Bas Hofland

Stone Stability in Non-uniform Flow

This paper presents the results of an experimental study on stone stability under nonuniform turbulent flow, in particular expanding flow. Detailed measurements of both flow and turbulence and the bed stability are described. Than various manners of quantifying the hydraulic loads exerted on the stones on a bed are extensively reviewed and extended. On the basis of the data, a new relationship between flow parameters and bed damage—expressed as a stone entrainment formula—has been established for nonuniform flow. As the present data is in line with existing data on other flows, the present relation seems applicable for other types of nonuniform flow as well. Such a relationship could provide more consistent design criteria and allow an estimate of the cumulative damage over time, which is important for making decisions regarding maintenance frequency and lifetime analysis of hydraulic structures.

A method for tsunami risk assessment: a case study for Kamakura, Japan

This paper presents a methodology for tsunami risk assessment, which was applied to a case study in Kamakura, Japan. This methodology was developed in order to evaluate the effectiveness of a risk-reducing system against such hazards, also aiming to demonstrate that a risk assessment is possible for these episodic events. The tsunami risk assessment follows these general steps: (1) determination of the probability of flooding, (2) calculation of flood scenarios, (3) assessment of the consequences and (4) integration into a risk number or graph. The probability of flooding was approximated based on the data provided by local institutes, and the flood scenarios were modeled in 1D using the Simulating WAves till SHore model. Results showed that a tsunami in Kamakura can result in thousands of casualties. Interventions such as improvements in evacuation systems, which would directly reduce the number of casualties, would have a large influence in risk reduction. Although this method has its limits and constraints, it illustrates the value it can add to existing tsunami risk management in Japan.

Stone Stability under Stationary Nonuniform Flows

AbstractA stability parameter for rock in bed protections under nonuniform stationary flow is derived. The influence of the mean flow velocity, turbulence, and mean acceleration of the flow are included explicitly in the parameter. The relatively new notion of explicitly incorporating the mean acceleration of the flow significantly improves the description of stone stability. The new stability parameter can be used in the design of granular bed protections using a numerical model for a large variety of flows. The coefficients in the stability parameter are determined by regarding the measured low-mobility entrainment rate of rock as a function of the stability parameter. Measurements of flow characteristics and stone entrainment of four different previous studies and many configurations (uniform flow, expansion, contraction, and sill) are used. These configurations have different relative contributions of mean flow, turbulence, and stationary acceleration. The coefficients in the parameter are fit to all ...

STONE STABILITY UNDER DECELERATING OPEN-CHANNEL FLOW

The current research is aimed at finding a proper relation between flow forces acting on the bed and the bed response. To this end, experiments were carried out in which both the bed response (quantified by a dimensionless entrainment rate) and the flow field (velocity and turbulence intensity distributions) are measured. The three available stability parameters, which are used to quantify for the flow forces, were evaluated using the measured data. The focus of the evaluation is on the correlation of these stability parameters with the measured bed damage expressed in terms of the dimensionless entrainment rate. The experimental results confirm that the Shields stability parameter fails to predict bed damage for non-uniform flow conditions (R2=0.18). In contrast, the stability parameters of Jongeling et al. (2003) and Hofland (2005) give better damage predictions (R2 = 0.77). The results confirm the strong influence of the velocity and turbulence intensity distributions on the stability of bed material.

Entrainment of Large Particles from Granular Bed Protections Under Low-Mobility Transport Conditions

In this paper the stability of bed protections is treated as a special form of sediment transport. The influence of turbulence fluctuations on the stability of bed protections is discussed and examined with the aid of measurements.