This study analyzes changes to the morphological characteristics of the Bishkhali Estuary in Bangladesh due to Cyclone SIDR in 2007 using the Delft3D morphology model. The study found the greatest changes at locations close to where the cyclone made landfall, with a maximum deposition of 2383.56 sqm found near the mouth. Overall, cross-sectional area and bed level elevation changes were greater downstream compared to upstream. The maximum bed shear stress during the cyclone was over 5 times higher than normal conditions near the landfall location, contributing to the more significant morphological changes observed downstream.

1. Title
Changes of Morphological Characteristics of
Bishkhali Estuary due to Cyclone SIDR
Authors
MD. WASIF-E-ELAHI
SAKIB MOHIUDDIN
FATIN NIHAL
REZAUL KARIM
ANISUL HAQUE
MUNSUR RAHMAN
International Conference on Climate Change and Water Security (ICCWS 2015)
ID: ICCWS-27

2. INTRODUCTION
River stability and response to changing environmental conditions are functions
of local watershed features and exposed environmental condition.
During hydrodynamic shock like severe cyclones, stream power increases above
normal levels, resulting in dramatic changes in the riverine landscape.
 High sediment transport occurs.
The geographical location and climatic condition make Bangladesh one of the
most cyclone prone countries in the world.
For Bangladesh coast also, not much information is available on changes of river
morphology due to cyclone induced storm surge.

3. INTRODUCTION
From literature:
• Excessive sediment transport occur during major river floods that reflect the
effectiveness of floods on shaping the river morphology.
• Several studies have shown that hyper concentrated flows can cause
extensive erosion of river beds because of enhanced boundary shear stress in
the sediment–water mixture.
As higher rate of bed erosion-deposition occur due to cyclonic event, so this
study mainly focus on cross sectional area, bed level elevation and bed shear
stress change during hydrodynamic shock resulting from a cyclonic event ‘SIDR’.

4. STUDY AREA

5. STUDY AREA
• The total length of the river is 96 km.
• The average width of the river from its origin to
first 30 km is about 1 km and the rest is about 2
km.
• The average depth is about 16m. The estuary is
dominantly influenced by tidal effects.
• SIDR made landfall at Bishkhali mouth.
• 7 locations are selected

6. THE MODEL
Year 2000 is considered as the base year
The open source Delft3D morphology model coupled with Delft
Dashboard is applied in the study region to compute the
morphological change due to cyclone SIDR.
By comparing the simulated bed level due to SIDR with the initial, the
net cross sectional change of river bed is computed.

7. MODEL BOUNDARIES

8. LOCATION OF MORPHOLOGICAL STATIONS FOR MODEL VALIDATION

9. River
name
Station name Measured erosion/deposition
rate (cm/month)
(For a long Period of data which is
more than 10 years)
Model
erosion/deposition
rate (cm/month)
(Only for the base year
condition)
Reliability %
Bishkhali
(CES)
BIS16 -0.0122 0.1912 49.26
BIS15 -0.1426 -0.1504
BIS14 -0.4637 -0.4430
BIS12 0.3157 0
BIS11 0.0083 -0.0837
Lower
Meghna
(EES)
ML2 -0.7832 0 41.80
ML5 0.2265 -0.0023
M12 -0.2471 -0.0010
Rupsa
(WES)
RP10 -0.9200 -0.2532 50.86
RP13 -0.6169 -0.3153
RP14 (n/a) -0.3492
Overall Model reliability over the estuarine systems 47.30
MODEL VALIDATION

10. STORM SURGE MODEL VALIDATION
• Performances of the model during the calibration exercises are evaluated by computing the model
reliability as described by Haque et al. (unpublished).
• Using this indicator, model reliability for the storm surge model for the ‘most acceptable model
parameters’ is obtained approximately 60%.
• The calibrated model is then validated where measured tidal water level was available during the
time of landfall of the cyclone.

11. RESULT AND DISCUSSION
Cross-section name Area (m2
) Change of Area (m2
)
Name Non-dimensional
distance factor
(NDF)
Initial (Before SIDR) After SIDR
Mouth 0.00 34237.71 34251.85 -14.14
Dw1 0.14 38381.25 35997.69 2383.56
Dw2 0.28 20564.27 20577.09 -12.81
Mid 0.42 17483.19 17483.17 0.02
Up1 0.57 14041.36 14042.44 -1.07
Up2 0.85 13321.40 13321.40 -0.01
Up 1.00 13778.03 13778.03 0
Non-dimensional distance factor = Distance with respect to mouth / Total length
(+ sign denotes deposition and - sign denotes erosion)

12. RESULT AND DISCUSSION

13. RESULT AND DISCUSSION
Cross-section name Depth (m)
Initial (Before SIDR) After SIDR
Name Non-dimensional
distance factor (NDF)
Maximu
m
Minimum Maximum Minimum
Mouth 0.00 8.49 7.85 8.49 7.85
Dw1 0.14
10.68 10.18 11.10 6.72
Dw2 0.28 5.37 5.08 5.38 5.08
Mid 0.42 6.52 6.36 6.52 6.36
Up1 0.57 8.55 8.50 8.56 8.50
Up2 0.85 8.11 7.97 8.11 7.97
Up 1.00 8.28 8.27 8.28 8.27
Non-dimensional distance factor = Distance with respect to mouth / Total length

14. RESULT AND DISCUSSION

15. RESULT AND DISCUSSION
• The maximum bed shear stress of 11.12 N/m2
is observed at the
mouth of the estuary.
• Due to this reason the maximum morphological changes are found in
this region.
• It gradually decreases from downstream to upstream of the estuary.
• At the upstream (NDF-1.00), maximum bed shear stress during
cyclone SIDR is 1.08 N/m2
whereas at normal condition i.e. 1.02 N/m2
.
• No change found at upstream (NDF-1.00)

16. CONCLUSION
• Impacts of cyclone SIDR was higher at landfall location compared to other places.
• Maximum bed shear stress during cyclone SIDR was almost five times higher than normal
condition at Dw1(NDF-0.014). (deposition of 2383.56 m2
)
• Maximum erosion of 14.14 m2
(cross-sectional area) was found at river mouth (NDF-0.00).
(maximum bed shear stress was seven times higher than normal condition)
• Cross sectional area and bed level elevation changes, resulting from bed shear stress due to
cyclone SIDR, was greater at downstream compared to the upstream of Bishkhali estuary.

17. ACKNOWLEDGEMENT
We would like to acknowledge to NERC, ESRC, UK DFID, ESPA for
funding this work as part of the ESPA Deltas project (Grant Reference
Code : NEJ0027551). We would like to acknowledge the contribution of
all the members associated with the research team who has direct and
indirect input to this article.

18. Thank
you