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Frequency Analysis of Flash Floods for Establishing New Danger Levels for the Rivers in the Northeast Haor

Region of Bangladesh

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Binata Roy

Lecturer, Institute of Water and Flood Management, BUET

binata_roy@iwfm.buet.ac.bd; binata_roy@yahoo.com

​

Binata Roy, A. K. M. Saiful Islam, G. M. Tarekul Islam, Md. Jamal Uddin Khan, Biswa Bhattacharya, Md. Hazrat Ali, Abu Saleh Khan, Md. Saiful Hossain, Gopal Chandra Sarker and Nahruma Mehzabeen Pieu

​

Technical Workshop on Flash Flood Forecasting and Early Warning System (FFEWS)

10 April 2019, Council Bhaban, BUET, Dhaka

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Necessity of Flash Flood Danger level

  • Danger Level the level above which the flood losses to nearby lives, crops and homesteads
  • Existing danger level is designed considering the monsoon flood only (June-September)
  • No danger level for flash flood of pre-monsoon season (March – May)
  • Flash Flood Danger level should be lower than monsoon flood danger level
  • False sense of protection to farmers
  • Flash flood Forecasting and Warning system is not properly working
  • Single cropped (Boro rice) agriculture of the northeast (NE) Haor region is severely damaged

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Damage of BORO CROP in Flash Flood

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Study Area

  • Sunamganj, Habiganj, Netrokona, Kishoreganj, Sylhet, Moulavibazar and Brahmanbaria
  • BWDB water level stations 40
  • Sunamganj, Kanaighat, Khalijhuri, Sarighat, Jaflong, Durgapur, Sylhet, Mohonganj, Habiganj etc

Selected 40 water level stations of the northeast region of Bangladesh

Data Collection

Data Type

Organization

Year

Water Level

BWDB

1996-2014

Cross section

BWDB/

IWM

Recent

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STUDY LAYOUT

Flood Frequency Analysis

River Bathymetry

The return periods at which the water level reach or is nearer to floodplain at the full stage

Selection of Best Probability Distribution Function

Lognormal

Lognormal – III

Pearson type III

Log-Pearson III

Gumbel

GEV

Probability plot correlations coefficient

​

Normalized Root Mean Square Error

​

Mean Absolute Error

Randomness Test

Independence Test

Stationarity Test

Run Test

Lag 1 Auto Correlation Test

Mann-Kendall Test

& Sen’s Slope

Mann-Whitney Test

Data Homogeneity and Adequacy Test

Data Processing

AM WL

(March 1 - May 15)

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Result : Regional Probability Distribution Function

  • The best fit regional distribution selected on Goodness of fits method
    • PPCC
    • NRMSE and
    • MAE

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  • Each distribution is assigned by a rank between 1 and 6

​

  • Rank 1 is for the best fitting distribution and rank 6 is for the worst fitting one

​

  • GEV the most suitable distribution for fitting the extreme flash flood events of the north east Haor region of Bangladesh

Goodness of fit

Distribution

Number of Sites Receiving Ranks

Total of Ranks

1

2

3

4

5

6

PPCC

Lognormal

0

2

3

14

18

3

177

Lognormal type III

2

7

3

9

14

5

161

Pearson type III

8

18

9

4

1

0

92

Log Pearson type III

2

9

20

7

1

1

119

Gumbel Distribution

2

0

2

2

5

29

215

GEV

26

3

4

4

1

2

77

NRMSE

Lognormal

2

5

2

17

13

1

157

Lognormal type III

2

3

5

8

3

19

184

Pearson type III

7

11

8

7

5

2

118

Log Pearson type III

4

14

17

3

2

0

105

Gumbel Distribution

0

2

2

3

14

19

206

GEV

24

6

6

2

2

0

72

MAE

Lognormal

4

5

3

13

11

4

154

Lognormal type III

4

2

7

5

3

19

178

Pearson type III

2

13

9

9

4

3

129

Log Pearson type III

1

9

19

9

1

1

123

Gumbel Distribution

1

3

1

3

16

16

198

GEV

27

8

1

2

2

0

64

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St. Name

St. ID

Flood level (m.PWD) with a return period of T (year)

St.

Name

 

St.ID

 

Flood level (m.PWD) with a �return period of T (year)

2.33

5

10

2.33

5

10

Brahmanbaria

SW3A

2.97

3.27

3.47

MonuRly.Bridge

SW 201

17.15

17.98

18.44

Nakuagaon

SW 34

20.9

21.6

22.2

Moulvi Bazar

SW 202

10.6

11.4

11.8

Jaria-jangail

SW 36

7.19

8.08

8.71

Jaflong_Spill

SW 233A

10.65

11.56

12.18

Mohanganj

SW36.1

5.81

6.38

6.78

Sarighat

SW 251

10.80

12.12

12.95

Kamalganj

SW 674

19.58

20.16

20.46

Salutikar

SW 252.1

8.26

9.37

9.85

Khaliajuri

SW 72

5.31

5.88

6.20

Durgapur

SW 263

11.50

11.74

11.92

Itna

SW 73

4.46

5.07

5.56

Kalmakanda

SW 263.1

5.40

6.86

8.02

Dilalpur

SW74

3.29

3.54

3.71

Kanairghat

SW 266

11.16

12.86

14.04

Laurergarh Saktiarkhola

SW 131.5

6.94

7.80

8.49

Sylhet

SW267

8.63

9.85

10.67

Juri Cont_Silghat

SW 135A

10.45

11.06

11.53

Sunamganj

SW 269

6.41

7.24

7.72

Sofiabad

SW 138

9.53

10.58

10.96

Markuli

SW270

6.71

7.59

8.20

Ballah

SW 157

21.93

23.06

24.06

Madna

SW 272

3.71

3.99

4.15

Chunarghat

SW158

14.92

15.62

16.08

Bhairab Bazar

SW 273

3.04

3.24

3.39

Shaistaganj

SW158.1

12.39

13.39

14.05

SutangRly.Brid

SW 280

5.24

5.97

6.51

Habiganj

SW 159

9.35

10.39

11.04

Ajabpur

SW 295

3.12

3.32

3.43

Amalshid

SW 172.5

13.34

15.31

16.60

Akhaura

SW 296

2.87

3.20

3.46

Sheola

SW 173

11.22

12.86

13.94

Netrokona

SW 310

4.73

5.63

6.32

Fenchuganj

SW174

8.67

9.71

10.33

Atpara

SW 311

5.17

6.42

7.27

Sherpur

SW 175.5

7.74

8.44

8.80

Lubachara

SW 326

11.55

13.11

14.12

Motiganj

SW192

8.70

8.97

9.08

Urargaon

SW 337

9.94

10.38

10.59

Result : Flood Frequency Analysis

  • Mean annual flood peak

​

  • Flood of 2.33 year return period
    • Gumbel
    • Generalized Extreme
    • Log-normal distribution

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  • Fixing the flood danger level at 2.33 year return period is referred by FFWC

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Result : Danger Level Determination

  • Its verified through the elevations of the cross sections of the rivers

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  • The elevations of the bank lines of each of the 40 forecasting stations are compared with the flooding scenarios of 2.33 to 100 years return period from frequency analysis

​

  • Danger levels fixed considering the return periods at which the water level reach or is nearer to floodplain at the full stage

​

  • 2.33-year or 5-year return period flood level is selected as the Flash Flood Danger Level

Water level at different Return period found from GEV are plotted against the cross section of the river

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Result : Recommended Flash Flood Danger Level

Station No

River Name

Station Name

Station ID

District

FFWC Monsoon Flood danger level (mPWD)

Proposed Flash Flood Danger Level (mPWD)

1

Anderson khal

Brahmanbaria Rly Bridge

SW 3

Brahmanbaria

5.5

2.97

2

Bhogai-Kangsa

Nakuagaon

SW 34

Sherpur

22.4

20.98

3

Bhogai-Kangsa

Jaria-Jhanjail

SW 36

Netrokona

9.75

8.08

4

Bhogai-Kangsa

Mohanganj

SW 36.1

Netrokona

-

5.81

5

Dhalai

Kamalganj

SW 67

Moulvi Bazar

19.82

19.58

6

Dhanu-Baulai-Ghorautra

Khaliajuri

SW 72

Netrokona

-

5.31

7

Dhanu-Baulai-Ghorautra

Itna

SW 73

Kishoreganj

-

4.46

8

Dhanu-Baulai-Ghorautra

Dilalpur

SW 74

Kishoreganj

-

3.29

9

Jadukata

Laurergarh Saktiarkhola

SW 131.5

Sunamganj

8.53

7.8

10

Juri

Juri_Cont_silghat

SW 135A

Moulavibazar

-

10.45

11

Korangi

Sofiabad

SW 1382

Habiganj

-

9.53

12

Khowai

Ballah

SW 1572

Habiganj

21.64

21.93

13

Khowai

Chunarghat

SW 158

Habiganj

-

14.92

14

Khowai

Shaistaganj

SW 158.1

Habiganj

-

12.39

15

Khowai

Habiganj

SW 159

Habiganj

9.5

9.35

16

Kushiyara

Amalshid

SW 172.5

Sylhet

15.85

13.34

17

Kushiyara

Sheola

SW 173

Sylhet

13.5

12.86

18

Kushiyara

Fenchuganj

SW 174

Sylhet

-

9.71

19

Kushiyara

Sherpur

SW 175.5

Sylhet

9

8.44

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Result : Recommended Flash Flood Danger Level

Station No

River Name

Station Name

Station ID

District

FFWC Monsoon Flood danger level (mPWD)

Proposed Flash Flood Danger Level (mPWD)

20

Lungla

Motiganj

SW 192

Moulvi Bazar

-

8.7

21

Monu

MonuRly.Bridge

SW 201

Moulvi Bazar

18

17.15

22

Monu

Moulvi Bazar

SW 202

Moulvi Bazar

11.75

10.57

23

Piyan

Jaflong_Spill

SW 233A

Sylhet

-

10.65

24

Sari-Gowain

Sarighat

SW 251

Sylhet

12.8

10.8

25

Sari-Gowain

Salutikar

SW 252.1

Sylhet

-

8.26

26

Someswari

Durgapur

SW 263

Netrokona

13

11.5

27

Someswari

Kalmakanda

SW 263.1

Netrokona

-

6.86

28

Surma-Meghna

Kanairghat

SW 266

Sylhet

13.2

12.86

29

Surma-Meghna

Sylhet

SW 267

Sylhet

11.25

9.85

30

Surma-Meghna

Sunamganj

SW 269

Sunamganj

8.25

6.41

31

Surma-Meghna

Markuli

SW 270

Habiganj

8.5

6.71

32

Surma-Meghna

Madna

SW 272

Habiganj

-

3.71

33

Surma-Meghna

Bhairab Bazar

SW 273

Kishoreganj

6.25

3.04

34

Sutang

Sutang Rly.Brid

SW 2802

Habiganj

-

5.97

35

Titas

Ajabpur

SW 295

Brahmanbaria

-

3.12

36

Titas

Akhaura

SW 296

Brahmanbaria

-

2.87

37

Mogra

Netrokona

SW 310

Netrokona

-

5.63

38

Mogra

Atpara

SW 311

Netrokona

-

6.42

39

Lubachara

Lubachara

SW 326

Sylhet

-

11.55

40

Noyagang

Urargaon

SW 337

Sunamganj

-

9.94

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Result : Comparison of Monsoon and Flash Flood DL

Existing monsoon flood danger level and proposed

flash flood danger level

  • The proposed flash flood danger level is much lower than the existing monsoon flood danger level
  • The differences are much prominent, range from 0.15 m to 3.21 m with an average difference of nearly 1.75 m
  • This difference causes severe destruction for Boro rice cultivation during a flash flood
  • Flash flood danger level should be fixed separately from existing monsoon danger level considering the flash flood period only and updated in FFWC website as soon as possible

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Summary of the Result

Proposed flash flood danger level (m.PWD) for the selected water level station based on the return period

of the 2.33-year or 5-year flood

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Conclusion

  • The existing danger level is not serving the purpose of Flash flood danger level
  • The Flash flood danger level is determined by
      • flood frequency analysis and
      • elevations of the river cross sections
  • DL is fixed at 2.33 year return period’s WL for 29 stations; 5 year return period is selected for the rest
  • Proposed flash flood danger level is much lower than the existing monsoon flood danger level
  • Sunamganj, the existing danger level is 8.25 mPWD, the recommended flash flood danger level is 6.41 mPWD
  • Sylhet, the existing danger level is 11.25 mPWD, the recommended flash flood danger level is 9.85 m PWD
  • Flash flood danger level is fixed separately from existing danger level considering the flash flood period only
  • BWDB in establishing a new flash flood danger level
  • FFWC in precise flash flood forecasting and early warning
  • Protect the Boro crop of NE region from the flash flood

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Recommendation

  • The crest levels of the submersible embankments are not considered
  • Incorporation of recent land topography and submersible embankment heights may give more accurate danger level
  • A free board of 0.3 m to 0.5 m needs to be subtracted from the recommended danger level for safety purpose depending on the topography, site condition and importance of the site for Boro cultivation

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  • Khan, M. N. H., Mia, M. Y., and Hossain, M. R. (2012). “Impacts of flood on crop production in Haor areas of two Upazillas in Kishoregonj.” J. Environ. Sci. Nat. Resour., 5(1), 193-198.
  • The Daily Star. (2010). Flood damages boro in huge areas of greater Sylhet. < http://www.thedailystar.net/news-detail-136901> (Dec. 19, 2016).
  • Deshwara, M. (2016). “FLASH FLOODS: Farmers in northeastern Bangladesh in trouble.” The Daily Star. < http://www.thedailystar.net/backpage/farmers-trouble-1218799> (Nov. 19, 2016).
  • Flood Forecasting and Warning Centre (FFWC) (2018). Bangladesh Water Development Board. Website link: http://www.ffwc.gov.bd/
  • IFCDR (Institute of Flood Control and Drainage Research). 1995. “Flood frequency analysis.” Vol. 1 of Component of the study on revisionof flood danger levels in Bangladesh, edited by J. U. Chowdhury, M. Y. Rana, and M. Salehin. Dhaka, Bangladesh: BWDB.
  • Karim, M. A., and J. U. Chowdhury. 1995. “A comparison of four distributions used in flood frequency analysis in Bangladesh.” Hydrol. Sci. J. 40 (1): 55–66. https://doi.org/10.1080/02626669509491390.
  • Center for Environmental and Geographic Information Services (CEGIS). (2012). “Master Plan of Haor Area. Volume 1, Summary Report.” Government of People’s Republic of Bangladesh, Ministry of Water Resources, Bangladesh Haor and Wetland Development Board.
  • Bangladesh Water Development Board (BWDB). (2006). “Mathematical Modelling with Hydrological Studies and Terrestrial Survey under the Haor Rehabilitation Scheme.” Bangladesh Water Development Board.

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References

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Download Link

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https://ascelibrary.org/doi/10.1061/%28ASCE%29HE.1943-5584.0001760

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Thank You