What is a 5 year flood event?
(The answer isn't simple...)
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 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:
| Flood Probabilities for any one year | |||
|---|---|---|---|
| "Year" | Probability | Percent | Flows |
| 500 | 0.002 | 00.2% | Extreme |
| 100 | 0.010 | 01.0% | Heavy to extreme |
| 50 | 0.020 | 02.0% | Moderate |
| 25 | 0.040 | 04.0% | Light to moderate |
| 10 | 0.100 | 10.0% | Light |
| 5 | 0.200 | 20.0% | Mild |
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.
As a rule, you'll probably have to engage the services of an engineer experienced in hydrology to estimate the flood level at a given site.
Please note that narrow, steep channels can cause flooding if debris or other barriers re-direct the water flow out of bank. The high water velocity can also cause serious erosion problems.