Precipitation

Computing IDF curves1. Computation from rainfall records

2. Wenzel method

3. Chen method

Computation from rainfall records

1. Choose duration t and determine maximum rainfall depths for this duration in each year.

2. Rank the precipitation amounts from high to low

3. Compute the return period.

4. Repeat for the next duration.

5. Interpolate to the desired return period

6. Compute intensity = depth/duration.

m

nT

n

mxxp m

1

1)(

Similar to Lec. 4

ExampleGiven: 39 year record, top two depths (mm) are

listed.

Find: Intensity vs. duration for 30-year stormt (min)

Rank 10 20 30 40

1 20 30 34 36

2 18 25 29 32

ExampleGiven: 39 year record, top two depths (mm) are

listed.

Find: Intensity vs. duration for 30-year stormt (min)

Rank T (yrs) 10 20 30 40

1 40 20 30 34 36

30 19 27.5 31.5 34

2 20 18 25 29 32

Example

Wenzel method10-year storm computed for some major U.S. cities

City a b1 c1

Atlanta 64.1 8.16 0.76

Chicago 60.9 9.56 0.81

Cleveland 47.6 8.86 0.79

Denver 50.8 10.50 0.84

Helena 30.8 9.56 0.81

Houston 98.3 9.30 0.80

Los Angeles 10.9 1.15 0.51

Miami 79.9 7.24 0.73

New York 51.4 7.85 0.75

Olympia 6.3 0.60 0.40

Santa Fe 32.2 8.54 0.76

St. Louis 61.0 8.96 0.78

1)( 1cbt

ai

Example:

Compute 10-year, 1-hour storm for Chicago.

t in minutes, i = in/hour

1. Determine from the contour maps, where is the rainfall with return period T (years) and duration t (min).

2. Find a1, b1, and c1 from the nomograph.

3. Compute

Chen method1024

1001

101 ,, RRR and

TtR

TT

RRx

TxRaa

bt

ai

p

p

c

/11ln/1

/

110/log)1(

)(

101

1001

101011

11

t in minutes, i = in/hour

where

Chen method: exampleDetermine the intensity of the 1-hour storm with 10-year return period for Hammonton, NJ

1. Determine from the contour maps, where is the rainfall with return period T (years) and duration t (min).

2. Find a1, b1, and c1 from the nomograph.

3. Compute

Chen method: example1024

1001

101 ,, RRR and

TtR

TT

RRx

TxRaa

bt

ai

p

p

c

/11ln/1

/

110/log)1(

)(

101

1001

101011

11

t in minutes, i = in/hour

where

Chen method: example

35.2101 R inches

Chen method: example3.510

24 R inches

Chen method: example4.3100

1 R inches

1. Determine from the contour maps, where is the rainfall with return period T (years) and duration t (min).

2. Find a1, b1, and c1 from the nomograph.

3. Compute

Chen method: example1024

1001

101 ,, RRR and

TtR

TT

RRx

TxRaa

bt

ai

p

p

c

/11ln/1

/

110/log)1(

)(

101

1001

101011

11

t in minutes, i = in/hour

where

Chen method: example

%44/ 1024

101 RR

b1 = 8.4

c1 = 0.77

a1 = 25.5a1 = 25.5

1. Determine from the contour maps, where is the rainfall with return period T (years) and duration t (min).

2. Find a1, b1, and c1 from the nomograph.

3. Compute

Chen method: example1024

1001

101 ,, RRR and

TtR

TT

RRx

TxRaa

bt

ai

p

p

c

/11ln/1

/

110/log)1(

)(

101

1001

101011

11

t in minutes, i = in/hour

where

Chen method: example

in/h