*Corresponding author’s e-mail: kn.ogbu@yahoo.com ASM Sc. J., 17, 2022 https://doi.org/10.32802/asmscj.2022.1008 Determination of Hydraulic Roughness Coefficients of Some Vegetated Species in Awka, Nigeria L.C Orakwe 1 , K.N Ogbu 1* , J.I Ubah 1 , C.P Nwachukwu 1 , R.C Akamonye 2 and U.P Okoro 3 1 Department of Agricultural and Bioresources Engineering, Nnamdi Azikiwe University PMB 5025, Awka, Nigeria 2 Department of Environmental Management, Glasgow Caledonian University, Cowcaddens Road, G4 OBA, Glasgow Scotland, United Kingdom 3 National Centre for Agricultural Mechanization, 240103 Ilorin, Nigeria In this study, selected types of grasses were studied to determine their hydraulic roughness coefficient and to select the most suitable grass for erosion control. The experiments were performed in twelve trapezoidal open channels measuring 5 m x 0.12 m x 0.03 m at different flow depths (0.001m, 0.002m, 0.003m, 0.004m and 0.005m) and at varying bed slope (0.2%, 0.3%, and 0.4%). Overall, Bahama grass showed the highest Manning’s n-value due to its deep root system and creeping nature. For each slope, the degree of submergence, Reynolds number, and flow depth increases as Manning’s n decreases. As the flow depth increases, the Reynolds number increased while the drag coefficient, Cd decreases. Keywords: Hydraulic Roughness Coefficient (n); Erosion; Discharge; Channel; Vegetation I. INTRODUCTION Soil erosion is known to be the major cause of environmental degradation in most developing countries. It appears to be the worst among the natural disasters especially in Nigeria (Onwuka et al., 2012). According to Ogunlela and Makonjuola (2000), soil erosion is simply the process of detachment, transportation, and deposition of soil particles (sediments) by erosion agents such as water and wind. It can be caused by both natural factors (water and wind) and human factors (e.g., man’s removal of the protective cover of vegetation). It causes continuing destruction of the fertile topsoil (contains plant nutrients and organic matter), thereby reducing agricultural productivity (Mahmoudzadeh, 2007). Vegetation can be used to control erosion due to its buttress and sprawling root systems that are responsible for increasing their resistance to erosion. Manning’s roughness coefficient (retardance co-efficient) n, is the property responsible for the flow resistance of vegetation. Vegetation roughness coefficients are the major parameters used to determine the flow characteristics which depend on the flow conditions (depth and velocity) and vegetation condition (type and density) (Ebrahimi et al., 2008). The vegetation in the channel influences channel flows and its extent of influence is based on the characteristics of the vegetation and the flow characteristics. The vegetation characteristics include; the vegetation species, degree of submergence (submerged or unsubmerged), density, distribution, and flexibility. The flow characteristics include; flow area, depth, and sidewalls of the channel. The velocity of flow is the main effect of vegetation in the channel, vegetation tends to increase the roughness or flow resistance or retardance (Fischenich, 2000). Rodney et al. (2011) stated that the roughness coefficient varies for different vegetation from season to season. In addition, the drag on the vegetation is known to be another important parameter that is related to flow resistance. Therefore, the main objective of this study is to evaluate the vegetation resistance in terms of Manning’s