Floods, Snow

Flash Flooding in Ellicott City

Overview of the Event

The discussion begins with a reference to the flash flooding event in Ellicott City, specifically identifying it as the first major flood in the area. A time-lapse video will later illustrate the rainfall patterns observed during the event.

Rainfall Intensity and Rate

At approximately 07:30, a staggering 3.1 inches of rain fell in just thirty minutes, equating to a rainfall rate of about 6 inches per hour. The instructor emphasizes the overwhelming nature of such precipitation, stating that urbanized areas with significant impervious surfaces can hardly manage that volume of water.

Impact on Annual Rainfall

The overall rainfall for the event reached about half a foot, which is notably equivalent to approximately 15% of the total annual rainfall experienced in the area over just two hours. This leads into a discussion about the standards that cities should consider when planning infrastructure.

Infrastructure Planning Challenges

The instructor addresses the difficulties in securing funding for infrastructure capable of handling such extreme rainfall events, citing that this specific flood qualifies as a one-in-a-thousand-year event, or a flood with a 0.1% probability of occurrence in any given year. This detail is particularly critical for students to understand regarding flood risk assessments and investment decisions in real estate.

Radar Technology for Meteorology

The lecturer provides a brief introduction to radar technology used in meteorology. Radar operates by sending out radio waves that reflect off atmospheric objects, allowing meteorologists to measure rainfall rates and droplet sizes. This segment includes a description of radar loops used by the National Weather Service to observe precipitation.

Visualization of Rainfall Rates

The discussion includes a visualization of rainfall rates using a radar image, where dark reds indicate the heaviest rainfall rates. The location of Ellicott City is marked on this radar image to provide context. The concept of 'training' is introduced, referring to the phenomenon where thunderstorms repeatedly pass over the same area, which significantly contributes to flash flooding.

Characteristics of the Flash Flood Event

Localized Rainfall Gradients

Ellicott City experienced a unique situation where two rounds of thunderstorms hit the region, causing severe flooding. The instructor notes that the rainfall gradient in this event was steep, with only a 40-mile distance resulting in significant differences (e.g., 3.1 inches in Ellicott City vs. around half an inch just 40 miles southwest).

Stream Gauge and Flood Stage

A stream gauge reading in the local river illustrates the rapid shift from stable conditions of about 4.5 feet to an alarming height of 21.5 feet during the flood within just a few hours. The river stages are defined as action, minor, moderate, and record floods, with the Ellicott City event reaching record levels.

Causal Factors of Flooding

Urban Development and Drainage

The instructor discusses how Ellicott City was historically chosen for development due to its advantageous location by a river and tributaries. However, increased urban development has led to the presence of many impervious surfaces, which compound runoff and exacerbate flooding.

Importance of Rainfall Location

The forecast that a flash flood would occur somewhere in the area was accurate, but pinpointing the exact location was more complex. Had the storm's path shifted slightly, other locations like Baltimore could have faced even greater flooding.

Time-Lapse Analysis of the Flood

A time-lapse video illustrates the progression of the flood. Initially, the streets appear calm, but within thirty minutes, significant flooding occurs. The video captures vehicles being swept away and reveals the extent of damage sustained by the area, emphasizing the rapid development of the flood.

Cost and Consequences of Flooding

The Ellicott City flash flood resulted in an estimated $25 million of damage, proving the financial implications of flooding. The situation also emphasizes the human cost associated with such events, with tragically noticed deaths during the flood.

Historical Context: The Johnstown Flood of 1889

Overview of the Event

The instructor shifts focus to another significant flooding event—the Johnstown flood of 1889. This flood is notable not only for its severity but also for its lessons in engineering negligence.

Human Error and Environmental Impact

In the 1840s or 1850s, a dam was constructed on one of the tributaries of a river in Johnstown, initially for recreational purposes. However, maintenance was neglected, and with time, the dam became incapable of preventing flooding due to accumulated debris and blocked spillways.

Catastrophic Outcome

During a period of intense rain from a decaying tropical cyclone, the dam failed, resulting in a 20 to 30-foot wave of water that devastated the city. This disaster was compounded by fires ignited by chemicals washed into the floodwaters.

Loss of Life

The death toll from the Johnstown flood was alarmingly high, with around 20% of the downtown population perishing in the disaster, either from the flood itself or subsequent fires. This event led to the establishment of the American Red Cross as a response to the need for organized disaster relief.

Lessons Learned

The underlying message from both flooding examples is clear: proper infrastructure maintenance and flood risk management are crucial in preventing such disasters. The Johnstown flood was a preventable disaster, highlighting the need for responsible environmental stewardship.

Snow and Winter Weather

Interesting Facts About Snow

The instructor introduces snowfall by noting a fun fact: no two snowflakes are exactly alike. This uniqueness parallels the diversity found within humanity and hints at the complexity of meteorological phenomena.

Types of Snowflakes and Their Significance

The characteristics of snowflakes are influenced by their formation temperatures. Snow that falls in colder climates typically has distinct crystalline structures, which affects accumulation and the overall impact on surface conditions.

Meteorological Considerations

The instructor discusses various types of snow based on temperature ranges at which they form, detailing how warmer temperatures result in compact snow while colder temperatures yield fluffier, more desirable snow for snow sports.

Challenges in Snow Forecasting

One significant challenge faced by meteorologists is accurately predicting snow accumulation. Forecasting models struggle because they do not calculate the physics at the micro-level of ice crystal formation. Instead, predictions are based on parameterized estimates that cannot guarantee precision.

Lake Effect Snow Phenomena

Definition and Requirements

Lake effect snow refers to the heavy snow that occurs in areas near large bodies of water, primarily those with significant temperature gradients between the warm lake water and the colder air above it. This phenomenon mainly takes place in regions like Buffalo, NY, around the Great Lakes.

Fetch and its Impact

The term 'fetch' describes the distance over which wind can blow across the lake's surface. Longer fetch increases the potential for substantial lake effect snow, as it allows more air to pick up moisture and energy from the lake before making landfall.

Localized Impact of Lake Effect Snow

Lake effect snow events are often highly localized, with some areas receiving several feet of snow within a short period, while nearby areas may receive little to no accumulation. This concentration of precipitation can lead to unique hazards like roof collapses due to heavy snow loads.

Notable Events

Historical examples demonstrate the extreme impacts of lake effect snow, such as 100 inches of snow reported in isolated events. This reality underlines the importance of understanding and forecasting lake effect snow characteristics and risks.

Conclusion and Recap

The lecture wraps up with a brief recap of critical concepts covered, including flash flooding dynamics, the historical context of past floods, and snow-related phenomena. Students are advised to reflect on the practical implications of extreme weather patterns and their forecasting methodologies as they prepare for upcoming assessments in the course.