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Statewide Map-based IDF Analysis. Norman Gonsalves for Caltrans. What are intensity-duration-frequency curves?. Plots of rainfall intensity against storm duration, for various “frequencies of occurrence”

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Statewide Map-based IDF Analysis

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## Statewide Map-based IDF Analysis

Norman Gonsalves

for

Caltrans

### What are intensity-duration-frequency curves?

• Plots of rainfall intensity against storm duration, for various “frequencies of occurrence”

• Frequency of occurrence is the chance of equally or exceeding an event in given period (usu. 1 year)

• Frequency of occurrence is the inverse of “return period”

• Storm duration is any time interval in a storm - not necessarily including the beginning or end.

### What are IDF curves used for?

• To estimate runoff from “small watersheds”

• Small watersheds are small enough for rainfall to be uniform everywhere

• Design storm is assumed to be uniform in time and space

• “Time” = life of storm

• Runoff gradually rises, then stabilizes at maximum

• Time to stable maximum is time of concentration

• Time of concentration set equal to design storm duration

• Design duration + IDF curve gives design intensity

• Return period is set by policy

### How are IDF curves developed?

• Empirically (educated guess!)

• From analysis of rain gage data (the hard way)

• Empirical methods err considerably on “safe side”

• OK for small projects and small agencies

• Results in waste for large projects

• IDF curves based on Ven Te Chow’s “Handbook of Applied Hydrology”

### Caltrans IDF equations

• Developed by California Dept of Water Resources

• Published in DWR Bulletin 195, in 1976

• Concluded that the mean annual precipitation is a reasonable basis for estimating short duration rainfall

• Pearson Type III distribution is the best model precipitation frequencies for storm durations of 5 minutes to 30 days

### Caltrans IDF equations

• Best rain gage data would be 5-minute totals

• Coefficients of variation and skew are important

• DWR derived these from 24-hour data and assumed they applied to all smaller durations

• Station skews good only from 100+ years of record; weighted skews for 25 to 100-year records

• Less than 25 years: use regional values (per Interagency Advisory Comm. On Water, 1982)

### Problems using Bulletin 195

• Maps of statistical values and gage locations, and tables of station data provided

• Hard to define project location accurately

• Hard to determine exact distance nearest gage

• Acceptable distance to gage = 3 miles

• Only 600 stations available statewide

• If no station nearby, estimated values on maps may be used

### Problems using software

• Software developed to compute distances

• Stations increased to 800, but still too few

• 1 station for all of Solano county!

• Interpolation between stations unsuccessful because of questions about “hydrological compatibility” and relative influence of stations

### Proposed solutions

• Number of stations must increase

• Groups of hydrologically compatible stations must be identified, and interpolation methods agreed to

• Estimates for areas with no stations must be agreed upon

### Proposed solutions

• Caltrans working with Jim Goodridge to try to use 4,000 non-recording gages for short duration work

• Caltrans to utilize PRISM mean annual precipitation maps to solve problems of “hydrological compatibility” between stations and estimate rainfall in areas with no gages

### Caltrans map-based IDF system

• Statistical parameters, gage locations to be mapped in Arcview, along with PRISM map

• Statistical maps to be copies of Bulletin 195 maps

• Revision of statistical parameters and update of maps to be subject of another project

• User clicks location, or enters coordinates and return period, and gets back IDF coefficients - all in Arcview

### Caltrans map-based IDF system

• For PCs without Arcview, Arcview maps to be gridded at 4 km intervals. Grid points to be dummy stations.

• Dummy stations and corresponding data to be accessed by computer program

• Caltrans hopes to make system available on the internet

### What does project hope to achieve?

• Enlarged rain gage database (from 800 to 4000 stations)

• Elimination of the need for user interpolation by creating dummy stations within 2 km of any point

• Credible precipitation estimation at high altitudes and in areas with no gages

• Reduced margin of error for IDF equations