Tag Archives: flooding

Euro 2016: France inundated by fans and floods

This week the final knockout rounds of Euro 2016 take place in France. Sadly, England has long since left the country and the tournament, largely due to some inept displays. But more miserable than England’s performance, was the weather at the start of the tournament, which caused concern in the capital as intense precipitation on top of an already saturated France, led to severe flooding.

Some areas of the country experienced the worst flooding they have seen in a century, with the floods across eastern and central France declared a natural disaster by French President François Hollande. River levels in the Seine were at their highest in nearly 35 years, impacting Paris, and leading to three of the capital’s best-known museums — the Louvre, the Grand Palais, and Orsay —closing their doors to the public, as staff moved priceless works of art to the safety of higher floors.

Source: The Guardian

There were also concerns surrounding how the flooding could impact the tournament. However, as you can see in the below image, which represents the RMS 1,000 year inland flood hazard extent, neither of the two stadia located in France’s capital (yellow markers) were really at any risk of flooding. The same can’t be said for the fan zone adjacent to the Eiffel Tower though (red marker). Continued intense rainfall, would have led to increased flood severity, meaning that 90,000 or so fans would have been in need of their waders.

Stade de France and Parc des Princes (yellow markers); Paris Fan Zone (red marker)

Paris wasn’t the only location in France to be impacted by the floods though; further south the town of Nemours observed severe flooding as the River Loing burst its banks. While devastating to the local community, this severity of flooding can be expected in the town. The RMS Europe Inland Flood maps demonstrate such flooding for events in excess of the 50 year return period, but as the below image of the 200 year flood extent demonstrates, the flooding could have been even more severe.

Rue de Paris, Nemours (yellow marker) and Château-Musée de Nemours (red marker)

The flooding in Nemours is a good example of why it is so important to understand the standard of protection offered by local flood defenses, in order to fully understand flood risk. The RMS Europe Inland Flood models and maps explicitly represent the impact of flood defenses and provide some noteworthy insights into the potential exposure at risk, if the standard of protection is not maintained or local flood defenses are overtopped.

Rue de Paris, Nemours. Source: The Guardian

If we removed all flood defenses and consider a 100 year return period level of flood hazard across France, the RMS analyses estimate that over €600 billion of insured exposure is at risk to flood damage. However, approximately 40 percent of this exposure at risk is protected against such levels of hazard by local flood defenses.

Source: Château-Musée de Nemours

And in the largest exposure concentrations, such as Paris and its surrounding area, the importance of local defenses is even more prominent. Looking at a similar 100 year return period level of flood hazard in this region, almost €60 billion of insured exposure would be at risk of flooding, but approximately 90 percent of that exposure is protected against this level of hazard.

Flood can be thought of as a polar peril; if you’re in the extent of a flood event, the costs are high but if you’re on the edge then you’re safe. And for this reason, an understanding of the impact of flood defenses is vital, because if they breach or become overtopped, the losses can be high. Knowing where exposure is protected allows you to write business smartly in higher risk zones. But understanding the hazard, should defenses fail, is also vital, enabling a more informed understanding of severe flood risk and its associated uncertainties.

This post was co-authored by Rachael Whitford and Adrian Mark.

Mangroves and Marshes: A Shield Against Catastrophe?

“We believe that natural ecosystems protect against catastrophic coastal flood losses, but how can we prove it?”

This question was the start of a conversation in 2014 which has led to some interesting results. And it set us thinking: can RMS’ models, like the one which estimates the risk of surge caused by hurricanes, capture the protective effect of those natural ecosystems?

The conversation took place at a meeting on Coastal Defenses organized by the Science for Nature and People Partnership. RMS had been invited by one of our leading clients, Guy Carpenter, to join them. The partnership is organized by The Nature Conservancy, the Wildlife Conservation Society, and the National Center for Ecological Analysis and Synthesis.

We were confident we could help. Not only did we think our models would show how biological systems can limit flood impacts, we reckoned we could measure this and then quantify those benefits for people who calculate risk costs, and set insurance prices.

RMS’ modeling methodology uses a time-stepping simulation, which relies on a specialist ocean atmosphere model, allowing us to evaluate at fine resolution how the coastal landscape can actually reduce the storm surge—and in particular lower the height of waves. In many buildings the real weakness proves to be the vulnerability to wave action rather than just the damage done by the water inundation alone.

The first phase of RMS’ work with The Nature Conservancy is focused on coastal marshes as part of a project supported by a Lloyd’s Tercentenary Research Foundation grant to TNC and UC Santa Cruz. Under the supervision of Paul Wilson, in the RMS model development team, and working with Mike Beck who’s the lead marine scientist for The Nature Conservancy, the project is focused on the coastlines, which were worst impacted by the surge from Superstorm Sandy. The irregular terrain of the marsh and resulting frictional effects reduce the surge height from the storm. Our work is showing that coastal marshes can reduce the flood risk costs of properties, which lie inland of the marshes by something in the range of 10-25%.

Tropical Defenses

So, that’s the effect of coastal marshes. But what about other biological defenses such as mangrove forests and offshore reefs (whether coral or oyster reefs)? Further research is planned in 2016 using RMS models to measure those likely benefits too.

But here’s a rather intriguing (if unscientific) thought: is there a curious Gaia-like principle of self-protection operating here in that the most effective natural coastal protections—mangroves and coral reefs—are themselves restricted to the tropics and subtropics, the very regions where tropical cyclone storm surges pose the greatest threat? Mangroves cannot withstand frosts and therefore in their natural habitat only extend as far north along the Florida peninsula as Cape Canaveral. And yet in our shortsightedness humans have removed those very natural features, which could help protect us.

Paradise Lost?

Between 1943 and 1970 half a million acres of Florida mangroves were cleared to make way for smooth beaches—those beautiful and inviting stretches of pristine sand which have for decades attracted developers to build beachfront properties. Yet, paradoxically, that photogenic “nakedness” of sand and sea may be one of the things, which leaves those properties most exposed to the elements.

With the backing of The Nature Conservancy it seems mangroves are making a comeback. In Miami-Dade County they’re examining a planting program to protect a large water treatment facility. Of course biological systems can only reduce part of the flood risk. They can weaken the destructive storm surge but the water still gets inland. To manage this might require designing buildings with water-resistant walls and floors, or could involve a hybrid of grey (manmade) and green defenses. And if we can reduce the destructive wave action, that might allow us to build earth embankments protected with turf in place of expensive and ugly, but wave-resistant, concrete flood walls.

On March 28, 2015 The Nature Conservancy organized a conference and press briefing in Miami at which they announced their collaboration with RMS to measure the benefits of natural coastal defenses. The coastline of Miami-Dade, already experiencing the effects of rising high tide sea levels, presents real opportunities to test out ways of combatting hurricane hazards and stronger storms through biological systems. Our continued work with The Nature Conservancy is intended to develop metrics that are widely trusted and can eventually be adopted for setting flood insurance prices in the National Flood Insurance Program.

Just How Unlucky Was Britain to Suffer Desmond, Eva, and Frank in a Single December?

Usually, it’s natural disasters occurring elsewhere in the world that make headlines in Britain, not the other way around. But you’d have to have been hiding under a rock to have missed the devastation wrought by flooding in the U.K. last month, thanks to the triple-whammy of storms Desmond, Eva, and Frank. Initial analysis from the Association of British Insurers suggests that the damage done could run to the region of £1.3bn.

But just how unlucky was the U.K.to suffer not just one, or two, but three big storms in one December, and for these three storms to interact in such a way as to produce the chaos that followed?

First it’s worth pointing out that floods in the U.K. are—as is usually the case elsewhere—subject to important seasonal variation (see chart below). The winter months bring the highest number of events, and December does in fact come out (slightly) on top, especially for flooding events of the sort seen last month, which tend to follow heavy rainfall leading to soil saturation (November 2015 received about twice the average climatological rainfall for November in the U.K.).

Source: RMS

The reason this matters is that, when soil is sodden following an extended period of heavy rains, further rains can more easily run off the surface, exacerbating the risk of pluvial flooding. The water will then follow natural and artificial drains until it reaches the closest river network, in which it can accumulate, potentially triggering river or “fluvial” flooding. The runaway effect of the masses of water can also cause what is known as ground-water flooding. This cumulative phenomenon means that—as we saw in December—flooding can persist for a significant amount of time, leading to several flood events in close succession.

A flood CAT model that properly captures these sorts of interactions between rainfall events and hydrological systems will allow not just for an assessment of the likelihood of a single severe event, but also a better understanding of the compounding factors that can lead to the sort of flooding seen in the U.K. last month. And based on our latest RMS pan-Europe flood model, the chances of having three rainstorms lead to major inland flooding over a single December are far from negligible.

Source: RMS Europe Flood Model

The chart above shows the probability of one, two, three, and four flood events for the month of December. What it means is that, on average, every second December in the U.K. has at least one flood event, and every third December has only one flood event. Around every eight years there are two flood events, and a cluster of three flood events happens once every quarter-century.

Now, this does not mean that flooding on the scale just witnessed happens on average every 25 years—just that this is the average period for seeing three flood events in one December. Even if it did, it wouldn’t mean that the U.K. can rest on its laurels until 2041… this is just a statistical average. It is quite possible for clusters to hit several years in a row—a so-called “flood-rich period”.

This gets to the real nub of the issue. The question of how often this sort of flooding takes place in the U.K. is almost by-the-by. The point is that it isn’t rare as hen’s teeth, and so the U.K. needs to be prepared. And what was most shocking about December wasn’t the flooding itself, so much as the sheer lack of resilience on display. A media storm has understandably been whipped up regarding the level of investment into flood walls and so on, but protective infrastructure is only part of the equation. What is needed is not just flood walls (which can actually be counterproductive on their own), but a wider culture of resilience. This includes things such as flood warning systems, regular evacuation drills, citizens having personal plans in place (such as being ready to move furniture to upper levels in the case of an alert) and, critically, the ability to respond and recover should the defences fail and the worst happen (which is always a possibility). The U.K. is the world’s sixth richest country—it has the resources to cope with flood events of this magnitude… whether they happen every five, ten or 25 years.

South Carolina Floods: The Science Behind the Event and What It Means for the Industry

South Carolina recently experienced one of the most widespread and intense multi-day rain events in the history of the Southeast, leaving the industry with plenty to ponder.

Parts of the state received upwards of 27 inches (686 mm) of rain in just a four day period, breaking many all-time records, particularly near Charleston and Columbia (Figure 1). According to the National Oceanic and Atmospheric Administration, rainfall totals surpassed those for a 1000-year return period event (15-20 inches (381-508 cm)) for parts of the region. As a reminder, a 1000-year return period means there is a 1 in 1000 chance (0.1%) of this type of event occurring in any year, as opposed to once every thousand years.


Figure 1: Preliminary radar-derived rainfall totals (inches), September 29-October 4. Source: National Weather Service Capital Hill Weather Gang.

The meteorology behind the event

As Hurricane Joaquin tracked north through the Atlantic, remaining well offshore, a separate non-tropical low pressure system positioned itself over the Southeast U.S. and essentially remained there for several days. A ridge of high pressure to the north acted to initiate strong onshore windflow and helped keep the low-pressure system in place. During this time, it drew in a continuous plume of tropical moisture from the tropical Atlantic Ocean, causing a conveyor belt of torrential rains and flooding throughout the state, from the coast to the southern Appalachians.

Given the fact that Joaquin was in the area, the system funneled moist outflow from it as well, enhancing the onshore moisture profile and compounding its effects. It also didn’t help that the region had experienced significant rainfall just a few days prior, creating near-saturated soil conditions, and thus, minimal absorption options for the impending rains.

It’s important to note that this rain event would have taken place regardless of Hurricane Joaquin. The storm simply amplified the amount of moisture being pushed onshore, as well as the corresponding impacts. For a more detailed breakdown of the event, please check out this Washington Post article.

Notable impacts and what it means for the industry

Given the scope and magnitude of the impacts thus far, it will likely be one of the most damaging U.S. natural catastrophes of 2015. Ultimately, this could be one of the most significant inland flooding events in recent U.S. history.

This event will undoubtedly trigger residential and commercial flood policies throughout the state. However, South Carolina has just 200,000 National Flood Insurance Program (NFIP) policies in place, most of which are concentrated along the coast, meaning that much of the residential losses are unlikely to be covered by insurance.


Figure 2: Aerial footage of damage from South Carolina floods. Source: NPR, SCETV.

Where do we go from here?

Similar to how Tropical Storm Bill reiterated the importance of capturing risk from tropical cyclone-induced rainfall, there is a lot to take away from the South Carolina floods.

First, this event underscores the need to capture interactions between non-tropical and tropical systems when determining the frequency, severity, and correlation of extreme precipitation events. This  combined with high resolution terrain data, high resolution rainfall runoff models, and sufficient model runtimes will optimize the accuracy and quality of both coastal and inland flood solutions.

Next, nearly 20 dams have been breached or failed thus far, stressing the importance of developing both defended and undefended views of inland flood risk. Understanding where and to what extent a flood-retention system, such as a dam or levee, might fail is just as imperative as knowing the likelihood of it remaining intact. It also highlights the need to monitor antecedent conditions in order to properly assess the full risk profile of a potential flood event.

The high economic-to-insured loss ratio that is likely to result from this event only serves to stress the need for more involvement by private (re)insurers in the flood insurance market. NFIP reform combined with the availability of more advanced flood analytics may help bridge that gap, but only time will tell.

Lastly, although individual events cannot be directly attributed to climate change, these floods will certainly fuel discussions about the role it has in shaping similar catastrophic occurrences. Did climate change amplify the effects of the flooding? If so, to what extent? Will tail flood events become more frequent and/or more intense in the future due to a rising sea levels, warming sea surface temperatures, and a more rapid hydrologic cycle? How will flood risk evolve with coastal population growth and the development of more water impermeable surfaces?

This event may leave the industry with more questions than answers, but one stands out above the rest: Are you asking the right questions to keep your head above water?

Reflecting on Tropical Storm Bill

After impacting coastal Texas and portions of the Plains and Midwest with rain, wind, and flooding for nearly a week, Tropical Storm Bill has dissipated, leaving the industry plenty to think about.

The storm organized quickly in the Gulf of Mexico and intensified to tropical storm status before making landfall in southeast Texas on June 16, bringing torrential rain, flash flooding, and riverine flooding to the region, including areas still trying to recover from record rainfall in May. Many surrounding towns and cities experienced heavy rain over the next few days, including some that recorded as much as 12 inches (30 cm). Thankfully though, most high exposure areas like Houston, TX, were spared of significant flooding.


Source: NOAA

Still, as damage is assessed and losses are totaled, Tropical Storm Bill reminds us of the material hazard associated with tropical cyclone (TC)-induced precipitation, and the importance of capturing its impacts in order to obtain a comprehensive view of the flood risk landscape. Without understanding all sources of flood hazard or their corresponding spatial and temporal correlation, one may severely underestimate or inadequately price a structure’s true exposure to flooding.

Of the $40 billion+ USD in total National Flood Insurance Program claims paid since 1978, more than 85% has been driven by tropical-cyclone induced flooding, approximately a third of which has come from TC-induced rainfall.

The most significant TC-rain event during this time was Tropical Storm Allison (2001), which pummeled southeast Texas with extremely heavy rain for nearly two weeks in June 2001. Parts of the region, including the Houston metropolitan area, experienced more than 30 inches (76 cm) of rain, resulting in extensive flooding to residential and commercial properties, as well as overtopped flood control systems. All in all, Allison caused insured losses of $2.5 billion (2001 USD), making it the costliest tropical storm in U.S. history.

Other notable TC-rain events include Hurricane Dora (1964), Tropical Storm Alberto (1994), Hurricane Irene (2011). In the case of Irene, the severity of inland flooding was exacerbated by saturated antecedent conditions. Similar conditions and impacts occurred in southeast Texas and parts of Oklahoma ahead of Tropical Storm Bill (2015).

Looking ahead, what does the occurrence of two early-season storms mean in terms of hurricane activity for the rest of the season? In short: not much, yet. Tropical Storms Ana and Bill each formed in areas that are most commonly associated with early-season tropical cyclone formation. In addition, the latest forecasts are still predicting a moderate El Nino to persist and strengthen throughout the rest of the year, which would likely suppress overall hurricane activity, particularly in the Main Development Region. However, with more than five months remaining in the season, we have plenty of time to wait and see.

4 Facts About California’s “Hellastorm”

California is bracing for a major storm this week. Many schools are closed and residents are hunkering down in preparation for potential flooding. Not to be outdone by the East Coast, which has come up with monikers like “snowmageddon” and “snowpocalypse” for their recent storms, some are referring to it as the “hellastorm.”

Source: twitter.com/AllyNgSF

So, what’s the deal with the so-called “storm of the decade?”

It’s getting rainy and windy on the West Coast.

Estimates this morning are predicting 1 to 5 inches of rain from Northern California up to Washington, 1 to 2 feet of snow in the Sierra Nevada mountains, and wind gusts over 50 miles per hour in the interior regions.

It will happen again.

Storms like these are not uncommon, occurring once every 5 to 10 years. So we could experience another one before the end of the decade.

The drought is partially to blame.

While drought conditions are not a necessity for these types of events, they can increase the impact of flooding because the ground cannot absorb water fast enough. The same can occur when the sustained heavy rain falls on ground is already saturated.

The current rain came all the way from Hawaii.

Storms like this are dependent on many variables. In this case, the excessive rain and snowfall is being driven by the position of the jet stream and what’s known as the Pineapple Express, an atmospheric plume of tropical moisture that flows from the sub-tropics near Hawaii to the U.S. West Coast. It generally occurs during El Nino years, but in this case, forecast El Nino conditions did not fully develop. In other words, it’s a weak one.

UPDATE: Northern California has gotten more than 8 inches of precipitation so far. Sustained winds were forecast to be up to hurricane force (70 to 80 mph) in the local mountains and up to 100 mph in the higher elevations across the Sierra summit. A wind gust to 147 mph was recorded at high altitude peak near Lake Tahoe, that had surfers catching 7-foot waves on the lake!

New Storms, New Insights: Two Years After Hurricane Sandy

When people think about the power of hurricanes, they imagine strong winds and flying debris. Wind damage will always result from hurricanes, but Hurricane Sandy highlighted the growing threat of storm surge as sea levels rise.

While Sandy’s hurricane-force winds were not unusual, the storm delivered an unprecedented storm surge to parts of the Mid-Atlantic and Northeast U.S. In total, Sandy caused insured losses of nearly $20 billion in the U.S., 65 percent of which resulted from surge-driven coastal flooding.

Considering the hazard and severity of the event, we used Sandy as the first real opportunity to validate our hydrodynamic storm surge model, which we released in 2011 and embedded in the RMS U.S. Hurricane Model. We verified the model against more than 300 independent wind and flood observations, the Federal Emergency Management Agency’s (FEMA) 100-year flood zones, and the FEMA best surge inundation footprint for New York City. The model captured the extent and severity of Sandy’s coastal flooding exceptionally well.

We also conducted extensive analysis of claims data from Sandy, which involved reviewing nearly $3 billion in location-level claims and exposure data across seven lines of business, provided by several companies. The purpose of the study was to deepen our understanding of the impacts of flooding on coastal exposures, particularly for commercial and industrial structures.

What struck us was how vulnerable buildings are to below-ground flooding. In many cases, damage to ground- and basement-level property and contents contributed a much higher proportion of the overall losses than expected, particularly for commercial structures in New York’s central business districts.

This insight has prompted us to improve the flexibility of how losses are modeled for contents and business interruption, specifically for basements. Early next year, we will release an update to our flagship North Atlantic Hurricane Models to provide the most-up-to-date view of hurricane risk with new vulnerability modeling capabilities based on insights gained from Sandy.

The model update includes new location-specific content triggers to enable users to make business interruption loss projections dependent on either contents or building damage, rather than on building damage alone. The model also allows users to assess the impact of multiple basement levels in a building, as well as the total value of contents stored within.

The claims data analysis also highlighted the importance of using high-resolution data to model high-gradient perils, such as coastal flooding. Flood losses are extremely sensitive to the locations of coastal exposures, as well as the surrounding topographical and bathymetrical features. Using high quality data with location-level specificity across a variety of building characteristics, as well as a high-resolution storm surge model that can accurately capture the flow of water around complex coastlines and local terrain, minimizes uncertainty.

At this time, RMS remains the only catastrophe modeling firm to integrate a hydrodynamic, time-stepping storm surge model into its hurricane models to represent the complex interactions of wind and water throughout a hurricane’s life-cycle, and we continue to implement lessons learned from new storms.