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Record rainfall in Texas prompts extensive flash flooding


April 19, 2016   by Canadian Underwriter


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Record rainfall prompted extensive flash flooding across parts of Texas on Monday, closing schools and city offices and overtopping levees, Impact Forecasting said in an Aon Cat Alert.

Impact Forecasting, Aon Benfield’s catastrophe model development team, said in the alert issued late Monday that rainfall rates fell at up to 4 inches (101.6 millimeters) per hour at times as rivers, creeks and bayous rapidly burst their banks and overtopped levees. Significant flood damage was noted in Harris County, and the county and city of Houston declared a disaster as local officials described the event as a “life-threatening emergency,” Impact forecasting said in the alert.

Lucky's Pub is not so lucky after flood water engulfed it Monday, April 18, 2016, in Houston. More than a foot of rain fell Monday in parts of Houston, submerging scores of subdivisions and several major interstate highways, forcing the closure of schools and knocking out power to thousands of residents who were urged to shelter in place. (AP Photo/Pat Sullivan)

Lucky’s Pub is not so lucky after flood water engulfed it Monday, April 18, 2016, in Houston. More than a foot of rain fell Monday in parts of Houston, submerging scores of subdivisions and several major interstate highways, forcing the closure of schools and knocking out power to thousands of residents who were urged to shelter in place. (AP Photo/Pat Sullivan)

The governor later declared a disaster for nine affected counties in southeast Texas. The flooding lead to more than 1,220 water rescues in the greater Houston metropolitan area alone and at least five fatalities were blamed on the floods.

As the day unfolded, the local Houston/Galveston National Weather Service offices declared a “rare Particularly Dangerous Situation Flash Flood Emergency” for parts of Harris, Colorado, Waller, Austin, and Fort Bend counties as rainfall rates of 2 to 4 inches (51 to 102 millimetres) per hour fell. This prompted rivers to exponentially increase in a matter of hours. The White Oak Bayou station at Houston rose from 8.3 feet (2.53 metres) at 2:45 UTC to 36.95 feet (11.26 metres) at 7:30 UTC. “Given the gauge stopped recording heights for a seven-hour period from 11:45 to 18:45 UTC, it is possible that the water height was even higher,” the alert said.

Given the extreme rainfall rates, it was determined by the National Oceanic and Atmospheric Administration and the University of Oklahoma’s FLASH (Flooded Locations and Simulated Hydrograph) program that some areas recorded a greater than 1-in-200 year rainfall return period event. A 1-in-200 year rainfall return period means that there is a 0.5% chance of such an event occurring in any particular year.

The rainfall at Houston Intercontinental Airport – 11.75 inches (298.5 millimetres) – on April 18 set the all-time daily record for the city of Houston. In fact, this record was broken before 10 a.m. local time, Impact Forecasting reported.

From an economic standpoint, total economic losses are expected to be significant, but it remains too early to provide any specific loss value, the alert said. For context, a small portion of the Houston metro region recorded water heights equal or slightly greater than what was seen during the floods resulting from 2001’s tropical storm Allison. That event caused roughly US$7 billion (2016) in economic damage in Harris County alone.

From an insurance perspective, Texas is home to the second-most number of active National Flood Insurance Program (NFIP) policies. Of the nearly 591,000 Texas policies in force, the city of Houston accounts for almost 121,000. “Despite the large number of policies, there are a substantial number of homes and businesses that either do not have NFIP coverage or are underinsured,” the alert said, noting that this will “lead to a large discrepancy between the overall economic cost and the total insured portion.”

During the flood, the most considerable damage was recorded in Harris County, Texas, where local officials reported that more than 70 neighbourhoods and subdivisions in the greater Houston metro region were inundated by multiple feet of water. “Multiple levees were overtopped that turned streets into rivers,” the alert said. “A preliminary tally suggested that a minimum of 1,000 homes were damaged, though it is expected that the final total will be far higher. Even more businesses and other public facilities were damaged.” Impacts were particularly significant in the Greenspoint area, where people at more than 1,500 residential units were stranded in multi-level apartment complexes. The Meyerland region was also heavily affected.

At its peak, more than half of the watersheds in Harris County – 13 of 22 creeks and waterways – experienced significant flooding as typically small flowing creeks and streams turned into raging torrents. More than 1,220 water rescues (897 in Houston city limits and 325 elsewhere) were conducted by emergency officials, many of which were via boat since water levels were too high or fast-flowing for vehicles to navigate. At least five people were killed, with nearly all of the fatalities resulting from drowning or vehicles being swept away. At the peak of the event, at least 121,000 customers were without electricity though a large portion had their power restored later in the day.

This is the fourth major flood event in the greater Houston metro region in the last 12 months, Impact Forecasting said, with previous events occurring in May, June, and October 2015. The May floods led to more than 6,660 claims filed to the NFIP, with a payout cost of US$442 million.

Elsewhere, heavy rains from the event also led to isolated flooding across parts of Oklahoma, Kansas, and Nebraska. Flash flooding was also possible in eastern Oklahoma, northern Texas, and the Lower and Middle Mississippi Valley.