The 6 th Int. Conf. on Hydroscience and Engineering (ICHE-2004), May 30-June 3, Brisbane, Australia 1 VARIATIONS AND CORRELATION OF ROUGHNESS INDICES OF NON-VERTICAL NON-RIGID PARTIALLY SUBMERGED VEGETATION IN OPEN CHANNELS Abdel-Azim M. Negm 1 ABSTRACT This study presents the preliminary results of an experimental investigation on the effect of partially submerged non-vertical non-rigid cylindrical vegetation on the roughness indices in open channels. The considered roughness indices are the friction factor f, the Chezy’s roughness coefficient C, and the Manning’s roughness coefficient n. Three model were tested. The vegetation elements were distributed in staggered way. It was found that the trend of variation of the roughness indices with the Froude number are similar for different slopes. The values of the the roughness indices were found to be a function of Froude number F n , the channel bottom slope S, and the roughness density I. Statistical prediction models were developed to estimate the roughness indices in terms F n , S and I using the multiple linear regression analysis. Good agreement were obtained between predictions of the developed models and the measured values. 1. INTRODUCTION Management of rivers and canals flows necessitates sufficient information on the type of vegetation, its density, degree of submergence and degree of resistance to the flow. Such information may be obtained through different kinds of studies either by various methods of simulation and modeling techniques like those done by Shimizu et al. (1992a) and (1992b) , Shimizu & Tsujimoto (1994), Kutija & Hong (1996), Nezu & Onitsuka (1999), Choi (2000) and Fischer-Antze et al. (2001), or through field measurements carried out by Bakry et al. (1992) and Dolgopolova (1998), or achieved by experimental tests of roughness elements simulating the vegetation either in rigid state, such as those accomplished by Shimizu et al.(1991) and Tsujimoto et al.(1992), or for flexible case, such as that done by Kouwen and Unny (1973), Christensen (1985), EL-Hakim & Salama (1992) and El- Samman & Attia (1999) or for non-flexible no-rigid state which was covered by Fahti-Maghadam and Kouwen (1997) and Wu et al.(1999). The resistance of vegetation to the flow may be measured by one of the roughness indices as the friction factor (f), the Chezy’s roughness coefficient (C), Manning’s roughness coefficient (n) or the so-called retardance coefficient (N). This paper presents the results of an experimental study on the effect of using non-rigid cylindrical roughness simulating vegetation under the effect of 1 Professor of Hydraulics, Dept. of Water & Water Structures Eng., Faculty of Engineering, Zagazig University, Zagazig, Egypt. E-mail: amnegm85@yahoo.com