Principles of Flash Flood Development: An Introduction to Hydrometeorology

Anthony PhillipsGEOG 490/590

Ball State University

Hazards associated with flash floods First law of Quantitative Precipitation Forecasting (QPF) Atmospheric and hydrologic contributors

Objectives

Definition of a flash flood Recognize the deadliest convective weather hazards, in order

of average annual fatalities, and identify trends over time Identify whether a hydrological factor (e.g., soil depth) will

increase or decrease the potential for flash floods Know the first law of QPF

Understand what factors favor precipitation duration, “D” Understand what factors favor average precipitation rate, “”

What is a Flash Flood?

What is the difference between a flood and a flash flood?

The term flood usually refers to a hydrological event that takes place over many hours or days.

Often associated with main-stem rivers and floodplains.

A flash flood often occurs within 6 hours of a convective rainfall event, or when a dam or levee fails.

Small streams and creeks are most susceptible.

What is a Flash Flood?

The NWS defines a flash flood as: “Within six hours (often within one hour) of a causative event such

as intense rain, dam break, or ice jam formation, one or more of the following occurs:

River or stream flows out of banks and is a threat to life or property. Person or vehicle swept away by flowing water from runoff that

inundates adjacent grounds. A maintained county or state road closed by high water. Six inches of fast-flowing water over a road or bridge. This includes low

water crossings in a heavy rain event that is more than localized (i.e., radar and observer reports indicate flooding in nearby locations) and poses a threat to life or property.

Dam break or ice jam causes dangerous out-of-bank stream flows or inundates normally dry areas creating a hazard to life or property.

Any amount of water in contact with, flowing into, or causing damage to a residence or public building as a result of above ground runoff from adjacent areas.

…”

Why Study Flash Floods?

Which hazard is most deadly? Wind & hail Hurricanes Tornadoes Lightning Flash floods

Flash floods are the deadliest hazard associated with con- vection worldwide. Flash floods cause more fatalities than tornadoes, wind, and hail combined!

Severe Weather Trends

Decrease in tornado- and hurricane-related deaths over the past 50 years. Better preparedness Better technology Increase in lead-times

However, fatalities from flash floods have not steadily decreased

Interaction of meteorology with hydrology

Flash flood risk is increasing Extreme (>2”) precipitation events Urbanization Population growth

Meteorological Factors

The development of flash floods depends on several factors, a few include: Storm type and precipitation Antecedent precipitation Drainage basin size Land use characteristics Soil composition

We’ll begin by reviewing the meteorological factors that can lead to the development of flash floods

First law of QPF

The greatest precipitation accumulations occur where it rains the hardest for the longest Known as the first law of Quantitative Precipitation Forecasting

(QPF) Can be stated as

P, precipitation accumulation , average precipitation rate D, duration of precipitation

Average precipitation rate is directly proportional to the duration of the precipitation

How can this be related to flash flooding?

First law of QPF

Maximum precipitation usually occurs only with intense or long duration convective systems Single- or multi-cell

convection MCCs or MCSs Squall lines Supercells

Stratiform precipitation is often too light, but can result in riverine flooding Long duration (D)

First law of QPF

Maximum precipitation usually occurs only with intense or long duration convective systems (

Usually, rainfall duration (D) increases with decreasing storm translation speeds Mean wind speed decrease

MCSs: maximum precipitation occurs when cell motion is opposed by propagation of new cells Results in quasi-stationary system

motion