Transcript Statewide Map-based IDF Analysis

Statewide Map-based IDF
Analysis
Norman Gonsalves
for
Caltrans
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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.
Intensity (in/hr)
What are intensity-duration-frequency curves?
5.0
4.0
3.0
10
0-y
r
25
-yr
10
-yr
2.0
1.0
0
5
10
15
20
25
30
35
40
Duration (minutes)
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?
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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
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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