Management of Riparian Woodlands to Management of Riparian Woodlands to Mitigate Flood Risk while Conserving Mitigate Flood Risk while Conserving the Environment in Hokkaido, Japanthe Environment in Hokkaido, Japan

May 8, 2008Atsushi YOSHII, CERI, PWRI

Flood Defence

Civil Engineering Research Institute,

PWRI

Background and Purpose of the Research

• It is important to manage riparian woodlands mitigating flood risk and conserving environment.

• Riparian woodlands obstruct flood flowing and could cause driftwood disaster.

• Riparian woodlands are indispensable for the river environment and waterfront scenery.

• It is necessary to research on the resistance of woodlands against flood flow and their transition.

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Field Site of the Research

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Toyohira River (622kmToyohira River (622km22))Flows through the Center Flows through the Center of Sapporo City with of Sapporo City with Steep current (Riverbed Steep current (Riverbed Slope : around 1/200 )Slope : around 1/200 )

Serious Flood in August 1981

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The Water level reached Horohira bridge girder

Woodlands Cause the Water level to Rise by Obstructing Flood Flow

38

36

34

32

HWL=36.55

0 50 100m

Levee

4.5m

Low Water Channel

Estimated Level of Water Raised as a Result of Obstruction by

Woodlands WL=37.28m

KP 17.0m

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The Toyohira River Improvement Plan by Hokkaido Development Bureau, MLIT

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Woodland Management Activity

• River authority have thinned woodlands to reduce resistant force.

• Willow trees can sprout shoots after cutting, so continuous management is required.

• Other native species of trees were left growing.

Sprouting branches

Thinned Woodland

Resistant Force of Trees against Flood Flow

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2

2

1UACF d

F： Resistant Force [N]ρ: Density of Water [kg/m3]A: Projected Area [m2] 　Cd: Drag CoefficientU: Flow Velocity [m/s]

Projected area “A” changes dramatically according to tree growth, and woodland management activities aim to decrease it.

Estimation of Projected Area

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WhHdhA )(2

111

S

AR i

i

d

145

1

A: Projected Area [m2] 　H: Tree Height [m]h1: Clearance Height under Tree Crown [m]W: Crown Width [m]d: Diameter of Tree [m]Rd: Resistance DensityS: Area Trees Growing [m2]

H

h1

W

4.5m

HWL

Toyohira River

Hokkaido, JAPAN

d : Diameter of Tree

4.5m

HWL

Toyohira River

Hokkaido, JAPAN

Resistance Density is calculated by accumulation of projected areas of each tree.

Estimated Projected Area at the Site

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4.5m

HWL

Toyohira River

Hokkaido, JAPAN

Projected area at the site

Stem: 26.8 m2

Tree crown: 399.0 m2

(The projected area of tree crown is diminished to half its original size in flood water)

Total A: 226.3 m2

WhHdhA )(2

111

H

h1

W

4.5m

HWL

Toyohira River

Hokkaido, JAPAN

d : Diameter of Tree

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Estimated Resistance density at the site

S

AR i

i

d

145

1

Annual growth of willow trees is Annual growth of willow trees is examined by stem analysisexamined by stem analysis

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Stem analysis: counting annual rings of trees to clarify the growing rate

H

h1

W

4.5m

HWL

Toyohira River

Hokkaido, JAPAN

d : Diameter of Tree

Projected Area Changes according to Trees growing taller

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WhHdhA )(2

111

A: Projected Area [m2] H: Tree Height [m]h1: Clearance Height under Tree Crown [m]W: Crown Width [m] d: Diameter of Tree [m]

Transition of Projected Area according to Tree Growth

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Annual growth of sprouting shoots after cutting

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Sprouting Shoots

Transition of Resistance Density

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Increasing resistance density in natural

condition

Recovery of resistance density after thinning

(80% cutting)

Resistance Density can be controlled by management Thinning

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Aiming to safer woodlands with less maintenance

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changing the woodland structure

Conclusions

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•The resistance density of the site was calculated as 1.13, which is quite high compared to ordinary woodlands because of the high density of the trees. •The resistance transition was simulated by projecting area of a standard tree model by stem analysis.•Recovery of the resistance density of thinned trees was also simulated using the model of sprouting shoots growing. •These simulations can be utilized for the future woodland management.

Further Researches Required

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(1) actual transition of projected areas in the future (2) resistance force of riparian woodlands and their behavior in flooding (3) transition of density and structure of woodlands in the future

32 native seedlings growing

under willows cover

Maple Tree