Week7 rainfall-runoff PDF

Preview

Summary

Week7 rainfall-runoff...

Description

24/04/2021

ENGR3851/8951 Hydraulics and Water Engineering

FLOOD ESTIMATION PART I

Thanks to: Cristina Solorzano-Rivas [email protected]

CONTENT  Design rainfall1  Critical rainfall duration1  Effective rainfall2

College of Science and Engineering

24/04/2021

What is a flood?

3

Australian flood definition (insurance): The covering of normally dry land by water that has escaped or been released from the normal confines of: any lake, or any river, creek or other natural watercourse, whether or not altered or modified; or any reservoir, canal, or dam. The most costly natural disaster in Australia ($377 mil/a) & USA.

4

24/04/2021

Importance of flood estimation: o Urban and rural structures (culverts, bridges, etc.) o Dam design o Town planning and land zonation o Emergency action plans

5

What makes a flood?

Causes: o Heavy rainfall o Snow and ice-melt o Blockages of streams or o Storm surge (low pressur high tide, sea-level rise) o Dam failure or landslide

6

24/04/2021

Flood estimation: Rainfall – runoff models

System Catchment (“black box”) Runoff production

Runoff production (Loss model, rainfall excess)

Hydraulic model 7

Flood estimation http://arr.ga.gov.au/arr-guideline

Version 1 – 1958 Version 2 – 1977 Version 3 – 1987 Current version – 2019

8

24/04/2021

Flood estimation: Models 1)

Source: ARR, 2019

9

Flood estimation: Models

Flood frequency analysis: o We’re interested in peak water levels/flows, o And recurrence intervals, o And the annual likelihood of particular events.

10

24/04/2021

Flood estimation: Models Event-Based Simulation

1)

2)

3)

Source: ARR, 2019

11

Flood estimation: Models Event-Based Simulation Design rainfall

Design rainfall (input)

Hydrograph formation output Losses 12

24/04/2021

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

Event-Based Simulation  Design rainfall estimation  One of the first steps in flood estimation analysis is the determination of the design rainfall  The most common approach to determine the design rainfall is based on the intensity-frequency-duration (IFD) analysis



IFD analysis provides a value for total rainfall (of an event) occurring at a single point



Real rainfalls show a temporal and a spatial distribution



Hyetographs represent the temporal distribution of the total rainfall during a storm

 (For designs that involve catastrophic economic damage or loss of life, the probable maximum precipitation (PMP) is generally used http://www.bom.gov.au/water/designRa infalls/pmp/index.shtml)

13

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

Design rainfall estimation  Intensity-Frequency-Duration (IFD) rainfall  IFD rainfall is derived from the statistical analysis of historical rainfall data to estimate how frequently rainfall events of a given duration and intensity occur at a single location

Source: BoM, 2019

14

24/04/2021

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

Design rainfall estimation  Intensity-Frequency-Duration (IFD) rainfall  The frequency terms used in ARR (2019):  EY, exceedances per year, generally used to describe Very Frequent rainfall events  AEP (%), annual exceedance probability, generally used to describe Frequent and Infrequent rainfalls  AEP (1 in x), is used for Rare design rainfalls  ARI, average recurrence interval, was generally used in previous versions of ARR. Currently, the use of EY, AEP (%) and AEP (1 in x) is recommended.

15

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

Design rainfall estimation  Intensity-Frequency-Duration (IFD) rainfall  The rainfall intensity decreases with duration for a given AEP  The rainfall intensity increases with decreasing AEP for a given duration

Source: BoM, 2019

16

24/04/2021

D (

Design rainfall estimation  Intensity-Frequency-Duration (IFD) rainfall  The design AEP depends on the consequences of exceedance  Standard engineering practices dictate the most appropriate rainfall frequency for a given situation.

Source: BoM, 2019

Design rainfall (hyetograph) INPUT

Effective rainfall

17

Design hydrograph OUTPUT

Design rainfall estimation  Intensity-Frequency-Duration (IFD) rainfall  BoM online IFD calculator: http://www.bom.gov.au/water/desi gnRainfalls/revised-ifd/

Source: BoM, 2019

18

24/04/2021

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

Design rainfall estimation  Intensity-Frequency-Duration (IFD) rainfalls  From the IFD curves the rainfall intensity or the total rainfall can be obtained Total depth = Intensity x Duration point rainfall

Source: BoM, 2019

Design rainfall (hyetograph) INPUT

Effective rainfall

19

Design hydrograph OUTPUT

Design rainfall estimation  Intensity-Frequency-Duration (IFD) rainfalls Example # 1 Determine the design rainfall depth and intensity for the 5% AEP storm of 30-min duration for a catchment in the Adelaide area.

Step 1. http://www.bom.gov.au/water/designR ainfalls/revised-ifd/

Coordinates: -34.9285° S, 138.6007° E

20

24/04/2021

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

Design ra  Intensi Example Determin depth an AEP stor a catchm Coordina 138.6007

21

22

24/04/2021

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

Design rainfall estimation  Intensity-Frequency-Duration (IFD) rainfalls Example # 1

Answer:

Determine the design rainfall depth for the 5% AEP storm of 30-min duration for a catchment in the Adelaide area.

Rainfall total : 23.3 mm Duration : 30 minutes .   Rainfall intensity :   46.6 ⁄  



Coordinates: -34.9285° S, 138.6007° E

23

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

Design rainfall estimation  Areal reduction factors  IFD values represent point rainfalls (rainfall design at a single point)  They are based on rainfall that occurs at rain gauges that sample a much smaller area than the analysed catchment area  The design rainfall to be applied in flood calculations is the average rainfall over the sub-catchment to the point of interest.  The average rainfall for a catchment is usually smaller than the point rainfall (Lower likelihood of a particular rainfall intensity over an area)

Source: BoM, 2019 24

24/04/2021

Design rainfall (hyetograph) INPUT

Design hydrograph OUTPUT

Effective rainfall

Design rainfall estimation Design rainfall to be applied in the flood estimation analysis

 Areal reduction factors Estimated according ARR (2019)

 The IDF rainfall point needs to be corrected in order to give the corresponding average rainfall over the studied catchment

 

  IDF design rainfall

 The correction is done by the use of the areal reduction factor (ARF), which relates the design rainfall depth for the studied catchment to the design depth at a point

  ARF x 

  Intensity

25

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

Design rainfall estimation  Areal reduction factors  In Australia, ARF depends on the catchment area and the rainfall duration.  For durations larger than 12 hours, the ARF estimation depends on the region classification

Source: ARR, 2019 26

24/04/2021

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

Design rainfall estimation  Areal reduction factors  ARF values for Australia are a function of the total area of the catchment, the duration of the design rainfall and its AEP Catchment area

Duration

ARF

≤ 1 km2

any

1

Between 1 and 10 km2

 ≤ 12 hours ARR equations  Between 12 and 24 hours  Between 24 hours and 7 days

Between 10 and 1000 km2

ARR equations  ≤ 12 hours  Between 12 and 24 hours  Between 24 hours and 7 days

Between 1000 and 30,000 km2

 Between 12 and 24 hours ARR equations  Between 24 hours and 7 days 27

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

Design rainfall estimation  Areal reduction factors For example the procedure to estimate ARF for catchment areas between 1 and 1000 km2 and duration ≤ 12 hours is: 1) AFR for an area of 10 km2 A10

3) Between 10 and 1000 km2:

A10 A10

A A

2) Between 1 and 10 km2

A

A10 = 10 km2 A = catchment area of interest in km2 Source: ARR, 2019

D = rainfall duration in minutes

28

24/04/2021

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

Design rainfall estimation  Areal reduction factors

6-hr

1-hr

30-min

29

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

Design rainfall estimation  Areal reduction factors Example # 2 Determine the design rainfall depth of Example # 1 (5% AEP storm of 30min duration) for a 70 km2 catchment in the Adelaide area.

6-hr

Answer: 1-hr

From the graph: ARF = 0.795

30-min

From Example # 1: Rainfall total : 23.3 mm x 0.795 = 18.5 mm Rainfall intensity :46.6 ⁄ x 0.795 = 37 ⁄ 30

24/04/2021

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

Design rainfall estimation  Rainfall temporal distribution  Temporal patterns describe how rainfall falls over time as a design input  Percentages of the total rainfall are defined in time-steps  Multiplying the percentages by the total rainfall produces a hyetograph in units of mm of rainfall falling in each time step.

Percentage of total rainfall

Example of a temporal distribution of a frequent storm (13.8% AEP), 20 min duration in Adelaide 35% 30% 25% 20% 15% 10% 5% 0% Temporal Distribution

0-5 min 17.34%

5-10 min 28.99%

10-15 min 30.43%

15-20 min 23.19%

 Dividing by the time-step duration gives hyetograph values in mm/time. 31

Design rainfall (hyetograph) INPUT

Design rainfall estimation

Percentage of total rainfall

Example of a front loaded storm – 0 to 40%

40% 30% 20% 10% 0%

0-5 min

Temporal Distribution 7.24%

0-5 min

Temporal Distribution 28.92%

5-10 min 23.14%

10-15 min 14.05%

15-20 min 0.83%

20-25 min 17.36%

25-30 min 15.70%

5-10 min 33.07%

10-15 min 21.18%

15-20 min 9.48%

20-25 min 7.54%

25-30 min 21.49%

Example of a back loaded storm – 60 to 100% Percentage of total rainfall

30% 25% 20% 15% 10% 5% 0%

Design hydrograph OUTPUT

Example of a middle loaded storm – 40 to 60%

 Rainfall temporal distribution

Percentage of total rainfall

Effective rainfall

40% 30% 20% 10% 0%

0-5 min

Temporal Distribution 17.34%

5-10 min 10.67%

10-15 min 1.33%

15-20 min 13.33%

20-25 min 33.33%

25-30 min 24.00%

24/04/2021

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

Design rainfall estimation  Rainfall temporal distribution  Rainfall temporal patterns are necessary for full hydrograph estimation, especially when the volume of flood is required  The temporal distribution is a significant factor in estimating a peak discharge and the hydrograph shape Source: University of Southern Queensland, 2017

33

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

Design rainfall estimation  Rainfall temporal distribution  ARR has grouped the rainfall temporal patterns across Australia into 12 regions with similar rainfall characteristics  Point temporal patterns should be used for catchment less than 75 km2. For larger catchments, areal temporal patterns should be used Source: ARR, 2019

34

24/04/2021

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

Design rainfall estimation  Point temporal patterns (< 75 km2)  Patterns are classified by AEP “bins”:    

Frequent Intermediate Rare Very Rare

 Each region includes 10 temporal patterns per AEP bin and duration

Source: ARR, 2019

35

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

Design rainfall estimation  Temporal pattern

36

24/04/2021

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

Design rainfall estimation  Areal temporal patterns (≥ 75 km2)  Areal temporal patterns are provided for 12 regions across Australia

Range of target catchment areas (km2)

Catchment area of designated areal temporal pattern set (km2)

75 – 140

100

140 – 300

200

300 – 700

500

 Areal temporal patterns are given for 9 catchment area ranges (as per the table)  Areal temporal patterns are independent of the AEP

700 – 1600

1000

1600 – 3500

2500

3500 – 7000

5000

7000 – 14,000

10,000

14,000 – 28,000

20,000

28,000 +

40,000 37

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

Design rainfall estimation  Rainfall temporal distribution: Using the ARR Data Hub  The ARR Data Hub provides the different rainfall temporal patterns for 12 Australian regions: http://arr.ga.gov.au/arr-guideline

38

24/04/2021

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

Design rainfall estimation  Rainfall temporal distribution: Using the ARR Data Hub Example #3

Step 1. http://data.arr-software.org/

Determine a design rainfall hyetograph for the rainfall depth of Example #2 (5% AEP storm of 30-min duration)

Source: ARR, 2019 39

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

40

24/04/2021

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT

41

Design rainfall (hyetograph) INPUT

Effective rainfall

Event-Based Simulation  Rainfall temporal distribution: Using the ARR Data Hub Example #3 Determine a design rainfall hyetograph for the rainfall depth and rainfall intensity of Example #2 (5% AEP storm of 30-min duration)

Design hydrograph OUTPUT

24/04/2021

Design rainfall (hyetograph) INPUT

Design hydrograph OUTPUT

Effective rainfall

Design rainfall estimation: Intensity  Rainfall temporal distribution: Using the ARR Data Hub Temporal distribution: Period

1st 5 min

%

2nd 5 min

3rd 5 min

4th 5 min

5th 5 min

6th 5 min

21.09

21.56

17.9

13.26

10.49

15.7

 Compute the amount of rainfall for each 5-min time step: Period Rainfall amount (mm)

1st 5 min 0.157  18.5mm = 2.90 mm

2nd 5 min

3rd 5 min

0.2109  18.5mm = 3.90 mm

4th 5 min

0.2156  18.5mm = 3.99 mm

5th 5 min

0.179  18.5mm = 3.31 mm

6th min

0.1326  18.5mm = 2.45 mm

0.1049  18.5mm = 1.94 mm

43

Design rainfall (hyetograph) INPUT

Design hydrograph OUTPUT

Effective rainfall

Design rainfall estimation: Intensity  Rainfall temporal distribution: Using the ARR Data Hub Temporal Distribution of 30 min storm of 5% AEP in Adelaide 4.00

Rainfall depth (mm)

3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00 Temporal Dist ribution (mm)

0-5 2.90

5-10 3.90

10-15 3. 99

15-20 3.31

20-25 2.45

25-30 1.94 44

24/04/2021

Design rainfall (hyetograph) INPUT

Design hydrograph OUTPUT

Effective rainfall

Design rainfall estimation: Intensity  Rainfall temporal distribution: Using the ARR Data Hub  Compute the rainfall rate (mm/hour) during each 5-min time step: Period

1st 5 min

2nd 5 min

3rd 5 min

4th 5 min

5th 5 min

6th min

Intensity (mm/hr)

(2.90/5)*60 =34.85

(3.90/5)*60 =46.82

(3.99/5)*60 =47.86

(3.31/5)*60 =39.74

(2.45/5)*60 =29.44

(1.94/5)*60 =23.29

45

Design rainfall (hyetograph) INPUT

Design hydrograph OUTPUT

Effective rainfall

Design rainfall estimation: Intensity  Rainfall temporal distribution: Using the ARR Data Hub Temporal Distribution of 30 min storm of 5% AEP in Adelaide 50.00 45.00

Intensity (mm/h)

40.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 Intensity (mm/h)

0-5 34.85

5-10 46.82

10-15 47.86

15-20 39.74

20-25 29.44

25-30 23.29 46

24/04/2021

Design rainfall (hyetograph) INPUT

Effective rainfall

Design hydrograph OUTPUT
...