Management of Riparian Woodlands to Mitigate Flood Risk while Conserving the Environment in Hokkaido, Japan May 8, 2008 Atsushi 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 (622km 2 ) Flows through the Center of Sapporo City with Steep current (Riverbed Slope : around 1/200 )

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

Woodlands Cause the Water level to Rise by Obstructing Flood Flow 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 F ： Resistant Force [N] ρ: Density of Water [kg/m 3 ] A: Projected Area [m 2 ] C d : Drag Coefficient U: 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 A: Projected Area [m 2 ] H: Tree Height [m] h 1 : Clearance Height under Tree Crown [m] W: Crown Width [m] d: Diameter of Tree [m] R d : Resistance Density S: Area Trees Growing [m 2 ] Resistance Density is calculated by accumulation of projected areas of each tree.

Estimated Projected Area at the Site Flood Defence Projected area at the site Stem: 26.8 m 2 Tree crown: m 2 (The projected area of tree crown is diminished to half its original size in flood water) Total A: m 2

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

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

Projected Area Changes according to Trees growing taller Flood Defence A: Projected Area [m 2 ] H: Tree Height [m] h 1 : Clearance Height under Tree Crown [m] W: Crown Width [m] d: Diameter of Tree [m]

Transition of Projected Area according to Tree Growth Flood Defence

Annual growth of sprouting shoots after cutting Flood Defence Sprouting Shoots

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

Resistance Density can be controlled by management Thinning Flood Defence

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

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.

Further Researches Required Flood Defence (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