FB
F. Bisschop
info
Please Note
<p>This page displays the records of the person named above and is not linked to a unique person identifier. This record may need to be merged to a profile.</p>
4 records found
1
This paper addresses the sediment pick-up process in the high-velocity range of 2-6 m/s. An existing sediment pick-up function was recalibrated and modified using data of new experiments in a closed pipeline circuit with sand diameters in the range of 50-560 μm. The new pick-up function was used to simulate the generation and passage of a turbidity current along the submarine Congo canyon offshore from the coast of Zaire in Africa.
...
This paper addresses the sediment pick-up process in the high-velocity range of 2-6 m/s. An existing sediment pick-up function was recalibrated and modified using data of new experiments in a closed pipeline circuit with sand diameters in the range of 50-560 μm. The new pick-up function was used to simulate the generation and passage of a turbidity current along the submarine Congo canyon offshore from the coast of Zaire in Africa.
Erosion of sand at high flow velocities
An experimental study
The safety level of a dike is expressed in terms of risk. Risk is defined as the product of the probability of inundation of a polder (after failure of a dike) and the expected damage (casualties, economic damage and damage to the infrastructure) caused by inundation. The rate of inundation determines the amount of casualties and depends heavily on the flow velocity through the breach and breach development in time. The flow velocity in a breach can become larger than 5 m/s. Due to these large flow velocities, the application of conventional sediment pick-up functions in breach growth models, leads to a significant overestimation of the breach growth and thus the rate of inundation.
...
The safety level of a dike is expressed in terms of risk. Risk is defined as the product of the probability of inundation of a polder (after failure of a dike) and the expected damage (casualties, economic damage and damage to the infrastructure) caused by inundation. The rate of inundation determines the amount of casualties and depends heavily on the flow velocity through the breach and breach development in time. The flow velocity in a breach can become larger than 5 m/s. Due to these large flow velocities, the application of conventional sediment pick-up functions in breach growth models, leads to a significant overestimation of the breach growth and thus the rate of inundation.
Dikes constructed from sand generally have a sand core and clay layers on the slopes and the crest to protect the core against erosion. In extreme hydraulic conditions, several failure mechanisms can lead to destruction of the clay layers, exposing the sand core to water. When water overtops the dike and the protective cover of the land-side slope or the crest fails, water flows over the core and erodes the sand. The dike starts to breach, and eventually the land behind the dike is flooded. The rate at which the dimensions of the breach grow influences the rate of inundation of the polder. Reduction of the inundation rate may be achieved by retarding the breaching process. This may reduce the number of casualties, resulting in increasing safety for the inhabitants. In order to achieve, for instance, a safety level ten times higher, mortality has to decrease by a factor of 10. The breaching process can be retarded by reducing the erosion velocity of the sand core. Experiments were executed to investigate the effect on the erosion velocity of adding bentonite to sand. The results of these experiments showed a significant reduction of the permeability and erosion velocity of the sand–bentonite mixtures compared with those of pure sand. The effect of adding bentonite on the breaching process was investigated by applying the Bres model (breach erosion in sand dikes model) to a sand dike tested in a large-scale field experiment (Zwin'94). It was found that adding a small percentage of bentonite reduces the rate of breach growth and the inundation rate in the polder. For the Zwin’94 dike it was determined that 5·4% of bentonite is sufficient to reduce the inundation rate below a threshold value of 0·5 m/h, leading to a significant increase in safety.
...
Dikes constructed from sand generally have a sand core and clay layers on the slopes and the crest to protect the core against erosion. In extreme hydraulic conditions, several failure mechanisms can lead to destruction of the clay layers, exposing the sand core to water. When water overtops the dike and the protective cover of the land-side slope or the crest fails, water flows over the core and erodes the sand. The dike starts to breach, and eventually the land behind the dike is flooded. The rate at which the dimensions of the breach grow influences the rate of inundation of the polder. Reduction of the inundation rate may be achieved by retarding the breaching process. This may reduce the number of casualties, resulting in increasing safety for the inhabitants. In order to achieve, for instance, a safety level ten times higher, mortality has to decrease by a factor of 10. The breaching process can be retarded by reducing the erosion velocity of the sand core. Experiments were executed to investigate the effect on the erosion velocity of adding bentonite to sand. The results of these experiments showed a significant reduction of the permeability and erosion velocity of the sand–bentonite mixtures compared with those of pure sand. The effect of adding bentonite on the breaching process was investigated by applying the Bres model (breach erosion in sand dikes model) to a sand dike tested in a large-scale field experiment (Zwin'94). It was found that adding a small percentage of bentonite reduces the rate of breach growth and the inundation rate in the polder. For the Zwin’94 dike it was determined that 5·4% of bentonite is sufficient to reduce the inundation rate below a threshold value of 0·5 m/h, leading to a significant increase in safety.
Two regimes can be distinguished for the pickup flux of sand. At a Shields parameter of less than about 0.5 (corresponding with flow velocities of 0.5–1 m=s), the erosion process is dominated by the size and density of the grains (grain by grain pickup). At higher flow velocities, the bulk properties of the sand bed start to influence the erosion process. Dilative behavior results in the inflow of water to the sand bed, which reduces the pickup flux (dilatancy-reduced pickup) because of the shearing of layers of sand. A pickup function was recently developed for this regime, incorporating the effect of bulk properties, such as permeability and porosity, on the pickup. This function agrees well with data of previous erosion experiments in which the permeability and porosity of the sand bed were varied. However, these experiments just met the condition for dilatancy-reduced pickup. The flow velocity during these previous experiments was between 1 and 1.5 m=s, while the Shields parameter varied between 1 and 2. In order to validate this pickup function for dike breaching and jetting of sand, during which the flow velocity ranges within 5–50 m=s, erosion experiments were executed at higher flow velocities. These experiments were executed in an adapted closed flume of the slurry circuit of the Dredging Research Laboratory at the Delft University of Technology (DUT) at flow velocities of 2–6 m=s (Shields parameter is between 50 and 1,000). The results of these experiments are consistent with the theory of dilatancy-reduced pickup. The porosity of the sand bed influences the erosion process, especially at flow velocities of more than 4 m=s bulk property.
...
Two regimes can be distinguished for the pickup flux of sand. At a Shields parameter of less than about 0.5 (corresponding with flow velocities of 0.5–1 m=s), the erosion process is dominated by the size and density of the grains (grain by grain pickup). At higher flow velocities, the bulk properties of the sand bed start to influence the erosion process. Dilative behavior results in the inflow of water to the sand bed, which reduces the pickup flux (dilatancy-reduced pickup) because of the shearing of layers of sand. A pickup function was recently developed for this regime, incorporating the effect of bulk properties, such as permeability and porosity, on the pickup. This function agrees well with data of previous erosion experiments in which the permeability and porosity of the sand bed were varied. However, these experiments just met the condition for dilatancy-reduced pickup. The flow velocity during these previous experiments was between 1 and 1.5 m=s, while the Shields parameter varied between 1 and 2. In order to validate this pickup function for dike breaching and jetting of sand, during which the flow velocity ranges within 5–50 m=s, erosion experiments were executed at higher flow velocities. These experiments were executed in an adapted closed flume of the slurry circuit of the Dredging Research Laboratory at the Delft University of Technology (DUT) at flow velocities of 2–6 m=s (Shields parameter is between 50 and 1,000). The results of these experiments are consistent with the theory of dilatancy-reduced pickup. The porosity of the sand bed influences the erosion process, especially at flow velocities of more than 4 m=s bulk property.