There are four major technical areas to be covered by our research: improved understanding of tsunami bore formation, runup and coastal inundation; improved modeling of sediment transport and scour; determination of the fluid forces imposed on structures; and understanding of structural response leading to guidelines for analysis and design for tsunami loading.
- Tsunami bore formation, runup, and coastal inundation
- Sediment transport and scour
- Fluid forces on structures
- Structural response, analysis and design
Runup Experiments and Modeling
Wave basin experiments will be performed to assist in the development of computational models for improved estimation of coastal inundation. These models will include the effects of site-specific bathymetry, fringing reefs, and surface roughness due to vegetation and coastal infrastructure. They will also be used to develop a better understanding of bore formation and the subsequent energy dissipation, as well as the fluid transport as the bore moves on land.
- Site-specific bathymetry
- Effect of fringing reefs
- Surface roughness
- Bore formation
- Energy dissipation
Tsunami wave basin will be modified to allow for three individual flumes with different bottom slopes (July - Dec 2007)
In order to perform coastal inundation experiments with different bottom slopes, the Tsunami Wave Basin will be separated into three two-dimensional wave flumes. Each flume will have a different bottom slope, at 1-to-5, 1-to-10 and 1-to-15. A series of experiments will be performed utilizing this setup during the second half of 2007. Depending on availability of instrumentation, multiple experiments may be run simultaneously in two or more of the wave flumes. Since it is necessary to wait for up to an hour between waves to allow the tank water to settle, the use of simultaneous experiments could significantly increase the number of tests that can be performed.
Run-up Experiments-Constant Slope
A cross-sectional view of the wave basin, with exaggerated vertical scale, shows the three bottom slopes in each of the wave flumes. These configurations will be used to study bore formation and subsequent energy dissipation on uniform slopes. An array of resistance wave gauges will be used to monitor the water surface profile, while numerous acoustic doppler velocimeters (ADVs) will be used to track the fluid velocity at critical locations. A series of waves from 5 cm to 65 cm height in 5 cm increments will be used to develop a benchmark for each bottom slope. These benchmarks will be used in the subsequent bed roughness, fringing reef, scour and structural loading experiments.
- Solitary waves with heights at 0.05m increments up to 0.65m
- Study bore formation and energy dissipation
- Resistance wave gauges and Acoustic Doppler Velocimeters (ADVs) will capture flow velocity
- Benchmark tests for bed roughness, fringing reef, scour and structural loading
Run-up Experiments-Fringing Reef
The upper end of the constant slope profiles will be curtailed to simulate a fringing reef. The water level will be varied so that the reef is either slightly below sea level, at sea level, or slightly above sea level. A similar array of instrumentation will be installed, and the same series of solitary waves will be used to investigate the effect of a fringing reef on bore formation.
- Fringing reef will be simulated by curtailing the beach slopes at –h2, water level, and +h2.
- Solitary waves with height at 0.05m increments up to 0.65m
In addition to the instrumentation mentioned previously, a laser altimeter will be used to augment the resistance wave gauge measurement of surface profile, especially in the white water region where air entrainment may distort the electrical readings. Particle Imaging Velocimetry (PIV) will also be used to monitor transition from solitary wave to a bore. In addition, high speed video cameras will be used to track floating markers on the water surface and the dry shoreline.
- Laser altimeter will track free surface when air entrainment distorts resistance gauges and ADV readings.
- Particle Imaging Velocimetry (PIV) will monitor transition to white water.
- High speed camera will track markers on still water and dry bed.
Sediment Transport and Scour
The experimental tests and computational modeling of sediment transport and scour are intended to develop improved understanding of four main topics: sediment transport mechanisms; pump up of sediments during wave collapse and bore formation; entrainment of sediment by instantaneous bed shear stress; and enhanced transport due to static liquefaction induced by high pore pressures during rapid drawdown as the tsunami inundation recedes.
- Develop and validate sediment transport mechanisms
- Pump up of sediments due to large-scale vortices created by bore collapse.
- Entrainment of local sediment by instantaneous bed shear stress.
- Enhanced transport due to soil instability (momentary static liquefaction caused by high pore pressure during drawdown)
A series of preliminary scour tests will be performed in Fall 2006 in the large wave flume. Although the wave flume cannot produce large solitary waves, these experiments will take advantage of sand already in the flume for a prior coastal erosion experiment. Velocity, sediment concentration and pore pressure measurements will be made with various instrumentation systems.
- Preliminary scour tests in Large Wave Flume (Fall 2006)
- Utilize existing sand bed from beach erosion experiment
Sediment Transport Experiments
Sediment transport experiments will also be performed in the two-dimensional flumes created in the tsunami wave basin, utilizing the 1-in-10 and 1-in-15 bottom slopes. These tests will utilize the same well-graded sand bed currently in the large wave flume. Instrumentation systems will include fiber optic backscatter sensors for sediment concentration measurements, ADVs and PIV for velocity measurements, and wave gauges and laser altimeter for surface profile monitoring.
- Repeat 1:10 and 1:15 bottom slope tests with moveable bed
- Well-graded sand bed (0.2mm median grain size)
A Plexiglas cylinder will be added to the sand slope with internal cameras to monitor scour around a typical pile or bridge pier.
- Include Plexiglas cylinder to simulate pile.
Fluid Forces on Structures
Fluid forces on structures will be determined using computational fluid dynamics validated by wave basin experimentation. The fluid forces to be considered will include horizontal and vertical hydrodynamic loads, and loads due to debris impact and debris damming. Horizontal hydrodynamic loads on columns and other elements due to laminar flow is well established, and there have been a number of studies on loads induced by a bore striking a column or pier. However, little attention has been paid to the uplift or vertical loads applied by tsunami waves passing below bridges, harbor piers, or building floors. There have been a number of experimental research projects involving impact between floating debris and structural members, particularly considering automobiles or timber poles as the debris. Larger debris such as shipping containers and barges have not been considered in enough detail. In addition, past research has not provided a consensus as to the force applied by debris on a structural element. Consideration must also be given to the damming loads induced by the change in geometry resulting from lodging of a shipping container or floating debris against a structure.
- Horizontal hydrodynamic loads
- Vertical hydrodynamic loads
- Debris impact loads
- Debris damming loads
Fluid Forces on Structures Experiments
The three fringing reef test setups in the tsunami wave basin will be used to study the effects of various size bores striking a simple structure or individual structural elements. PIV and high speed cameras will be used to monitor flow patterns around the structure to validate the computational fluid dynamics modeling of the same scenario. The load on the structural elements will be monitored using load cells and pressure sensors.
- Utilize fringing reef setup to produce bore.
- Monitor loading on structural elements and simple structural systems
The same structural test setup will also be used to evaluate the effects of debris damming against the front structural members. Particularly for open column frame systems, this damming effect will change the exposure to hydrodynamic loads and increase the loads on the structural members.
- Monitor debris damming effects
Computational fluid dynamic simulations will utilize Reynolds Averaged Navier Stokes, or RANS, to model various fluid-structure interactions. The modeling assumptions will be validated using the experimental test results for the simple structures tested in the wave flumes. A combination of FLUENT and ABAQUS will be used to study both the fluid dynamics and structural response respectively. An alternative integrated model can also be developed using COMSOL, previously known as FEMLAB, which uses a fully-coupled fluid-structure model, which tends to be more computationally demanding.
- Use Reynolds Averaged Navier Stokes, RANS fluid models with the experimental data to improve fluid-structure interaction modeling
- Combination of FLUENT + ABAQUS
- Possible use of COMSOL (FEMLAB)
Structural Response and Design
Structural response of typical building and other structural elements will be determined based on the hydraulic and impact loads determined from the experimental and simulation studies. Elements of progressive collapse prevention design will be incorporated to protect against disproportionate collapse in the event of unforeseen damage to a local area of the structure due to larger than anticipated impact loads. The results of these structural evaluations will be incorporated into prescriptive design guidelines presented in code adoptable form. In addition, the project will develop the methodology for performing a site specific Performance Based Tsunami Engineering design.
- Structural response to hydraulic and impact loads
- Progressive collapse prevention
- Prescriptive design
- Methodology for site-specific PBTE
Borrowing from the earthquake community, and Ron Hamburger in particular, the performance levels required for tsunami design can be represented schematically with reference to a typical coastal hotel. For a minor tsunami, the building should be available for immediate occupancy after the event. This can be achieved by appropriate siting or the addition of buffer systems to protect the building. Under a design level tsunami, extensive non-structural damage is anticipated at the lower levels of the building. In fact, lower level non-structural walls will be designed to break away so as to reduce the loads on the structural members. However, the upper levels of the building must remain intact so that occupants who have evacuated vertically will be safe. In many areas of the West Coast, and Hawaii, vertical evacuation is the only alternative for near-source tsunamis. For areas like Waikiki, where there are too many people to evacuate horizontally even for a far-source tsunami, vertical evacuation is the only viable alternative. The tsunami evacuation maps in the front of the Honolulu phone books currently instruct building occupants that if they are in a reinforced concrete or steel framed building over six stories tall, they should evacuate vertically to the third floor or above in the event of a tsunami warning. Finally, when subjected to the maximum credible tsunami, it is intended that progressive collapse preventive design will protect against total collapse even if individual members are damaged by unanticipated loads. People who have sought protection in the upper levels should still be protected against injury.