Velocity distribution and Retardance Coefficient in Vegetated Channels

• Main Authors: Su-Pai, Kao, 高蘇白
• Other Authors: 陳彥璋
• Format: Others
• Language: en_US
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/e376bv

博士 === 國立臺北科技大學 === 工程科技研究所 === 97 === The vegetation in channels results in loss of energy and retardance of water flow. However, this vegetation helps to stabilize the slopes and bottoms of open channels. Recently, vegetated channels have been used to improve the surface water quality and to reduce the delivery of sediment and nutrient to rivers and swales. Therefore, it is important to understand the velocity distribution of vegetated channels. The velocity distribution above canopy is usually assumed to follow the well accepted logarithmic law, whereas the velocity distribution below canopy is expressed by exponential law or power law. However, logarithmic, exponential and power laws can be affected by some characteristics of vegetation resulting in the sufficiently inaccurate description of velocity distributions in vegetated channels. In this study, and velocity distribution equation based on probability is developed to simulate the velocity distribution of vegetated channels in the submerged condition. An extensive series of laboratory experiments planted with Egeria densa Planch in the channel bed of the flume are carried out to devise and validate the velocity distribution equation. The results indicate that the developed model of velocity distribution have the applicability and capability to simulate the velocity distribution effected by aquatic vegetation. The other important parameter of vegetated channel is retardance coefficient. Manning Equation is widely adopted to estimate open channel flows, and selecting retardance coefficient is always one of the most difficult task for estimating velocity and discharge. In cases of estimating accurate retardance coefficient values of vegetated channels, countless trial and error are to be made before reaching conclusive results due to conditions created by various aquatic plants. The majority of past studies on this subject, however, are established based on terrestrial plants and plastic moulds as laboratorial factors, and only few are done with natural aquatic vegetations. Hence, in this study, two different types of natural aquatic plants are applied to estimate retardance coefficients; and the result indicates that each type of plant affects differently in terms of flow resistance. Analysis of hydraulic parameters indicates there are strong correlations between the retardance coefficient and the Froude number. The Froude number is most important and commonly used parameter of open-channel hydraulics. The retardance coefficient and Froude number are exponentially related. Therefore, the Froude number can replaces the product of velocity and hydraulic radius (VR) to estimate the retardance coefficient, in which VR lacks a physical mean. Additionally, it can be used accurately to estimate velocity and discharge during river and wetland restoration.