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Basic Hydrology – Precipitation and Rainfall Distributions By: Paul Schiariti, P.E., CPESC Mercer County Soil Conservation District
Basic Hydrology –Precipitation and Rainfall
DistributionsBy: Paul Schiariti, P.E., CPESC
Mercer County Soil Conservation District
SCS Storm Return Periods• For example, if a flood has a 20 percent chance of being
equaled or exceeded each year, over a long period of time the flood will be equaled or exceeded on an average of once every five years.
• This is called the return period or recurrence interval (RI). • The Storm Return Period is the inverse probability of the
storm event recurrence interval being equaled or exceeded in any given year.
• For instance a 25 year storm has a 4 % chance of being equaled or exceeded in any given year.
• The probability can be computed by dividing 100 by the return period as follows:
100 = 4 %25
SCS Storm Return Periods (Continued)
• Therefore as the precipitation depths get higher, the chance of occurrence gets lower.
• A 1 year storm event has a 100% chance of being equaled or exceeded in any given year:
100 = 100 %1
• A 100 Year Storm event has a 1 % chance of being equaled or exceeded in any given year:
100 = 1 %100
• It really makes sense. The higher the precipitation depths the less frequently storms of that magnitude actually occur.
NRCS Rainfall Distributions
• Rainfall Distribution is the variability of the intensity throughout a storm….. although the overall depth for a storm will be the same for a given duration no matter which Distribution is chosen.
• There are four (4) different types of rainfall distributions throughout the U.S. – Type I, Type 1A, Type II and Type III
NRCS Rainfall Distributions
The entire State of New Jersey is located within the Type III Rainfall Distribution.
New Jersey is located within the Type III rainfall distribution
boundary
Comparison of the different rainfall distributions
Note that approximately 50% of the rainfall occurs between the 11th and 13th hour. Ever notice that the peak flow rate from a TR-55 generated runoff hydrograph
occurs at around the 12th hour?
5.00 Inch Precipitation Distribution
SCS Type III Rainfall Distribution
0%10%20%30%40%50%60%70%80%90%
100%
0.00
1.20
2.40
3.60
4.80
6.00
7.20
8.40
9.60
10.8
0
12.0
013
.20
14.4
0
15.6
016
.80
18.0
0
19.2
0
20.4
0
21.6
0
22.8
024
.00
Time in Hours
Cum
mul
ativ
e %
of R
ainf
al
5.00 Inch Precipitation Distribution
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
0.00
1.20
2.40
3.60
4.80
6.00
7.20
8.40
9.60
10.8
0
12.0
0
13.2
0
14.4
0
15.6
0
16.8
0
18.0
0
19.2
0
20.4
0
21.6
0
22.8
0
24.0
0
Time in Hours
Prec
ipita
tion
in In
chesApproximately 2.50 Inches or
50% of the 5.00 Inch Rainfall falls within a 2 hour period from the 11th to the 13th hour.
SCS Type 3 Distribution Cumulative Rainfall Table0.3 hour time interval
1.00000.99800.99500.99200.98900.98600.98300.97900.97600.97300.96900.96600.96200.95800.95500.95100.94600.94200.93800.93300.92900.92400.91900.91300.90800.90000.89200.88300.87400.86400.85300.84200.82900.81600.80100.78500.76600.74300.71400.67300.50000.32800.28700.25800.23500.21600.19900.18500.17100.15900.14800.13700.12800.11800.10900.10000.09300.08700.08200.07700.07200.06700.06300.05800.05400.05000.04600.04200.03800.03500.03100.02800.02400.02100.01800.01500.01200.00900.00600.00300.0000
Approximately 50 %of the total rainfalloccurs within a 1.8hour time increment
From hour 11.1 to 12.9
The Majority of the Precipitation Falls between hours 11.00 and 13.00
Incremetal Change in Precipitation vs. Time forType III Distribution and 5.00 Inches Precipitation
0.00
0.100.20
0.300.40
0.50
0.600.70
0.800.90
1.00
0.0
1.5
3.0
4.5
6.0
7.5
9.0
10.5
12.0
13.5
15.0
16.5
18.0
19.5
21.0
22.5
24.0
Time in Hours
Prec
ipita
tion
in In
ches
IncremetalChange inPrecipitation
Precipitation Rates as shown on Isopluvial Maps
These maps are contained within Appendix B of the Tr-55 Manual for Storm Frequencies of 2, 5, 10, 25, 50 and 100 year return periods.
Comparison of Previous vs. Newly Adopted 24 Hour Design Storm Rainfall Depths
The values contained within this table correspond to the “contours” on the Isopluvial Maps shown on the previous slide.
Comparison of Runoff Hydrographs between the different Rainfall Distributions
Lets examine an example:Drainage area = 25.00 AcresRunoff Curve Number = 74
Time of Concentration = 0.75 Hours
QPeak Type I = 20.03 cfs @ 10.32 HoursQPeak Type IA = 9.84 cfs @ 8.33 Hours
QPeak Type II = 38.07 cfs @ 12.36 HoursQPeak Type III = 31.36 cfs @ 12.55 Hours
The range of the peak rates goes from 9.84 cfs 38.07 cfs or 28.23 cfs. The range in the time to peak goes from 8.33
Hours to 12.55 Hours or 4.22 Hours
Runoff Hydrographs Generated from WinTr-55 Software for Each Distribution
Comparison of Hydrographs generated from different Rainfall Distributions
Effects of Rainfall Distribution on Peak Flow Rates
0
5
10
15
20
25
30
35
40
5.16 6.2
7.24
8.28
9.33
10.4
11.4
12.5
13.5
14.5
15.6
16.6
17.7
18.7
19.7
20.8
21.8
22.9
23.9 25
Time in Hours
Flow
Rat
e in
cfs
Type IIIType IIType IAType I
I-D-F Curves for use with the Rational and Modified Rational Method
Rainfall Intensity is Obtained by:
1. Entering the “Duration of Storm in Minutes” with the computed “Time of Concentration”.
2. Intersecting the Frequency of Recurrence Curve.
3. Drawing a horizontal line to the Rainfall Intensity.
Example: Find the Rainfall Intensity in Inches per Hour for a TC of 25 min. and a 10
Year Storm Frequency.
Answer: 3.6 In/Hr
