Analysis of Roughness Characteristics in Open Channel Flow due to Vegetation

Abstract

Roughness coefficients of the river are very important to accurately predict the flood level of the river. As river restoration projects are activated in Korea, if training an existing river to a close-to-nature river, it is very necessary to accurately predict vegetative roughness when determining roughness for the entire river channel. In this study, the quantitative variation of roughness coefficients depending on the characteristic factors by each vegetation is examined by performing hydraulic experiments for grass and woody vegetation. Based on this, the applicability of n-VR relationship typically presented for vegetative roughness in previous studies was reviewed and the effect of each new applied dimensionless parameter was analyzed based on the experimental data measured to replace VR and new evaluation equation for vegetative roughness coefficient n4 is presented. The results for vegetative roughness element n4 based on n-VR relation equation were reviewed by performing experiments based on various hydraulic conditions for grass vegetation such as Zoysia matrella, Pennisetum alopecuroides (L.) Spreng., Phragmites communis Trin., Phragmites japonica Steud., Miscanthus sacchariflorus (Maxim.) Benth. typically found in domestic rivers. According to the results of the review, the effect of relative depth and in vegetation rigidity depending could not be considered properly with a single n-VR relation equation. Therefore, in order to propose a new relation equation that can take the place of existing n-VR relationship in this study. Several dimensionless parameters are selected through the dimensional analysis. Those are MEI that can take into account the rigidity of vegetation simultaneously and considering height of vegetation as main characteristic factors, and stem Reynolds number which can consider the effect of diameter of vegetation for flow conditions. In this study, evaluation equations of vegetative roughness as a single relation equation are proposed in consideration of all of changes in the rigidity of vegetation due to seasonal distinction, relative inundation depth of vegetation, and influence of flow conditions by vegetation and coefficient of determination. of the presented relation equations are more than 0.76 and convenience, accuracy and applicability for evaluating vegetative roughness are improved compared to existing relation equation. Therefore, when comprehensively reviewing the equation presented for each vegetation in this study, relationship that has been traditionally used is considered to be replaced based on the results of this study. Stems with the canopy form were extracted from Salix gracilistyla Miq., Salix subfragilis Andersson, Salix koreensis Andersson found a lot in the Central Region of Korean peninsular in trees growing in river channels. The variation of biomass depending on inundation depth was identified by performing an experiment for various hydraulic conditions. The relationship between vegetative roughness element and dimensionless parameters , , selected through the dimensional analysis was examined as in grasses. According to the results of the review, when considering , for roughness coefficients, is more than 0.97, indicating that the influence of both parameters is very large but the increase of explanatory power depending on the addition of a dimensionless parameter is very small (0.00 or 0.01) so in the case of shrubs, the effect of for vegetative roughness was found to be insignificant in this study. Therefore, in the case of shrubs, the usability of the equations presented based on dimensionless parameters , is considered to be very high compared to the existing roughness selection method using references widely used now. However, the applicability of the presented evaluation equation of woody vegetative roughness was examined and as a result, it was found that although the fully submerged condition is implemented, the magnitude of total roughness keeps increasing so when considering the experimental conditions, the application range of the evaluation equations presented for woody vegetative roughness is , that is, it should be applied restrictively for the emergent condition and just submerged condition.