GIS at its Finest: Storm Surge Analysis

Earlier this week, First American Spatial Solutions (FASS) released a report detailing the risk of 13 coastal cities in the United States to storm surge. It’s a fantastic example of how GIS analysis can be used in a very practical way and on a very broad scale. Did you know that there is a difference (in the insurance company’s eyes) whether an insured residential structure is destroyed by hurricane-force winds or by a storm surge (flooding) caused by the hurricane? The first is covered, while the second might not be.

I had the opportunity to speak with author Dr. Howard Botts about his report this morning; a link to the 10-minute conversation comes at the end of this article. Dr. Botts is executive vice president and director of database development at First American.

First of all, from a GIS perspective, FASS has a database of approximately 124 million U.S. parcels. That equates to about 87 percent of the parcels in the United States. FASS estimates there are somewhere around 144.3 million total parcels. Dr. Botts said the biggest challenge for FASS, from a GIS perspective, is automating the management of this massive dataset.

“There is no national mandate for cadastral or parcel data. So you have about 4,000 different collection units, primarily cities and counties, each of which has their own projection, data standards, different attributes,” said Dr. Botts. “So the first couple of challenges are normalizing the data in terms of projection and coverage and then georectifying it. Some stuff is survey-quality, some stuff is so bad we just put it on the shelf because we can’t use it. Then we also normalize the attributes.”

“We are using ArcInfo to do the processing and rectification along with some various air photos and other kinds of information and then we manage the entire dataset in Oracle Spatial,” he said. “Literally on a daily basis, we are getting cities and counties feeding us new parcels that have been created. We are constantly refreshing that data and so we do all of that data management within Oracle.”

The source of the land-elevation and bathymetric data, the heart of the data needed to run analyses such as this, was developed using techniques proprietary to FASS. Dr. Botts wouldn’t go into further detail about the source of the data, but he did say that developing and maintaining that data is more complex than people would think. For example, the bathymetric properties of the sea floor and coast line can change substantially after a storm event, to the point that FASS’s models need to be updated, which they do on an annual basis at the end of April.

Storm Surge Graphic Courtesy of First American Spatial Solutions

On to the FASS report

“Retired insurance agent Norman Broussard and his wife Genevieve lived at 154 Brady Drive in Biloxi, Mississippi. Their home, which they built in the 1960s, was just a short stroll from the picturesque Mississippi Sound. When Hurricane Katrina hit, they sought refuge with their daughter. But when they returned, there was nothing left but the concrete slab — the slab the home used to sit upon. The Broussards filed a claim with State Farm. They argued that their home and its contents were destroyed by the hurricane-force winds. State Farm, Mississippi’s largest insurer, denied the claim. The company asserted that the house had been destroyed by the storm surge, or flood, for which coverage was excluded. The Broussards sued. Theirs was the first of the so-called “slab suits” to go to trial. The court’s decision, rendered on January 17, 2007, was a warning to all carriers handling coastal property. In a directed verdict, Judge L.T. Senter found in favor of the Broussards, reasoning that the insurer could not prove what portion of the loss was due to flooding and what portion to wind. The judge then sent the question of punitive damages to the jury, which promptly awarded the plaintiffs $2.5M (later reduced to $1M).”

After reading the above, you can understand that this is a serious issue involving billions of dollars in residential real estate value — more precisely $234 billion in 13 high-risk U.S. cities, according to FASS. Furthermore, the $234 billion value only includes “current structure value” and does not consider replacement costs, contents, vehicle loss, loss of life or business interruption.

The 13 cities were determined, using GIS, to be the highest-risk cities to storm surge. The only properties included in the report were those that would be “wet” in a storm-surge event. FASS storm-surge polygons assume the worst-case scenario for each category of hurricane based on the following factors, according to FASS:

1. Maximum wind speed for each category at time of landfall.

2. Right-front quadrant of the storm at landfall.

3. Peak high tide at time of landfall.

As mentioned previously, FASS has a database of ~124 millions parcels. FASS then identified every property contained within each category of the storm-surge polygon and matched the structure valuation for each residential structure identified.

The FASS report states that storm surge can damage in several ways:

1. Water depth. The higher the water level, the greater the damage.

2. Storm surge velocity. A cubic yard of sea water weighs nearly a ton (1,728 lbs)! Wind-driven water moving at 10-15 mph can cause a tremendous amount of damage.

3. Transportation of debris. Storm surge can move large objects such as trees, vehicles, boats, and other large objects, and sweep them up against other structures.

4. Standing period. After the storm surge event, standing water can turn into a toxic lake of debris. Generally, the longer it stands, the more damage is caused.

Several factors were considered when determining which geographic areas to study:

1. Hurricane probability.

2. Vulnerability. Storm surge is most pervasive in coastal areas where there is shallow offshore bathymetry, low-lying coastlines or river estuaries.

3. Residential density. Most of the nation’s population density is located along the coast.

Based on the above criteria, FASS selected the following 13 geographic areas:

*Brownsville, TX
*Corpus Christi, TX
*Myrtle Beach, SC
*Wilmington, NC
*Long Island, NY
*Jacksonville, FL
*New Orleans, LA
*Charleston, SC
*Houston-Galveston, TX
*Tampa, FL
*Virginia Beach, VA
*Miami-Dade, FL

The report provides substantial detail on each geographic area such as storm history, number of properties affected, and value of properties affected. For each geographic area, three different maps    illustrate the affected area. Following is an example that shows some of the summary data. Further details of the Gulf-Shores area and the rest of the geographic areas are provided in the full report.

Gulf-Shores, AL

Category 5 Exposure: $1,154,467,296.00
Hurricane Probability: High
Storm-surge Vulnerability: High
Residential Density: Medium

Overhead View of Categories 1-5 Storm Surge Impacting Gulf-Shores and Mobile, AL

The full 18-page FASS report can be downloaded from the FASS website by clicking here. Note that the report is free of charge, but you’ll have to re
gister before downloading.

Listen to my 10-minute conversation with Dr. Botts by clicking here. It’s about a 2.5mb audio file.

See you next week.

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