Flood Hazard Overview

What is a Flood?

FEMA defines a flood as “A general and temporary condition of partial or complete inundation of 2 or more acres of normally dry land area or of 2 or more properties” (Federal Emergency Management Agency, 2017).

The US Army Corps of Engineers states that a flood as an “abnormally high water flow or water level that overtops the natural or artificial confining boundaries of a waterway” (US Army Corps of Engineers, 2006).

The primary factors that determine the severity of a flood include:

• Rainfall intensity and duration – A large amount of rain over a short time can result in flash flooding. – Small amounts may cause flooding where the soil is already saturated.

• Topography – Water runoff is greater in areas with steep slopes and little vegetation.

Flooding types are riverine, flash, and urban flooding.


Kentucky Flood Risk Assessment Overview

Flooding is Kentucky’s #1 most frequent and costly natural disaster.  Not only is flooding Kentucky’s most common disaster, but its risk of happening can change over time.  These changing risks can be due to new development in the watershed, changes in weather patterns, or new and better data used in creating the maps.

Kentucky’s topography contains 13 major drainage basins to accommodate 40-50 inches of average rainfall (maximum during winter and spring, minimum during late summer and fall). The state contains 89,431 miles of rivers and streams, 637,000 acres of wetlands, 18 reservoirs over 1,000 acres in size, and 228,382 acres of publicly-owned lakes and reservoirs.



Rainfall and Topography Vary Considerably across the Commonwealth

Kentucky’s topography contains 13 major drainage basins to accommodate 40-50 inches of average rainfall (maximum during winter and spring, minimum during late summer and fall), The state contains 89,431 miles of rivers and streams, 637,000 acres of wetlands, 18 reservoirs over 1,000 acres in size, and 228,382 acres of publicly-owned lakes and reservoirs.

Eastern Kentucky generally has steep slopes and narrow valleys throughout, making it susceptible to flash flooding events. The speed with which these events develop gives little time for warning and can potentially lead to loss of life.

Central Kentucky has rolling hills and several medium to large river systems and their associated small tributaries. These areas tend to have increased warning times but the smaller tributaries are still susceptible to flash flooding events.

Northern Kentucky along the Ohio River and Western Kentucky are generally flatter with larger, well defined floodplains. These areas tend to have the greatest amount of warning time in the Commonwealth which provides communities and citizens the time to get out of the way. These areas are less vulnerable to loss of life due to flooding but have a higher risk of economic losses.

Other factors that contribute to flood impacts include storm types, soil types, channel slope, karst areas, soil saturation, infrastructure development, impervious These variations in local conditions can cause extreme variability in flood levels, duration, and impacts.


Types of Flooding

There are many factors that can lead to different types of flooding with each type of flooding having different effects on communities.  The types of floods can be grouped into eight categories:

1) Regional flooding can occur when seasonal rain events, coupled with melting snow, fill river basins with too much water too quickly.   

2) Riverine flooding is a high flow or overflow of water from a river, stream, or similar body of water, occurring over a long period of time.  T

3) Flash floods are quick-rising floods that occur as the result of heavy rains over a short period of time.   

4) When a dam fails, an excess amount of water is suddenly released downstream, destroying anything in its path.  

5) Debris flooding can occur on rivers or streams that are either totally or partially blocked due to buildup of debris or ice, causing a rapid rise in stream stage both at the point of the jam and upstream. 

6) Storm-surge flooding occurs when water is pushed up onto otherwise dry land by offshore winds.  This flooding is generally due to wind action associated with large storms over an open body of water.   

7) Mudflows originate as sheet flow across a land area that cannot retain runoff, such as areas that have been deforested by wildfires.  The mudflow starts when sheet flow picks up soil and debris, and once wet enough, begins flowing in mass. 

8) Urban drainage (stormwater) flooding is a combination of both natural and man-made elements. 


 

Link to the Kentucky Flood Risk Assessment for complete details. The Flood Risk Assessment defines Kentucky’s overall flood risk and identifies potential mitigation actions that can be implemented to reduce this overall flood risk.