How to Identify a Flood Plain

Identifying floodplains, low areas that are prone to flooding during rains or waterway changes, is far from an exact science. You can use several methods to determine the probability of flooding in low areas. The Federal Emergency Management Agency (FEMA) in the United States, for instance, uses the 100-year-rain standard (the probability that land will flood determined by the highest rates of rain recorded in the past 100 years); other standards can be used.

Things You'll Need

• Topographic maps of the area
• Waterway maps of the area
• Historical records of land-use and flooding patterns
• Index of local flora

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Instructions

• 1

  Consult flooding maps for the area. Many local and national government agencies keep detailed records on flooding and floodplain areas to organize land-use plans. In the United States, FEMA offers full maps of floodplains to determine eligibility for national flood insurance.

• 2

  Analyze plant life in the area. The species of plants that flourish in an area can be a good indication of the hydrological history and behavior of that area. Trees like silver maples and hackberry trees or shrubs like silky dogwood and buttonbush thrive in areas with humid soil and occasional flooding. If the area you are assessing has a large percentage of water-loving plant life, the ground is likely prone to flooding. Trees in particular, due to their longer life-spans, can be a good indication of the areas's flooding history.

• 3

  Measure the distance between the land you are assessing and waterways. Many floodplain maps consider any land along the edges of rivers or streams, regardless of the topography, to be part of the waterway's floodplain. In extreme situations like heavy storms or inadequate rainwater management from nearby populations, these areas can be prone to flooding.

• 4

  Look at the topography of the area. Consider whether the area is downhill from a major river, or is the lowest point of a valley where water could accumulate. You should be able to predict whether or not your area will be where those excess waters settle. Remember that water will always flow downhill and accumulate at low altitudes.

  Be aware that drainage patterns can be altered by man-made structures like irrigation and damming. Consider hydrological infrastructure in the area when trying to predict which way water will flow and drain.

• 5

  Look at the land-use history of the area. The history of human occupation of an area can be as telling as plant life. If the area has historical buildings that have never been flooded or has been used for economic activities that require dry conditions, such as cattle-ranching, for long periods of time, the area clearly does not have a history of severe flooding. If, on the other hand, historical structures are built on stilts or you know of periods when the area was used for aquatic resources like fishing, you clearly have a historical precedent of regular or long-term flooding.

  Of course, human intervention and changes in climate can alter these factors, so be sure you can put together a reliable time-line of periods of flooding and alterations to the terrain.