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”

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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)


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


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