This can actually be quite complicated because it has its roots in hydrology, engineering, statistical analysis and probability theory. So I am going to over simplify. A common problem in many fields such as geophysics, economics, meteorology and many other fields is a determination of periodicity. That is to decide if certain data taken over time provides evidence of periodic behavior which can then be used to improve prediction of future behavior for planning purposes. Examples would be how often might we expect a crop failure, an earthquake of a given magnitude, a hurricane direct hit, or a flood of X level.

So in the case of flooding, the total record of known rainfall occurrences in the past and data from past floods are combined with knowledge of the flood plain in question, the topography, land-use, vegetation, soil types, elevation and land slope of the river basin etc. and put into mathematical formulas to determine what amount of rain, over what area, over what period of time, will lead to what level of flooding in various locations. For example, when determining whether to issue a flood watch. The forecaster first determines the amount of rain expected, then consults charts of current soil moisture and river levels. Thus a 6 hour rainfall rate, 12 hour 24 hour etc. rain flood criteria is found and if it will be met or exceeded a watch is issued.

This also allows for the development of a so-called 100 year flood plain map or base flood and a 500 year or extreme flood plain map used for urban planning. A 100-year flood is major flooding. A 500 year flood is extreme and historic. Lesser flood levels can also be calculated.

Mother nature of course does not follow statistics because it is a non-linear or chaotic system.  Nature can give us three 100 year floods in one year or two 500 year floods in a single decade, or a 500 year drought followed by a 500 year flood within a 3 year period or anything she wants, as the extreme climate variation Georgia has seen over the last 5-15 years testifies.

Specifically the term a 100 year flood does NOT mean such a flood should be expected only once in a lifetime or once every 100  years. It means such a level of flooding from the aforementioned formulas has a 1% chance of occurring in ANY given year and therefore is a 1/100 year event, statistically speaking it should occur once in every 100 years. The 500-year flood has a 00.2% chance of occurring in any given year and therefore statistically would be expected to occur once every 500 years.

FEMA publishes flood insurance rate maps (F.I.R.M.) that show various categories of flood hazard zones. However, the calculations, engineering, and surveying needed to determine those zones are done by engineers working in the private sector. This work might be done under these circumstances:

1. An area-wide study (maybe an entire county) to update a set of F.I.R.M.s, such as for an urbanizing area. This will normally be contracted by the local community, often with Federal and State funding.
2. Study of a specific river or stream, within a single community, to assist with planning and orderly development. The community will normally contract with the engineer.
3. Study of a short stretch of a waterway that is to be developed (or has been developed) to determine the impact of the development on the flood-carrying capacity. This is typically contracted and paid for totally in the private sector.
4. Occasionally FEMA itself, or another branch of the Federal or State government will contract with private sector engineers for a major flood study. This is most typically true for levee systems or flood control dams and reservoirs.

Hydrology and Hydraulics

For this flood plain work, the engineer must complete certain calculations. Hydrology calculations predict rainfall and compute the resulting stream flow. This is determined by the physical characteristics of the drainage basin--area, slope, shape, soil type, amount of development--and the regional climate--probable rainfall pattern and intensity based on years of historical records.

FEMA procedures require that all flood mapping be based on what is called the 100-year flood. This is the rainfall amount and associated stream flow that have a 1 percent chance of occurring in any given year (and thus statistically should occur once every hundred years). FEMA calls this the base flood. Calculations are also done for the 500 year flood, although that is less important in flood insurance issues than is the base flood.

Hydraulic calculations take the run-off determined from hydrology and compute the depth and spread of flood waters. The physical characteristics of the stream, such as slope, main channel dimensions, overbank dimensions, roughness, obstructions, and development on the overbank, are all factored into the calculations.

The hydraulics of most interest are for the base flood. The calculations determine the height of flood waters from the 100 year rainfall, which is then called the base flood elevation (BFE). This is the elevation of greatest regulatory significance within the flood plain.

Flooding occurs when an existing stream (such as a river or creek) can't handle the waterflow. The cause of the high waterflow varies, but generally comes from high amounts of precipitation, or from snowmelt. The existing channel is overwhelmed, and the water "comes out of bank" to enter what is termed the flood plain.

The floodplain is a part the terrain adjacent to the channel where water doesn't normally flow; a floodplain is often not obvious to the inexperienced observer. A floodplain is simply a natural storage reservoir for flood waters, and has been created by nature, the master engineer, through thousands of years of water flow and floods.

When water leaves the normal flow channel, you have a flood event. The flood level is defined by the amount of water present. There are two general factors affecting the level of the flood:

• The amount of water in the channel.
• The shape and characteristics of the channel at a given point.

The amount of water is governed by local precipitation: snowpack, rainfall, and sometimes storage capacity in a reservoir.

The channel and shape characteristics control how fast the water flows. A narrow, steep channel tends to move water quickly, while a wide, flat channel moves water slowly. When water moves slowly, it tends to rise, or back up. This can cause over bank flooding. Other factors, such as vegetation and soil, will also affect water flow.

Flood events are defined by the probability that a certain amount of water is possible any one year. For example, the infamous "100-year flood" is in fact the level of water with a 1-percent chance (1 in 100) occurring any one year. The amount of water actually varies from river to river. In fact, that amount can vary along a river. The use of "n year flood" is technical jargon that has caused endless problems with the public. The term does not mean that a flood occurs every n years, but that it has a chance of 1/n of occurring any one year. Water volume increases as the probability decreases. The table below shows how this might affect you:

In short, the infrequent floods tend to high and violent water flows (and a good thing, too!). The common floods are much smaller, although damage is still possible.

"Great!" you say, "But how high is a 5-year flood?" The answer, as noted earlier, is not simple. That's because the climate varies, and conditions affecting water flow along a channel change. Therefore, each site must be examined to determine the potential water elevations! This has been done extensively across the country by FEMA, for 100 and 500 year flood plains in selected communities. Such a study is rarely made for lesser floods, although data from 100 flood plain studies can yield estimates for lesser floods.