Flooding Basics

Flooding is the most common and widespread of all natural disasters in the United States, except fire. Most communities experience some kind of flooding after spring rains, heavy thunderstorms, or winter snowmelt.

Many factors contribute the magnitude and size of a flooding event, including:

  • Quantity of precipitation;
  • Duration and intensity of a precipitation event;
  • Snowmelt contribution;
  • Antecedent soil moisture conditions (wet soils are less able to accept additional precipitation, resulting in increased stormwater runoff); and
  • Land use make-up and extent of impervious surfaces in the watershed.

What is a Floodplain?

Floodplains are the land areas adjacent to rivers, streams, ponds, and other water bodies that are subject to recurring inundation. Flooding occurs when these water bodies receive a larger quantity of water than they can contain within their banks, causing water to overflow and flood the adjacent low lying areas.


Floodplains are a key component of the Town of Winchester’s natural environment. Understanding and protecting the natural functions of floodplains can help reduce flood damage and protect resources. When flood waters spread out across the floodplain, their energy is dissipated, which results in lower flood flows and elevations downstream, reduced erosion of the stream bank and channel, and improved groundwater recharge. Floodplains also provide valuable wildlife habitats.

Poorly planned development in floodplains can lead to loss of valuable property, increased risk of flooding to downstream properties, stream bank erosion, degradation of water quality, and imperiled public safety.

100-year flood plain FEMA image

Flood Frequency

Surface-water flooding associated with over-topped rivers, streams, and ponds is typically described in terms of its statistical frequency, or “recurrence interval”. Surface-water flooding may result from many different circumstances, and not all floods are equal in magnitude, duration, or effect. Therefore, statistical analysis allows society to evaluate the potential risks associated with various flooding events, and to address issues such as risk to life and property.

The most commonly referenced event is the “100-year flood”, which is the standard used by the National Flood Insurance Program (NFIP) administered by the Federal Emergency Management Agency (FEMA) for the purposes of requiring the purchase of flood insurance and regulating new development. The term “100-year flood” is a bit of a misnomer, as it implies that a flood of this magnitude will only occur once every 100-years. Rather, from a statistical standpoint, this extreme hydrologic event has a 100-year recurrence interval. In other words, a flood of that magnitude has a 1% chance of being equaled or exceeded in any given year. FEMA also refers to the 100-year flood as the “base flood”. Similarly, a 25-year storm has a 1 in 25, or 4%, chance of happening in any given year, and so on.

Flood Frequency Statistics

  • Structure in the 100-year floodplain being flooded in a given year: 1 in 100
  • Matching 1 number plus the Powerball in the Powerball lottery: 1 in 124
  • Structure in the 500-year floodplain being flooded in a given year: 1 in 500
  • Being struck by lightning in a given year: 1 in 600,000
  • Winning the Powerball lottery jackpot (matching 5 numbers plus the Powerball): 1 in 120,526,770