Flood Barrier USA

The Rapid City Flash Flood

Author: Flood Expert Donna Conneely

Rapid City, South Dakota was a medium-sized city of about 40,000 residents in 1972. The city had been founded a century earlier, after an expedition led by General George Custer had discovered gold in the surrounding Black Hills. A town sprang up in service to the hordes of fortune seekers who toiled for gold in the inhospitable terrain, and it was named after the Rapid Creek, which ran straight through the new town. Rapid City was touted as the ‘Gateway to the Black Hills’ by a group of promoters who were seeking opportunities other than digging for gold themselves. Almost exactly 100 years after the town was founded, the normally inconsequential creek that ran through Rapid City would all but destroy its namesake.

By early June of 1972, the ground in and around Rapid City had become fully saturated by heavy rainfall from the previous few days. Then, a series of conditions — each of which would have amounted to nothing on its own — began to collide. First, a low-altitude flow of moist air was forced up the steep, rocky slopes of the Black Hills, producing a condition called an ‘orographic lift’. An orographic lift happens when air is pushed from a low elevation to a high elevation, rapidly cooling the air as it gains altitude. This sudden cooling of the air creates clouds, quickly raises humidity to 100%, and produces rain. On this day of June 9, 1972, the entire set of variables was complicated by the lack of winds that are normally present at high altitudes over the Black Hills. So now, as this fateful day drew to a close, an orographic lift was setting up thunderstorms that would hover immobile over the area- and all that precipitation was about to fall on soggy, super-saturated soil.

Aftermath of the flash flood of June 9-10, 1972, Rapid City, South Dakota. Photo: NOAA

When rain began to fall on the afternoon of June 9, it received little notice. The past few days had been rainy, so this was just a little more rain during an already rainy week. The fact that the rain began during the late afternoon was one more unlucky detail in a very catastrophic series of events. The downpour continued through the afternoon and evening, and into the night. But because night had fallen, people were unaware of just how much water was falling from the sky, and could not see how much runoff had been produced, and how quickly it was filling creeks, reservoirs, and any low-lying gulches. Most of the residents of Rapid City were sound asleep in their beds when disaster struck around midnight.

In a few short hours between the start of the storms at 3:30 and the explosive arrival of the flash flood at midnight, a lot had been happening. The main problem was the sheer volume of the precipitation: up to an incredible 12” of rain had been dumped on the sodden soil in and around Rapid City, an amount that even dry, absorbent soil would not be able to soak up. All that water immediately filled every creek and gully in the area, and began pulsing downstream, pushing before it a massive amount of debris. This debris piled up against any dam, bridge, or embankment that stood in its way, and in the case of the Canyon Lake Dam, created an unstable blockage that caused the lake to rise 11 feet in a few hours. Then it broke.

The massive wall of water that struck the sleeping town caused an unimaginable amount of damage. Trees, vehicles, boulders, bridges, and even sturdy buildings were swept away in minutes. There was no warning, and 238 people lost their lives as their homes were washed away like sand castles on the beach. Over 3000 others were injured, many grievously. The town of Rapid City was scraped clean, and it all happened in minutes. Five hours later, when the dawn revealed the horrors of the night, the creek had already returned to its normal level — this one of the incredible hallmarks of a flash flood. On June 10th, witnesses described scenes of incomprehensible damage. Over 1000 homes had been swept away, and almost 3000 others were damaged. 5000 cars had also been carried off, as well as all but a couple of the many bridges that served the area. The damage in terms of monetary loss was dire — estimated at around $165 million. This, combined with the considerable loss of human lives and livelihoods, means that the Rapid City Flash Flood still stands to this day as one of the worst flash floods in US history.

October 7th, 2021  |  Posted in Flood Protection  |  No Comments »

Catastrophic Floods Strike New York and New Jersey

Author: Flood Expert Donna Conneely

August 19, 2021 was the 16th anniversary of the most destructive and deadly hurricane on record for the USA: Hurricane Katrina. On this day, as if to acknowledge the anniversary, Hurricane Ida made landfall in Louisiana, coming in as a dangerous CAT-4 storm. In a few days, Ida would go on the record books as the 2nd most destructive storm in US history, after Katrina. The passage into the record books was a strange and circuitous journey.

With massive preparation efforts and evacuation orders arranged before the landfall, officials were hoping against hope that there would not be another disaster like Katrina. But the signs were not looking good, and with wind gusts of up to 172mph, this storm would be a monster in its own right. After striking shore in Louisiana, Ida maintained its CAT-4 status over land for about four hours, and then it weakened to a still-dangerous CAT-3. As the storm unleashed an almost incomprehensible amount of precipitation on the area, the hurricane-force winds tore homes and businesses to splinters. Power lines went down like dominoes, and those who had defied the mandatory evacuation order- as well as all those who had been unable to leave- began to suffer through the sweltering heat without AC, refrigeration, gasoline, medications, and perishable food staples like milk and meat. But still, Ida was not done- not by a long shot!

Long Island Expressway in New York City shut down due to flash flooding from Post-Tropical Storm Ida’s landfall. Photo: Tommy Gao via Wikimedia Commons

As FEMA officials in the Deep South began to release a sigh of relief that the initial death toll appeared to be very low, Hurricane Ida veered to the northeast- a heavily populated region for which large hurricanes are a relatively rare event. As the storm moved on, through Mississippi, Alabama, the mid-Atlantic states into Virginia and Maryland, the main concern was tornado activity. This fear was not unfounded, and at least seven tornadoes touched down in Alabama alone. Another tornado in Maryland caused significant damage to the Capitol city of Annapolis. But even now, Ida had not wrought even half of the damage and loss of life that would ultimately be tallied to this storm. It was in the states of New York and New Jersey where the storm would make its deadly mark.

Almost as soon as the storm arrived in New York, Mayor Bill DeBlasio pronounced it, “…an historic weather event”, and for good reason. In just a couple of hours, 6-10” of rain fell in the NYC area, and the consequences were dire. Brooklyn, Manhattan, and Queens were almost immediately flooded. Rainwater gushed into the subway system and produced nightmarish scenes of underground stations being quickly filled with water. The trains attempted to push through the water to get out of the subterranean death traps that had once been safe, dry tunnels. City buses were seen plowing through water so deep that passengers had to stand on the seats to stay above the water. Central Park was pounded with over 3” of rain in just one hour, breaking a record that had stood since 1913. That same historical record had also been broken two weeks earlier by Tropical Storm Henri, and now that brand-new record from Henri was already obliterated. It was truly a 100-year flood event in New York City!

Both New York and New Jersey declared a state of emergency before the day was out. But why was Hurricane Ida causing even more destruction in this northern area, more than a thousand miles from where the storm made landfall? The answer is this: bad luck. By the time Ida reached the New York/New Jersey region, the hurricane-force winds had long since lost strength. But the core of the storm was still packed with rain, and then it unfortunately merged with another localized storm front. When this happens, massive amounts of precipitation is often the result, and we saw this in New York and New Jersey. The damage in the New York City alone will top $50 million. But more importantly, 18 people lost their lives, and many of those unfortunates lived in basement apartments that lacked any type of flood barriers.

Ten years ago, Hurricane Sandy laid waste to the New York area, and the aftermath of that storm led to calls for better planning and improvements in flood defenses. Many new mitigation efforts have since been put into place, but Hurricane Ida has exposed the wide gap between what has been done and what still needs to be done. This month, during Hurricane Ida, New York City faced its first-ever Flash Flood Emergency. It will surely not be the last.

September 28th, 2021  |  Posted in Flood Protection  |  No Comments »