What is a flash flood? A civil engineer explains

A bridge and road submerged by floodwaters from the North Fork of the Kentucky River in Jackson, Kentucky, July 28, 2022. Leandro Lozada/AFP via Getty Images

by Janey Camp, Vanderbilt University

Flash flooding is a specific type of flooding that occurs in a short time frame after a precipitation event – generally less than six hours. It often is caused by heavy or excessive rainfall and happens in areas near rivers or lakes, but it also can happen in places with no water bodies nearby.

Flash floods happen in rural and urban areas, as in late July 2022 in St. Louis and eastern Kentucky. When more rainfall lands in an area than the ground can absorb, or it falls in areas with a lot of impervious surfaces like concrete and asphalt that prevent the ground from absorbing the precipitation, the water has few places to go and can rise very quickly.

If an area has had recent rainfall, the soil may be saturated to capacity and unable to absorb any more water. Flooding can also occur after a drought, when soil is too dry and hardened to absorb the precipitation. Flash floods are common in desert landscapes after heavy rainfalls and in areas with shallow soil depths above solid bedrock that limits the soil’s ability to absorb rain.

Since water runs downhill, rainfall will seek the lowest point in a potential pathway. In urban areas, that’s often streets, parking lots and basements in low-lying zones. In rural areas with steep terrain, such as Appalachia, flash flooding can turn creeks and rivers into raging torrents.

A home security video shows floodwaters rising rapidly in Waverly, Tennessee, in August 2021.

Flash floods often catch people by surprise, even though weather forecasters and emergency personnel try to warn and prepare communities. These events can wash away cars and even move buildings off their foundations.

The best way to stay safe in a flash flood is to be aware of the danger and be ready to respond. Low-lying areas are at risk of flooding, whether it happens slowly or quickly and whether it’s an urban or rural setting.

It’s critical to know where to get up-to-date weather information for your area. And if you’re outdoors and encounter flooded spots, such as water-covered roadways, it is always safer to wait for the water to recede or turn back and find a safer route. Don’t attempt to cross it. Flood waters can be much faster and stronger than they appear – and therefore more dangerous.

Building for a wetter future

Engineers design stormwater control systems to limit the damage that rainfall can do. Culverts transfer water and help control where it flows, often directing it underneath roads and railways so that people and goods can continue to move safely. Stormwater containment ponds and detention basins hold water for release at a later time after flooding has ceased.

Many cities also are using green infrastructure systems, such as rain gardens, green roofs and permeable pavement, to reduce flash flooding. Restoring wetlands along rivers and streams helps mitigate flooding as well.

Often the design standards and rules that we use to engineer these features are based on historic rainfall data for the location where we’re working. Engineers use that information to calculate how large a culvert, pond or other structure might need to be. We always build in some excess capacity to handle unusually large floods.

Now, however, many parts of the U.S. are experiencing more intense storm events that drop significant amounts of rainfall on an area in a very short time period. The recent St. Louis and Kentucky floods were both on a scale that statistically would be expected to occur in those areas once in 1,000 years.

With climate change, we expect this trend to continue, which means that planners and engineers will need to reconsider how to design and manage infrastructure in the future. But it’s hard to predict how frequent or intense future storm events will be at a given location. And while it’s extremely likely that there will be more intense storm events based upon climate projections, designing and building for the worst-case situation is not cost effective when there are other competing demands for funding.

Right now, engineers, hydrologists and others are working to understand how best to plan for the future, including modeling flood events and development trends, so that we can help communities make themselves more resilient. That will require more, updated data and design standards that better adapt to anticipated future conditions.

Janey Camp, Research Professor of Civil and Environmental Engineering, Vanderbilt University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Flood maps show US vastly underestimates contamination risk at old industrial sites

Maywood Riverfront Park was built on the site of eight former industrial properties in Los Angeles County. Luis Sinco/Los Angeles Times via Getty Images

by Thomas Marlow, New York University; James R. Elliott, Rice University, and Scott Frickel, Brown University

Climate science is clear: Floodwaters are a growing risk for many American cities, threatening to displace not only people and housing but also the land-based pollution left behind by earlier industrial activities.

In 2019, researchers at the U.S. Government Accountability Office investigated climate-related risks at the 1,571 most polluted properties in the country, also known as Superfund sites on the federal National Priorities List. They found an alarming 60% were in locations at risk of climate-related events, including wildfires and flooding.

As troubling as those numbers sound, our research shows that that’s just the proverbial tip of the iceberg.

Many times that number of potentially contaminated former industrial sites exist. Most were never documented by government agencies, which began collecting data on industrially contaminated lands only in the 1980s. Today, many of these sites have been redeveloped for other uses such as homes, buildings or parks.

For communities near these sites, the flooding of contaminated land is worrisome because it threatens to compromise common pollution containment methods, such as capping contaminated land with clean soil. It can also transport legacy contaminants into surrounding soils and waterways, putting the health and safety of urban ecosystems and residents at risk.

A boat sits by a dock outside a new building along the waterway.
New York developers are planning thousands of housing units along the Gowanus Canal, a notoriously contaminated industrial area and waterway. Epics/Getty Images

We study urban pollution and environmental change. In a recent study, we conducted a comprehensive assessment by combining historical manufacturing directories, which locate the majority of former industrial facilities, with flood risk projections from the First Street Foundation. The projections use climate models and historic data to assess future risk for each property.

The results show that the GAO’s 2019 report vastly underestimated the scale and scope of the risks many communities will face in the decades ahead.

Pollution risks in 6 cities

We started our study by collecting the location and flood risk for former industrial sites in six very different cities facing varying types of flood risk over the coming years: Houston; Minneapolis; New Orleans; Philadelphia; Portland, Oregon; and Providence, Rhode Island.

These former industrial sites have been called ghosts of polluters past. While the smokestacks and factories of these relics may no longer be visible, much of their legacy pollution likely remains.

In just these six cities, we found over 6,000 sites at risk of flooding in the next 30 years – far more than recognized by the EPA. Using census data, we estimate that nearly 200,000 residents live on blocks with at least one flood-prone relic industrial site and its legacy contaminants.

Without detailed records, we can’t assess the extent of contamination at each relic site or how that contamination might spread during flooding. But the sheer number of flood-prone sites suggests the U.S. has a widespread problem it will need to solve.

The highest-risk areas tended to be clustered along waterways where industry and worker housing once thrived, areas that often became home to low-income communities.

Legacy of the industrial Northeast

In Providence, an example of an older industrial city, we found thousands of at-risk relic sites scattered along Narragansett Bay and the floodplains of the Providence and Woonasquatucket Rivers.

Over the decades, as these factories manufactured textiles, machine tools, jewelry and other products, they released untold quantities of environmentally persistent contaminants, including heavy metals like lead and cadmium and volatile organic chemicals, into the surrounding soils and water.

Map with dots, primarily along waterways.
Flood-prone relic industrial sites in Providence, R.I. Marlow, et al. 2022, CC BY-ND

For example, the Rhode Island Department of Health recently reported widespread drinking water contamination from PFAS, often referred to as “forever chemicals,” which are used to create stain- and water-resistant products and can be toxic.

The tendency for older factories to locate close to the water, where they would have easy access to power and transportation, puts these sites at risk today from extreme storms and sea-level rise. Many of these were small factories easily overlooked by regulators.

Chemicals, oil and gas

Newer cities, like Houston, are also vulnerable. Houston faces especially high risks given the scale of nearby oil, gas and chemical manufacturing infrastructure and its lack of formal zoning regulations.

In August 2017, historic rains from Hurricane Harvey triggered more than 100 industrial spills in the greater Houston area, releasing more than a half-billion gallons of hazardous chemicals and wastewater into the local environment, including well-known carcinogens such as dioxin, ethylene and PCBs.

Maps with dots widespread in the city.
Flood-prone relic industrial sites in Houston. Marlow, et al. 2022, CC BY-ND

Even that event doesn’t reflect the full extent of the industrially polluted lands at growing risk of flooding throughout the city. We found nearly 2,000 relic industrial sites at an elevated risk of flooding in the Houston area; the GAO report raised concerns about only 15.

Many of these properties are concentrated in or near communities of color. In all six cities in our study, we found that the strongest predictor of a neighborhood’s containing a flood-prone site of former hazardous industry is the proportion of nonwhite and non-English-speaking residents.

Keeping communities safe

As temperatures rise, air can hold more moisture, leading to strong downpours. Those downpours can trigger flooding, particularly in paved urban areas with less open ground for the water to sink in. Climate change also contributes to sea-level rise, as coastal communities like Annapolis, Maryland, and Miami are discovering with increasing days of high-tide flooding.

Keeping communities safe in a changing climate will mean cleaning up flood-prone industrial relic sites. In some cases, companies can be held financially responsible for the cleanup, but often, the costs fall to taxpayers.

The infrastructure bill that Congress passed in 2021 includes $21 billion for environmental remediation. As a key element of new “green” infrastructure, some of that money could be channeled into flood-prone areas or invested in developing pollution remediation techniques that do not fail when flooded.

A large brick housing complex with people sitting in lawn chairs outside. A sign on the lawn is in Spanish.
The West Calumet Housing Complex in East Chicago, Ind., was built on the site of an old lead refinery. It was closed down after children there were found to have elevated levels of lead in their blood. The sign reads: ‘Do not play in the dirt or next to shredded wood mulch.’ AP Photo/Tae-Gyun Kim

Our findings suggest the entire process for prioritizing and cleaning up relic sites needs to be reconsidered to incorporate future flood risk.

Flood and pollution risks are not separate problems. Dealing with them effectively requires deepening relationships with local residents who bear disproportionate risks. If communities are involved from the beginning, the benefits of green redevelopment and mitigation efforts can extend to a much larger population.

One approach suggested by our work is to move beyond individual properties as the basis of environmental hazard and risk assessment and concentrate on affected ecosystems.

Focusing on individual sites misses the historical and geographical scale of industrial pollution. Concentrating remediation on meaningful ecological units, such as watersheds, can create healthier environments with fewer risks when the land floods.

Thomas Marlow, Postdoctoral Fellow in the Center for Interacting Urban Networks (CITIES) at NYU Abu Dhabi, New York University; James R. Elliott, Professor of Sociology, Rice University, and Scott Frickel, Professor of Sociology and Environment and Society, Brown University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Climate change is making flooding worse: 3 reasons the world is seeing more record-breaking deluges and flash floods

Fast-moving floodwater obliterated sections of major roads through Yellowstone National Park in 2022. Jacob W. Frank/National Park Service

by Frances Davenport, Colorado State University

Heavy rainfall turned into dangerous flooding in rugged Appalachia in late July, sweeping away homes and killing more than three dozen people, Kentucky’s governor announced. The destruction followed flooding a few weeks earlier in the mountains of Virginia and Tennessee.

In June, flooding hit mountains in the Western U.S., where rain combined with melting snow can be particularly destructive. Storms dumped up to 5 inches of rain over three days in and around Yellowstone National Park, rapidly melting snowpack. As the rain and meltwater poured into creeks and then rivers, it became a flood that damaged roads, cabins and utilities and forced more than 10,000 people to evacuate.

The Yellowstone River shattered its previous record and reached its highest water levels recorded since monitoring began almost 100 years ago.

Although floods are a natural occurrence, human-caused climate change is making severe flooding events like these more common. I study how climate change affects hydrology and flooding. In mountainous regions, three effects of climate change in particular are creating higher flood risks: more intense precipitation, shifting snow and rain patterns and the effects of wildfires on the landscape.

A woman with work gloves and clothing covered in mud walks through a muddy residential street filled with mud-covered furniture and other damaged belongings people are throwing out after a flood.
Extreme rain storms triggered flooding and mudslides in Western Europe in July 2021, killing more than 200 people. Thomas Lohnes/Getty Images

Warmer air leads to more intense precipitation

One effect of climate change is that a warmer atmosphere creates more intense precipitation events.

This occurs because warmer air can hold more moisture. The amount of water vapor that the atmosphere can contain increases by about 7% for every 1.8 degrees Fahrenheit (1 degree Celsius) of increase in atmospheric temperature.

Research has documented that this increase in extreme precipitation is already occurring, not only in regions like Yellowstone, but around the globe. The fact that the world has experienced multiple record flooding events in recent years – including catastrophic flooding in Australia, Western Europe India and China – is not a coincidence. Climate change is making record-breaking extreme precipitation more likely.

The latest assessment report published by the Intergovernmental Panel on Climate Change shows how this pattern will continue in the future as global temperatures continue to rise.

More precipitation falling as rain

In colder areas, especially mountainous or high-latitude regions, climate change affects flooding in additional ways.

In these regions, many of the largest historical floods have been caused by snowmelt. However, with warmer winters due to climate change, less winter precipitation is falling as snow, and more is falling as rain instead.

This shift from snow to rain can have dramatic implications for flooding. While snow typically melts slowly in the late spring or summer, rain creates runoff that flows to rivers more quickly. As a result, research has shown that rain-caused floods can be much larger than snowmelt-only floods, and that the shift from snow to rain increases overall flood risk.

The transition from snow to rain is already occurring, including in places like Yellowstone National Park. Scientists have also found that rain-caused floods are becoming more common. In some locations, the changes in flood risk due to the shift from snow to rain could even be larger than the effect from increased precipitation intensity.

Changing patterns of rain on snow

When rain falls on snow, as happened in the recent flooding in Yellowstone, the combination of rain and snowmelt can lead to especially high runoff and flooding.

In some cases, rain-on-snow events occur while the ground is still partially frozen. Soil that is frozen or already saturated can’t absorb additional water, so even more of the rain and snowmelt run off, contributing directly to flooding. This combination of rain, snowmelt and frozen soils was a primary driver of the Midwest flooding in March 2019 that caused over US$12 billion in damage.

While rain-on-snow events are not a new phenomenon, climate change can shift when and where they occur. Under warmer conditions, rain-on-snow events become more common at high elevations, where they were previously rare. Because of the increases in rainfall intensity and warmer conditions that lead to rapid snowmelt, there is also the possibility of larger rain-on-snow events than these areas have experienced in the past.

A large two-story building is collapsing after fast-moving water eroded the land under nearly half of it.
The 2022 Yellowstone flood inundated communities and swiftly eroded the land beneath this cabin that housed park employees. Gina Riquier via National Park Service

In lower-elevation regions, rain-on-snow events may actually become less likely than they have been in the past because of the decrease in snow cover. These areas could still see worsening flood risk, though, because of the increase in heavy downpours.

Compounding effects of wildfire and flooding

Changes in flooding are not happening in isolation. Climate change is also exacerbating wildfires, creating another risk during rainstorms: mudslides.

Burned areas are more susceptible to mudslides and debris flows during extreme rain, both because of the lack of vegetation and changes to the soil caused by the fire. In 2018 in Southern California, heavy rain within the boundary of the 2017 Thomas Fire caused major mudslides that destroyed over 100 homes and led to more than 20 deaths. Fire can change the soil in ways that allow less rain to infiltrate into the soil, so more rain ends up in streams and rivers, leading to worse flood conditions.

Two men point stand on a deck overlooking a neighboring house where mud has flowed through the yard and is mounded half way up the side of the home.
A 2021 rainstorm that hit the denuded landscape of a burn scar sent mud flowing into streets and yards in Silverado, California. Paul Bersebach/MediaNews Group/Orange County Register via Getty Images

With the uptick in wildfires due to climate change, more and more areas are exposed to these risks. This combination of wildfires followed by extreme rain will also become more frequent in a future with more warming.

Global warming is creating complex changes in our environment, and there is a clear picture that it increases flood risk. As the Yellowstone area and other flood-damaged mountain communities rebuild, they will have to find ways to adapt for a riskier future.

This article was updated July 31, 2022, with Kentucky’s governor announcing a higher death toll.

Frances Davenport, Postdoctoral Research Fellow in Atmospheric Science, Colorado State University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Shifting sands: Carolina’s Outer Banks face a precarious future

Read the full story at e360.

Despite the risks of building on barrier islands, developers kept constructing homes on North Carolina’s Outer Banks. Now, as sea level rises and storms become more frequent and powerful, the famed vacation spot is fighting an increasingly difficult battle to keep from washing away.

New flood maps clarify the risk homeowners face

Read the full story from the University of Georgia.

Flooding in urban areas cost Americans more than $106 billion between 1960 and 2016, damaging property, disrupting businesses and claiming lives in the process. Determining which areas are most likely to flood amid ever-changing land use and shifting rainfall and climate patterns can be expensive and complicated – and past methods of drawing flood maps fail to capture the inherent uncertainty in flood predictions.

Now, new research from the University of Georgia outlines a simplified, cost-effective method for developing flood maps that reflects the uncertainty in flood predictions. Published in the journal Water, the study was led by engineering professor Brian Bledsoe, director of UGA’s Institute for Resilient Infrastructure Systems (IRIS), and Tim Stephens, a UGA and IRIS alumnus now with Dynamic Solutions LLC, an engineering, planning and research firm that specializes in water resources.

‘White gold’: why shrimp aquaculture is a solution that caused a huge problem

Read the full story at The Guardian.

In the 1980s, farmers in Bangladesh went from paddies to ponds, letting salt water flood their land. Now millions are left counting the cost

Urban flooding presents challenges for electric vehicle charging stations

Read the full story at Smart Cities Dive.

Considerations around sea level rise and intense rains come as the Federal Highway Administration weighs where to effectively locate chargers for the future as more people drive EVs.

Yellowstone flooding rebuild could take years, cost billions

Read the full story from the Associated Press.

Created in 1872 as the United States was recovering from the Civil War, Yellowstone was the first of the national parks that came to be referred to as America’s best idea. Now, the home to gushing geysers, thundering waterfalls and some of the country’s most plentiful and diverse wildlife is facing its biggest challenge in decades.

Floodwaters this week wiped out numerous bridges, washed out miles of roads and closed the park as it approached peak tourist season during its 150th anniversary celebration. Nearby communities were swamped and hundreds of homes flooded as the Yellowstone River and its tributaries raged.

The scope of the damage is still being tallied by Yellowstone officials, but based on other national park disasters, it could take years and cost upwards of $1 billion to rebuild in an environmentally sensitive landscape where construction season only runs from the spring thaw until the first snowfall.

Based on what park officials have revealed and Associated Press images and video taken from a helicopter, the greatest damage seemed to be to roads, particularly on the highway connecting the park’s north entrance in Gardiner, Montana, to the park’s offices in Mammoth Hot Springs. Large sections of the road were undercut and washed away as the Gardner River jumped its banks. Perhaps hundreds of footbridges on trails may have been damaged or destroyed.

Data from the sky inform flood planning

by Tiffany Jolley, Prairie Research Institute

Hydrologic models based on lidar data can be created to see how water flows over a landscape, under bridges, and through culverts. Modeling can predict where bottlenecks might occur and where that potential overflow water would flood into. Modeling can also reveal the locations of flood plains, indicating what structures would be affected by a flood event and help to map out evacuation routes that would not likely be underwater.

The Illinois State Water Survey’s the Coordinated Hazard and Assessment Mapping Program (CHAMP) is at the helm of one of the largest 2D models in Illinois, spanning five southern Illinois counties – Johnson, Pope, Massac, Pulaski, and Alexander. The extremely flat topography of this region, known as the Cache River Valley, follows the Cache River system and the historic path of the Ohio River that’s been heavily manipulated by humans over time.

“Water moves in different directions during the course of a flood and that makes the hydraulic flow patterns really complicated to study flooding in this area,” said Chris Hanstad, a CHAMP project engineer. “The Water Survey works with regulatory groups in this region a lot because of this reason.”

Pictured in the model above are the complex hydraulics and hydrology of the Cache River. Flow paths and auxiliary channels adjacent to the Cache River can flow in the opposite direction of the Cache River. At other times, flow can all be moving in the same direction, sometimes even the main Cache River can backup and flow to the East.

The models rely on detailed light detection and ranging (lidar) data captured by the Illinois State Geological Survey (ISGS). Lidar data is acquired from a sensor that is attached to the belly of a plane that flies an average of 2,000 to 1,500 meters above the ground. The sensor emits pulses of light and measures the time it takes for each pulse to return to the sensor. That measurement is then used to compute distances to objects (latitude, longitude, and elevation) with accuracy within centimeters both vertically and horizontally. As the light returns to the sensors several times, it travels through soft targets such as trees, power lines, and bushes until it encounters the ground or a building.

In flood mitigation, lidar data allows the researchers to “see beneath the trees.” Scientists are able to look at only the bare earth returns and triangulate specific points to create a seamless mosaic representation of the ground.

“The Cache River valley has an interesting geologic and man-made history that has affected flooding,” said Hanstad.

At the turn of the 19th century, agriculture interests, and then later in the 1950s the Army Corps of Engineers built levees that forced the Upper Cache and Lower Cache River to become separated.

CHAMP and Hanstad are still waiting on input for the modeling of the Reevesville Levee in southern Illinois. As their analysis work winds down, FEMA will begin to produce new floodplain maps for the area.

FEMA requires floodplain maps to be based on current risks and current conditions, developed using data from recent climate assessments, which has been a challenge for communities in emphasizing today’s risk and future risk 25 to 50 years from now.

Flood model of a storm in March 2008 that produced between 7 and 12 inches of rainfall in about 40 hours over the region. This event was the flood of record for the Upper Cache River.

“A lot of floodplain maps were based on older rainfall data, but our new studies are using Bulletin 75 rainfall data which was published in 2020,” said Hanstad.

Models and simulations can also aid in post-disaster recovery. Ongoing work will help bolster mitigation efforts by looking at damages and prioritizing high-risk areas for mitigation against future disasters.

This story first appeared on the Prairie Research Institute News Blog. Read the original story.

Resilience of U.S. coastal wetlands to accelerating sea level rise

Buchanan, Maya K.; Kulp, Scott; Strauss, Benjamin. (2022) Environmental Research Communication 4 061001

Abstract: Coastal wetlands provide a wide array of ecosystem services, valued at trillions of dollars per year globally. Although accelerating sea level rise (SLR) poses the long-term threat of inundation to coastal areas, wetlands may be sustained in two ways: by positive net surface-elevation change (SEC) from sediment and organic matter buildup and by accumulation, or horizontal migration into refugia—low-lying, undeveloped upland areas that become inundated. Using a simple model together with high-resolution elevation data, we provide, across the contiguous United States, analysis of the local effects of SLR, maximum SEC rates, and coastal development on the long-term resilience of coastal wetlands. We find that protecting current refugia is a critical factor for retaining wetlands under accelerating SLR. If refugia are conserved under an optimistic scenario (a high universal maximum SEC rate of 8 mm/yr and low greenhouse gas emissions), wetlands may increase by 25.0% (29.4%–21.5%; 50th, 5th–95th percentiles of SLR) by the end of the century. However, if refugia are developed under a more pessimistic scenario (a moderate universal maximum SEC rate of 3 mm/yr, high greenhouse gas emissions, and projections incorporating high ice-sheet contributions to SLR), wetlands may decrease by −97.0% (−82.3%–99.9%). These median changes in wetland area could result in an annual gain of ∼$222 billion compared to an annual loss of ∼$732 billion in ecosystem services in the US alone. Focusing on key management options for sustaining wetlands, we highlight areas at risk of losing wetlands and identify the benefits possible from conserving refugia or managing SEC rates.