The following excerpt is from the new book Curbing Catastrophe: Natural Hazards and Risk Reduction in the Modern World, by Timothy H. Dixon (Cambridge University Press, 2017).

GRACE [The Gravity Recovery And Climate Experiment, a joint satellite mission of NASA and the German Aerospace Center] makes a map of the Earth’s gravitational ï¬eld roughly once month, giving a pretty good idea of how Earth’s mass distribution is changing. While the rocks don’t change much in a month (except during big earthquakes such as the ones that struck Sumatra in 2004 and Japan in 2011), Earth’s ï¬‚uid envelope does. Groundwater can change a lot (think of India’s seasonal monsoon, or wet and dry seasons in the Amazon basin). More importantly for this chapter, the mass of Greenland and Antarctica changes due to melting of their respective ice sheets. They change on an annual cycle, reï¬‚ecting summer loss and winter growth of ice, and on longer time scales as well (Figure 7.6). The biggest change observed by GRACE so far is Greenland. Greenland’s ice sheet is melting away much faster than experts had predicted even 15 years ago, so fast that it will likely be the main contributor to global sea-level rise within a decade or two.

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Figure 7.6. GRACE data showing monthly mass estimates for the Greenland ice sheet (small black circles) between 2002 and 2014, arbitrarily setting the starting point (the average of the ï¬rst year’s measurements) to zero. Note the annual changes, reï¬‚ecting summer melting and winter growth, and the longer term loss of ice (the downward trend of the graph). Note also that the losses are accelerating (the graph curves downward). During this particular period, Greenland lost more than 2,500 gigatons (2500 billion tons) of ice, roughly 0.1 percent of its total. During the ï¬rst ï¬ve years of the mission, Greenland lost mass at an average rate of about 200 gigatons (GT) per year. During the next ï¬ve years, the average loss rate increased to about 300 gigatons per year. The thin line shows a model ï¬t to the data assuming a constant annual cycle (winter growth, summer loss) and constantly accelerating long term mass loss at 20 GT per year per year; i.e. every year, the rate of loss increases by 20 GT/yr. Modiï¬ed from Yang et al. (2016).

Slow sea-level rise and rapid flooding

What will the effects of sea-level rise look like in the next few decades? Probably much like the last few decades. Individual ï¬‚ood events will strike several low-lying places, with potentially devastating conse- quences. In 1991, a massive cyclone struck Bangladesh. Much of this country’s population lives on a low-elevation river delta, where the Ganges and Brahmaputra Rivers empty into the Bay of Bengal. As with the Mississippi Delta, it is slowly subsiding. Winds from the cyclone drove storm surge far inland. The combination of high winds, ï¬‚ooding, and the subsequent disease and starvation led to approximately 130,000 fatalities and devastating economic losses for this already impoverished nation. Massive ï¬‚ooding again devastated the country in 1998, this time associated with high rainfall upstream. Deforestation in the upstream areas also contributed to ï¬‚ooding. Densely forested hill slopes reduce rapid runoff in high rainfall events, encouraging absorption of rain into the soil. Cutting down the trees allows rainwater to ï¬‚ow rapidly downhill, worsening ï¬‚ooding down- stream. As sea level continues to rise and storm intensity increases (see Chapter 8), ï¬‚ooding events in Bangladesh similar to the 1991 and 1998 catastrophes are virtually certain to increase in frequency and severity.

The 2005 ï¬‚ooding of New Orleans in the aftermath of Hurricane Katrina, and the 2012 ï¬‚ooding of New York and New Jersey associated with tropical storm Sandy, give clues to what the future will look like for coastal parts of the US. Many levees in New Orleans were rebuilt in the 1960s after extensive ï¬‚ooding, and for decades, they seemed to be working. Geologists, engineers, and urban planners warned that such defenses were insufï¬cient in the face of rising sea level and subsiding land. For many years, nothing happened, and the optimists seemed to have the upper hand. When Katrina struck in 2005, the associated storm surge, starting from a higher base level (several decades worth of sea-level rise) attacked levees that were now too low (several decades worth of subsidence). In the end, coastal defenses were overwhelmed from over-topping and other failure modes, including erosion at the base of levees. Experts had warned of New York’s and coastal New Jersey’s vulnerability to storm surge for a long time before Hurricane Sandy struck in late October 2012. As with Bangladesh, these events will hit the US more frequently in the future as sea level slowly rises and storms become more intense. Areas that also experience coastal subsidence, such as cities built on river deltas or coastal areas built on dredged ï¬ll, are more likely to be hit ï¬rst and suffer more extensive damage when storms do hit.

Most people (including many scientists) assume that this is all that most coastal communities will have to deal with for the next 50 years or so in terms of hazards and costs related to sea-level rise – the occasional violent storm and associated ï¬‚ooding. Catastrophic, to be sure, but rare enough that individual communities can recover and rebuild. Rebuilding costs will be steep, but bearable: more than $100 billion for New Orleans and more than $50 billion for New York and New Jersey. Even the most pessimistic estimates predict that sea level will rise by less than 0.2 meters (a little less than 1 foot) by 2050, hence the amount of rise and additional risk is small compared to the short term but much larger effects from 5–6 meter storm surge. It’s the rapid storm surge, not the slow sea-level rise, that’s important, at least in the short term.

However, there are also moresubtle hidden costs associated with sea-level rise. In their book The Battle for North Carolina’s Coast, Dr. Stanley Riggs, a professor at East Carolina University, and several colleagues investigated the effects of sea-level rise on the health of their state’s beaches, a major tourist attraction and recreation resource for the public. In many places, private housing was built behind the public beaches decades ago. However, rising seas are eroding the beaches, bringing new coastline to the edge of private developments, often protected behind sea walls and sandbags. The net effect is that the public beaches are increasingly lost. Long-term loss of tourist revenue is a likely outcome.

Miami Beach, Florida, a beautiful, art-deco tourist destination and a major source of income for South Florida, illustrates another example of hidden costs. When I was a professor at the University of Miami, a graduate student I knew lived in Miami Beach from 2005 to 2008 and parked her car on the street near her apartment. On several occasions, street-level ï¬‚ooding was bad enough to rise above the door panels of the car and ï¬‚ood the interior, requiring professional cleaning. On one occasion, ï¬‚ooding was severe enough that the entire engine had to be replaced. The car’s electrical system also became heavily corroded, requiring additional repairs. While insurance covered most of her losses, over a four-year period she experienced several thousand dollars in unreimbursed costs from deductibles, co-payments, and lost time.

Parts of Miami Beach and Fort Lauderdale now experience ï¬‚ood- ing several times a year. These events used to be restricted to periods of intense rainstorms. The storm sewer system does not have much gradient, so during intense rain, the water has no place to go and ï¬lls the streets. In the last decade, a new phenomenon called sunny-day ï¬‚ooding has started to occur (Figure 7.7). If high tide corresponds with a period of offshore wind, local sea level rises enough to ï¬‚ood some of the streets, even without rain. Saltwater comes up through the storm sewers, and brackish water (mixed freshwater and saltwater) wells up through the porous ground.

If you were living in Miami Beach and wanted a safe place to park your car, you might consult a topographic map (a map with elevation information), or if you have access to a computer or smartphone, you could consult Google Earth. Google has digitized the topographic maps of many areas, so it’s possible to read off the elevation of a speciï¬c location. If you are using an older map, the elevations will be in feet, and many areas of Miami Beach will lie on or close to the three-foot level (slightly lower than one meter). On Google Earth, they show up at or near the 1 m elevation mark. The elevation measurements are relative to mean sea level, deï¬ned as the average sea level over several decades, using a ground reference or “datum” called NAVD-29 (North American Vertical Datum 1929). You might think that as long as you were lucky enough to ï¬nd a parking space for your car that was higher than 3 feet in elevation, which means three feet above the average height of the ocean, you would be safe most of the time, except during hurricanes or other extreme events. But, you would be wrong for several reasons.

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Figure 7.7 (top and bottom): Two pictures of Miami Beach during the “king tide” of October 2014. In the bottom picture, note that sea level is already quite close to the ground level – even a moderate storm or onshore wind would lead to signiï¬cant inundation. Courtesy of S. Wdowinski (top) and Q. Yang (bottom).

First, the datum is now incorrect. Topographic maps are constructed with the assumption that the Earth is static – neither the Earth’s surface nor sea levels are supposed to change. For most places most of the time, static Earth is a good assumption. Most people (except geologists) think of the Earth in this way. But the Earth is actually changing all the time. In the more than 80 years since NAVD-29 was deï¬ned and heights in Miami Beach were measured relative to it, certain parts of Miami Beach (the ones built on artiï¬cial ï¬ll) have subsided, reï¬‚ecting compaction of the ï¬ll (by one or two feet), while sea level has risen by at least a foot. So places originally deemed to be 3 feet above mean high tide are now much closer to average sea level (i.e. zero elevation). If the tide is higher than average, the streets will ï¬‚ood. Mean sea level is updated every few decades, but maps tend to be updated more slowly. Many buildings in Miami Beach, including most of its art-deco hotels, were built in the 1920s and 1930s, using maps based on NAVD-29 or earlier datums.

Second, as the surface of a street gets closer and closer to sea level (approaching zero elevation) details become important in terms of ï¬‚ood potential. Small height differences (a foot or two) can make all the difference. Local depressions or high points may not be recorded on a typical topographic map. Also it is no longer enough to know the average elevation. This close to zero elevation, we also need to know the detailed time variation of sea level, which can change several tens of centimeters (1 to 2 feet) in an hour or less, due to tides and local weather. Miami Beach does not have levees like New Orleans to hold back high water, so high water conditions are felt almost immediately, with water often forcing its way up through the storm sewers or porous ground. High tide actually varies quite a bit. For Miami Beach, a spring high tide (when the moon is full or new, aligning its gravitational pull with the sun) will be a foot or two higher compared to a neap high tide (when the sun and moon are at right angles). The difference between spring and neap tides does not matter if you live in Colorado, much higher than sea level, but if you are living close to the ocean, that extra foot or two can make the difference between ï¬‚ooding and no ï¬‚ooding. Sometimes your parking space is actually going to be one or two feet below that day’s average water level, and you need to take that into account if you are looking for a safe place to park. In effect, you need a weather forecast for local sea level. If you knew sea level was going to be especially high over the next 24–48 hours, due to either a spring tide or an offshore wind, and you had the option, you might choose to pay extra to park your car overnight on the second or third ï¬‚oor of a parking structure.

The third reason topographic maps are misleading in terms of ï¬‚ood potential is related to the second. As the street surface gets lower and lower, it no longer drains very well. A heavy rainstorm can lead to signiï¬cant local, short-term ï¬‚ooding until excess water has time to drain away. If the rain happens to coincide with high tide, things get much worse since there is nowhere for the water to drain until the tide recedes.

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Figure 7.8 Frequency of ï¬‚ood events in Miami Beach, Florida. Modiï¬ed from Figure 3a of Wdowinski et al. (2016).

Perhaps this will be the next new thing for software developers – an app for coastal residents that details the local high spots, and predicts actual sea level and ï¬‚ood susceptibility for various locations for a given time of day, similar to a weather forecast. The prediction would be based on local elevation and drainage, short-term changes from tides and weather, and longer-term changes from sea-level rise and land subsidence. Available data suggest a signiï¬cant increase in the rate of sea-level rise in the last decade, but it is still not completely understood. Dr. Shimon Wdowinski, a professor at Florida International University in Miami, has studied the frequency of local ï¬‚ooding in Miami Beach using media reports, insurance claims, and weather records (Figure 7.8). There has been a rapid increase in the number ï¬‚ood events over the last two decades.

Frequent ï¬‚ooding of Miami Beach with water that is increas- ingly saline does more than damage parked cars. It is starting to wreak havoc with all sorts of infrastructure, from building foundations to buried cables and underground pipes for water and sewage. Gravity- drained storm sewers have become ineffective and require costly pumps, similar to those used in New Orleans. The present and future cost of these repairs and infrastructure upgrades is very high. In 2013 and 2014, Miami Beach spent millions of dollars installing new pumps to ï¬‚ush ï¬‚oodwaters into nearby Biscayne Bay. Unfortunately, this “ï¬x” has had unintended economic consequences. Biscayne Bay is an important tourist attraction, famous for its swimming, boating, and ï¬shing. The untreated ï¬‚oodwater is hundreds of times higher in enterococci (the kind of bacteria that indicates fecal contamination) compared to levels recommended by the US Environmental Protection Agency. This is not exactly a great tourist draw.

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