Principles of Flash Flood Development: An Introduction to Hydrometeorology Anthony Phillips GEOG...
-
Upload
jean-walker -
Category
Documents
-
view
219 -
download
0
Transcript of Principles of Flash Flood Development: An Introduction to Hydrometeorology Anthony Phillips GEOG...
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