1. IDF Curve Revisions Using New Regional Rainfall Data
National Hydraulic Engineering Conference
August 9-12, 2016
Rippling Water
(Basic)
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presented by
David Baker, PE

2. IDF Curve Revisions IDF Curve Data

3. IDF Curve Revisions National Oceanic and Atmospheric Administration
NOAA Atlas 2
NOAA Atlas 14 update
Remaining states (not scheduled)

4. IDF Curve Revisions Our own private Idaho
here
Ada County

5. IDF Curve Revisions IDF Curve Development in Idaho
Idaho – NOAA Atlas 2, Volume 5
published 1973
data ended in 1970
that was 46 years ago!
IDF Curves from NOAA Atlas 2
Isopluvials for 6 and 24 hour storms
Technical Paper 40 for shorter durations

6. IDF Curve Revisions NOAA TECHNICAL PAPER 40
TP-40
published 1961
methodology for short durations

7. IDF Curve Revisions Ada County Highway District
Ada County Highway District (ACHD)
Fun Facts:
Est. 1972
Boise, Meridian, Kuna, Eagle, Star, Garden City
2,100 miles of urban streets and rural roads
Infiltration is primary stormwater outfall method
Ponds
Seepage Beds

8. IDF Curve Revisions Ada County Highway District
IDF curve use
Stormwater conveyance design
Detention facility design
Retention facility design

9. IDF Curve Revisions ACHD Policy
Policy changes
0.6 in. retention (24 hr event) - NPDES
Infiltration rates F.S. 1 to 2
Use of GSI alternatives

10. ACHD IDF Curve Development Green Stormwater Infrastructure

11. ACHD IDF Curve Development Green Stormwater Infrastructure

12. ACHD IDF Curve Development Policy Changes
Design Storm changes
Conveyance – 100-yr to 25-yr
Retention/Detention – 100-yr to 25-yr (with emergency conveyance)
Existing facilities appear to be oversized
Update IDF Curves

13. IDF Curve Revisions When it rains…
Available Data
9 Stations
Which are best?
Eliminated 3
Record Length
Location
Applicability
L moments test

14. ACHD IDF Curve Development And then there were 6
Final Station Selections
National Climatic Data Center (NCDC) Global Database FTP site ftp://ftp.ncdc.noaa.gov/pub/data/ghcn/d aily/all/

15. IDF Curve Revisions Number Crunchin’
Daily precipitation data from 6 stations
Screen data
Partition data into homogenous sub-regions
Fit probability distributions
Software – L-moments package (Hosking 1996)

16. IDF Curve Revisions Old vs. New
Precipitation comparison
23% to 30% difference
Why?
no significant trends in data
Largest observed event 0.9 in. less than Atlas 2
data sets – fewer/regional
statistical models

17. IDF Curve Revisions More Old vs. New
Full 6 station comparison
15 to 24% difference

18. IDF Curve Revisions But what if?
Facilities have a design life of 25 years +/-
Predict IDF curve for design year
Climate change models
based on projected change in global greenhouse gas (GHG) emissions
published emissions scenarios

19. IDF Curve Revisions Back to the Future
Climate change prediction software: SIMCLIM (CLIMsystems)
L-moments results utilized
Design Year 2050
3.6 to 10% intensity reduction

20. IDF Curve Revisions IDF Curve Option
ACHD selected Option 1
Acceptance of climate change model

21. IDF Curve Revisions Anticipated Savings
Infrastructure Costs
right-of-way
13 to 38% reduction in footprint
construction
maintenance

22. IDF Curve Revisions Questions and Comments
David Baker, PE
322 E. Front Street
Suite 200
Boise, ID 83702
(office)
(cell)