dec, 2015 mndot bridge hydraulics · mndot bridge hydraulics ... distributions based on atlas 14...
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Dec, 2015
MnDOT Bridge Hydraulics
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A rainfall distribution can be developed directly from the Atlas 14 Rainfall Information
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Determine ratios of x hour / 24 hour rainfall.
Place the rainfall ratio for the shortest duration in the center of the distribution.
Symmetrically place each larger duration to include the shorter durations.
(From NRCS Presentation)
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24 Hour total = 6.67
3 Hour total = 4.09 ◦ 4.09/6.67 = 0.613
◦ Centered around Precipitation ratio of 0.5 and Time of 12, ordinates are
(10.5,0.193) and (13.5,0.806)
6 Hour total = 5.23 ◦ 5.23/6.67 = 0.784
◦ Ordinates are: (9,0.108) and (15,0.0.892)
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Example of a Nested Distribution
0
0.1
0.2
0.3
0.4
0.5
0.6
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0.8
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0 3 6 9 12 15 18 21 24
Pre
cip
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Hours
Atlas 14 24 hour- 100 year nested distribution
24 hour curve
3 Hour
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Example of a Nested Distribution
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 3 6 9 12 15 18 21 24
Pre
cip
itati
on R
ati
o
Hours
Atlas 14 24 hour- 100 year nested distribution
24 hour curve
3 Hour
6 Hour
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Old NRCS Type II distribution developed in 1960’s based on TP-40
NRCS is adopting NOAA Atlas 14 in the states where it is applicable
WinTR-20 has been updated to use Atlas 14 rainfall distribution
NRCS has developed new rainfall distributions, Minnesota NRCS office recommends using MSE-3 distribution
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National NRCS developed typical storm distributions based on Atlas 14 data
Developed for
Midwest and
Southeast US
(Atlas 14 volumes
7 and 8)
MSE6
MSE5
MSE4
MSE3
MSE2
MSE1
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Recommend using Atlas 14 derived rainfall distribution or MSE-3 when using NRCS method
See MnDOT Tech Memo 15-10-B-02
See Minnesota NRCS Web Site http://www.nrcs.usda.gov/wps/portal/nrcs/detail/mn/technical/?cid=nrcs142p2_023722
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HydroCAD 10 will import PFDS data and can create rainfall distributions from the Atlas 14 data
Good Help Information on HydroCAD Web Site http://hydrocad.net/rainfall/pfd.htm ◦ Help Sheet
◦ Webinar on Rainfall