Challenges from a low-energy river in Bangladesh

(1)

Delft University of Technology

Challenges from a low-energy river in Bangladesh

Planform changes, stratigraphy, sharp bends and response to constriction (PPT)

Mosselman, Erik

Publication date 2020

Document Version Final published version

Citation (APA)

Mosselman, E. (2020). Challenges from a low-energy river in Bangladesh: Planform changes, stratigraphy, sharp bends and response to constriction (PPT). 1-19. Why river managers should care about the alluvial subsurface, Wageningen, Netherlands.

Important note

To cite this publication, please use the final published version (if applicable). Please check the document version above.

Copyright

Other than for strictly personal use, it is not permitted to download, forward or distribute the text or part of it, without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license such as Creative Commons. Takedown policy

Please contact us and provide details if you believe this document breaches copyrights. We will remove access to the work immediately and investigate your claim.

This work is downloaded from Delft University of Technology.

(2)

Erik Mosselman

Symposium “Why river managers should care about the alluvial subsurface”

Wageningen

Planform changes, stratigraphy, sharp

bends and response to constriction

18 February 2020

Challenges from

a low-energy river

in Bangladesh:

N

a

m

A

(3)

Finding a low-energy river in Bangladesh

2

2 10 W/m

P 

2 10 W/m

Q

g

i

B





= 1000 kg/m

3 ≈ 10 m/s

2 < 10

-4

= 0.1 m/km

(4)

Finding a low-energy river in Bangladesh

3

4

10 0.1 m/km

i



−

=

2 10 m /s

Q

B



i = 0.07 m/km

Q = 50 000 m

3 /s

B = 10 000 m

Brahmaputra!

(5)

Brahmaputra

Ganges

Bay of Bengal

(6)

this is not the other

side of the river

(7)

10 km

Rhine or

Danube

(8)

(9)

(10)

(11)

Bank protection and river training

(12)

Stratigraphy

(13)

Stratigraphy

(14)

Sharp bends

− Result of river training

− Scour holes up to 30-40 m deep (15-storey building)

(15)

(16)

(17)

Bank stabilization and constriction

Bank stabilization

− Prevention of misery of people losing everything

(no other choice than

to move to city slums)

− Prevention of breaching of dikes by erosion

(the main benefit in

cost-benefit analyses)

− Stabilization of offtakes for water supply

Constriction

− Strong lobbies and agendas

(not justified by cost-benefit analyses)

− Land reclamation

(for economic zones … and ecotourism)

− Development of inland waterway by narrowing and deepening

− Dredging and sand mining

− ... Singapore offered to take out the sediment for free

(18)

Response to constriction

Theory:

Morphological computations by Institute for Water Modelling:

− About 5 m bed degradation in 25 years

Yet … in Brahmaputra less extreme than in other rivers:

− Width taken away more from islands than from channels

− Formula relatively insensitive due to low b:

• common Engelund-Hansen: b = 5

• calibrated for Brahmaputra: b = 3.66

• (physically realistic: b > 3)

− More sinuous alignment for main channels

17 1 3/b

new

old

B

i

B

−





=

_

_





(19)

New approach to bank stabilization

(20)