高灘地上之植物增加水流阻力，因而減緩流速，增加水深，遲緩洪水宣洩，也因流速減緩及植物根系之作用而有減少沖刷及固土護岸的功能。近年來，則因河流景觀塑造及生態工法之推動，有關浸沒植物及露出水面植物之莖葉間的水流現象日益受到重視。

　　本文之目的以定量緩變速流況下，植物尚露出水面時之水理現象為研究對象，經由理論分析及水工試驗研究，植物莖間流速與植物密度的關係，以及植生地與裸地間之交互作用。因植物莖狀多近似圓柱形，因此水工試驗時，將以適當的圓柱群模擬植物莖幹群。

　　本研究並由理論分析得出，未潛沒圓柱群水流流入裸地時，不發生超臨界流及水躍現象之最大密度計算方法。由試驗結果之分析證明上述公式及最大密度計算方法之適用性。

　　由理論分析與試驗結果得知，水流流經植物群中所受到之阻力，隨植物密度增加而上升，故在較密的植生群中，因產生較大之水流阻力，因而會減緩流速，增加水深，遲緩洪水宣洩。如植生群密度過大，會因過度束縮在植生地與裸地間產生超臨界流與水躍現象，造成水流不穩定，影響護岸固土效果，故應該避免植生草地下游有裸地之情況，或應用本研究建議之計算方法研訂植生密度，對於高灘地之護岸固土效果較佳。

　The vegetation in the floodplain and along the bank can increase the flow resistance and flow depth, so that retard the flow velocity and conveyance. Hence, it can prevent bed erosion and enhance bank stability due to decrease of flow velocity and effect of roots system of the riparian vegetation. In recent years, the hydraulic phenomenon about flow resistance for submerged vegetation and emergent vegetation is more and more important cause of landscaping and the ecological engineering methods were used.

　The purpose of this paper was to investigate the hydraulic phenomenon about emergent vegetation, that include both the interrelation between vegetation density and velocity and the interaction between bare zones and vegetation regions, in steady gradually varied flow by laboratory experiment and theoretical analysis. The cylinders were used to simulate the stems of vegetation because of their shapes are similar.

　In this paper, the results were analyzed by the theory; flow without critical flow or hydraulic jump when through the emergent cylinders into zones of without emergent cylinders, maximum vegetation density was calculated. The application of the mean velocity formulas can be performed by experiment.

　The result of the analysis and experiments indicate that the flow resistance by flow through the emergent vegetation, the flow resistance increase with vegetation density. Hence, larger flow resistance by dense vegetation which can increase flow depth and decrease the flow velocity and conveyance. If the vegetation density is excessively, local subcritical flow and hydraulic jump occurred between the emergent cylinders and zones of without emergent cylinders to reduce bank stability. In order to enhance the bank stability, it should avoid the zones of without emergent vegetations in the downstream of emergent vegetations region, the method of calculating vegetation density can use the method suggested in this paper.

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