“Erosion to Deposition” - JICA - 国際協力機構 Erosion to Deposition”...

“Erosion to Deposition”Tractive force reduction

1) Bamboo dragons for depth reduction2) Single groyne and bamboo basket3) Multiple groynes for roughness installation

1Ibo River, Japan

Presentation 3 24 March 2014 Dr. Matsuki Hirotada: JICA Expert for Disaster Risk Reduction

How to make the riverbank stable against erosion?

Thalweg shifts downward and outward due to tractive force.

If the tractive force gets smaller, the thalweg shifts less.

Spiraling flow and thalweg shift

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Theory 1

Tractive force reduction

Tractive force develops by the weight of water in the direction of the flow.

τ = ρ x g x H x S

τ : tractive forceg : gravity acceleration

H : water depthS : bed slope

v = 1/n x R2/3 x I1/2

v : flow velocityn : roughness coefficient

R : hydraulic radiusI : energy gradient

To reduce tractive force,increase roughness.

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To reduce tractive force,decrease water depth.

Theory 2

River facilities must be safe and also

easy-to-maintain within the lowest cost.

River maintenance

= combination of flow control and dyke protection

= groyne+ foot protection + foundation + slope protection

4

Flow control and bank protection

Total planning

1) Bamboo dragons for depth reduction

Ta Trach River/ Kim Ngoc, Hue, Vietnamin JICA project for disaster resiliency (phase-I), 2009

1) Bamboo dragons for depth reduction

Ta Trach River/ Kim Ngoc, Hue, Vietnamin JICA project for disaster resiliency (phase-I), 2009

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Bamboo dragons were set under the water.

The cost was 10,000,000JPN for 210m (520USD/m)

That keeps water depth shallower during floodso that tractive force doesn’t get larger.

Tributary

Stream

Eroded slope

Existing Bank Protection Works(stone placing)

Flow direction

Community RoadResidential area of Kim Ngoc Ward (120 households)

Ferry service

Flow direction

Kim Ngoc Ward in Hue (2009)

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Tributary

Stream

Riverbak Protection Works

Existing Bank Protection Works(stone placing)

Flow direction

Community Road

Residential area of Kim Ngoc Ward (120 households)

Ferry service

Flow direction

(1)

(2)

(1) Slope Protection Works with Bamboo Stone Box (total length : 250m )

(2) Foot Protection Works by Bamboo Stone Gabion or Stone Riprap

Kim Ngoc Ward in Hue (design)

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mainly by slope protection with bamboo stone box and gabion

ＢａｍｂｏｏＳｔｏｎｅ Box

( 2m wide * 2m long * 1m deep stone size max 20cm)

Bamboo Stone Gabion (stone size max 20cm)

(Length ? : Depeding of cross section)

Bamboo Pile

(diameter 10- 12cm, 2- 2.5m long ?)

Weave (connected) bamboo piles with

bamboo outer skin

Lowest water level

Stone Placing

Sodding or other vegetations (willow, Malaysia grass, etc)

TS40 to 50 (matt)

Community Road

1:2

Bamboo Planting

Bamboo Gabion Net (diameter 0.6 m, mesh interval of

mbamboo outer skin 10cm)

Length ?

Diameter 0.6m ?

TS40 to 50 (matt)

Kim Ngoc Ward in Hue (design for shallow river)

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9


9Eroding bank

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Bamboo dragon for foot protection


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11


11After completion

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12After 2 years


13After 4 years


14Sedimentation for plants


15Nature-friendly riverbank

Recommendation! Generate BAMBOO DRAGON using pig container art

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Bamboo dragon (in a museum)

Bamboo snake (in Kim Ngoc)

2) Single groyne and bamboo basket

Thu Bon River/ Thanh Xuyen, Quang Nam, Vietnamin JICA project for disaster resiliency (phase-I), 2009

2) Single groyne and bamboo basket

Thu Bon River/ Thanh Xuyen, Quang Nam, Vietnamin JICA project for disaster resiliency (phase-I), 2009

1717

A masonry groyne was installed to block direct flow attack.Bamboo baskets were sunk behind design thalweg line.

Timber fences were set to reduce flow velocity.

The cost was 10,000,000JPN for 270m (400USD/m)

These facilities keeps riverbed shallow and reduces water velocityto get sediment which covered the eroded bank.

Thanh Xuyen Ward in Quang Nam (2009)

18Eroding bank


19Bamboo fence (working as an attracting groyne)


20Rock outcrop (can be a guide groyne)

Sand bar

Sand bar Flow direction

Natural stone groin (existing)

Bamboo groin (existing)(sand bar length by spur dyke : 50 m in long Eroded slope

Flow direction

Community road

Residential area of Thanh Xuen Ward (250 households)

Rock (existing)Rock(existing)

Stream (existing)

(rebound water flow direction)


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……. ………. ………. …..………. …. ………

……….

………. …

....

Sand bar

Sand bar

Flow direction

Natural ? stone groin(existing)

Bamboo groin (existing)(sand bar length by spur dyke : 50 m in long Eroded slope

Flow direction

Community roadResidential area of Thanh Xuen Ward (250 households)

Rock(existing)

Rock(existing)

(1) Stone Groin (total length : 20m )

(2) Bamboo Stone Box(1m wide*1m long* 1m deep)

(3) Bamboo Groin (18m long* 5 sets with 50m internal)

(4) Bamboo Stone Gabion (1.5 m diameter* 1m high)

(5) Slope Protection (leveling and sodding, site works mainly by community people)

(1)

(2)

(3)

(4)

(5)

(rebound water flow direction)

Thanh Xuyen Ward in Quang Nam (design)

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Top Width3m

Height4 or 5 m ?

Slope Gradient1:1.0 or 1:1.5

Bottom Width13 m or 18 m ?

Slope Gradient 1:20

Slope Gradient 1:1.0 or 1.5

Stone groin

Stone size: 20 - 40 cm ?

Community people: access to the top of groin(for monitoring of dyke condition after construction of stone groin)

Upstream and downstream of dyke: Planting of bamboo

(1) Stone Groin (Total Length : 20m): Image Plan

Direct angle(90 degree)

Lowest water level

Depth 2.0- 2.5 m ?

Height 2.0- 2.5 m ?

1. Surface above low water level is filled up by concrete?

Downstream View

Groin length 20m

Foundation of groin ?see below points

Points to be reconsidered Foundation of groin: can be replaced by gravel bag (not only sand, material of bag ?)

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(4) Bamboo Stone Gabion : Image Plan

Bamboo Stone Gabion

Diameter: 1.5m

Height 1.0m

Prefablication of bamboo gabion net : It is simple net. Can be purchased at market.However, this gabion net can be prefablicated by community people.(strongly desired in view of livelihood improvement)

Lowest water level

Beacon sign

Stone size 10- 20 cm

Downstream View

Points to be reconsidered

Bamboo gabion in the first layer in the above Figure :

Gabion net : can be replaced by wire netStone to be filled in the net: can be replaced by gravel (not only sand) Material of bag ？？？

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25After completion


26After completion


27After 4 years


28After 4 years (over 1m of sedimentation)

3) Multiple groynes for roughness installation

Mekong River/ Ban Tonpheung, Bokeo, Laos since 1998

3) Multiple groynes for roughness installation

Mekong River/ Ban Tonpheung, Bokeo, Laos since 1998

2929

A series of groynes were installed in 7 years.These had impermeable masonry and permeable vegetation.

The cost was 4,178,000,000Kip for 2km (237USD/m)

Groynes on the riverbank works as roughnessto turn off the thalweg and to get sediment.

Sediment comes in flood seasons and protects riverbank in dray seasons.

Mekong groynes at Ban Tonpheung

Mekong groyne project started in1998 at Ban Tonpheung, Laos.

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Eroded (left) riverbank was anaked cliff of 10-meters heightwith no vegetation before 1998.On the opposite (right) side, finesand was deposited thickly.

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Riverbank erosion before 1998

Chiang Saen

Ban Tonpheung

1km

THAI LAO


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b) Dried-up cliff slides down after heavy rain fall.

High water level

a) Strong shear force leaves a sheer cliff under the water.

Fluctuation

5m

10m

Low water level

c) Collapsed soil are moved away by flow year by year.


Cyclic erosion in monsoon fluctuation

a) Cliff formation under the waterAt a concave bank of a river bend,

spiraling flow washes both riverbank andriverbed. Especially in high-waterseasons, strong shear force removes smallparticles away. After repeating high-water, Mekong finally leaves a sheer 10-meterm cliff of undisturbed diluvial layersunder the water.

b) Cliff emergence and circular slipIn the next low-water season, arid air

dries up the cliff to keep its stability. Inthe end of the season, however, heavyrain seeps into the cliff top and suddenlyupsets the gravity balance to causecircular slips. The bank shoulder lineretreats and collapsed soil clump supportsstability of the bank.

c) Collapsed soil to be swept awayThe collapsed soil under water is easy

to be swept away. Spiraling flow incoming high-water seasons sweeps it littleby little. In several years at 5-meter set-back position, a 10-meter cliff will standagain.

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Design parameters (masonry)

Rock size: over 20 cm diameterReferring an empirically-derived formula6)

to keep still in 3.0 m/s flow. Same withexisting concrete blocks and 4 times largerthan riverbed gravels.

Length: 40 mFor dry-work workability in the low waterseason. Equivalent to one-tenth of the riverwidth.

Height: 6mHalf of the cliff heights to be submergedduring the high water season.

Crest width: 3mTo plant willows on clogged masonries.

Slope: 1:3 and 1:1.5Imitation of stable natural riverbank of 1:3for the tip/downstream side. Half of it for theupper.

Spacing: 100 m2.5 times longer than the masonry length tocover the 3km early. Predetermining anadditional short groyne for another collapsebetween groynes.

Work sequence: downstream firstTo prevent scouring due to diffraction flow.

Inspection and rehabilitation: every yearTo repair quickly for smaller damage.

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Groyne to welcome sand sedimentation form fluctuating Mekong river

(impermeable masonry with permeable vegetation)


1st groyne (May 1998) 4 groynes (February 1999)

352.5 groynes (October 1998) 4 groynes (April 2000)

No.12

No.10

No.7

No.11

No.8

No.20

No.3

No.9

No.6

No.4

No.5

No.18

No.17

No.16

No.14

No.13

No.19No.1

No.15

No.2

Gro

yne

fiel

d (2

km)

(ALOS satellite image by JAXA on 26 February 2010)


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The 1st groyne has beennaturally covered by Kok Khaiand tall trees, which make flowslower and slower.

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Groynes are trapping sedimentbetween them and also workingas a jetty.

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Riverbank keeps stability due tosedimentation and growingplants are making themsustainable.

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Total cost was 474,522 USD for 20 groynes and the groynes had 100-meters interval, which means initial cost was 237 USD per meter.And it is almost free for maintenance and rehabilitation.

Year Number Cost in Lao Kip Equivalent in USD

1998-1999 3 540,000,000 75,949

1999-2000 1 338,000,000 47,539

2000-2001 2 600,000,000 63,830

2001-2002 4 800,000,000 85,106

2002-2003 4 700,000,000 74,468

2003-2004 4 800,000,000 85,106

2004-2005 2 400,000,000 42,253

total 20 4,178,000,000 474,552


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(Data source: Muraoka, former JICA Expert for Lao)


41Eroding bank in February 1998


42Eroding bank in February 1998


43The first groyne in February 2014


44Stable bank in February 2014

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