The dynamics of cohesive mud under wave action is important to the geological processes along some coastlines. Because of the complex rheological properties there have been vastly different models for predicting the effect of mud on wave attenuation. This paper focuses on a non‐Newtonian behavior of highly concentrated mud, whereby shearing strain occurs only if the shear stress exceeds a certain threshold called the yield stress. The wave‐induced motion in a thin mud layer beneath a clear water layer is studied. There are circumstances in which the yield stress is dominant and acts like a Coulomb friction. Mud motion can change from continuous to intermittent as the wave attenuates. The effects of the non‐Newtonian behavior on wave damping are compared to other more common causes of dissipation.

A numerical program developed for field application is presented in this paper. We use the generalized Julien and Lan [8] rheological model to simulate debris flows. Due to the derivative discontinuous nature of the constitutive law, flow is separated into plug region and bottom region (with stress greater than yield stress). The program solves the plug flow layer solution first, and then corrects the solution with the bottom layer approximation. Numerical scheme with upwind method and central difference in space and AdamYBashforth third-order scheme in time is used for both layers. The scheme is tested against analytical solutions and laboratory experiments with very good results. Application to a field case with more complicated geometry also achieves good agreement, with error less than 5% compared to field measurements. The final example demonstrates how this numerical program is used in a preliminary design.

Debris flow disasters are usually accompanied by serious loss of lives and properties. However, debris flows are also part of earth's natural phenomenon, and so what is the reasonable budget to be spent on mitigation measures becomes an important issue for the budget allocation processes. This article utilizes economic concepts to propose a reasonable estimation of the hazard damage and the cost of proposed mitigation measures. The proposed method is composed of four steps, namely, delineating the area of the disaster with different return periods, itemizing the land use within those areas, calculating the hazard loss using official values, and computing the expected probable maximum loss with a probability distribution. The comparison between the assessment of hazard and the economic gains of any proposed mitigation measures can be used as a reference for future decision-making process.

The time required to pull a large object from a sandy seabed is estimated by assuming that the seabed is porous but rigid. The phenomenon of breakout (i.e., sudden release) is shown to occur without the assumption of elasticity of the soil skeleton (Foda 1982). A new case of wedge-shaped gap is also studied, and compared to a uniform gap, Laboratory experiments are shown to support the theory.

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