Drainage Manual - County of Santa Clara

Drainage Manual 2007
County of Santa Clara, California
Flood Frequency – The frequency of flood and precipitation events are described in
practice and in this manual using complementary terminology. “Exceedance frequency”
refers to the probability of any individual precipitation or runoff event in any water year
exceeding a certain threshold. Ten‐percent peak discharge is that flow rate with a ten
percent chance of being equaled or exceeded during any water year. A “one percent, 6‐
hour precipitation depth” is that 6‐hour depth of rainfall that has a one percent
probability (chance) of being equaled or exceeded during any water year.
An alternate terminology used in practice is the “recurrence interval,” which is also
referred to as a “return period.” This terminology uses a number of years to specify a
flood event. For example, a “ten‐year peak discharge” would be that discharge expected
to be equaled or exceeded once every ten years on the average. The “100‐year, 6‐hour
precipitation depth” is the 6‐hour depth of rainfall expected to be equaled or exceeded at
a location once every 100 years on the average. Annual hydrologic events are considered
to be independent of one another; so there is a finite probability of exceeding the 100‐
year runoff in back‐to‐back water years. Experiencing a rainfall or runoff event of a
certain magnitude does not lessen the chance (probability) of experiencing another event
of equal or greater magnitude in subsequent years.
These two frequency terminologies are related as the reciprocal of one another. The ten‐
percent event (0.10 probability) is equivalent to the ten‐year event because the reciprocal
of 0.10 is 10. Similarly the one‐percent event (0.01 probability) is the 100‐year event.
Freeboard – The vertical distance between an elevation of interest (e.g. water surface,
hydraulic grade line, or energy grade line) and the elevation of containment, such as the
top of stream bank, street grade, or floodwall. Freeboard is intended to provide for a
factor of safety in the design of stormwater storage and conveyance facilities.
Frequency Interval – The frequency interval (or recurrence interval) of a peak flow is the
number of years, on average, in which the specified flow is expected to be equaled or
exceeded one time. Exceedance probability and frequency interval are mathematically
inverse of each other; thus, an exceedance probability of 0.01 is equivalent to a frequency
interval of 100 years. For example, a peak flow with a 100‐year frequency interval will,
on average, be equaled or exceeded once every 100 years and has an exceedance
probability of 0.01 (a 1‐percent chance of being exceeded in a given year). Frequency
intervals refer to the average number of occurrences over a long period of time; for
example, a 100‐year flood is statistically expected to occur about 10 times in a 1,000‐year
period, rather than exactly once every 100 years. Additionally, it should be noted that
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