Saturday, November 30, 2013

What is your flood risk tolerance?

I recently read an article in the December 2013 issue of Money magazine titled "After the Flood", which chronicled the financial difficulties of a Staten Island couple in the wake of Hurricane Sandy. Sandy made landfall on October 29, 2012 and when it was all said and done, caused $70 billion in damages across 24 states. The couple stayed put during the storm, which inundated their house up to the second floor and floated their cars several blocks down the street. Their home value decreased from $303,000 to $204,000 post-Sandy (likely due to flood damage and higher cost to insure) and they still owe $275,000 on their mortgage - putting them literally and figuratively underwater. Even worse for the couple (but perhaps better in the long term), new post-Sandy FEMA building standards will require the couple to elevate their home by 10 feet to place it above the new flood elevation - a very costly proposition which they cannot afford. These are the costs and risks of living in flood prone areas.

Predicted and observed tide levels at the Battery, NY during Hurricane Sandy. October 29, 2012 was the highest ever recorded tide at this location due to coincidence of high tide and peak storm surge.

The Money article provides more details on the financial struggles of the couple and the steps they are taking to get their lives back on track. The article got me thinking about our attitudes towards risk and the value we place on home ownership, even if we do live in disaster-prone, yet desirable, areas. This was highlighted the most clearly by the very last paragraph of the article, which noted:
Still, they're eager to move on and to finally be able to put Hurricane Sandy behind them. Says Motherway (the wife): "You just hope it's the kind of storm that happens only once every 100 years."
As an engineer, I consider flood risk from a statistical perspective and not one based on wishful thinking - if you live in a flood zone, you are playing the odds and betting that either: (1) you will not encounter a storm severe enough to flood you, (2) you have adequate insurance to cover your losses, or (3) the state and/or federal government will come to your aid in the event of a natural disaster. For example, if you live in a low-lying area behind a levee that provides 100-year flood protection, you are presumably protected from all less severe flood events (e.g., 10-year, 50-year, etc); however, for any storm more severe than a 100-year event, you will be flooded. That is a level of risk that many Americans live with every day. The problem with the terminology "the 100-year flood", as illustrated by the Staten Island couple's statement, is that it implies such a storm will occur only once every 100 years. More accurately stated, the 100-year flood is an event that has a 1% chance of occurrence in any individual year (the "1% annual chance" terminology is preferred by risk analysts). Over a very long period of time, this severity of storm will indeed occur, on average, once every 100 years, but over the short term it could occur more or less frequently. For example, during El Ninos in California, we can have back-to-back severe weather events (e.g., rain, water level, and wave) during a single winter storm season, separated by years of calmer conditions.

Using a statistical concept known as "encounter probability", we can estimate the likelihood of experiencing a storm of a particular severity over a specified time frame. For example, the commonly quoted statistic that the San Francisco Bay area has a 63% chance of experiencing a 6.7 magnitude earthquake or greater over the next 30 years is a type of encounter probability analysis. In terms of flooding, there is about a 25% chance that a homeowner will experience 100-year flooding or greater over the lifespan of a standard 30-year mortgage. Now, I don't know about you, but that seems like a pretty high likelihood to me! It's actually a bit frightening to think that our nation's flood protection infrastructure in any one location generally has a 25% chance of being overwhelmed by flood waters over such a short length of time. Using the concept of encounter probability, I produced a set of curves (shown below), which illustrates the likelihood of experiencing a storm of a given severity over a specified length of time. By selecting a storm severity curve and project lifespan, the likelihood of flooding (encounter probability) can be determined. For example, using the green curve below, there is an 80% chance of experiencing a 25-year flood or greater over a 40 year time span.

Encounter probability of an extreme storm event over a given duration.

FEMA's National Flood Insurance Program uses the 100-year floodplain as the standard to assess whether or not a homeowner is required to purchase flood insurance. Most mortgage companies require borrowers to purchase flood insurance to protect their loans until homeowners pay off their mortgages (after that, insurance is recommended, but not required). I calculated the encounter probabilities for storms of different severity over a standard 30-year mortgage period and a 75-year lifetime of a typical American (see table below). As you can see, the chances of experiencing a moderate to strong storm (say, 25 to 50-year event) over these time periods are quite high. For reference, I also estimated the encounter probability for two recent extreme storms - Hurricanes Katrina (300 to 400-year event) and Sandy (1,000 to 1,500-year event). Estimates of the exact frequency of these types of storms are uncertain given our relatively short period of record, which is why I show a range of likelihood. Based on these estimates, it seems fair to say that Hurricane Sandy was a "once in a lifetime" storm, whereas Hurricane Katrina, while rare, has a reasonable chance of recurrence (approximately 20%) during our lifetimes (especially considering the effects of climate change, which could increase sea levels and storm frequency and intensity - but that's another topic!).

Likelihood of flooding (encounter probability) for different storm severities over a 30-yr mortgage period and 75-yr lifetime:

Return Period
30-yr Mortgage
Encounter Probability
75-yr Lifetime
Encounter Probability

I polled my girlfriend, Allison, and her sister, Tina, to see what level of risk they would take on assuming a 30 year habitation in their home. Tina indicated she would assume a 20% risk of flooding (comparable to FEMA's acceptable risk level) and Allison would accept a 10% risk (on the order of a Katrina-type storm). I plotted those points on the figure below (blue and red triangles) and they roughly translate to the 125 and 300-year flood events, respectively. Personally, I would tolerate a much lower risk of flooding, probably more along the lines of 5% (a 600-year flood event). This indicates that all three of us would not feel comfortable owning property within FEMA's 100-year floodplain.

Personal risk tolerance relative to likelihood of flooding during a 30-year mortgage period.

So what is your risk tolerance? If you purchased a home in a river or coastal flood prone area, what likelihood of flooding would you accept? It's not a simple question - what other factors would you consider?

Money Magazine

Climate Central

Significance Magazine

The Lens

Tuesday, October 8, 2013

Coastal Flood Processes Along the California Coast

As the Atlantic hurricane season draws to a close and the Pacific storm season picks up, I thought it would be interesting to consider the differences in coastal flood processes between these two coastlines and remind us what's on the way this winter. The post below is based on an article I prepared as part of FEMA's coastal flood mapping along the California coast, currently underway. That article can be found here, but this one has pictures!

Recent media coverage of catastrophic coastal flood events, such as Hurricanes Katrina (2005) and Sandy (2012), has increased the public’s awareness of coastal flood vulnerabilities along the nation’s shorelines. These large storm systems, with their powerful winds and overwhelming storm surge and rainfall, can devastate coastal communities. Due to these recent events, the public has seen firsthand the damage wrought by large tropical storm systems along the Gulf and Atlantic coastlines. But what about coastal storms in California? Coastal storm systems and impacts along the California coast differ significantly from the Atlantic and Gulf Coasts due to the characteristics of the Pacific Ocean basin, storm types, and steep coastal topography.

As we know, hurricanes are a relatively common occurrence along the Atlantic and Gulf shorelines. In contrast, the likelihood for a hurricane to make direct landfall along the California coast is very remote, although offshore tropical storms can affect coastal communities through wind, rain, and remote swell impacts. Due to the oceanographic conditions in the northeast Pacific Ocean and the narrow continental shelf, it is not possible to generate the large storm surges seen in the warmer and shallower Gulf and Atlantic waters. Instead, coastal flooding along the California coast is typically a result of the combination of high tides, modest storm surge, and moderate to high wave energy. Unlike the Gulf and Atlantic coasts, where storm surge in excess of 20 feet is possible, storm surge along the California coast rarely reaches 3 feet and is typically on the order of 1-2 feet during winter storm events. Instead, wave effects, such as wave setup and runup, typically dominate flood levels at the shoreline. The majority of coastal flood events in California occur during the late fall through early spring and are the result of extratropical storm systems that originate offshore in the northeast Pacific Ocean. During El Niño winters, tides are further elevated along the coastline and storms follow a more southerly track, exposing the California coast to abnormally high tides, wave-induced flooding, and coastal erosion.

The summary below explains various types of coastal flood processes along the California coast that are typically responsible for flood impacts, ranging from King Tides to extratropical storms to tsunamis:

King Tide – Abnormally high, but predictable, astronomical tides that occur approximately twice per year, typically during the winter months. King Tides are the highest tides that occur each year and typically exceed 7 feet (relative to mean lower low water). Coastal flood impacts include nuisance flooding and inundation of low-lying roads and paths. High tides can exacerbate coastal and riverine flooding, especially in inland bay areas such as San Francisco and Newport Bays.

A 7 ft King Tide at San Francisco's Embarcadero (January 21, 2012). During the 1983 El Nino, the water level was 1.5 ft higher.

Extreme High Tide – When Pacific Ocean storms coincide with high astronomical tides, storm surge due to meteorological effects can further elevate water levels along the coast to produce extreme tides. El Niño conditions along the coast can also contribute to storm surge and produce extraordinarily high water levels (for example, January 1983 and February 1998). Extreme high tides can exceed 7.5 to 8.5 feet in southern and central California and 10 feet in northern California. Impacts include severe inundation of inland bay shorelines, intensification of upstream riverine flooding, and inhibited drainage from stormwater outfalls in tidally influenced areas.

The table below shows the typical tide range and highest observed tide at various points along the California coastline*. As you can see, the tide range increases from south to north. Additionally, the magnitude and frequency of storm surge events increases as well. These factors combine to produce more extreme tides in the north than the south.

Station Daily Tide Range (ft) Highest
Observed (ft)
Port Orford, OR 7.3 11.5
Crescent City, CA 6.9 10.7
North Spit, Humboldt Bay, CA 6.9 9.7
Arena Cove, CA 5.9 8.6
Point Reyes, CA 5.8 8.5
San Francisco, CA 5.8 8.7
Ocean Beach, San Francisco, CA 5.9 8.7
Princeton, Half Moon Bay, CA 5.7 8.5
Ano Nuevo Island, CA 5.4 8.3
Monterey, CA 5.3 7.9
Port San Luis, CA 5.3 7.7
Santa Barbara,  CA 5.4 7.4
Rincon Island, CA 5.5 7.8
Santa Monica, CA 5.4 8.5
Los Angeles, CA 5.5 7.9
La Jolla, CA 5.3 7.7
San Diego, CA 5.7 8.1
*All tide heights reported relative to the MLLW tidal datum.

Wind Wave Event – Pacific Ocean storms or strong thermal gradients can produce strong winds that blow across sheltered water bodies and inland bays (for example, San Francisco Bay, Tomales Bay, etc.). When the wind blows over long reaches of open water, large waves can be generated that impact the shoreline and cause damage to coastal structures such as levees, docks and piers, wharfs, and revetments. Locally generated wind waves in the southern California bight can also cause flood and erosion issues along the open coast shoreline.

Locally generated wind waves at Point Pinole in San Francisco Bay

Pacific Winter Storm – During the winter, storm systems from the Aleutian Islands, Hawaii (“Pineapple Express”), and other parts of the North Pacific impact the California coastline. Storms generally approach from the west or northwest, although “southeaster” events can also occur in southern California. These low pressure systems generate large waves and elevated tide levels along the coast. Impacts include beach and bluff erosion and damage to homes and coastal structures.

Winter storm waves batter the Pacifica shoreline (January 2011)

El Niño Winter Storm – During El Niño winters, atmospheric and oceanographic conditions in the Pacific Ocean produce severe extratropical winter storms that impact the California coast. Storms follow a more southerly track and bring intense rainfall and storm conditions. Rainfall and elevated tide levels persist through the winter and often coincide to produce upstream riverine flooding. Impacts are widespread but sheltered south facing beaches are particularly vulnerable. The El Nino cycle oscillates on approximately a 5-7 year timelime. El Ninos correspond to warmer than normal surface ocean temperatures and La Ninas correspond to cooler than normal surface ocean temperatures in the equatorial Pacific. The Oceanic Nino Index quantifies the strength of the El Nino/La Nina based on the magnitude of this temperature anomaly.

The El Nino-Southern Oscillation Index, which indicates the strength of El Nino (red) and La Nina (blue) events. Recent significant El Ninos include 1982-83, 1997-98, and 2009-2010.
Remote Swell – Remote swell is generated by storms in the Pacific Ocean and from other regions such as Baja California and more distant areas such as New Zealand. Storm types include offshore extratropical storms, tropical storms, hurricanes, and southern hemisphere storms. Remote swell events can be difficult to predict since waves travel from distant source regions. Impacts include wave damage and overtopping along the shoreline, particularly to coastal structures such as breakwaters, piers, and revetments. Wave overtopping can also cause inundation and ponding of water in backshore areas, such as low-lying roads and parking lots. 

Tsunami – Tsunamis are extremely long period waves generated primarily by earthquakes, but can also be caused by volcanic eruptions or landslides. Tsunamis can be generated from far-field source regions such as Chile, Alaska, or Japan and from near-field source regions along the Pacific coast. Impacts include strong currents and long lasting water level oscillations in harbors which can damage docks, piers, and boats moored within the harbor. For larger tsunami events, impacts could include shoreline inundation and overland flow of water that damages structures in low-lying areas.

Damage at the Crescent City harbor after the March 2011 Japan tsunami (Photo: Nicole Metzger)
Numerical modeling predictions of tsunami wave height and arrival times across the Pacific for the Japan earthquake in 2011. Also of note is the focusing of wave energy in the vicinity of Crescent City in northern California. Source: NOAA Center for Tsunami Research, Pacific Marine Environmental Laboratory