Design storms

1
Design storms

• Manuel Gómez Valentín
• Universitat Politècnica de Catalunya
• E.T.S. Ing. de Caminos, Canales y Puertos

2
Rain data necessary for the study

• Information to level of hyetograph
• Minimal level of information a curve IDF/DDF
• In general, there is little suitable information
  for direct use in studies of urban hydrology

3
Level of usual information

• Information of rainfall in 24 hours
• Level of insufficient detail (problems in urban
  areas in a few hours or even minutes)
• Difficulty of disagregrating information
• To propose relationships between rains of
  duration D and those of duration 24 hours
• Problems due to that the information of rain in
  24 hours can be a consequence of 2 or more rain
  events

4
How to obtain a curve IDF

• Available in some observatories
• Obtained from the analysis of a historical time
  series of rain events (20/30 years)
• If it is not available
• Options to establish synthetic IDF (Spanish
  procedure in 5.2.IC regulation)

5
Synthetic IDF

• Instruction 5.2. I.C.
• Use of the maximum rainfall in 24 hours for the
  considered return period P24
• Regionalización of the rain from a certain
  factor, variable according to the geographical
  situation of the observatory

6
Expression of the synthetic IDF

• Expression proposed (studies of the CEH)
• Relation between duration and average maximum
  intensity

7
Maximum rainfall in 24 hours

• Information from the information provided in the
  meteorological zonal centers
• Utilization of a statistical extreme
  distributions (Gumbel, SQRT-ET)
• All of them give practically equal results for
  low return periods

8
Regional factor

• Ratio between the maximum hourly average
  intensity, and the daily average intensity

9
Design storms

• 1.- Historical rains or time series of
• rains generated from statistical methods
• 2.- Design stor, obtained from
• information included in the shape of curve
• Intensity - Duration - Frequency.

10
Rainfall patterns

• Defined the duration of rain, and known the
  quantity of rainfall associated for the return
  period T, it is necessary to to distribute the
  temporarily above mentioned quantity of rain
• To study guidelines of temporal distribution of
  rainfall that repeat themselves in different rain
  events
• Huff's distribution
• Distribution of the Soil Conservation Service

11
Huff's distribution
12
Distribution of accumulated rainfall depending on
time that fits to the first quartil, and for
different probability values
Probability
13
Rainfall patterns - Huff's temporal distribution
14
S.C.S's distribution.
The IIIrd
Acumulated rainfall
The IInd
IA
I
Time (hours)
15
Rainfall patterns of the SCS
16
Rainfall patterns summary

• Interesting approach, but with proper pattterns
  extracted in the local observatory
• Do not use patterns derived from different
  observatories reflex of meteorological
  situations that do not have anything to see with
  our zone of study
• Obtaining of local patterns from registers of
  rain events of few years (5-8)

17
Design storms from IDF curves

• 2.1. constant Rain or in block
• 2.2. triangular Storm
• 2.3. storm type Sifalda
• 2.4. Double Triangle rainfall
• 2.5. Keifer-Chu rainfall
• 2.6. method of the Alternated Blocks

18
Constant rain or in Block
19
Triangular rain
20
Sifalda rain type
21
Double triangle rainfall
I
P1 (10 years P.R.)
P2 (3 years P.R.)
30 mint
3 h.
D
4 h.
22
Keifer-Chu type
f (ta) f (tb) for any Td, then
Also
Differing
Defining
Replacing
23
Example Calculation with Keifer-Chu
r 0.50 Td 60 min c 4,477.44 e 1 f 19.031
24
Hietograma of the obtained rain
25
Alternating blocks method
I5 '
I10 '
I15 '
26
Calculation process
I5 ' I1

• Rain of the block 1
• I5 ' I1
• Rain of the block 2
• I10 x 10 ' I1 x 5 ' I2 x 5 '
• Rain of the block 3
• I15 x 15 ' I10 x 10 ' I3 x 5 ' I1 x 5 '
  I2 x 5 ' I3 x 5 '

I2I10 x 10- I1 x 5
27
Example Calculation with the Alternated Blocks
method
28
Hyetograph of Intensities
200
186.32
150
122.14
100
86.25
Intensity (mm/h)
64.15
49.58
39.47
50
32.16
26.72
22.54
19.28
16.67
14.56
0
1
2
3
4
5
6
7
8
9
10
11
12
Blocks of 5 minutes