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DISASTER 101 – FLOODING

When most people think of natural disasters, the first thing to come to mind is not likely flooding. However, flooding is the most common natural disaster. Flooding occurs in all 50 states, accounts for 40% of natural disasters, averages 5 billion dollars in damage each year, and claimed an average of 75 lives per year over the last 30 years.

Worldwide, statistics are similar.

According to the 30 year average, flooding is responsible for the most weather-related fatalities.

TYPES OF FLOODING

River and Lake Flooding

River and lake flooding is probably what most people envision when they think of floods. Heavy rainfall or snowmelt can cause water levels to rise overflowing banks and levees. River flooding is common in the Midwest as rain and snowmelt swells the tributaries that feed into larger rivers downstream. Once the water level crests the river banks, the area that is inundated can be widespread. Low lying areas, saturated soils, and urban areas can further exacerbate the effects of the overflow and take days to dissipate.

California experienced flooding in February when a weekend of storms increased runoff from Anderson Lake and flooded low-lying areas of San Jose. The early year storms also prompted the evacuation of over 88,000 people near the weakened Oroville Dam.

In May of 2017, heavy rainfall over the Midwest caused widespread flooding. Nearly 15 inches of rain fell over multiple states, saturating soils, and swelling multiple rivers above historic levels. Numerous levees were breached which flooded towns causing an estimated $1.7 billion dollars in damages to homes, businesses, and infrastructure.

40% of all natural disasters are flooding.

Coast Guard overflight of South Carolina flooding

Coastal Flooding and Storm Surge

A form of flooding happens regularly along coastlines due to the cycle of rising and lowering tides. Tides are a regular rise and fall of sea level caused by the gravitational interplay of the sun, moon, and earth. Occasionally, these tides can be exceptionally high. An increase of just a few feet is more than enough for tides to breech natural and man-made barriers. Many coastal cities are very near, or in some cases lower than, sea level making them especially prone to any change in sea level.

Storm surge can also cause extreme coastal flooding. The surge develops during severe weather, hurricanes, and tropical storms raising sea level as much as 25 feet. Sea level rise is the result of the low atmospheric pressure found in these storms which has a similar effect on sea level rise as the gravitational pull on tides. High winds common with these storms also cause large waves to batter the coast and push water farther inland. In worst case scenarios, the storm surge strikes the coast during a high tide cycle, increasing the flooding exponentially. Storm surge flooding is responsible for 90% of hurricane related deaths and the majority of the damage to structures.

In August of 2017, Hurricane Harvey alone caused over $125 billion dollars in damage and killed 89 people. The majority of the devastation caused by Harvey was a direct result of the widespread flooding of the Houston area.

Flash Floods

Flash floods can result from a variety of causes, but the common denominator is that they develop quickly and are normally caused by heavy rainfall. These floods can also be the result of snow melt, dislodged ice, inadequate urban drainage, or dam breaks. The actual volume of water carried in a flash flood is usually less than other flood types but the water is channeled down confined spaces which causes it to move with devastating force and speed. Because of this speed, flash floods are very dangerous, easily carrying mud, rocks, and trees in its flow. A Weather Channel article stated that, “water flowing at 7 mph has the equivalent force per unit area as air blowing at EF-5 tornado wind speed.” Whereas, “water moving at 25 mph has the pressure equivalent of wind blowing at 790 mph, faster than the speed of sound.”

 

BASIC SAFETY AND PREPAREDNESS

Ready.gov provides many helpful tips.

Be Mindful

  • Stay tuned to phone alerts, TV, or radio for weather updates, emergency instructions, or evacuation orders.
  • Avoid walking or driving through flood waters. 6 inches of moving water can knock a person over, and one foot of moving water can sweep a vehicle away. According to the Environmental Protection Agency, the leading cause of flood-related injury and death is individuals attempting to drive through flood waters.
  • Do not drive over bridges that are over fast-moving floodwaters. Floodwaters can scour foundation material from around the footings and make the bridge unstable.
  • If there is a chance of flash flooding, move immediately to higher ground.

Protect your home

  • Know your flood risk, avoid building in flood plains, and consider buying flood insurance.
  • If you have to evacuate due to flooding, and if safety permits, turn off all the utilities to your home and attempt to move valuables to the highest possible level.

SOURCES

https://www.ready.gov/floods
https://www.nssl.noaa.gov/education/svrwx101/floods/types/
https://www.ncdc.noaa.gov/billions/events/US/2017-2018
http://www.floodsite.net/juniorfloodsite/html/en/student/thingstoknow/hydrology/floodtypes.html
https://weatherology.com/articles/106/The+Dangers+of+Flash+Floods.html
https://weather.com/storms/severe/news/power-flood-water-20130704
https://www.livescience.com/23913-flood-facts.html

san jose flood

Areas of San Jose Flood After Nearby Dam Overflows

Low-lying areas of San Jose have flooded due to increased runoff from Anderson Lake after a weekend of multiple rain storms.  The lake, which is 15 miles southeast of San Jose, had been slowly filling to capacity.  Water levels eventually spilled over Anderson Dam into Coyote Creek which flows northwest, directly into the heart of San Jose.  The influx of extra runoff caused the creek to crest at a height of over 13 feet, causing widespread flooding to many areas between Gilroy and San Jose.

The worst of the Flooding appears to be centered on the Nordale neighborhood of San Jose. Many homes and streets in that area are under water. As of Tuesday afternoon (2/21), 186 residents had been evacuated via boat and helicopter. San Jose mayor Sam Liccardo announced that up to 500 more residents were under voluntary evacuation as the creek continues to receive the dam’s excess water. The city itself saw over two more inches of rain on top of a very wet winter. The area has had measurable precipitation on all but five of the days so far this month.

San Jose Flood at a Glance

• Recent storms have caused Coyote Creek to become a spillway for Anderson Reservoir, which is overflowing beyond its capacity.
• Rescue efforts have been focused on the Nordale Neighborhood in the heart of San Jose where dozens of people were aided to safety by boat and helicopter.
• Officials are concerned with potential contamination of the water due to potentially overflowing sewage lines, oil and gas from vehicles trapped in the water, or household chemicals that may have leaked into the flood waters.
• San Jose’s mayor announced a voluntary evacuation for low-lying areas along Coyote Creek between the I-880 and the Capitol Expressway due to the risk of continued flooding.
• So far, 16 of February’s 21 days have involved measurable precipitation in San Jose.
• Multiple zoo animals had to be relocated in the nearby Happy Hollow Park.

San Jose Flood Outlook

The persistent rains are forecasted to cease overnight, and dryer weather is expected at least through Saturday evening.  Despite the release of countless gallons of water, Anderson Dam will still sit well above its recommended level of 68% for the foreseeable future.  Evacuated residents will have to wait until the floodwaters subside to return to their homes, and may be delayed by the potential of polluted water.

In the past few weeks, most of the rainfall worries have been centered on Butte County and the Oroville Dam Spillway situation.  If the relentless rain trend continues, reservoirs across the state of California could see further rising levels, which in turn could increase the risk of more flooding events.

Sources: Weather Channel, ABC7 News: Bay Area, Wunderground

Landslide Concerns in Fire-Ravaged Gatlinburg TN

While the community of Gatlinburg, Tennessee grieves their losses and starts planning how to recover and rebuild, a new threat confronts the area. An inch and a half of winter rain helped put out the horrific fire over the weekend, but as we’ve covered in the past, wildfire-scorched areas often have an increased risk of landslides and mudslides. Local fire crews have thus far reported several small landslides that are slowing their ability to access damaged areas.

wildfire landslides

Before and After Image of a Burn Scar from the Chimney Tops 2 Fire

How a Fire Can Increase Landslide Risk?

Depending on soil type and topography, vegetation and land cover have a significant impact on the stability of the soil. Under normal conditions, leaf litter and other surface vegetation slow the rainfall water moving down a given slope.  This allows much of that moisture to permeate through the soil and drain into the water table or aquifer below, leaving the surface soil relatively stable.  Even during heavy rainfall when surface soil becomes saturated, root systems from brush and trees help to keep the soil from moving downhill.

However, when vegetation is lost due to wildfire (or other reasons such as construction), the factors that keep soil in place are minimized, and there is greater risk that the soil’s surface tension in a given area is overcome by gravity and washes down the slope.

Gatlinburg is in the Great Smoky Mountains, a very old mountain range within the Appalachian Mountain region. Due to the age of the mountains and the region’s climate, the mountains themselves are very weathered, and have much deeper soils than the mountains in the Western United States. The region is also heavily wooded, so the roots of the dense vegetation help to stabilize the nearby soil. When the wildfires recently moved through the area, the vegetation and leaf litter was burned out, and the stabilizing root systems were compromised. Storms then came and assisted in firefighting efforts, but the lack of vegetation due to the recent burns caused a few small slides. Before winter brings snow to the normally wet area, an increased likelihood for flooding, mudslides, and landslides remains a worrying possibility. As of December 7th, there is little rain forecasted in the region for several days.

Visit ready.gov to learn about how to better protect yourself, your family, and your property from landslides and other hazards.

Hurricane Matthew Brings Dangerous Conditions to the Coast

Hurricane Matthew Update:  October 7, 2016.

Hurricane Matthew continues to push north paralleling the east coast of Florida.  The storm has yet to make landfall, but powerful wind gusts of over 100 mph have downed trees and caused power outages for nearly 1,000,000 people.  Bands of heavy rain currently stretch as far north as South Carolina with flash flooding likely across the lowland areas.  The hurricane has been reduced to a category 2 storm, staying just offshore, but the threat of storm surge flooding remains a critical concern.

Hurricane Matthew

Hurricane Matthew predicted path, as shown in the RZAlert Dashboard.

Unease Growing Over Storm Surge

The storm is currently off the northeast coast of Florida, dousing the coastal town of Jacksonville.  Storm surge flooding combined with heavy rain has already effected much of northeast Florida and Georgia, with–maybe–the worst conditions  yet to come.

In St. Augustine (FL), flooding preceded the storm inundating much of the city’s historic downtown with knee-deep water.  The storm surge in St. Augustine is projected to top 8 feet in some places as the water continues to rise. Nearby, the city of Jacksonville fears for an unprecedented event, warning residents of the potential for catastrophic damage. Officials are expecting storm surges as high as 9 feet and residents began evacuating days ago. According to the city’s mayor, anything over 3 feet is life-threatening.  A major tropical storm has not impacted the city of Jacksonville in over a century.

Charleston Susceptible

Fears are also mounting for the Charleston area where the eye-wall is projected to potentially center itself over the Coastal Carolina city come Saturday morning. Officials are worried the flood levels could be near or even surpass those experienced in the October 2015 flood event which set historic records in terms of damage and lives lost.  The nowCoast™ modeling of the potential storm surge by the National Oceanic and Atmospheric Administration shows dangerous conditions far inland due to the numerous inlets and intercostal waterways. Interestingly, the storm is projected to move clockwise away from the Southeastern US coast and curl back around toward Cuba. Time will tell what else this storm has in store for an area that has historically escaped major hurricane impact.

Hurricane

Storm surge forecast for Charleston, NC.

Sources:

http://www.pbs.org/newshour/rundown/heres-hurricane-matthew-might-cause-worst-flooding/?utm_content=buffer1cf9c&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer

https://weather.com/storms/hurricane/news/hurricane-matthew-bahamas-florida-georgia-carolinas-forecast

https://weather.com/news/news/south-carolina-historic-flood-rainfall-record-extreme

http://www.cnn.com/2016/10/07/us/hurricane-matthew-florida/

 

Another Epic Flood Hits Texas in May

A year to the day since the devastating flood of 2015 pummeled the Austin & San Antonio areas of Texas, other parts of Texas received in excess of 9″ of rain in 3 hours around Houston, accumulating over 12″ over 12 hours as the storm passed through the region. As of Friday, May 27th, 2016, two fatalities have been reported – 1 from drowning, 1 from heart attack after driving through high water. Local highways remain closed as riverbanks are not high enough to contain the resulting floodwaters. These images were taken just 24 hours apart in Brazos County.

During & After of neighborhood drainage flooding

 

More flooding anticipated

While many streams and rivers are seeing floodwater levels diminish, those areas downstream of the harder hit areas are now dealing with the added water flowing throughout the watersheds racing toward the Gulf of Mexico. The Advanced Hydrologic Prediction Service (AHPS) from NOAA observes and predicts river stage levels across the country. Each gauge has a chart that indicates what water levels would cause the river to overrun its banks and flood the surrounding areas. As shown below, the Brazos River is forecast to continue dropping while the West fork of the San Jacinto River (farther east, toward the Gulf of Mexico) is expecting to reach ‘major flood stage’ levels prior to returning to normal. The recently observed readings are shown on the left, leading up to the forecasted levels along the right side of each chart.

AHPS of a couple Texas river levels

Observed and forecasted river level charts from AHPS-NOAA.

 

Additional damage and dangers

In addition to the widespread flooding, many reports of hail and tornado touchdowns spanned the area. Some local residents in Bryan, TX took cell phone videos as the tornado passed through the Wheeler Ridge neighborhood. As the weather clears and river levels return to normal, the damage and impact can begin to be addressed. More flooding and tornadoes are forecast across the Southeastern US in the next few days as the storm system continues moving east.