Study on influence of bank vegetation on river flood discharge
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Abstract:
Urban river channel is an important element in city economy and ecology development. Considering soil and water conservation, landscaping or ecological factors, the river bank is usually planted with grass, shrub and other vegetation. The plant coverage has changed the flow surface of the river bank, as well as the flood discharge capacity. Flood control and management is the most crucial factor for the planning of urban rivers, thus, it is quite necessary to study the influence of bank vegetation on river flood discharge.In order to analyze the effect of bank vegetation on river flood discharge, physical model experiments were carried out in laboratory. An experimental platform was built with the length of 40 m, the width of 8 to 12 m, the water depth of 4 to 8 cm and the width depth ratio of 100 to 300. The water levels of no vegetation, sparse vegetation, sparse vegetation + shrub and dense vegetation, were tested under typical flow. Then, the prototype parameters and water level difference corresponding to the tested conditions, were deduced according to the similarity principle. Mathematical analysis was further conducted on basis of the classical composite roughness calculation formulas, including Pavlovskij formula and Einstein-Banks formula. An equation was proposed to determine the increase of composite roughness while the river bank changed from no vegetation state to planted vegetation state. A practical procedure was also given to analyze the influence of bank vegetation on river flood discharge.The results showed that, the water level of the tested channel increased by 0.3 to 1.3 mm due to the dense vegetation, while it increased by 0.1 to 0.4 mm due to the sparse vegetation and sparse vegetation + shrub. The denser the bank vegetation, the higher the water level. For a river with the length of 400 m, the water depth of 0.4 to 0.8 m, and the roughness of 0.026, the water level increased by 2 to 15 mm. For a river with the length of 2200 m, the water depth of 2.2 to 4.4 m, and the roughness of 0.035, the water level increase could be large as 70 mm. When the river bank planted vegetation, the increase of composite roughness was related to two factors, namely, the ratio of the covered wetted perimeter to the total wetted perimeter and the roughness of the covered area. It increased with the growth of the roughness of the vegetation covered area, and decreased with the increase of width-depth ratio. While the width-depth ratio was larger than 60 and 100, the growth of composite roughness due to planted vegetation was no more than 10% and 7%, respectively.Urban river bank is usually planted with grass, shrub, or other vegetation, and the water level increases with the growth of the vegetation density. The water level growth can be large as 70 mm in the river length of 2200 m. The water level increase should be given special consideration. The composite roughness increase is related to the ratio of the planted wetted perimeter to the total and the roughness of the planted area. For the river whose width-depth ratio is larger than 100, the increase of composite roughness is no more than 7%. The formula was proposed to calculate the variation of composite roughness while the river bank vegetation changed. Further, an analysis procedure was given to guide further design.
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