Flooding Basics

What is Flooding?

FEMA definition

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 (at least 1 of which is the policyholder's property) from:

  • Overflow of inland or tidal waters; or
  • Unusual and rapid accumulation or runoff of surface waters from any source; or
  • Mudslides (i.e., mudflows) …
  • Collapse or subsidence of land along the shore of a lake or similar body of water as a result of erosion or undermining caused by waves or currents of water exceeding anticipated cyclical levels that result in a flood as defined above.

Types of flooding:

A flood inundates a floodplain. Most floods fall into three major categories: riverine flooding, coastal flooding, and shallow flooding. 

  • Types and Causes of flooding

    • Riverine flooding 
      • Occurs when streams or rivers exceed their capacity by rainfall and runoff from upstream and overflow their banks flooding adjacent lands.infographic-fluvial-floods

        Image Credit: zurich.com

    • Coastal flooding
      • Occurs when high winds associated with tropical cyclones (hurricane) push water onshore; conditions are worsened by high tides.
      • “Storm Surge”, measured by rise of water level over normal astronomical tide. Not including waves.
      • Storm surge in combination with waves can cause extensive damage.
      • “Sunny day” flooding also called high tide flooding is a type of coastal flooding that occurs when local sea level temporarily rises above an identified threshold height for flooding, in the absence of storm surge or riverine flooding.infographic-coastal-floodImage Credit: zurich.com
    • Surface (pluvial) flooding 
      • Occurs when heavy rainfall exceeds the capacity of natural and urban drainage systems. Expected to increase due to impacts of over-development and climate change. infographic-pluvial-floods    Image Credit: zurich.com


IS MY PROPERTY IN THE FLOODPLAIN? 

Determine if your property is located within the floodplain - Floodplain Map


WHAT ARE FLOOD ZONES? 

Flood hazard areas identified on the Flood Insurance Rate Map as identified as a Special Flood Hazard Area (SFHA)  SFHA are defined as the area that will be inundated by the flood event having a 1-percent chance of being equal to exceeding in any given year. The 1-percent annual chance flood is also referred to as the base flood or 100 yr. flood.  (FEMA)


WHAT TYPES OF FLOOD ZONES ARE WITHIN THE CITY?

Zone AE - Special flood hazard areas subject to inundation by the one-percent annual chance flood; base flood elevation is determined. (FEMA) 

Zone VE - Special flood hazard areas subject to inundation by the one percent annual chance flood and subject to high-velocity wave action (also referred to as "coastal high hazard areas")  (FEMA) 

Zone X (shaded) - Areas subject to inundation by the 0.2& annual change; areas subject to the 1% annual chance flood with average depths of less than one foot or with contributing drainage area less than one square mile; and areas protected by levees from the base flood. (FEMA) 

Zone X (unshaded) - Areas determined to be outside the 1% annual chance flood and outside the 0.2% floodplain.   (FEMA) 

LiMWA - The inland limit of the area affected by waves greater than 1.5 ft. during the base flood.  Base flood conditions between the Zone VE and the LiMWA will be similar to, but less severe than, those in the Zone VE.   (FEMA)


What is the Base Flood Elevation? 

According to FEMA base flood elevation (BFE) is the elevation of surface water resulting from a flood that has a 1% chance of equaling or exceeding that level in any given year. Basically, BFE is a reference point to determine flood protection requirements.


What is Freeboard? 

According to FEMA, freeboard is the following:

  • “An additional amount of height above the Base Flood Elevation used as a factor of safety (e.g., 2 feet above the Base Flood) in determining the level at which a structure's lowest floor must be elevated or floodproofed to be in accordance with state or community floodplain management regulations.
  • Freeboard is a factor of safety usually expressed in feet above a flood level for purposes of floodplain management. "Freeboard" tends to compensate for the many unknown factors that could contribute to flood heights greater than the height calculated for a selected size flood and floodway conditions, such as wave action, bridge openings, and the hydrological effect of urbanization of the watershed. Freeboard is not required by NFIP standards, but communities are encouraged to adopt at least a one-foot freeboard to account for the one-foot rise built into the concept of designating a floodway and the encroachment requirements where floodways have not been designated. Freeboard results in significantly lower flood insurance rates due to lower flood risk.”


What is Sea Level Rise? 

Sea levels are rising today at a rate of about one inch every decade. As the Earth warms, sea levels will rise at a faster pace, resulting in tide levels that could be between 1.6 and 4.9 feet higher than they are today. Additionally, the mid-Atlantic region is simultaneously experiencing land subsidence, or sinking, as the result of the last ice age.  The combination of higher tides, land subsidence, and storm surges can result in flooding of low-lying areas causing losses to private property, agricultural areas, and habitats such as wetlands.  Furthermore, rising sea levels may impact septic systems and drinking and irrigation water due to saltwater intrusion and potentially higher water tables. For information on sea level rise and projections for Delaware visit the Delaware Geologic Survey webpage or the frequently asked questions about the Delaware sea level rise planning scenarios.