Vol. 27 (2) 2011_02

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Pak. J. Agri., Agril. Engg., Vet. Sci., 27 (2), 2011

115

ISSN 1023-1072

Pak. J. Agri., Agril. Engg., Vet. Sci., 2011, 27 (2): 115-124

ESTABLISHING INTENSITY DURATION FREQUENCY CURVESFOR SISTAN AND BALOCHISTAN PROVINCES OF IRAN

1

M. A. Zainudini1, M. J. Marriott

2M. S. Mirjat

3, and A. S. Chandio

3

1Faculty of Fisheries, Chabahar Maritime University, Iran

2School of Computing, Information Technology and Engineering,

University of East London3Department of Irrigation and Drainage, FAE, Sindh Agriculture University,

Tandojam. Pakistan

ABSTRACT

Rainfall intensity-duration-frequency (IDF) curves encompass the estimates ofrainfall intensities of different durations and recurrence intervals. These curvesare used by engineers to create design of a risk assessment of dams andbridges, flood plain management systems, roof and storm water drainagesystems, storm sewers, storage structures, runoff canals, etc. They can beused as a prediction tool to identify the likelihood with which a certain rainfallrate or a specific volume of flow will recur in future, that will create floodinghavoc in an area. In the present study, IDF curves using rainfall data fromSistan and Balochistan, different durations have been developed andpresented. The results have been compared with analysis of data from othercountries. The results based on shorter duration rainfall data are plausible and

can potentially be useful for design purposes; however, a detailed analysisusing comprehensive data sets is needed.

Keywords: Flooding, frequency, hydrograph, intensity, rainfall, runoff

INTRODUCTION

In many hydrologic design projects the determination of the rainfall event is aprerequisite. The most common approach to determine the design storm eventinvolves a relationship between rainfall intensity (or depth), duration, and thefrequency (or return period) appropriate for specific location. Rainfall intensityduration frequency (IDF) relationship comprises the estimates of rainfall

intensities of different durations and recurrence intervals. The IDF curves aregraphical presentation of average rainfall depth that falls per time increment.Rainfall intensity is plotted on the y-axis and is measured in inches per hour or inmm/hr. Storm duration is plotted on x-axis and is measured in minutes. Onceestablished, these curves can be used as a prediction tool for an area that may

Corresponding author: [email protected]

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be flooded, to identify the likelihood with which a certain rainfall rate or a specificvolume of flow will recur in the future. IDF curves usually represent 2, 5, 10, 25,50 and 100-year return periods. They are used to characterize rainfall pattern fora given area and determine the intensity of a rain that falls over that area. Forexample, high rainfall intensity indicates that its raining hard and low rainfallintensity indicates that its raining lightly. The IDF curves are generated withrainfall records collected at a specific monitoring location. The rainfall eventsfrom the past are analysed and the re-occurrence periods are determined forvarious standard return periods, for example, rainfall event that statisticallyoccurs every 10 years. An intensity-duration-frequency curve graph representsthe probability of occurrence of rainfall event thus a graph with linerepresenting10-year would mean that the rainfall event would have a probabilityof occurrence once every 10 years. Another way to put it is that the probability ofa 10-year magnitude storm (or greater) occurring in any given year is 1/10 or10%, and of a 50-year storm occurring 1/50 or 2%. It should be noted that theinformation presented in the graph is based on statistical analysis, not a

prediction of actual storms.

Design applications of IDF curves

The IDF curves are most often used to express the severity of a single rainfallevent. They are required for the design and risk assessment of dams andbridges, for flood plain management systems, design of roof and storm waterdrainage systems, design of storm sewers, design of storage structures andrunoff canals, etc. They are also useful in soil conservation studies. The curvesare used in design with the assumption that past rainfall statistics continue torepresent rainfall statistics into the future. Using IDF curves, runoff generatingfrom catchments is usually calculated by methods such as rational or modifiedrational. The duration of rainfall to provide peak runoff is usually selected as thetime of concentration of the catchment, although for storage analysis for longerdurations and hydrograph methods are required. The durations relevant to localdrainage and flooding are likely to be measured in minutes or hours for smallstreams and in days for larger river catchments. Longer duration data is thusmore relevant to water resources considerations.

A typical system will require a standard design infrastructure that carriesmaximum capacity in terms of rainfall return periods. In big cities, storm sewersare typical designed to carry a minimum of the 5-year storm. It means that all therunoff from a 5-year storm from the area upstream of the sewer system must fit in

the sewer without overflowing onto the road. For a specific location and set ofsite specific characteristics, particular storm duration will produce the greatestrain effect, usually the highest peak runoff flow or greatest rainfall volume.Circumstances can include whether an area is forest or urban or whether thelocation is in the mountains or in the middle of a prairie. Testing the various stormdurations will determine which statistical storm will produce the greatest effect(governing storm duration). Once the duration of governing storm recurring inperiod of time is known then storm water management facilities or floodelevations structures could be designed well in advance in order to make surethat the worst case scenario dealt with.

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The work in this study was carried out near the border of Iran and Pakistan,extending south from Afghanistan to the Gulf of Oman. This paper analysisshorter duration rainfall intensity frequency curves, as obtained by Zainudini(2007). Such curves will help to determine peak runoff calculations for drainageand flood alleviation purposes. The present work is continuation/extension of aprevious study (Marriott and Zainudini, 2006) that considered the monthly rainfalltotals for the Iranian Province of Sistan and Balochistan.

MATERIALS AND METHODS

The study was conducted in the Sistan and Balochistan provinces of Iran at 12selected locations. The area is located near the borders of Iran and Pakistan,extending south from Afghanistan to the Gulf of Oman. The exact locations areshown in Figure 1.

Figure 1. Location of study area in south of Iran (Sistan and Balochistan)

The short duration rainfall data for 15 min, 30 min, 1 hr, 3 hr, 6 hr and 12 hr were

collected from 12 different stations in the study area as shown in the Figure 1.Since insufficient data was available at the time from any one site forconventional frequency analysis the data were pooled taking a station-yearapproach to extend the effective record length. Data were independent and froma meteorologically homogeneous zone, thus yielded as equivalent to a singlelong record. The annual maximum values, for a given duration were ranked, andfitted to a Gumbel distribution, using the Gringorten plotting position formula. Thisapproach has also been used by Wheater and Bell (1983) in their analysis usingthe data from Northern Oman. Nouh (1987) also mentioned this approach, buthe favored an alternative method for analysis of rainfall in Saudi Arabia. He

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produced growth curves for return periods of 5 to 100 years, and presented arealreduction factors for areas greater than 10 km

2. Wheater and Bell (1987)

presented an IDF relationship for return periods of 2 to 500 years, with the latterdotted as a more tentative result. They noted that the results for 1-year returnperiod are not possible by this method, as the Gumbel reduced variate isundefined at this value.

The derivation of a preliminary intensity duration frequency relationship isimportant as it enables us to assign a return period that a flood storm will recurand also allows to analysis the intensities for the short duration rainfall for a

range of return periods which could subsequently be used in the flood protectionworks. Generally, three steps are required to the formulation and construction ofIDF curves using data from the recording stations (Le et al., 2006). A Pearson

type III distribution is usually used for frequency analysis. In this distribution aProbability Distribution Function or Cumulative Distribution Function to eachgroup comprised of the data values for a specific duration is fitted. It is possible

to relate the maximum rainfall intensity for each time interval with thecorresponding return period from the cumulative distribution function. Given areturn period T, its corresponding cumulative frequency F can be calculated

using relation: F = 1-1/T or T = 1/1-F. Once the cumulative frequency is known,the maximum rainfall intensity is then determined using a chosen theoreticaldistribution function (e.g. GEV, Gumbel, Pearson type III distributions). Following

this procedure, the rainfall intensities for 15 min, 30 min, 1 hr, 3 hr, 6 hr and 12 hrduration were calculated for selected return periods of 2, 5, 10, 20, 50, 100years, in this study. This was done by using the probability distribution functions

derived. Finally, the empirical formula was used to construct the rainfall IDFcurves.

The derivation of short duration rainfall was carried out using the station yearapproach. Marriott and Zainudini (2006) have explained the use of frequencyanalysis for the abstracted data for the short durations. A type III Gumbeldistribution was fitted by moments, for a given duration, accordingly. The plotting

position assigned to each data item was calculated according to the Gringortenformula (Wheater and Bell 1983).

RESULTS AND DISCUSSION

Analysis of rainfall data collected at Sistan and Balochistan

The intensity-duration-frequency values for various durations and return periodsfor Sistan and Balochistan are summarized in Table 1. The Gumbel distributionlines for shorter duration rainfall i.e. 15, 30 and 60 min are plotted in Figure 2while, those for longer duration i.e. 3, 6 and 12 hr rainfall are shown in Figure 3

for Sistan and Balochistan. The ranked values plotted in these figures against theGumbel reduced variate, were calculated using the Gringorten plotting position

formula. Intensity values for each return period were then calculated from theresulting best fit straight line, using the appropriate values of the reduced variate.

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Table 1. Intensity-duration-frequency values (mm/hr), with suspectedanomalous figures in brackets, for various durations and return periodsfor Sistan and Balochistan

Return

Period(years)

Duration

15 min. 30 min. 1 hour 3 hour (6 hour) (12 hour)

2 53 36 22 9 (9) (8)

5 74 53 35 14 (13) (11)

10 88 65 44 18 (15) (13)

25 105 79 55 22 (19) (16)

50 118 90 63 25 (22) (18)

100 131 101 71 28 (24) (20)

Figure 2. Gumbel distributions for 15, 30 and 60 min duration in Sistan and

Balochistan.

The regression equations for fitted lines for a given duration and their respectiveR2

value are shown in the Figures 2 and 3. The details on Gringorten formulaeand calculation procedures have been given in the standard texts such asMarriott et al. (2009). The combined data set yielded maximum values for each of

6 durations.

The rainfall intensity versus duration have been illustrated in Figure 4 for all

durations (i.e. 15 min, 30 min, 1 hr, 3 hr, 6 hr and 12 hr) and for the shorter

durations (i.e. 15 min, 30 min, 1 hr) with smaller return periods are shown in

Figure 5.

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The trends show that the rainfall intensity generally decreases with increasingduration. However, with small duration, it changes quickly but the line tends toflatten for longer duration. However, for short durations and smaller returnperiods it seems more reliable.

Comparison with data from other countries

The rainfall data from London area, Kuala Lumpur (KL) and Northern Oman werecompared with those from Sistan and Balochistan. The London data representsthe Wallingford Procedure (NWC, 1981) while, the KL data represents theMalaysian Urban Storm Water Management Manual (DID 2000) and is based ondata for the years from 1953 to 1983. The Intensity-Duration-Frequency curvesfor 5 year return period were generated and are shown in Figure 6. Resultsshowing plausible values up to 3 hour duration and anomalous values for 6 hrand 12 hr both for Sistan and Balochistan provinces,

The results from this study further show plausible agreement for shorterdurations with the data from Northern Oman, but they did not agree for longerdurations where the IDF curves bend and flatten out that could be seen in thefigure. The results for shorter durations appear to be acceptable thus may beuseful for the design purposes or during the analysis of facilities that are requiredto alleviate flooding hazards, particularly when protective conveyance structuresare designed and constructed. Not surprisingly, the tropical Malaysian data (DID,2000) showed considerably higher intensities. Figure 6 also shows a likelyanomaly in the longer duration results in this study. The curves for 6 and 12 hourdurations tend towards the KL line, against the general trend of results for thedata from Sistan and Balochistan provinces. However, the shorter duration

results up to 3 hours appear reasonable so it might be useful when detailedanalysis of longer data sets is done.

Figure 6. Comparison of Intensity-Duration-Frequency curves for 5-year returnperiod with other countries.

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There are considerable problems associated with excessive rainfalls in area thatcause heavy flooding. The impacts of flooding hazard pose a large and yet,untamed problem for both the regulatory bodies (public) and the local authorities.Furthermore, with increased development in the floodplain the economicimplications are likely to grow significantly then a number of issues are paramountto the area of Sistan and Balochistan. Therefore using short duration rainfall datafor the Sistan and Balochistan correlations and curves of intensity-duration-frequency (IDF) are determined, which are thus used to produce intensities forshort duration rainfall over a range of return periods. The intensity durationfrequency correlations and curves for Balochistan are useful to illustrate that thereturn period of storms over the Balochistan region is of the order of 100 years,Intensity values for each return period are calculated from the resulting best fitstraight line, using the appropriate values of the reduced variate. Significant ofresults is illustrated in Figure 7 that shows return periods from 5 to 100 years.Flood risk zones deemed acceptable and remained unsafe as they have a 1%chance of being flooded according to changes in climate this is likely to increase to

4% or 5% chance within a decade, thus if a 1 in 100-year (1% chance ofoccurrence) is likely to occur as a 4% or 5% chance (1 in 25 or 20-year flood), thenthe costs associated with this will increase by the respective magnitude that needsto be looked into before designing any flooding structure (Viner and Agnew, 2001).

Figure 7. Rainfall intensity-duration-frequency relationship for Sistan andBaluchistan for a range of return periods up to 100 years

CONCLUSION

For many hydrologic analyses, planning or design problems, reliable intensity

estimates are necessary. When local rainfall data are available, intensity-duration-frequency (IDF) curves can be developed using frequency analysis. In

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the present study, the rainfall data from sites in Sistan and Bolochistan have

been used to establish intensity-duration-frequency curves, covering returnperiods from 2 to 100 years, and durations from 15 min up to 12 hrs. This is oneof the outcomes of work carried out by (Zainudini, 2007) in a preliminary study

covering data of various durations, which could usefully be extended with largersets of data. The results for shorter durations appear plausible and may be usefulfor the design or analysis of facilities to alleviate flooding hazards, particularly byproviding provision of protective diversion, conveyance and storage structures.

Flood alleviation by means of providing increased storage would requireadditional hydrograph data, to give not only the peak discharge but also the

volume. The results suggest that the analysis presented here should bereworked using more extensive data. The Government and the relevantauthorities should maintain and keep the rainfall and the runoff records available,for detailed analyses.

ACKNOWLEDGEMENTS

The authors acknowledge theiremployers support for their research activity, but

note that opinions expressed in the paper are the authors personal views, andnot necessarily those of the organizations mentioned. The authors are gratefulfor the data obtained from Iran for this research work carried out at University ofEast London. Colleagues at Kolej Linton, Legenda Group, Malaysia are thanked

for their assistance regarding data from Kuala Lumpur.

REFERENCES

DID. 2000. Drainage and Irrigation Department Malaysia. Urban Storm WaterManagement Manual (Manual Saliran Mesra Alam, MSMA).http://www.water.gov.my, www.msmam.com

Le, M. N., T. Yasuto and T. Kaoru. 2006. Establishment of intensity-duration-frequency curves for precipitation in the monsoon area of Vietnam. Annals of

Disas. Prev. Res. Inst., Kyoto Univ., No. 49 B, pp. 93-103.

Marriott, M. J. and M. A. Zainudini. 2006. A review of rainfall data from theIranian province of Sistan and Balochistan. Proceedings of the Advances in

Computing and Technology Conference, London. ISBN-0-9550008-1-5, pp.113-

118. www.uel.ac.uk/act/proceedings/index.htm.

Marriott, M. J., Nalluri and Featherstones. 2009. Civil Engineering Hydraulics, 5th

edition, Wiley-Blackwell, Chichester and Oxford, 2009. http://www.wiley.com/go/marriott

NWC. 1981. Design and analysis of urban storm drainage, the WallingfordProcedure, Volume 4: The Modified Rational Method, National Water Council

(NWC) Standing Technical Committee Report No. 31.

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Nouh, M. A. 1987. Analysis of rainfall in the south-west region of Saudi Arabia.Proceedings of the Institution of Civil Engineers, P.2, (83): 339-349.

Viner, D. and M. Angrew. 2001. The climate system and its implications, climatechange and insurance. CII Research Report, the Chartered Insurance Institute,London. pp. 15-26.

Wheater, H. S. and N. C. Bell. 1983. Northern Oman flood study. Proceedings ofthe Institution of Civil Engineers, P. 2 (75): 453-473.

Zainudini, M. A. 2007. Sistan and Balochistan flood study, M.Sc. Thesis,University of East London.

(Received 18 November, 2011; Revised 31 December, 2011)

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