TOPIC 3B: RAINFALL MEASUREMENT AND DESIGN RAINFALL · PDF fileUnit CIV2262: Waterway...

Unit CIV2262: Waterway Engineering Topic 3B: Rainfall Measurement and Design Rainfall Estimation

TOPIC 3B: RAINFALL MEASUREMENT AND DESIGN RAINFALL ESTIMATION

1 Preview......................................................................................................................2

1.1 Introduction..........................................................................................2

1.2 Objectives ............................................................................................2

2 Rainfall data .............................................................................................................. 4 2.1 Rainfall Measurement ..........................................................................4

2.2 Historical data ......................................................................................7

2.2.1 Patched Point Data...............................................................................8 2.2.2 Data Drill .............................................................................................8 2.3 Rainfall statistics..................................................................................9

3 Estimating point and areal rainfalls from measured data.......................................12 3.1 Estimating point rainfall ....................................................................12

3.2 Estimating areal rainfall .....................................................................13

4 Rainfall temporal patterns.......................................................................................13 5 Design Rainfall .......................................................................................................14

5.1 Introduction........................................................................................14

5.2 Australian Rainfall and Runoff and its history ..................................14

5.3 Selection of design standards.............................................................15

5.4 Specification of Design Storms .........................................................16

6 Rainfall Intensity-Frequency-Duration (IFD) Analys ...........................................17 6.1 Introduction........................................................................................17

6.2 Australian Rainfall and Runoff procedure for calculating IFD curves18

6.3 Example: calculation of IFD curves for Boggy Creek at Angleside .20

7 Design Areal Rainfall .............................................................................................21 8 Design Temporal patterns.......................................................................................22 9 References ...............................................................................................................23 10 Key Concepts and Definitions................................................................................24 11 Review Questions ...................................................................................................24 12 Answers to Review Question .................................................................................25

Department of Civil Engineering, Monash University Edition Date: 9:03


1 PREVIEW

1.1 Introduction

Rainfall is a key part of the hydrologic cycle as it contributes to:

surface runoff and streamflow;

soil moisture storage and aquifer recharge; and

the amount of water available for drinking and to grow crops.

The amount of rain also contributes to the need for drainage, and high rainfalls are the major cause of floods in Australia.

There is currently a national system for the collection of rainfall data throughout Australia that is run by the Bureau of Meteorology, a Federal Government Department set up specifically for the collection and analysis of meteorological information.

Current rainfall observations are available from the Bureau's web site and historical data can be ordered for particular rain gauges. Rainfall data can also be obtained at any location in Australia using products from the Bureau of Meteorology including Patch Point data set and the Data Drill. Satellite rainfall estimates can also be obtained for most areas in the world.

As well as current rainfall observations and historical rainfall data, there is the need for design rainfalls, that is, standard rainfall depths and temporal patterns that can be used in the analysis and design of drainage and flood protection systems. A statistical analysis of historical data was used to develop a procedure to provide estimates of design rainfall for storms:

of any duration from 6 minutes to 72 hours;

with average recurrence intervals from 1 year to 100 years; and

for any geographical location in Australia.

Procedures (both algebraic and graphical) are available to carry out this task by use of 9 parameters available from master charts provided in Volume 2 of Australian Rainfall and Runoff.

1.2 Objectives

1) To know how rainfall is measured.



2) To be familiar with sources of rainfall data.

3) To know how to estimate rainfall at points located away from rain gauges.

4) To know how to estimate rainfall depths over catchments based on measured data at rain gauges.

5) To be able to determine rainfall intensity for given recurrence intervals and durations for any location in Australia, using procedures from Australian Rainfall and Runoff.

6) To know about (but not how to use) design temporal patterns and areal reduction factors.

SUGGESTED

Shaw, E. M. Hydrology in practice. Chapman and Hall 2nd Edition Section 2.3 Precipitation Networks and Chapter 3, Precipitation Australian Rainfall and Runoff: 1998 or 2001 editions - Book II Sections 1 and 2; 1987 edition - Chapter 2 and Chapter 3

OPTIONAL

Any of the following provide a good introduction to precipitation Ward, R. C and Robinson, M. Principles of Hydrology McGraw-Hill Publishing Company. Chapter 2 Precipitation Viessman, W. and Lewis, G. L. Introduction to Hydrology, Prentice Hall, 5th Edition. Section 2.3 Hydrologic Measurements and Chapter 4 Precipitation Maidment, D. R. (1993) Handbook of Hydrology. McGraw-Hill. Chapter 3



2 RAINFALL DATA

2.1 Rainfall Measurement

The first rain gauges were used about 300 to 400 BC in India and there were other early developments in Korea around 1441. Modern rain gauges appeared in Europe in around 1660-1680 and there have been at least 250,000 precipitation gauges set up around the world. In Australia, rainfall information was first collected at the settlement of Sydney and reliable rainfall estimates for Melbourne date back to 1855.

The standard type of storage rain gauge is shown in Figure 1. A key feature is the funnel which magnifies the rainfall to allow more precise reporting. The scale on the measuring tube automatically adjusts for the diameter of the funnel and rainfall collected in both the overflow tube, and the measuring tube, can be measured. Standard rain gauges are usually read at 9 am each day and a daily total rainfall is reported. A daily rainfall bulletin is provided for Victorian stations by the Bureau of Meteorology.

There are strict rules about setting up rain gauges to ensure data is accurate. For example, the rain gauge must be twice as far from an obstruction such as a tree or house, as the height of the obstruction. The height of the water must be read at eye level.

It's easy to make sure the initial set up is correct, but trees grow and often rain gauges become obstructed, and readings less accurate over time.

• Figure 1 Rain gauge (source: http://ccc.atmos.colostate.edu/~hail/howto/help/pics/smgauge.jpg)


http://ccc.atmos.colostate.edu/~hail/howto/help/pics/smgauge.jpg

Unit CIV2262: Waterway Engineering Topic 3B: Rainfall Measurement and Design Rainfall Estimation A tipping-bucket rain gauge is used to collect data at a much finer time scale than the daily measurements provided by storage gauges ( ). When rain fills a bucket, which typically has an equivalent capacity of about 0.2 mm, it will tip up, creating an electrical impulse which is logged. Maximum rain rates of up to 625 mm/hour can be recorded.

Figure 2

• Figure 2 Tipping bucket rain gauge (source: www.environment-products.com/ waterlog-h-340-sdi/)

Recent advances in rainfall measurement include collection of data using ground-based or satellite based radar. Radar estimates of rainfall over Melbourne are available online and updated every 10 minutes.

Radar and tipping bucket rain gauges can provide a measure of rainfall intensity (rainfall per unit of time) as well as rainfall quantity.


http://www.environment-products.com/waterlog-h-340-sdi/

http://www.environment-products.com/waterlog-h-340-sdi/


• Figure 3 Weather radar shows rainfall during a thunderstorm over Sydney (Source Bureau of Meteorology http://mirror.bom.gov.au/weather/radar/)



Activity 3B.1. (20 minutes)

Rainfall can be continuously estimated from satellites and radar and is available on line. Check out the US Navy's estimates of rainfall for Australia: http://www.nrlmry.navy.mil/sat-bin/rain.cgi?GEO=aus Look at the radar rainfall estimates for Melbourne which are updated every 10 minutes: http://mirror.bom.gov.au/products/IDR023.shtml The information on the weather at Monash University is available in real time:

For more traditional data, check the Bureau of Meteorology's daily rainfall bulletin: http://www.bom.gov.au/cgi-bin/wrap_fwo.pl?IDV60003.html

http://www.arts.monash.edu.au/ges/research/climate/weather/realtime.html

2.2 Historical data

It is possible to obtain historical daily rainfall data, but traditionally it has been difficult and time consuming to process the information. Common problems include:

Rainfall not being recorded on the correct day or at the correct time;

The rain gauge not being read on weekends or while the operator was on holiday;

The rain catch changing over time because of the rain gauge being obstructed by trees and new buildings;

Missing months and years; and

Data not being available where it is needed.

The typical estimate was that it took about 1 day's work to clean up the data from one gauge. All this has recently be greatly improved by two products from the Bureau.


http://www.nrlmry.navy.mil/sat-bin/rain.cgi?GEO=aus

http://mirror.bom.gov.au/products/IDR023.shtml

http://www.bom.gov.au/cgi-bin/wrap_fwo.pl?IDV60003.html

Unit CIV2262: Waterway Engineering Topic 3B: Rainfall Measurement and Design Rainfall Estimation 2.2.1 Patched Point Data

The Patched Point dataset provides the daily rainfall data for 4650 meteorological stations maintained by the Bureau. Any gaps in the record have been filled by interpolation. Data is available form 1st January 1889 to the present at every site. The greatest accuracy is for data from 1957 to the present.

Use of the Patched Point dataset is appropriate when rainfall data is required near to a Bureau of Meteorology rainfall station.

2.2.2 Data Drill

The Data Drill is also a dataset of daily rainfall data but information is available at any location on the Australian continent. Information for rain gauges has been interpolated to provide data for a grid network across Australia about 5 km by 5 km.

Although the data drill dataset is a very useful tool, there are limitations. Data prior to 1957 is based on long term climate means because most of these old records have not been computerised. Data from 1957 until the year before the current year have been quality checked by the Bureau of Meteorology so results are expected to be reliable. Very recent data is less reliable as checking and reporting from all stations is still taking place.

The Data Drill dataset may not be reliable where:

There are few rainfall stations, such as in remote areas of Australia;

There are steep gradients of climate such as near the coast or in mountainous areas.


Read about the Patched Point and Data Drill datasets of daily rainfall on the internet http://www.nrm.qld.gov.au/silo/ppd/PPD_frameset.html

http://www.nrm.qld.gov.au/silo/datadrill/datadrill_frameset.html These services require payment but you can order a small dataset for free.


http://www.nrm.qld.gov.au/silo/ppd/PPD_frameset.html

http://www.nrm.qld.gov.au/silo/datadrill/datadrill_frameset.html

Unit CIV2262: Waterway Engineering Topic 3B: Rainfall Measurement and Design Rainfall Estimation 2.3 Rainfall statistics

Notable rainfall statistics are presented in for Victoria and the World. It is useful to be aware of the approximate size of these numbers so you can check if the answers of your calculations are reasonable. As would be expected, record rainfall depths increase over time as shown in Figure 4. What is perhaps less obvious, is that record rainfall intensity decreases with duration as shown in Figure 5. This is a common feature of rainfall intensity relationships at any location. Rainfall statistics for a few Australian cities are shown in Table 2.

Table 1

• Table 1- Rainfall records for Victoria and the world (adapted from WMO (1994).

World Victoria Duration Rainfall

(mm) Rainfall Intensity (mm/hr)

Rainfall (mm)

Rainfall Intensity (mm/hr)

1 min 38 2280

6 min 32 320

8 min 126 945

15 min 198 792 39 156

60 min (1 hour) 401 401 102 102

120 min (2 hour) 105 52.5

180 min (3 hour) 137 45.7

240 min (4 hour) 153 38.3

360 min (6 hour) 840 140 157 26.2

720 min (12 hour) 203 16.9

1440 min (1 day) 1,825 76 375 15.6

2880 min (2 days) 2,467 51.4 461 9.6

4320 min (3 days) 3,130 43.5 588 8.2

4 days 3,721 35.8

7 days 5,003 29.8

14 days 6,433 19.1

1 month 9,300 12.5

2 months 12,767 8.7


Unit CIV2262: Waterway Engineering Topic 3B: Rainfall Measurement and Design Rainfall Estimation 6 months 22,454 5.1

12 months 26,461 3.02

2 years 40,768 2.3

1

10

100

1000

10000

100000

0.01 0.1 1 10 100 1000 10000 100000Duration (hour)

Rai

n (m

m)

World Victoria

• Figure 4 Record rainfalls for Victoria and the World (adapted from WMO, 1994)



1

10

100

1000

10000

0.01 0.1 1 10 100 1000 10000 100000Duration (hour)

Inte

nsity

(mm

/hr)

World Victoria

• Figure 5 Rainfall intensity records for Victoria and the World (adapted from WMO 1994)

• Table 2 Selected rainfall statistics for Australian cities (source: Bureau of Meteorology http://www.bom.gov.au/climate/averages/)

City Average Annual Rainfall

(mm)

Average no. of rain days per year

Hobart Hobart Botanical Gardens

571 131

Melbourne BOM regional office

653 147

Sydney Airport

1102 129

Darwin Darwin Post Office

1535 93



3 ESTIMATING POINT AND AREAL RAINFALLS FROM MEASURED DATA

A rain gauge only samples a small amount of the total rain that falls. A rain gauge may have an area of about 10-4 m2 compared to catchment areas that will be 106 m2 or more (that is, we are sampling 1 part in 1010). Rainfall also varies spatially and temporally (Figure 3) and the rain gauge catch can be influenced by factors such as wind. In short, it pays be to cautious about using and extrapolating rain gauge data!

3.1 Estimating point rainfall

Often, as part of hydrologic analysis, rainfall totals are required at locations other than at the sites of rain gauges. Consider a location A (Figure 6), some distance from two rain gauges. The simplest recommended procedure is to estimate the rainfall at A by weighting the rainfalls R1 and R2 by the inverse of the square of the distance, d1 and d2, these gauges are from A (equation 1).

1 22 21 2

2 21 2

1 1

1 1A

R Rd dR

d d

+=

+ (1)

where: RA is the rainfall at A

R1 is the rainfall at gauge 1

R2 is the rainfall at gauge 2

d1 is the distance from A to gauge 1

d2 is the distance from A to gauge 2

Equation 1 can be extended to apply to any number of gauges and to take account of elevation as well as planar distances. It can also be used to estimate rainfall totals at gauges where there are missing data.



R1 Catchment boundary

d1 R2

d2

y

A

x

• Figure 6 Rainfall data from two gauges is to be used to estimate the rainfall at A.

3.2 Estimating areal rainfall

It is common to need an estimate of rainfall across a whole catchment from a network of rain gauges that provide point measurements. This is basically, the same task as discussed in 3.1 except that the rainfall must be estimated at every point in the catchment, not just at a single point of interest. The computational requirement can be simplified by dividing the catchment into small areas and calculating the rainfall for the centroid of each area.

4 RAINFALL TEMPORAL PATTERNS

Rain rarely falls at a constant intensity throughout a storm. Thunderstorms usually reach a high peak intensity very early in the storm and then tail off (see a). General or frontal storms are more variable in their temporal patterns but most often reach a moderate peak intensity just before the mid-point of the storm (see Figure 7b). The plot of rainfall against time is called a hyetograph. For a given depth of rainfall, an intense, but short duration storm will usually produce larger flood peaks than a longer, more uniform rainfall.

Figure 7



Time, hours

Mea

n In

tens

ityIn

tens

ity5

4

3

2

1

0 21Time, hours

Mea

n In

tens

ityIn

tens

ity

5

4

3

2

1

0 26 41812

• Figure 7 Typical storm temporal patterns, (a) Thunderstorm, (b) General Storm

5 DESIGN RAINFALL

5.1 Introduction

The intensity and duration of rainfall will determine the performance of schemes that provide drainage or protection against floods. Usually the procedure is to design a scheme to a chosen standard recognising that it will not work well, be damaged or fail, if larger events occur. The performance of schemes is analysed based on their response to design rainfall events; hypothetical events that are used to analyse some critical "loading" on a catchment, hydraulic structure or scheme.

Standard procedures for generating design rainfall events have been developed by the Institution of Engineers, Australia and are specified in Australian Rainfall and Runoff - the standard guide to professional practice for flood estimation in Australian hydrology.

5.2 Australian Rainfall and Runoff and its history

Infrastructure to accommodate and control floods is a significant cost to the population, both rural and urban. To provide guidance and set out “state of the art” methodology the Institution of Engineers, Australia has provided a lead in the field by the development and publication of Australian Rainfall and Runoff.

The 1st Edition was published in 1958 with two significant rewrites in 1977 (2nd Edition) and 1987 (3rd Edition). Each new addition has provided a significant improvement and increased information. In particular, the estimation of design rainfall has changed (improved) significantly in each edition. This in part has been due to the expanding data bank of rainfall records, but also represents improved computational and statistical analysis of the data and the methodology of presentation of the data for engineering design.


Unit CIV2262: Waterway Engineering Topic 3B: Rainfall Measurement and Design Rainfall Estimation A further revision of Australian Rainfall and Runoff has been released in 1998 which has reorganized the text into 8 separate “books” although most of the material has only had minor changes. This will allow individual “books” to be progressively updated as the need arises rather than attempting to revise the complete text. This process is already in progress with the section on estimation of extreme floods being revised.

5.3 Selection of design standards

The selection of appropriate design standards for hydraulic structures varies considerably and depends on cost of infrastructure and the consequences of failure. In general, the cost of infrastructure will increase as the design standard increases - designing for a larger flood means that pipes have to be bigger, levees higher, and spillways larger. Benefits will usually be larger as well since damage will occur less frequently. Initially, every dollar spent on increasing the standard of infrastructure provides more than one dollar’s worth of benefit in protection against flooding or drainage problems but, as the design standard increases, there are diminishing marginal returns and escalating costs ( ). The optimal standard is where the marginal costs and benefits are equal, that is, where the slope of the benefit and cost curves are equal.

Figure 8

• Figure 8 The optimal design standard is where marginal costs and benefits are equal.

Ben

efits

and

cos

ts ($

) Benefits

Costs

Optimal standard is where the slope of the curves are equal

A 1 in 1000 10 1 in 10 1 in 100

Design standard (AEP)

Although the calculation of a theoretical optimal design standard is possible, often damage costs are highly variable and uncertain, particularly if loss of life is possible. Reliance on standards which have become established over time as acceptable (to both engineers and the public) for different types of infrastructure are generally employed, particularly where the individual projects (components) are relatively minor so that


Unit CIV2262: Waterway Engineering Topic 3B: Rainfall Measurement and Design Rainfall Estimation detailed estimates of cost/benefit are not justified. Standards from previous surveys of Australian Practice are shown in Table 3.

• Table 3 Standards for design events

Infrastructure Typical AEP Range Spillways – major 1 in 10,000 to 1 in 106

Spillways - minor 1 in 100 to 1 in 1,000

Flood mitigation 1 in 100

Bridges/culverts - major 1 in 50 to 1 in 100

Bridges/culverts - minor 1 in 5 to 1 in 50

Rural, urban-residential 1 in 5 to 1 in 20

Urban-business 1 in 20 to 1 in 100

5.4 Specification of Design Storms

Design storms are used in the analysis and design of infrastructure to measure its performance against a chosen design standard.

To define a design storm, the following factors must be specified:

1) duration of storm rainfall;

2) intensity of rainfall;

3) temporal pattern of rainfall;

4) areal pattern of rainfall

Strictly, all four of these factors are random variables, which are partially correlated with each other. Theoretically, their conditional probability distributions could be combined but the task is impossibly complex. It is simplified by:

ignoring correlations between the factors and treating them as independent;

ignoring the variability of some of the factors and representing them by “design values” or “design patterns”.

Usually, a design storm is specified with factors (1) to (4) above determined as follows:



1) A range of storm durations is trialled to find the storm that is has the critical duration for the scheme being designed or analysed.

2) Rainfall intensities for frequencies (that correspond to the AEP of the design standard) are calculated for the range of durations using procedures specified in ARR (see section 6).

3) Temporal patterns of the trial design storms are found using procedures in ARR (see Section 8).

4) Areal pattern of rainfall is often assumed to be uniform or, for larger catchments, rainfall intensity may diminish with area.

6 RAINFALL INTENSITY-FREQUENCY-DURATION (IFD) ANALYS

6.1 Introduction

The aim of Intensity-Frequency-Duration analysis is to provide the rainfall Intensity data for design storms, of a particular Frequency (measured as AEP or ARI) for a range of Durations for use in flood estimation.

Rainfall intensity-duration curves have been introduced in section 2.3. The statistics associated with record rainfalls shows that, for Victoria, and the World, as the duration of rainfall increases, the intensity decreases ( ). Also note that the intensity and duration curves for Victoria, and the world are approximately parallel. The general form of IFD curves is similar ( ). The basic relationship between intensity and duration remains, but there is a family, of approximately parallel curves, that specify the influence of frequency (AEP or ARI).

Figure 5

Figure 9



Rai

nfal

l int

ensi

ty (m

m/h

r)

Larger events

AEP = 1 in 100

AEP = 1 in 2

Duration (time)

• Figure 9 - General form of an Intensity-Frequency-Duration curve

6.2 Australian Rainfall and Runoff procedure for calculating IFD curves

A procedure has been developed to calculate IFD curves and is based on 9 parameters that have been mapped for each location in Australia ( ). Once a location is specified, the required values can be read from the maps. An example of one of the maps from Australian Rainfall and Runoff, Volume 2, is shown in Figure 10.

Table 4

Once the 9 parameters have been read from maps, the IFD curves can be calculated using graphical or algebraic procedures specified in Australian Rainfall and Runoff. In practice, it is common to use software that is freely available, to complete this task.



• Table 4 - Parameters used in Australian Rainfall and Runoff to create IFD curves

Parameter Explanation Determined from ARR Volume 2

2i1 Rainfall intensity for ARI = 2 years and 1 hour duration Map 1 2i112 Rainfall intensity for ARI = 2 years and 12 hour duration Map 2 2i72 Rainfall intensity for ARI = 2 years and 72 hour duration Map 3 50i1 Rainfall intensity for ARI = 50 years and 1 hour duration Map 4 50i12 Rainfall intensity for ARI = 50 years and 12 hour duration Map 5 50i72 Rainfall intensity for ARI = 50 years and 72 hour duration Map 6 G Skewness Map 7 F2 Geographical Factor F2 for obtaining 6 minute duration

intensities, ARI = 2 years Map 8

F50 Geographical Factor F2 for obtaining 6 minute duration intensities, ARI = 50 years

Map 9

• Figure 10 - Design rainfall isopleths 72 hour duration, 2 year ARI. Map 3.8 from Australian Rainfall and Runoff Volume 2.


Unit CIV2262: Waterway Engineering Topic 3B: Rainfall Measurement and Design Rainfall Estimation 6.3 Example: calculation of IFD curves for Boggy Creek at

Angleside

Boggy Creek is a tributary of the King River in north east Victoria, south of Wangaratta. The location of the centroid of the catchment upstream of Angleside is at 36.75oS, 146.3oE. The 9 parameters have been estimated from the maps in Australian Rainfall and Runoff, Volume 2 as shown in Table 5. The IFD curves for Boggy Creek, calculated using the program AusIFD are shown in Figure 11.

• Table 5 - Design rainfall input parameters for Boggy Creek

Input Value Boggy Creek Latitude -36.75

Longitude 146.3 2i1 22.5

2i12 4.9 2i72 1.4 50i1 44.8 50i12 8.1 50i72 2.5

G 0.22

F2 4.3

F50 15.2



• Figure 11 Design rainfall frequency curves for Boggy Creek (AUSIFD output)

7 DESIGN AREAL RAINFALL

The design rainfall values derived by the procedures described above relate to point rainfalls. For all but the smallest catchments, the corresponding catchment design rainfalls are somewhat smaller, reflecting the fact that storm rainfall is unlikely to occur uniformly over the whole area of a large catchment. This can be allowed for by the use of an Areal Reduction Factor (ARF) that have the general form shown in Figure 12. Details of ARFs, for Australia, are provided in Grayson et al. 1996.



(Rai

nfal

l dep

th fo

r are

a)/(R

ainf

all d

epth

for a

poi

nt)

1.0

24 hr

2 hr

0 Area (km2)

• Figure 12 - General form of areal reduction factors for design rainfall

It should be noted that for the application of design rainfall intensities with the Rational Method (Topic 3E), areal reduction factors should not be applied.

8 DESIGN TEMPORAL PATTERNS

Design temporal patterns for various rainfall durations and various regions of Australia for use in design storm synthesis are given in ARR (1987), Ch. 3 or ARR (1998) Book II, Section 2. Australia is divided in 8 zones, with a different set of temporal patterns for each zone. For a design storm, the total amount of rain is determined from the IFD curve by choosing a particular intensity, frequency and duration. A design rainfall burst can then be created by dividing the rainfall up into a standard temporal pattern (provided in ARR Volume 2).



0

5

10

15

20

25

30

5 10 15 20 25 30 35 40 45Time (min)

Perc

enta

ge o

f rai

n in

tim

e pe

riod

• Figure 13 Design rainfall burst for zone 8, 45 min duration, ARI ≤ 30 years


Download the AusIFD program from the internet and explore IFD curves and temporal patterns for Australia's capital cities http://www.ens.gu.edu.au/eve/Research/AusIfd/AusIfdVer2.htm

9 REFERENCES

Bureau of Meteorology (2003) Bureau of Meteorology website http://www.bom.gov.au (accessed 22 August 2003).

Grayson, R. B., R. M. Argent, R. J. Nathan, T. A. McMahon, and R. G. Mein. (1996) Hydrological Recipes. Estimation Techniques in Australian Hydrology. Cooperative Research Centre for Catchment Hydrology.


http://www.ens.gu.edu.au/eve/Research/AusIfd/AusIfdVer2.htm

http://www.bom.gov.au/

Unit CIV2262: Waterway Engineering Topic 3B: Rainfall Measurement and Design Rainfall Estimation Linsley, Kohler & Paulhus, Hydrology for Engineers, Sections 3-11, 3-14, and 13-13, to 13-17.

Maidment, D. R. (1993) Handbook of Hydrology. McGraw-Hill.

New, M. Todd, M., Hulme, M. and Jones, P. (2001) Precipitation measurements and trends in the twentieth century. International Journal of Climatology 21: 1899-1922.

Shaw, E. M. (1988) Hydrology in practice. 2nd edition. Chapman and Hall.

Viessman, W. and Lewis, G. L. (2003) Introduction to Hydrology 5th edition. Prentice Hall.

Ward, R. C. and Robinson, M. (2000) Principles of Hydrology 4th Edition. McGraw-Hill Publishing Company, London.

WMO (World Meteorological Organisation) (1994) Guide to Hydrological Practises(1994), 5th edition, WMO No. 168. (cited by the Bureau of Meteorology http://www.bom.gov.au/hydro/has/notables.shtml (accessed 1 September 2003).

10 KEY CONCEPTS AND DEFINITIONS

Areal rainfall pattern - The distribution of rainfall over the land surface.

Australian Rainfall and Runoff - A guide to flood estimation developed by the Institution of Engineers, Australia.

Design Storm, or design rainfall event - A hypothetical rainfall event for a given recurrence interval that has a specified intensity, duration and temporal pattern.

Hyetograph - a graph of rainfall against time

IFD (Intensity-Frequency-Duration) - Statistically derived design rainfall intensities for a given frequency and duration.

Isohyet - a line joining locations of equal rainfall depth

Isopleth - a line joining locations of equal rainfall intensity

Rainfall intensity – Quantity of rainfall per unit of time, usually expressed as mm per hour (mm/hr).

Temporal pattern -The variation of rainfall intensity during a storm or rainfall burst.

11 REVIEW QUESTIONS

1) Compare the use of Patched Point data, with that from the Data Drill.


http://www.bom.gov.au/hydro/has/notables.shtml



2) The variability in design rainfalls can be demonstrated from the 6 key rainfall estimates of the capital cities in Australia. Tabulate these values and comment on the differences and suggest possible reasons for these variations.

3) When would you use design rainfall instead of measured rainfall from historical data?

12 ANSWERS TO REVIEW QUESTION

1) Patched Point Data is generally used if there is a rain gauge nearby. Data from the Data Drill is synthetic - it is based on interpolation of measured values but values of rainfall at a location that contains a rain gauge, will not equal the historical values recorded at the rain gauge.

2) e.g. Canberra (mm/hr): 2i1 = 22.0, 2i12 = 4.30, 2i72 = 1.14, 50i1 = 43.0, 50i12 = 8.0050i72 = 2.25

The quickest way of looking at the rainfall IFD curves from a number of locations around Australia is to download the free program AUSIFD, which has many Australian cities already entered.


Reasons for differences are predominately meteorological e.g. storm type, aspect, air temperature & humidity which determines moisture volume which can be released as rainfall.

3) Design rainfall events can be created to match a specified intensity, frequency and duration which is useful to examine the performance of a drainage or flood mitigation system. Historical events can be a useful for design, but information is required on the event frequency.

Often historical rainfall data will not be available at a point of interest. Design rainfall events can be calculated anywhere in Australia.


TOPIC 3B: RAINFALL MEASUREMENT AND DESIGN RAINFALL · PDF fileUnit CIV2262: Waterway...

Documents

Transcript of TOPIC 3B: RAINFALL MEASUREMENT AND DESIGN RAINFALL · PDF fileUnit CIV2262: Waterway...