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Restoration of coastal resilience through tidal river management
Rezaur Rahman, Munsur Rahman, Anisul Huq (Bangladesh University of Engineering and Technology),
&
Hajime Nakagawa (Kyoto University)
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The coastal region
Cyclone tracks
Sidr (2007) Aila (2009)
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Difficulties in rehabilitation
• Very old polders
• Severe labor shortage – Migration
– Lack of proper living condition
– Lack of drinking water
• Morphological changes – Silted up and wider channels because of
polderization
– New channels formed to convey storm surge
Polders 31/32/33
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Satellite image 1989
Satellite image 1997
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Satellite image 2014
Change in bank line
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A typical storm surge can exert a force on the channels which is almost 100 times larger than the force that is exerted on the same channel in the normal condition.
To study the effect on channel morphology of this huge accelerated force during the storm surge, a numerical model (Delft 3D Dashboard) is applied for a typical cyclonic storm surge (Aila-like) in the coastal region of Bangladesh.
Location where the model results are shown
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Yearly resultant erosion / sedimentation in meter in normal flow condition
Negative value - erosion
Positive value- sedimentation
Zero value – No erosion/sedimentation
White Lines - Polder
Resultant erosion / sedimentation in meter in few hours during a storm surge
Negative value - erosion
Positive value- sedimentation
Zero value – No erosion/sedimentation
White Lines - Polder
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Observations
• The storm surge is causing a huge morphological changes in few hours
• This is mainly due to the accelerated force acting on the water body due to storm surge
• It is unlikely that the river will attain its pre-storm surge equilibrium even after a long term normal flow condition
• The initial model run supports this hypothesis.
Difficulties in rehabilitation
• Very old polders
• Severe labor shortage – Migration
– Lack of proper living condition
– Lack of drinking water
• Morphological changes – Silted up and wider channels because of polderization
– New channels formed to convey storm surge
Loss of system resilience
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Give space to the rivers
• Retiring embankments
• Retiring polders
• Rotating tidal basins
Khulna-Jessore Drainage Rehabilitation Project (KJDRP)
Ha
ri r
iver
Upper Bhadra river
Ko
be d
ak
rive
r
Arp
anga
sia
river
Sh
ibsa
riv
er
Pa
ssu
r ri
ver
Pa
ssur
riv
e r
Mal
anch
a r
ive
r
#
Study area
5 0 5 10 15 km
Project area
Intermediate area
Sundarbans
Bay of Bengal
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Context of KJDRP
• Coastal Embankment Project in the 60s
• Good crop production in the 70s
• Drainage congestion in the 80s
• KJDRP undertaken in the 90s
• Tidal basin operation in 00s
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Slide - 24 September 26, 2014
Ebb Tide
(Fresh water & erosion)
TBM Concept
5. Sustainable drainage system
High Tide
(Water & Sediment)
1. Water allowed to enter tidal basin during high tide
2. Sediments get deposited in the basins
3. Water carrying less sediment cuts its way back through rivers during low tide
4. Conveyance capacity of river increases
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Slide - 25
Tidal basins at KJDRP
Bhaina
Khuksia
Bokor
Kadaria
Madhugram
Dakatia
Hari
River
Jessore
Lower Bhadra
Dakatia
Bhaina
Khuksia
Kadaria
Bokor
Madhugram
Ghangrail
Kaji B
acha
Back
Size of tidal basins
O’Brien formula (1969)
• A = 41.5 10 -6 V
Required size
• 400 ha in Kedaria Beel for Hari river
• 200 ha in Buruli-Panjia-Pathra beel for Upper Bhadra river
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Potential of TBM – 3R
• Replication elsewhere
• Restoration of resilience of the coast
• Rising land to face rising sea level
Let us move forward towards a resilient coast
