005
Project Title: Submarine Slope Stability Risk
MMS Project: 491 TO Numbers: 73648/35986
PI: Stephen G. Wright
COTR: A. Konczvald
Estimated Completion Date: September, 2006
Project Description: The objective of this project is to study the failure and post-failure sliding of submarine slopes. The potential travel distances of subaqueous slides are much larger than for subaerial slides. In this project hydroplaning has been recognized and is being studied as the mechanism most likely causing submarine slides to travel large distances. A theoretical basis, including analytical and numerical models is now being developed to simulate the process of sliding including possible hydroplaning. Current research has been focused on the interaction of the sliding mass and surrounding fluid. Once a theoretical basis and model are developed they will be validated through comparisons of solutions with results of bench-scale experiments and larger scale experiments in a model test tank or basin.
Progress: Research in the early stages of this project showed that forces applied on a sliding soil mass by the surrounding fluid are important in predicting the sliding process. These forces are determined by the flow conditions around the slide mass whose shape and velocity change as the mass moves.
This report focuses on progress since May 2004. The interaction between the slide mass and surrounding fluid is currently being investigated by numerical modeling. A numerical model has been constructed for the steady 2-D flow around a slide mass, where currently the slide mass is assumed to have a constant shape and velocity. The commercial software, Fluent 5/6, is being used for the numerical modeling. A series of analyses has been performed for slide masses with different shapes to approximate both natural slides and small scale experiments. The shapes include a simple rectangle with a length-height ratio varying from 10 to 100, a block with rounded corners, and a smooth shape similar to that of a hydrofoil. Both laminar and turbulent flow models have been applied to simulate the flow. For turbulent flow the Reynolds-Stress model has been used for turbulent flow. Geometric characteristics such as the length-to-height ratio of the slide mass and the ratio of the height of the slide mass to the thickness of the underlying water film have been found to present challenges in the numerical modeling. Also the memory and computational capacities of available computers impose practical limits on the total number of elements that can be used in the numerical modeling - over one-million elements have been used in some cases.
Results of the numerical modeling for experimental cases have shown that both laminar and turbulent flow exist simultaneously depending on the particular region of the flow domain. The results have also confirmed that dynamic lubrication theory is applicable for characterizing the flow within the water film between the moving slide mass and underlying base material. Finally, the numerical results show that a negative kinetic pressure develops at the upper surface of the front of the slide mass. This negative pressure produces additional “lift”, which helps to explain the onset condition of hydroplaning.
While numerical modeling to date has provided useful information and insight into hydroplaning of slide masses, it appears that the very large length-to-height ratios of natural slides may make numerical modeling unfeasible for at least some practical applications. However, even in these cases numerical modeling can provide insight into what simplifications are appropriate for modeling actual slides.
Reports & Publications: None for this reporting period.
Date: December, 2004Project Title: Submarine Slope Stability Risk
MMS Project: 491 TO Numbers: 73648/35986
PI: Stephen G. Wright
COTR: J. McNeil
Estimated Completion Date: September, 2005
Project Description: The objective of this project is to assess the risk of submarine slope failures, including predicting the subsequent extent of sliding once a slope failure is initiated. Submarine slides are known to often travel much larger distances than typical subaerial slides. These large potential travel distances pose some of the greatest uncertainty and, thus risk, in assessing submarine slope stability. This project is focused on slide movements where the slide mass "hydroplanes" by moving along a layer of water. The phenomenon of hydroplaning can explain why many subaqueous slides travel large distances while their counterparts on land do not. The research currently underway focuses on the interaction between the moving slide debris and the surrounding water when hydroplaning occurs.
Progress: Existing experimental data and models for hydroplaning of subaqueous debris flows have been reviewed and limitations have been identified. A series of analyses has been performed using simple numerical models. The results obtained using these models have been compared with experimental and actual field data. It has been shown that the hydrodynamic pressures on the exposed surfaces of a moving slide mass have a significant effect on the onset of hydroplaning and on the movements once hydroplaning develops.
A numerical model has been developed and is currently being used to understand better the hydrodynamic pressures and flow around the surface of moving debris. The current model employs the follow simplifying assumptions with the primary focus being on the fluid pressures on the exposed surface of the debris: (1) the debris body is a rigid block, (2) the ground surface beneath the moving debris is smooth, (3) the surrounding fluid is water with constant physical properties, (4) the thickness of the hydroplaning water layer between the moving debris and underlying surface is constant, and (5) the flow is two-dimensional, incompressible, steady and laminar.
Some difficulties have been encountered with the current numerical model and these are currently being addressed. The numerical analyses suggest that the shape of the slide debris may have an important effect on the flow of the surrounding water and this aspect is now being examined. Results from these analyses will ultimately by used to develop an improved model for initiation of hydroplaning and the subsequent slide movement.
Reports & Publications:
Hance, J.J. (2003). “Development of a database and assessment of seafloor slope stability based on published literature.” M.S. Thesis. University of Texas at Austin.
Hu, Hongrui, and Wright, Stephen G. (2004), "Preliminary studies and numerical modeling of hydroplaning of submarine slide masses," OTRC Research Report.
Date: June 2004
Project Name: Risk Assessment for Submarine Slope Stability
Project Number: 491 Task Order: 73648
Principal Investigators: Dr. Stephen G. Wright
Estimated Completion Date: September, 2005
Project Description:
The objective of this project is to assess the risk of submarine slope failures, including predicting the subsequent extent of sliding once a slope failure is initiated. Submarine slides often travel large distances that far exceed the distances that typical subaerial slides travel. These large potential travel distances currently pose some of the greatest uncertainty and, thus risk, in assessing submarine slope stability. This project is currently focused on the mechanisms that cause submarine slides to travels large distances. A theoretical basis, including analytical and numerical models is now being developed to predict slide runout with special emphasis on hydroplaning and similar mechanisms of movement. After a theoretical basis is developed it will eventually be validated through bench-scale experiments and larger scale experiments in a model test tank or basin.
Progress:
An analytical solution for the steady-state motion of a moving slide mass has been implemented. The model is based on a rigid slide mass moving on a fluid layer, i. e. "hydroplaning". The initial model was based on work by Harbitz et al (2003) and has been implemented in both C++ computer code and Matlab. Our experience with the model revealed that the model is limited primarily to very flat slopes (generally less than one percent) because of the assumptions of steady state behavior and the only resistance being viscous shear resistance along the slip plane. For steeper slopes the slide mass becomes physically unstable, which is reflected in numerical instabilities in the analytical solution.
Work is now focused on non-steady motion of the slide mass and the interaction between the slide mass and the surrounding fluid, especially at the leading edge of the slide mass. The analytical solution is being extended to account for these aspects and additional numerical models are being explored. In particular a numerical model is being used to examine the velocity profile in the fluid at the interface between the moving slide mass and the underlying slope surface.
This current work is focused on the movement of a slide mass once the movement has been initiated and hydroplaning has fully developed. Eventually this work will be extended to modeling the transition between the initiation of a submarine slide (which can already be modeled) and the movement of the slide mass due to hydroplaning.Reports & Publications: None
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Date: December 2003
Project Name: Risk Assessment for Submarine Slope Stability
TEES Project Number: 32558-60291 MMS Task Order: 73648 MMS Project Number: 491
Principal Investigators: Dr. Stephen G. Wright
Estimated Completion Date: September, 2006
Project Description:
The objective of this project is to assess the risk of submarine slope failures, including predicting the subsequent extent of sliding once a slope failure is initiated. Submarine slides often travel large distances that far exceed the distances that typical subaerial slides travel. These large potential travel distances currently pose some of the greatest uncertainty and, thus risk, in assessing submarine slope stability. This project is currently focused on the mechanisms that cause submarine slides to travels large distances. A theoretical basis, including analytical and numerical models is now being developed to predict slide runout with special emphasis on hydroplaning and similar mechanisms of movement. It is anticipated that once such a theoretical basis is developed it will be validated through bench-scale experiments and eventually larger scale experiments in a model test tank or basin.
Progress:
Currently there are four main achievements to report: (1) a workshop on risk assessment for submarine slope stability was organized and held in Houston, Texas in May of 2002, (2) a database of past submarine slope failures has been compiled and analyzed for the interrelationships among causes and subsequent extent of slide movement, (3) potential mechanisms for slide initiation and movement have been identified, and (4) existing models of predicting slide movement after a slide is initiated have been reviewed.
Based on the published literature, a database has been developed that includes 534 slide events with information on the geographic location, soil properties, cause of failure, and extent of failure and other pertinent information. Fourteen triggering mechanisms have been identified for the slides examined. This work was summarized by J. J. Hance (see below). It is particularly important to note that the study revealed that a relatively large number of submarine slides have occurred on much flatter (less than 10 degree) slopes and have traveled much greater distances than slope failures on land. This strongly suggests that different mechanisms are prevalent for submarine slides, compared to those on land.
Existing analytical and numerical models of slope failure and movement are being reviewed with particular emphasis on models for post-initiation slide movement. Lumped mass models of debris flow such as glide block models and continuum models, including rheological, porous media, inertial grain flow, hydraulic and mixture theory models are being studied comparatively. Various theories for hydroplaning and dynamic lubrication are also being examined. Results of experiments of tire and debris flow hydroplaning have also been examined to determine if correlations can be made between submarine sliding and the well-understood hydroplaning of vehicle tires.
Reports & Publications:
Hance, J.J. (2003). “Development of a database and assessment of seafloor slope stability based on published literature.” M.S. Thesis. University of Texas at Austin.