management of riparian woodlands to mitigate flood risk while conserving the environment in...
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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.
Flood Defence
Field Site of the Research
Flood Defence
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 )
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
Flood Defence
The Toyohira River Improvement Plan by Hokkaido Development Bureau, MLIT
Flood Defence
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
Flood Defence
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
Flood Defence
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
Flood Defence
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
Annual growth of willow trees is Annual growth of willow trees is examined by stem analysisexamined by stem analysis
Flood Defence
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
Flood Defence
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 Resistance Density
Flood Defence
Increasing resistance density in natural
condition
Recovery of resistance density after thinning
(80% cutting)
Conclusions
Flood Defence
•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.