Damascus University Journal Vol. (27) - No. (1) 2011 Mohammed, Al-taee Al-Talib 47 Gravel Roughness and Channel Slope Effects on Rectangular Free Overfall Dr. Mwafaq Y. Mohammed 1 Ahmed Y. Al-taee 2 and Azza N. Al-Talib 3 Abstract The experiments where made in hydraulic laboratory, water resources Dept., Mosul University, Iraq. In this paper the effect of bed roughness for free overfall on flow characteristics in open channel are studied. The characteristics include, water depth over free overfall, critical and brink depth at lip of free overfall. The aim of the study is to know the effect of bed roughness of free overfall on flow depths (normal, critical and brink depth) we obtained a relation between these depths and compared the those with results without roughness for the same discharges. Six empirical formulas were found for describing the relationship between the critical and brink depths, and their relation to slope, channel bed roughness as well as to the of roughness distribution. The study shows that the full bed roughness at steeper slopes has greater effect. Keywords: Overfall, brink depth, bed roughness 1 Dept. of Water Resources Engineering, Coll. of Eng., Mosul Univ., Mosul, Iraq. 2 Dept. of Water Resources Engineering, Coll. of Eng., Mosul Univ., Mosul, Iraq. 3 Dept. of Water Resources Engineering, Coll. of Eng., Mosul Univ., Mosul, Iraq.