Ruidoso flood history

RUIDOSOTODAY TODAY'S EVENTS

Ruidoso mid-town Sky Cam

UPCOMING EVENTS

WEATHERNOW RUIDOSO

Ruidoso's Flood History Analysis

By Frank Potter

Floods: Recurrence intervals and 100-year floods

When flood conditions occur, you might hear reports on the radio or TV say something like, “This storm has resulted in a 100-year flood on the Rio Ruidoso, which crested at a stage of 12 feet.” This means that the river reached a peak stage height that happens only once every 100 years, right? Well, not exactly. Hydrologist don't like to hear a term like “100-year flood” because, scientifically, it is a misrepresentation of terminology that leads to a misconception of what a 100 year flood really is.

Instead of the term “100-year flood”, a hydrologist would rather describe this extreme hydrologic event as a flood having a 100-year recurrence interval. According to historical rainfall and stream stage data, the probability of the Rio Ruidoso reaching a stage of 12 feet is once in 100 years. In other words, a flood of that magnitude has a 1 percent chance of happening in any year.

What is a recurrence interval?

Statistical techniques, through a process called frequency analysis, are used to estimate the probability of the occurrence of a given precipitation event. The recurrence interval is based on the probability that the given event will be equaled or exceeded in any given year. For example, there is a 1 in 50 chance that 6.60 inches in a consecutive 48 -hour period is said to have a 100-year recurrence interval. Likewise, using frequency analysis, there is a 1 in 100 chance that a stream flow of 1000 cubic feet per second will occur during any year at a certain stream flow-measurement site. Therefore, that site is said to have a 100-year recurrence interval.

Ten or more years of data are required to perform a frequency analysis for the determination of recurrence intervals. Ruidoso has 67 years of historical data and hydrologist have more confidence in an analysis of at least 30 years of records. Based on the historical data, the following chart depicts the recurrence interval, or what percent of probability, of future flooding of the Rio Ruidoso.

Ruidoso's data is typically taken at the reservation line in the upper canyon at the USGS station and not the Hollywood USGS station due to the fact that officials are measuring initial impact on the village. When rainfall data is collected at a point within a stream basin, it is highly unlikely that the same amount of rainfall occurred uniformly throughout the entire basin. During intensely localized storms, rainfall amounts throughout the basin can differ greatly form the rainfall amount measured at the location of the rain gage. Ruidoso experienced this in almost all known floods, especially the 1965 flood.

The 100-year flood level can change.

Since the 100-year flood level is statistically computed using past and current data, as more data comes in, the level of the 100-year flood will change. (Especially if a huge flood hits in the current year.) As more data is collected, or when the river basin is altered in a way that affects the flow of water in the river, scientist re-evaluate the frequency of flooding necessitating the redesign of structures, such as bridges, to meet the 1000 cubic feet per second flow. Ruidoso's past history indicates that bridges have been designed to accommodate 300 to 500 cubic feet per second.

Based on the 100-year flood assumption, every year there is a 1% chance of recurrence. However, 100-year floods can happen two years in a row and can be analyzed using the frequency analysis process for a given area.

Ruidoso Flood History

1500