沖積河流洪水平原之植物會增加水流阻力，因而改變超過滿岸水位之洪水流量分配，增加深槽流量，影響深槽河相演變與建造物安全。本研究旨在研究高水位時之複式斷面流量分配。
本研究提出摩擦能量坡度假設，即假設高水位時之深槽及洪水平原之摩擦能量坡度，與全斷面摩擦能量坡度相等，應用坡度-面積法理論發展植生複式斷面河槽流量分配演算模式，以應用於由洪水位推估洪水流量及深槽流量。
本研究進行植生洪水平原之複式斷面水工試驗，以分析摩擦能量坡度假設及植生複式斷面河槽流量分配演算法之適用性。
依據實驗結果之分析，深槽摩擦能量坡度與全斷面摩擦能量坡度相等之假設，雖會高估深槽摩擦能量坡度，而低估洪水平原摩擦能量坡度，因而高估深槽流量及全斷面流量，但由模式演算結果可看出全斷面流量與實驗值流量之比值約為1.0～1.5，而深槽流量與全斷面流量之比值，演算結果約為量測結果之1.0～1.3倍，摩擦能量坡度假設與演算模式仍具適用性。
本研究並分析洪水平原植生對摩擦因子及流量之影響。因莖幹間隙寬度影響水流之流通，此一流通特性以莖間流通率因子Fr表示，Fr之定義如下：
（3-2）
上式中，Bv為植生洪水平原寬度，nb為Bv寬度之植生莖幹數，d為莖幹直徑。由實驗結果分析，Fr與摩擦因子fv之關係如下：
（4-10）

The vegetation on the floodplains of alluvial streams can increase the flow resistance. Flood dominant discharge may thus change its distribution over the bankfull stage and effect mainstream geomorphology evolution and construction safety. The purpose of this research was to investigate the discharge distribution in compound channels of high water stage.
Slope-area method theory was applied to develop the discharge distribution calculation model in vegetated compound channels based on the assumption that the friction energy slope of mainstream and floodplain equals with that of the full cross-section during high water stage. Flood discharge and mainstream discharge could then be estimated.
This research also conducted laboratory experiments of vegetated compound channel to test the friction energy slope hypothesis and the discharge distribution calculation method. Based on the experimental results , although the mainstream discharge and the full cross-section discharge were overestimated due to overestimated mainstream friction energy slope and underestimated floodplain friction energy slope by assuming equivalence of friction energy slope , the suitability is satisfactory. Ratio of the full cross-section discharge and the experimental discharge by the calculation is approximately 1.0～1.5. The calculated ratio of the mainstream discharge and the full cross-section discharge shows approximately 1.0～1.3rd times of that of gauging data.
This research also analyzed the influences of floodplain vegetation on the friction factor and the discharge. Because the stem clearance width influences flow condition , this circulation conductivity factor can be indicated as：
（3-2）
In the above equation , Bv is vegetated floodplain width , nb is the vegetated stem number of width Bv , d is the stem diameter. From the experimental results , the relationship of Fr and the friction factor fv can be shown as follows：
（4-10）

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