In a recent newsletter, we took a look at how roadway engineers design roads that can stand up to traffic, particularly heavy trucks and buses. While these are usually the prime causes of roadway damage and deterioration, our transportation infrastructure can be subjected to more unpredictable forces, and these have the potential to be devastating. In the last year, countries across the world have had to deal with natural disasters including earthquakes, tsunamis, hurricanes, tornadoes, and floods. Each of these events have the potential to cause serious damage to roadway infrastructure. In the event of a disaster, roads provide a vital link to emergency services, relief, and evacuation routes. Because of this, transportation agencies need to be prepared to respond quickly to road damage. In this edition of the RoadReady Newsletter, we’re going to take a look at how different natural disasters can damage roadways, and how to prepare for it.
Earthquakes can be particularly damaging to roadway systems, and seismic considerations are an important component of roadway design in certain areas. While bridges and similar structures are the most at risk during an earthquake, surface roads can sustain damage as well. Roadway damage is generally caused by ground failure, or faulting. Ground failure includes events such as landslides, as well as less extreme events such as differential settling and loss of soil strength. Roads depend on the strength of the soil below them for support. If the soil below them loses strength or is ripped apart, the roads will follow suit. Faulting occurs when surface faults coincide with the roadway surface, causing it to shift, and potentially buckle. In addition, earthquakes can also cause debris from buildings, or masses of soil to be deposited on the roadway, which can block traffic.
Flooding is considered one of the most common natural disasters, and a sudden excess flow of water can damage a road through several mechanisms. This can occur by introducing water to upland areas, or by increasing the flow depth and velocity in existing channels. When the increased flows from a heavy rainfall are confined to an existing riverbed, damage to roadways and roadway structures can still occur. When water velocities in existing rivers increase, the soil surrounding bridge supports can be washed downstream, a process known as scouring. This weakens the bridge foundation, which can ultimately lead to failure. In addition, debris washed down the river can damage bridge structures as well. Heightened flows also lead to increased erosion of the soil surrounding the river, which can undermine the subgrade of roads adjacent to a river.
Outside the riverbed, water can also damage roadways. In some cases, errant flows are powerful enough to completely wash away a pavement structure. Pooled water can also weaken pavement structures and the subgrade below. Finally, debris washed onto a roadway from a flood event can make a road temporarily impassable.
Natural Disaster Relief and Management
Over time, traffic loads degrade highways and slowly decrease the level of service they provide. While regular maintenance and rehabilitation is crucial to a healthy roadway network, the gradual nature of standard roadway damage allows careful planning of these activities, and allocation of the necessary funds ahead of time. In contrast, natural disasters can leave a roadway completely unusable, with little or no warning. If a road becomes impassable, the negative effects to society are much more significant than say, a road with substantial cracking that is still usable in the short term. In addition to the economic issues associated with a disabled roadway network, roads are critical in mitigating a disasters’ other impacts.
As a result, timely repair of roads damaged or incapacitated in such events is critical. Because of this, disaster relief programs for highways are established at various levels of government. In many cases, these programs allow agencies to quickly begin recovery efforts, for which they will later be reimbursed. In the United States, these programs exist in some state DOTs, as well as at the federal level. Federal relief is administered by the Federal Highway Administration, under the Federal-aid Highway Emergency Relief Program.
Funds for the federal relief program are obtained through the Highway Trust Fund, which is fueled primarily by the gasoline tax. These funds are used for repair on Federal-Aid highways, which encompasses most major roads. In general, any state can be allocated up to $100 million for a given natural disaster. In addition to natural disasters, catastrophic failures of infrastructure, such as the 2007 collapse of the I-35 bridge in Minneapolis, are also covered under this program. Funds in excess of this amount can still be provided, but require approval by congress. In some cases, the financial burden is shared by the state or local agency and the federal government. However, for immediate costs to restore highway traffic, the federal government pays 100% of the cost. The cost breakdown for different kinds of restoration projects is summarized in the figures below.
Gulf Coast Hurricanes
In 2005, the combination of hurricanes Katrina, Rita, and Wilma, did an unbelievable amount of damage to the networks of federal-aid highways on the gulf coast. Roads in coastal areas suffered a wide range of calamities. Roads in certain areas were completely washed out, and winds were strong enough in some instances to lift up bridge decks and remove them completely from their structures. In other cases, roads that were covered with either water or debris remained impassable once the storm had passed. These incidents were most severe in Mississippi and Louisiana, but Texas, Alabama and Florida were affected as well. In total, the FHWA allocated just under $3 billion in funds to aid relief efforts in these areas, dwarfing the theoretical limit of $500 million that could be distributed to these five states in a single year.
Another notable case, as previously mentioned, is the 2007 collapse of the I-35 bridge in downtown Minneapolis. Though not caused by natural disaster, this catastrophe was completely unexpected. After the collapse, the National Transportation Safety Board determined that the collapse was likely due to a design error on the part of the structural engineer, and additional weight from construction crews who were working on the bridge deck when it failed. The bridge was a vital transportation artery, connecting two neighborhoods in downtown Minneapolis and carrying 140,000 cars each day. The importance of the bridge made it a clear candidate for federal relief, and ultimately upwards of $370 million was provided by the federal government. This allowed rapid planning and reconstruction of the new bridge, which was completed just over a year after the collapse.
Funding Disaster Aid
The appropriation of federal funds for recovery of highway systems following natural disasters has existed since 1934. At the outset of the program however, no additional funds were set aside for such expenditures, and states were only allowed to use funds that had already been apportioned to them for the federal government. This meant that any state that wished to use this money for disaster recovery would have to forgo other planned highway projects to make ends meet.
The emergency relief program was first given its own set of funds in 1956. Initially, 60% of the funds were drawn from the highway trust fund, while the remaining balance came from the general treasury. Since 1979, the entirety of the money automatically appropriated to relief has come from the HTF, supported by the gas tax. Today, the fund is given a $100 million dollar annual authorization from the HTF, and any additional expenditures must be approved by congress and come from the general fund. These appropriations have been made virtually every year, and the need to make these requests to congress results in a delay in the availability of funds to relieve transportation networks.
The increasing constraints and decreased revenues of the Highway Trust Fund means that today, the emergency relief program is now mostly funded by general fund revenues, calling into question the sustainability of the program. Without a permanent solution in place, there is uncertainty where the money to deal with the next Katrina scale disaster would come from.
Hope for the best, plan for the worst
The unpredictable nature of natural disasters and other catastrophes present a challenge to transportation agencies, and throw a wrench into well designed pavement management programs. With the use of relief programs, agencies can be ready to react as quickly and effectively as possible to unforeseen circumstances. Maintaining a healthy, robust transportation network requires anticipation of unexpected events in addition to the traffic loads that roadways are designed for. The importance of our highways during times of crisis makes it vital that we design them to last, and plan to keep them functioning in any contingency.