A flood detention basin is one of the structural measures used for the mitigation of floods. Its design consists of determining the required flood storage and the dimensions of bottom outlet(s) and overflow spillway. Hydraulic performance of a detention basin is dictated with the achievement of the desired degree of attenuation which is affected by storage and capacities of outlet facilities. In this paper, a reliability based model is presented for the preliminary design of flood detention basins having dual outlets using a static loading-resistance interference. In the analysis, loading and resistance of the system are assumed as the maximum lake elevation and the crest elevation of the embankment, respectively. By examining the relevant statistical information for some existing flood detention basins in Turkey, the lognormal probability density function is observed to represent the randomness of loading and resistance. Use of the model is illustrated on a case study and a set of relationships are derived among reliability, attenuation, safety factor, and return period for various entrance elevations of bottom outlet and length to crest elevation ratios of overflow spillway, With the execution of the model, various alternatives can be compared from hydraulic conformity, economy, and safety viewpoints.