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.

Inte

nsit

y (i

n/hr

)

Duration (minutes)0 5 10 15 20 25 30 35 40

1.0

2.0

3.0

4.0

5.0

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)

To simplify computations, DWR developed maps of regional coefficients of skew and variation

(See maps in handout)

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

PRISM precipitation map

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