The flood plain is generally a flat or gently sloping area of land beside a river channel which is inundated during periods of high water. The history of flood-prone rivers shows a recurring pattern of overflow in the flood plain area.
The Floodway is that portion of the flood plain, including the area normally occupied by the river, in which most of the floodwaters are conveyed. This is the area where water velocities are typically high during a flood and where damages are often greatest. The floodway is defined as that area flooded on average once in 20 years (or an area which has a 5% chance of being flooded in any given year).
The Flood Hazard Area is defined as that area flooded on average once in 100 years (or an area which has a 1% chance of being flooded in any given year). Alternatively, the area in a specific community which was inundated by a major flood in the past may be designated as the Flood Risk Area, providing the water levels were equal to, or higher than, the 1 in 100 year flood.
The Floodway Fringe is that portion of the flood plain between the floodway and the outer limit of the Flood Risk Area, whether it is the 1 in 100 year flood line or a higher historic flood line.
It is important to note that the above definitions are based on open water flooding conditions. Frequency projections cannot be estimated for ice-jam flooding, which may occur at any time, given the right conditions.