Precipitation Maps

Precipitation maps are used by engineers and other hydrology modelers to assess the size of a particular frequency storm for the project location. Engineers model their site’s rainfall response to different size storms that are expressed in inches over a 24-hour period. Each frequency has a different application. Not all storms are used in every jurisdiction, so check with yours to see which storm sizes are significant. NOTE: the "6-month, 2-year, 10-year, 25-year and 50-year" terms refer to a frequency analysis and DO NOT mean that this size storm only returns once in that particular time period.  

Storm Frequency Significance
6-month design storm (pdf) Typically, a jurisdiction will conduct a frequency analysis of storms and identify the size of storm that is equal or greater than 80 - 90% of the storms that occur. This size storm is referred to as the "water quality storm". LID facilities designed to infiltrate runoff from this size storm using design standards (based on current research) are assumed to meet EPA water quality requirements.The 6-month design storm has been found to be a conservative estimate for a "Water Quality Storm" in Oregon and could be used when a city or county has not identified a specific size "water quality" storm.
2-year design storm (pdf)

Channel Shaping Storm: Streams adjust to additional water (either via runoff or seepage) by readjusting their shape. Exceeding the duration, volume, or frequency of the pre-development 2-year flow will cause channel erosion. The frequency of this storm may vary from 18 months to 2.5 years, depending on the watershed, but in the absence of better information, this is a good surrogate. As a rule of thumb, using LID to completely reduce runoff from the 2-year storm tends to attenuate flows from the 25-year storm. Using this strategy, LID would replace a detention basin.

10-year design storm (pdf) The size of this storm dictates conveyance design of pipes, swales, ditches, etc. Designers might model one of these storms to keep what most would consider "all" stormwater on site using LID or to attenuate flooding if using a detention basin. Depending on built and natural conditions, an agency may use the 10-year or the 25-year storm.
25-year design storm (pdf)
50-year design storm (pdf) Conveyance or Flood Storm: The size of this storm dictates conveyance design of pipes and bridges for primary roads. Agencies may use this size storm or a 100-year storm depending on the importance of using the road/highway during major flood events.

Geographic Boundaries for SCS/SBUH Rainfall Distributions


Patterns of rainfall are modeled as types, which in Oregon include Type IA, Type I, and Type II. Type IA is the most gentle, longest storm while Type II storms are shorter, most intense storms. Many landowners prefer to see their facilities designed to empty within 24 to 30 hours; however, depending on rainfall distribution, facilities should be empty in at least: 

Type IA    30 hours
Type I      72 hours
Type II     72 hours

Precipitation varies greatly in Oregon. Know your storm patterns (distribution) and which storms to use for what.
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