NOAA Atlas 14 Rainfall Depths, NRCS Rainfall Distributions .
NOAA Atlas 14 Rainfall Depths, NRCSRainfall Distributions, and DimensionlessUnit HydrographsAMANDA SMITH,P. E .NRCS STATE HYDRAULIC ENGINEER
ObjectivesHistoryNOAA 14 Rainfall DepthsNRCS Rainfall DistributionsDimensionless Unit HydrographsNRCS Hydrology Policy
HistoryU.S. Weather Bureau Technical Paper 40 Initial publication in 1961Rainfall durations: 30 min to 24 hourRainfall frequency: 1 year to 100 year eventsUniversally used and widely acceptedDeveloped using available rainfall information from much fewer stationsthan what exists today
History
History
NOAA Atlas 14There was concern within the state that TP-40 was not representative ofthe precipitation of todayWorked through a FHWA pooled fund project to coordinate with 11other states to fund the regional study completed by NOAAWithin Minnesota the project was funded by: MPCA MnDOT City State Aid
NOAA Atlas 14
NOAA Atlas 14Volume 8 of NOAA Atlas 14 Precipitation-Frequency Atlas of the UnitedStates was released in 2013405 Data sets for MN were used in the analysis (vs. 140 in TP-40)Average record length now over 50 years (more than double the recordused in TP-40)Rainfall durations: 5 min to 60 daysRainfall frequency: 1 year to 1000 year events
TP-40 Daily StationsNOAA Atlas 14 Daily Stations
NOAA Atlas 14 Rainfall DepthsGIS was used to tabulate county wide average 24-hour rainfall depths byfrequency for each county in MNCountyAITKINANOKABECKERBELTRAMIBENTONBIG STONEBLUE RWATERCOOKCOTTONWOODCROW WINGDAKOTARainfallTypeMSE 3 MNMSE 3 MNMSE 3 MNMSE 3 MNMSE 3 MNMSE 3 MNMSE 3 MNMSE 3 MNMSE 3 MNMSE 3 MNMSE 3 MNMSE 3 MNMSE 3 MNMSE 3 MNMSE 3 MNMSE 3 MNMSE 3 MNMSE 3 MNMSE 3 .566.636.496.535.577.046.257.41
NOAA Atlas 14 Rainfall DepthsSome surprises with large changes over short distances: Difference in 100-yr, 24-hr rainfall depths: Minneapolis, MN to St. Cloud, MN : 7.9” to 6.1” 1.8” difference Duluth Airport to Duluth Harbor: 6.65” to 6.0” 0.65” difference
TP-40 vs. NOAA Atlas 14 Rainfall DepthsSome rainfall depths increased, some did not change, and somedecreased. Depends on: Frequency Duration Location
TP-40 vs. NOAA Atlas14 Rainfall DepthsTP-4010-year/24 hour Itasca County: 3.67” Pine County: 3.95” Isanti County: 4.05”100-year/24 hour Itasca County: 5.25” Pine County: 5.5” Isanti County: 5.75”NOAA 1410-year/24 hour Itasca County: 3.79” Pine County: 4.06” Isanti County: 4.13”100-year/24 hour Itasca County: 6.05” Pine County: 6.49” Isanti County: 6.68”
TP-40 vs. NOAA Atlas 14 Rainfall DepthsTP-40 100 YR, 24 HRNOAA 14 100 YR, 24 HR
What is a Rainfall Distribution?Rainfall distributions are syntheticarrangements of a total rainfall amount overtime such that it includes maximum rainfallduration intensities for the selected designfrequency that is critical for producing peakrunoff.
NOAA vs. NRCS Rainfall DistributionsNOAA Rainfall Distributions – Statistical Analysis of Individual Storms,Huff Distributions, etc.NRCS Rainfall Distributions – Used for designs. Example: A 10-yr, 24 hrrainfall design storm needs to contain: 10-yr – 5 min intensity10-yr – 30 min intensity10-yr – 1 hr intensity10-yr – 2 hr intensityEtc.Etc.10-yr – 24 hr intensity
NRCS Rainfall DistributionsThe NRCS Water Quality and Quantity Team developed temporal stormdistribution regions for the Midwest and Southeast US (NOAA Atlas 14volumes 7 and 8)Distributions were based on ratios of the NOAA Atlas 14(25-yr, 1-hr)/(25-yr, 24-hr) precipitation depthsSince these were developed for the Midwest and Southeast US, thedistribution regions were titled MSE (MSE 1 through MSE 6)
NRCS Rainfall DistributionsMSE6MSE5MSE4MSE3MSE2MSE1
NRCS Rainfall Distributions
NRCSRainfallDistributionsMinnesota NRCS will useMSE 3 for the entire state
4 hours70% of the storm rainfall
Dimensionless Unit HydrographWhat is a Dimensionless Unit Hydrograph?From NEH, Part 630, Chapter 16 - a dimensionless unit hydrograph is ahydrograph developed to represent several unit hydrographs; plottedusing the ratio of the basic units time to peak and peak rate.What is a Unit Hydrograph?Discharge hydrograph resulting from 1 inch of direct runoff distributeduniformly over the watershed resulting from a rainfall of a specifiedduration.
SCS Dimensionless Unit HydrographWhat is SCS Dimensionless Unit Hydrograph?Derived from many natural unit hydrographs from watershed varyingwidely in size and geographical locations.
Peak Rate FactorWhat is Peak Rate Factor?Essentially controls the volume of water on the rising and recessionlimbs of a dimensionless unit hydrograph.The higher the peak rate factor the higher the peak discharge will befrom the watershed.The standard SCS dimensionless unit hydrograph has a peak rate factorof 484.According to NEH 4, Part 630, Chapter 16, peak rate factors can rangefrom below 100 to more than 600.
Tr time from the peak to the end of the triangular hydrographTp time from the beginning of the triangular hydrograph to its peakTc time of concentrationqp peak discharge
Peak Rate Factor 100Peak Rate Factor 600
MN NRCS Hydrology PolicyUse NOAA Atlas 14 Rainfall Depths.Use NRCS Rainfall Distribution MSE 3 for the entire state.Use a dimensionless unit hydrograph with a peak rate factor of 400.
MN NRCS Hydrology PolicyMN NRCS Homepage – Topics – Technical Resources – Engineering –Engineering Resources – Engineering Field Handbook (EFH) MinnesotaSupplements – Chapter n/technical/engineering/?cid nrcs142p2 023722Google: MN NRCS EFH
How will this impact design?GRASSED WATERWAYCass CountyBackground:244 Acre Watershed, RCN 72, Watershed Slope 2.4%,Watershed Length 5600’Design:Initial Design Rainfall (10 yr-24 hr) 3.73” (TP-40 – Type II)Peak Q 83 cfsRevised Design Rainfall 3.84” (NOAA 14 – MSE 3 MN)Peak Q 83 cfs
How will this impact design?GRADE STABILIZATION STRUCTUREBenton CountyBackground:264 Acre Watershed, RCN 68, Watershed Slope 2.8%,Watershed Length 5900’Design:Initial Design Rainfall (50 yr-24 hr) 5.13” (TP-40 – Type II)Peak Q 139.5 cfsRevised Design Rainfall 5.53” (NOAA 14 – MSE 3 MN)Peak Q 151 cfs
Questions?
Amanda Smith(651) 602-7884amanda.smith2@mn.usda.govAn Equal Opportunity Provider and Employer
NOAA Atlas 14 Rainfall Depths. Some surprises with large changes over short distances: Difference in 100-yr, 24-hr rainfall depths: Minneapolis, MN to St. Cloud, MN : 7.9” to 6.1” 1.8” difference Duluth A
24-HOUR RAINFALL DEPTH DATA FOR VIRGINIA. Table of Contents . TABLES . Table 11-B.1. NRCS Implementation of NOAA’s ATLAS 14 Rainfall Data . for Virginia 11-B-2 . FIGURES . Figure 11-B-1. NRCS Rainfall Zone Map for Albermarle County, Virginia 11-B-6 . Figure 11-B.2. NRCS Rainfall Zone
RAINFALL FREQUENCY ATLAS OF THE MIDWEST by Floyd A. Huff and James R. Angel Title: Rainfall Frequency Atlas of the Midwest. Abstract: This report presents the results and methodology of an intense study of rainfall frequency relationships throughout the Midwest (Illinois, Indiana, Iowa, Kentucky, Michigan, Minnesota, Missouri, Ohio, and Wisconsin).
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