By Steven Baldridge, PE, SE, LEED AP
With nearly one-third of recorded large tsunamis occurring in Japan, the phenomenon is reflected extensively in the region’s culture and history. Thus, the Japanese word “tsunami,” which means “harbor wave,” is recognized globally for an event that can occur along any coastline.
Tsunamis adjacent to harbors can be very destructive due to several factors, including the presence of ships and containers that can float inland, damaging nearby structures. Many harbors also have a shape both above ground (topography) and below the water’s surface (bathymetry), that can force the incoming tsunami wave to rise higher as it is squeezed into a smaller space.
The risk of the potential devastating effects from tsunamis is not limited to Japan or harbors for that matter. The Pacific coast states and provinces of the U.S. and Canada have both the risk and history of tsunamis. These include deadly tsunamis in Liuya Bay, Alaska, in 1958; Prince William Sound, Alaska, in 1964; and Hilo, Hawaii, in 1946. A deadly tsunami also occurred on the U.S. Atlantic coast in Cape Hatteras, North Carolina, in 1883.
The most common triggering events of tsunamis are earthquakes below or near the ocean floor, but a tsunami can be created by volcanic activity, landslides, undersea slumps, and even meteorite impacts. The Tohoku 9.0 earthquake in March 2011, which is considered one of the most powerful earthquakes ever recorded, occurred on the east coast of Japan and caused a massive tsunami that killed thousands and sent 30-foot waves of water across much of the countryside, destroying entire towns and leveling hundreds of low-rise buildings. The tsunami caused the Fukushima Daiichi nuclear disaster.
Tsunamis are rare but devastating events; as such, strategies to mitigate risk have usually involved horizontal evacuation to areas of naturally occurring high ground outside the tsunami inundation zone. This can be the most efficient and safest way to protect life when there is enough warning. For some coastal communities, additional options can include a vertical evacuation strategy, utilizing buildings or man-made mounds or hills that have sufficient height to evacuate the level of tsunami inundation and the strength and resiliency needed to resist the effects of tsunami waves.
As coastal populations continue to grow in the U.S. , horizontal evacuation cannot be the only strategy, especially for Risk Category III and IV buildings. Lessons learned from Japan showed that consideration for hardening infrastructure can be critical to recovery. Essential services such as water supply, wastewater treatment, and power facilities, often located near the coasts, can be crippled by the effects of tsunami wave inundation. The Fukushima nuclear disaster, for example, was caused by tsunami damage to the backup generators that were needed for reactor cooling. Emergency facilities and hospitals are additional candidates for hardening.
Far- and Near-Sourced Tsunamis
Tsunamis are categorized by the location of the triggering event and the time it takes the waves to reach a site. A far-source-generated tsunami is one that originates from a source that is far away and takes two hours or longer after the triggering event to arrive. A near-source-generated tsunami originates from a source that is close and could have 30 minutes or less advanced warning. A mid-source-generated tsunami is one that originates from a source that is close to the site of interest but not close enough for the effects of the triggering event to be felt at the site and would be expected to arrive between 30 minutes and two hours after the triggering event.
Wave propagation times from near-source-generated tsunamis can strike suddenly with little to no warning while far-source-generated tsunamis can allow for advanced warning to distant coastal communities. The 2004 Indian Ocean Tsunami, for example, devastated coastal areas both near and far. Coastlines near the tsunami were inundated in as little as 15 minutes, while those far away waited seven hours for the tsunami to reach their shores. On average, 20 tsunamigenic earthquake events occur each year worldwide, with five large enough to generate tsunami waves capable of causing structural damage and loss of life. With the trend toward increased habitation in coastal areas, more populations will be exposed to tsunami hazard.
There is significant uncertainty in the prediction of hydrodynamic characteristics of tsunamis because they are influenced by tsunami waveform and surrounding topography and bathymetry. Although exceptions exist, field research and surveys indicate that tsunamis have the following characteristics:
- The magnitude of the triggering event determines the period of resulting waves, and generally—but not always—the tsunami magnitude and damage potential.
- A tsunami can propagate more than several thousand miles with little loss of energy. This is due to the large wavelengths of tsunamis, which can be more than 100 miles.
- Tsunami energy propagation has strong directivity—it tends to propagate in a direction normal to the major axis of the tsunami force. Direction of approach can affect tsunami characteristics at the shoreline because of the sheltering or amplification effects of other land masses and offshore bathymetry.
- For a locally generated tsunami, the first leading wave is often a receding water level followed by an advancing positive heave or elevation wave. This may not be the case if the coastal ground subsides by co-seismic displacement. For far-source-generated tsunamis, the leading wave is often an elevation wave. This trend may be related to the pattern of seafloor displacement resulting from a subduction-type earthquake.
Tsunami Hazard Versus Risk
Tsunami hazard is a measure of the potential for a tsunami to occur at a given site. It also is a measure of the potential magnitude of site-specific tsunami effects, including extent of inundation, height of runup, and velocity of tsunami flow. Tsunami risk is a measure of consequence given the occurrence of a tsunami, which can be characterized in terms of damage, loss of function, injury, and loss of life. Risk depends on many factors including vulnerability and population density.
Evaluation of tsunami risk will depend on several factors including the presence of a tsunami warning system, existence of a local emergency response plan, availability of various evacuation alternatives, the vulnerability of the existing building stock, and location of existing short and long-term shelter.
Survivability of Structures to Tsunami Effects
Evidence points to the ability for multi-story concrete and structural steel structural systems to survive tsunami inundation with little more than nonstructural damage in the lower levels and to continue to support the levels of a building above the inundation depth. This can be attributed in part to these buildings being designed for lateral wind and seismic forces that exceed the force of a tsunami rolling through the lower levels of those buildings. Recent data, including those from the 2004 Indian Ocean Tsunami, 2009 Samoa Tsunami, and 2011 Tohoku Japan Tsunami, where no six-story or higher concrete or steel buildings collapsed due to the tsunami, support these conclusions. Building survivability does, however, vary with construction type. For example, for a given tsunami height, wood or other light-framed construction can experience considerably more damage and frequently be destroyed.
In Hawaii, the potential survivability of multi-story buildings is recognized in its tsunami evacuation messaging, informing the public that vertical evacuation in buildings taller than 10 stories is an option if horizontal evacuation is not feasible.
Structural Design Criteria
Despite a long history of tsunamis around the world, because they are relatively rare, there has not been good guidance on designing to mitigate their effects. For the U.S., this started to change in 2004 when the Applied Technology Council (ATC) was awarded a contract by the Federal Emergency Management Agency (FEMA) to help develop design guidance for special facilities for vertical evacuation from tsunamis. That seminal document FEMA P-646 would become the forebearer of a new chapter to ASCE 7-16: Chapter 6 Tsunami Loads and Effects, which includes tsunami design requirements specific to Risk Category III and IV structures.
Several good resources now provide guidance for the design to resist the loads induced by tsunami as noted here:
FEMA P-646 Third Edition—“Guidelines for Design of Structures for Vertical Evacuation from Tsunamis,” 3rd Edition, provides guidance on planning for vertical evacuation from tsunami. This document includes determination of tsunami and earthquake loads along with the structural design criteria necessary to address them, and structural design concepts and other considerations including using existing structures. It provides additional guidance and commentary on the structural design criteria contained in ASCE 7.
ASCE/SEI Standard 7-22—The American Society of Civil Engineers/Structural Engineering Institute (ASCE/SEI) Standard 7-22 Chapter 6 “Tsunami Loads and Effects” provides minimum requirements for tsunami-resistant design and construction of Risk Category III and IV structures located in a tsunami inundation zone.
ASCE Press—The book “Tsunami Loads and Effects: Guide to the Tsunami Design Provisions of ASCE 7-16” is a good companion document to ASCE 7 and includes detailed commentary and example problems.
Determining Wave Height and Flow
While detailed modeling can be performed, the primary method referenced in ASCE 7-22 for determining tsunami flow depth and velocity at a location is Energy Grade Line Analysis (EGLA). This method is based on ASCE Tsunami Design Zone (TDZ) maps which can be found at asce7tsunami.online. The EGLA includes establishing topographic transects, a straight line that cuts through the topography to determine the impact of topography in a given direction of tsunami flow. The EGLA is based on looking at three transects where the center transect is perpendicular to the orientation of the shoreline with the other two located +/- 22.5 degrees from the principal inflow direction.
This initial step is based solely on topography and does not consider the potential impact of adjacent structures on flow velocity. ASCE 7-22 allows the velocity increase that may occur due to adjacent structures to be considered by three possible approaches: two requiring detailed site-specific analysis and one simpler method approximating their impact utilizing a roughness coefficient applied to the EGLA. That coefficient is indicated in ASCE 7-22 Table 6.6-1.
Structural Impacts
Once the tsunami inundation depth and flow velocity are established, the extensive task of determining structural load criteria can begin. The primary structural impacts that can result in damage to a structure and have specific design requirements include the following:
Hydrostatic Loads (ASCE 7-22 6.9)—These include determining potential loads that might occur from buoyancy, unbalanced lateral hydrostatic forces, residual water surcharge load on walls and floors and hydrostatic surcharge pressure on foundations.
Hydrodynamic Loads (ASCE 7-22 6.10)—These forces are due to moving water and include the global forces on the overall structure along with the local loads on structural components of the structure. Because water-borne debris could potentially accumulate against vertical elements of the structure, consideration is given in both the global forces and individual member loads. This is considered in a minimum closure ratio (6.8.7) and in the drag coefficients for individual members in the hydrodynamic calculations. Also included in this section is consideration of potential hydrodynamic forces, including surge uplift on elevated slabs in a structure.
Debris Impact Loads (ASCE 7-22 6.11)—The force and extent of a tsunami wave not only causes damage to structures, the debris from that damage, along with other floatable objects, can cause additional debris impact forces as those elements continue to move with the wave. These include scenarios such as impact by floating vehicles, tumbling concrete debris, wood logs or utility poles, shipping containers and if near harbors, floating vessels.
Foundation Design (ASCE 7-22 6.12)—This includes potential scour and slope/foundation failure. The rushing water can undermine shallow foundations. Deep foundations that have positive connections to the structure they support have a reduced potential for catastrophic impact.
Community Resiliency
While ASCE 7-22 Chapter 6 is intended specifically for Risk Category III and IV buildings, communities can consider encouraging incorporating tsunami design into conventional buildings in areas with fewer horizontal evacuation options. Unfortunately, this comes with added costs during design and construction. For communities with longer warning times for tsunami, it may not even be necessary to design some low-rise Risk Category III buildings if those buildings do not require design per the Tsunami Risk Category and can’t be used for shelter or there is a well-developed horizontal evacuation plan. That money may be better spent elsewhere.
With recent construction cost inflation conflicting with the growing needs for affordable housing, it is critical that communities look to partner with their builders and designers to offset the costs if additional resiliency measures such as tsunami design are considered for these types of buildings. As all developers of affordable housing will attest to, any increase in regulations or design requirements directly decreases the production of affordable housing units.
When considering requiring conventional construction to meet design requirements intended for essential facilities, it is critical for those communities to consider ways of keeping added costs to a minimum. FEMA recommends several methods to assist in mitigating the additional costs of this type of construction. These include offering tax incentives, modifying zoning requirements to increase height restrictions and FAR, or applying for federal or state grants.
Tsunami Resistant Design Concepts
Since no six-story or higher concrete or steel buildings collapsed in the devastating Tohoku Japan tsunami, it begs the question, “Is it worth using elaborate and time extensive analysis and design for multi-story buildings unless they are essential buildings?” A more cost-effective approach may be to just include tsunami-resistant design concepts in the construction of these buildings.
In performing tsunami design on several projects, Baldridge & Associates Structural Engineering (BASE) found multi-story buildings designed to current codes can meet the intent of ASCE 7-22’s tsunami design requirements without the expense of extensive modeling and additional structural analysis by simply enhancing them with a few structural design concepts, including:
- Provide structural systems that have inherent redundancy and utilize structural integrity concepts. This can include seismically designed cast-in-place concrete systems or structural steel framing, especially those utilizing perimeter moment frames for lateral resistance.
- Incorporate deep foundation systems where possible. For many coastal locations this may be required anyway.
- Consider making the ground floor as open as possible and detailing non-load bearing walls connections to the structure with little if any overstrength for wind and seismic requirements. The more open the ground floor is, the less load and damage that may occur. A great example of this is podium style construction with open parking at grade level and residential uses starting at the 2nd or 3rd floor levels. With podium construction, the elevated residential levels of a building may not need to consider tsunami requirements at all and can utilize conventional wood or other light framing methods.
- The wild card in tsunami design is the potential for water-borne debris impact loading that might happen on lower-level structural elements. This can be addressed with more robust construction for those structural elements only. This could include increasing column vertical and tie reinforcement at critical concrete columns, concrete encasement of lower-level steel wide flange columns or filling tube columns to make them composite columns.
- Detail slabs to be able to relieve potential buoyancy or uplift forces. This can include areas of slab that can pop out if pressures start to build. ■