IDF Curve Revisions Using New Regional Rainfall Data

IDF Curve Revisions Using New Regional Rainfall Data National Hydraulic Engineering Conference August 9 -12, 2016 presented by David Baker, PE

IDF Curve Revisions IDF Curve Data

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

IDF Curve Revisions Our own private Idaho • Idaho here Ada County

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

IDF Curve Revisions NOAA TECHNICAL PAPER 40 • TP-40 − published 1961 − methodology for short durations

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

IDF Curve Revisions Ada County Highway District • IDF curve use − Stormwater conveyance design − Detention facility design − Retention facility design

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

ACHD IDF Curve Development Green Stormwater Infrastructure

ACHD IDF Curve Development Green Stormwater Infrastructure

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

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

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/

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)

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

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

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

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

IDF Curve Revisions IDF Curve Option • ACHD selected Option 1 − Acceptance of climate change model

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

IDF Curve Revisions Questions and Comments David Baker, PE 322 E. Front Street Suite 200 Boise, ID 83702 208. 383. 6282 (office) 208. 340. 0351 (cell) david. baker@ch 2 m. com