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May 8, 2008 Atsushi YOSHII, CERI, PWRI

Civil Engineering Research Institute, PWRI. Management of Riparian Woodlands to Mitigate Flood Risk while Conserving the Environment in Hokkaido, Japan. May 8, 2008 Atsushi YOSHII, CERI, PWRI. Background and Purpose of the Research.

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May 8, 2008 Atsushi YOSHII, CERI, PWRI

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  1. Civil Engineering Research Institute, PWRI Management of Riparian Woodlands to Mitigate Flood Risk while Conserving the Environment in Hokkaido, Japan May 8, 2008 Atsushi YOSHII, CERI, PWRI Flood Defence

  2. 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

  3. Field Site of the Research Toyohira River (622km2) Flows through the Center of Sapporo City with Steep current (Riverbed Slope : around 1/200 ) Flood Defence

  4. Serious Flood in August 1981 The Water level reached Horohira bridge girder Flood Defence

  5. Woodlands Cause the Water level to Rise by Obstructing Flood Flow The Toyohira River Improvement Plan by Hokkaido Development Bureau, MLIT Flood Defence

  6. 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. Thinned Woodland Flood Defence Sprouting branches

  7. Resistant Force of Trees against Flood Flow F： Resistant Force [N] ρ: Density of Water [kg/m3] A: Projected Area [m2] Cd: Drag Coefficient U: Flow Velocity [m/s] Projected area “A” changes dramatically according to tree growth, and woodland management activities aim to decrease it. Flood Defence

  8. Estimation of Projected Area 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 Density S: Area Trees Growing [m2] Resistance Density is calculated by accumulation of projected areas of each tree. Flood Defence

  9. Estimated Projected Area at the Site 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 Flood Defence

  10. Estimated Resistance density at the site Resistance Density at the site Rd=1.13 It is much larger compared to the previous study. The density of the tree count seems to affect the resistance. Flood Defence

  11. Annual growth of willow trees is examined by stem analysis Stem analysis: counting annual rings of trees to clarify the growing rate Flood Defence

  12. Projected Area Changes according to Trees growing taller A: Projected Area [m2]H: Tree Height [m] h1: Clearance Height under Tree Crown [m] W: Crown Width [m]d: Diameter of Tree [m] Flood Defence

  13. Transition of Projected Area according to Tree Growth Flood Defence

  14. Annual growth of sprouting shoots after cutting Sprouting Shoots Flood Defence

  15. Transition of Resistance Density Increasing resistance density in natural condition Recovery of resistance density after thinning (80% cutting) Flood Defence

  16. Resistance Density can be controlled by management Thinning Flood Defence

  17. Aiming to safer woodlands with less maintenance changing the woodland structure Flood Defence

  18. Conclusions • 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. Flood Defence

  19. Further Researches Required (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 Flood Defence

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